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LEES 2021

International Conference on Low Electrodynamics in June 28 - July 8, 2021 Table of Contents

Code of Conduct ...... 3

Support Organizations...... 4-6

Program...... 7

Abstracts...... 15

• Monday, June 28...... 15

• Tuesday, June 29...... 22

• Wednesday, June 30...... 30

• Thursday, July 1...... 36

• Monday, July 5...... 42

• Tuesday, July 6...... 50

• Wednesday, July 7...... 57

• Thursday, July 8...... 64

2 Code of Conduct

The organizers of the LEES 2020 conference are fully committed to it being an inclusive event, and as organizers we will do everything in our power to make it a safe, productive, and welcoming space to all attendees. All participants, including , but not limited to, attendees, speakers, volunteers, exhibitors, -fac ulty, staff, students, service providers, and others are expected to abide by the LEES code of Conduct.

The mission of the LEES conference is to provide a forum for interdisciplinary research on low energy electrodynamics in solids and in exotic condensed phases. The discussions generated may arouse debate and strong disagreements may occur. Our goal is to encourage the discussion while maintaining respect for others.

All participants are expected to: • Treat each other with respect • Communicate openly with respect, critiquing ideas rather then individuals • Alert LEES organizers or staff if you see a dangerous situation or someone in distress

Unacceptable behavior includes but is not limited to: • Harassment, intimidation, or discrimination of anyone in any form • Verbal abuse in any form • Unwelcome sexual advances or comments • Intimidating or hostile comments or conduct

Consequences: Anyone requested to stop unacceptable behavior is expected to comply immediately. LEES staff (or their designee) or security/local police may take any action deemed necessary and appropriate, including im- mediate removal from the meeting without warning or refund.

All unacceptable behavior should be reported to the conference chair (Kenneth S. Burch), Staff, (Andrea Wherry) or members of the local organizing committee. Incidents may be reported via email or via the chat function in zoom.

3 Supported By

Thank you to the companies and organizations that have made LEES possible.

4 Supported By

TeraHertz

Modern Quality Control and Failure Analysis

 Quality Control Confirm the identity and quality of both raw materials as well as finished products. Also offers the possibility of examining coatings and layer thicknesses.

 Failure Analysis Determine the chemical reason behind product failure: Identify contaminations and detect wrong compositions.

 Surface Analysis Check technical cleanliness and analyze microscopic contaminants. Identify inclusions and pollutants to draw conclusions about the causes of failure.

 Reverse Engineering Investigate competitor´s products and obtain valuable information about the materials used.

Contact us for more details: www.bruker.com/optics

Spectroscopy Innovation with Integrity

5 Supported By

GigaHertz

MegaHertz

6 Program - Monday, June 28

*All times on the Program are Eastern Standard Time

Novel Magnets Chair: Kamaras Katalin 10:00 AM Evan Constable Vibronic processes in the quantum ice candidate Tb2Ti2O7

10:30 AM Rolando Valdes Aguilar Magnetic dynamics of honeycomb antiferromagnets

11:00 AM Natalia Drichko Time reversal and lattice symmetry breaking in Nd2Ir2O7 observed by Raman scattering spectroscopy

11:20 AM Itzik Kapon Magnetic field tuning of the valley population in the Weyl semi- of Nd$_2$Ir$_2$O$_7$ 11:40 AM Break 12:30 PM Poster Session 1 Berend Zwartsenberg Spin-orbit coupling effects in Sr2IrO4 Carina Belvin Revealing the soft electronic modes involved in magnetite’s Verwey transition Giorgio Sangiovanni Design and realization of topological Dirac on a triangular lattice

Kirill Amelin Experimental observation of E8 particles in Ising-chain compounds Martin Dressel Dielectric Catastrophe at the Mott Transition Nimi Bachar Unconventional free charge in the correlated Weyl Nd2Ir2O7 Severino Adler From Mott to Dirac Fermions via Van der Waals Stacking Shiming Lei Magnetic skyrmions in non-Gd based centrosymmetric magnet Valentina Brosco Rashba-metal to Mott- transition - spectral signatures Wibke Bronsch Time- and Angle-Resolved Photoemission Study on Bulk VSe2 Yuanyuan Xu Importance of dynamic lattice effects for crystal field excitations in quantum spin ice candidate Pr2Zr2O7 2:00 PM Long Break New Results in Mott Systems Chair: Dmitri Basov 7:00 PM Jie Shan in 2D moiré superlattices 7:30 PM Yiping Wang Modulation Doping via a 2D Crystalline Acceptor 8:00 PM Masahiro Sato Nonlinear Optical Responses in Quantum Spin 8:20 PM Shuqiu Wang Scattering Interference Signature of a Pair Density Wave State in the Cuprate Pseudogap Phase 8:40 PM Break

7 Tuesday, June 29

Novel Optics in 2D Materials Chair: Marc Scheffler 10:00 AM Ingrid Barcelos Probing in 2D Materials with Synchrotron Infrared Nanospectroscopy 10:30 AM Hanan Herzig Sheinfux Bound in the continuum modes in indirectly-patterned hyperbolic media

11:00 AM Jeremy Levy Gate-Tunable Optical Nonlinearities and Extinction in Graphene/LaAlO3/ SrTiO3 Nanostructures 11:30 AM Alexey Kuzmenko Infrared nanoscopy of polaritons in functional oxides and interfaces 11:50 AM Break 12:30 PM Poster Session 2 Amalia Coldea Anomalous magnetotransport of the nematic FeSe and related chalcogenides Claudio Giannetti Non-thermal metallic phase emerging from nanoscale complexity in a photo-excited Mott material Daniele Nicoletti Radiating Stripes Ivan Fotev Ultrafast Pump-Probe Spectroscopy of BaFe2As2 under High Pressures Jean-Côme Philippe Orbital dichotomy of Fermi properties in Sr2RuO4 probed by Raman spectroscopy Jonathan B. Curtis Spectroscopic signatures of time-reversal symmetry breaking Marco Marciani Resistivity anisotropy in nematic FeSe from multiorbital Boltzmann equation Michele Buzzi Photo-molecular high temperature superconductivity Min-Cheol Lee Ultrafast strain modulation of superconductivity in cuprate heterostructures Roberta Citro Topological superconductivity by orbital confinement in oxide nanowires 2:00 PM Long Break Unconventional Superconductivity 1 Chair: Setsuko Tajima 7:00 PM Johnpierre Paglione Exotic superconductivity in nearly ferromagnetic UTe2 7:30 PM Peter D. Johnson Time Reversal Symmetry Breaking in the FeTe1-xSex family of high Tc superconductors 8:00 PM Ryan Day The Three-Dimensional Electronic Structure of LiFeAs: Strong-coupling Superconductivity and Topology in the Iron Pnictides 8:20 PM Masamichi Nakajima Effects of electronic correlations and nematicity in FeSe1-xTex studied by optical spectroscopy 8:40 PM Break

8 Wednesday, June 30

Optics in Quantum Matter 1 Chair: Andrea Perucchi 10:30 AM Tommaso Cea , and superconductivity in twisted bilayer graphene 11:00 AM Milan Orlita Suppressed Auger scattering and tunable light emission of Landau- quantized massless Kane electrons 11:20 AM Prineha Narang Predicting Correlated Light-Matter Interactions 11:50 AM Méasson Marie-Aude Collective mode of the Hidden Order State in URu2Si2: Degeneracy and Symmetry 12:10 PM Long Break Magnetic Topological Systems Chair: N. Peter Armitage 7:00 PM Ilya Sochnikov Microscopy of tunable magnetic domains in noncentrosymmetric ferromagnetic 7:30 PM Shingo Toyoda Nonreciprocal second harmonic generation in a magnetoelectric CuB2O4

7:50 PM Changyoung Kim Sign-tunable anomalous induced by two-dimensional symmetry- protected nodal structures in ferromagnetic perovskite oxide thin films

8:20 PM Artem Strashko Crescent states in charge-imbalanced condensates 8:40 PM Break 9:00 PM Poster Session 3 Changmin Lee Observation of a within the domain walls of the magnetic Weyl semimetal Co3Sn2S2 Gael Grissonnanche T-linear resistivity from an isotropic Planckian scattering rate Jae Hoon Kim Terahertz Electrodynamics of Superconducting Nb Films in External Magnetic Field Kazuya Shinjo Effect of phase string on single-hole dynamics in the two-leg Hubbard ladder Ran Jing Terahertz response of monolayer and few-layer WTe_2 at the nanoscale Takahiro Ito Angle-resolved photoemission study of MAX phase compound Ti2SnC Yinming Shao Nonlinear Nano-electrodynamics of a Weyl semimetal Yue Sun Mapping Domain Wall Topology in the Magnetic Weyl Semimetal CeAlSi 10:30 PM Break/Poster Session Ends

9 Thursday, July 1

9:00 AM LEES 2022 Announcement 9:20 AM Genzel Prize Ceremony 9:30 AM Genzel Prize Invited Talk 10:00 AM Break 10:30 AM Poster Session 4 Bastien Michon The spectral weight of hole doped cuprates across the pseudogap critical point. Erik van Heumen Disentangling carrier density and relaxation in cuprate superconductors Fabio Boschini Non-monotonic interactions in the copper oxide plane Florence Observation of sub-gap superconducting modes with Kinetic Inductance Levy-Bertrand Detectors Francesco Barantani Temperature dependence of d-d in hole-doped cuprates Francesco Gabriele Coupled waves in layered cuprates Marco Grilli Strange metal behaviour from charge density fluctuations in cuprates Marta Zonno Ubiquitous suppression of the nodal coherent spectral weight in Bi-based cuprates Petr Adamus "Pseudogap in the c-axis (interplane) conductivity of a pair of t-J planes with a single doped hole" 12:00 PM Long Break Novel Magnets 2 Chair: Matteo Mitrano 7:00 PM Xiao-Xiao Zhang Ultrafast spin dynamics in 2D antiferromagnet 7:30 PM Md Mofazzel Hosen Anomalous Raman response of a Charge Density Wave in a high mobility, 2D antiferromagnet 7:50 PM Xiaodong Xu Intertwined Topological and Magnetic Orders in Atomically Thin Chern Magnet 8:20 PM In-Sang Yang Spin Excitation in Hexagonal LuMnO3 8:40 PM Break

10 Monday, July 5

Unconventional Superconductivity 2 Chair: Dmitrii Maslov 10:00 AM Nurit Avraham Visualization of Topological Boundary Modes Manifesting Topological Nodal-Point Superconductivity 10:30 AM Andrea Cavalleri Advances in Optically Driven Superconductivity 11:00 AM Denitsa Baykusheva Ultrafast of the onsite Coulomb repulsion in an underdoped cuprate 11:20 AM Stuart Brown Even parity superconducting state of the strongly correlated Fermi Liquid Sr2RuO4 11:50 AM Break 12:30 PM Poster Session 5 Alexander Boris Approaching 2D Superconductivity in Ultrathin DyBa2Cu3O7-δ Films Fabian Mooshammer Ultrafast low-energy dynamics of excitons in twisted van der Waals bilayers Federico Cilento Decoupling electronic and lattice contributions to the unconventional charge-density-wave transition of NbSe2 by time-resolved optical spectroscopy Götz Seibold Third harmonics generation from collective modes in disordered superconductors Jan Gospodaric Energy spectrum of semi-metallic HgTe quantum wells Miguel-Ángel Sánchez- Optical signatures of multifold fermions in the chiral topological Martínez RhSi and CoSi Riccardo Arpaia Doping evolution of charge density fluctuations across the entire dome of high-Tc superconductors Rocco Vitalone Cryogenic THz Nano Imaging and Spectroscopy Graphene/RuCl3 Heterostructures Grégory Setnikar Structural instability and superconducting state of V3Si: a Raman study Stefano Dal Conte Interlayer charge transfer and spin/valley dynamics in TMD heterostructures 2:00 PM Long Break Graphite, Graphene and Flat Bands Chair: Jason Hancock 7:00 PM MengXing Na Time-and Angle-Resolved Photoemission studies of electron- coupling in graphite 7:30 PM Yaxian Wang Ab initio signatures of phonon-mediated hydrodynamic transport in semimetals 7:50 PM Bohm Jung Yang Wave function geometry and anomalous Landau levels of flat bands 8:20 PM Sandeep Joy Transparent mirror effect in twist-angle-disordered bilayer graphene 8:40 PM Break

11 Tuesday, July 6

Correlated States 1 Chair: Dirk Van Der Marel 10:00 AM Prachi Sharma Optical conductivity of Dirac Fermi liquid 10:30 AM Fahad Mahmood Observation of a marginal Fermi glass using THz 2D coherent spectroscopy 10:50 AM Elena Bascones Correlated states in graphene based moiré systems 11:20 AM Sharareh Sayyad Pairing and non-Fermi liquid behavior in partially flat-band systems

11:40 AM Break 2:00 PM Long Break Topological Systems 1 Chair: Justin Wen 7:00 PM Robert Kirby Ultrafast Dynamics in the Topological Nodal-Line Semimetals ZrSiX (X = S, Se, Te)

7:30 PM Ming Yi Room-Temperature Topological Phase Transition in Quasi-1D Bi4I4 7:50 PM Ece Uykur Optical fingerprints of unconventional carriers in kagome 8:20 PM Graham Baker Non-local microwave electrodynamics in ultrapure PdCoO2 8:40 PM Break 9:00 PM Poster Session 6 Abhishek Kumar Zero-field electron spin resonance in graphene with proximity-induced spin- orbit coupling Alfred Zong Disentangling fluctuations from long-range order in a light-induced phase transition Calvin Pozderac Magnetoresistance from Guiding Center Drift of Two-Dimensional Electrons in a Moiré Potential Daniel J. Rizzo Charge-Transfer Polaritons at Graphene/α-RuCl3 Interfaces David Barbalas Deviations from Matthiessen’s rule in PdCoO2 Thin Films Dominik M. Juraschek Highly confined phonon polaritons in monolayers of oxide perovskites Kota Katsumi Photoexcited nonequilibrium state of underdoped YBa2Cu3Oy studied by the terahertz nonlinear optical responses Pavel Volkov Electronic phase diagram of the excitonic insulator candidates Ta2Ni(Se1- xSx)5 probed by Raman scattering Yang Yang Raman scattering in spin-orbit coupled Mott insulators: application to β-Li2IrO3 Yasunori Toda Systematic study of photoinduced dynamics in Bi-based cuprates with out-of-plane disorder

12 Wednesday, July 7

Correlated States 2 Chair: Natalia Perkins 10:00 AM Alain Sacuto Energy scales in cuprate superconductors revealed by electronic Raman spectroscopy 10:30 AM Sean Hartnoll Planckian electrons and phonons in strange metals 11:00 AM Avraham Klein A critical theory of quantum ferroelectric metals 11:30 AM Assa Auerbach Hall anomalies in strongly correlated metals and superconductors 11:50 AM Break 12:30 PM Poster Session 7 Dániel Datz Polariton-enhanced molecular absorption in boron nitride nanotubes: experiments and numerical calculations Hector Pablo Ojeda Emergent parametric resonances and time-crystal phases in driven BCS Collado systems Ivan Mohelsky Landau level spectroscopy of Bi2Te3 Jan Wyzula Magneto optical spectroscopy of Dirac nodal line semimetal NbAs2 Luca Tomarchio THz and Optical Spectroscopy Study of Magnetic Topological Materials Mark D. Thomson Towards a complete, broadband picture of collective modes in incommensurate charge-density-wave systems Mattia Udina THz pump-probe spectroscopy: instantaneous response and coherent oscillations Niklas Wagner Resistivity Exponents in 3D-Dirac Semimetals From Electron-Electron Interaction Philipp Eck Design and realization of a triangular QSHI: Indenene Stephan Bron Imaging the insulator-to-metal transition of thin-film VO2 with sSNOM and AFM IR Xuanbo Feng Elucidating the role of electronic correlations in Van der Waals materials VSe2 and Se doped TaS2 with optical spectroscopy 2:00 PM Long Break Nonlinearity in Quantum Matter 1 Chair: Mengkun Liu 7:00 PM Qiong Ma Geometry and Topology in 7:30 PM Bolin Liao Time-resolved Imaging of Photocarrier Dynamics with Scanning Ultrafast Electron Microscope 7:50 PM Darius Torchinsky Probing Weyl Semimetal Physics with Nonlinear Photocurrents 8:20 PM Zhiyuan Sun Light induced order parameter steering in excitonic insulators 8:40 PM Break

13 Thursday, July 8

Nonlinearity in Quantum Matter 2 Chair: Andrei Pimenov 10:00 AM Adolfo G. Grushin linear and nonlinear optics in chiral topological metals 10:30 AM Lara Benfatto Third-Harmonic Generation from plasma waves in cuprates 11:00 AM Angel Rubio Floquet and cavity QED materials engineering

11:30 AM Stefano Lupi Plasmons in Topological Materials and Highly Conductive Oxides: Linear and Non Linear Optical Properties

11:50 AM Break 12:30 PM Poster Session 8 Aaron Sternbach Programmable hyperbolic polaritons in Van der Waals Abhay Kumar Nayak Resolving topological classification through topological defects Adamya Prakhar Intrinsic Optical Absorption in Dirac Metals Goyal Alexey Shuvaev Solution of MIRO polarization immunity problem Artem Pronin Faraday rotation due to topological Hall effect in Mn2−xPtSn Dorri Halbertal Nano-optical studies of Moiré super-lattice domains in twisted bilayer graphene heterostructures Gergely Németh Ultrasensitive molecule detection via tip-launched graphene plasmons Rebecca Cervasio Probing thin films of functionalized materials by SR infrared and THz spectroscopy Tamaghna Hazra Band inversion and topology of the bulk electronic structure in FeSe_{0.45} Te_{0.55} Vaisakh Chelod Ultrafast plasmon thermalization in epitaxial graphene probed by time- Paingad resolved THz spectroscopy Yinan Dong Fizeau drag in graphene plasmonics 2:00 PM Long Break Novel Optics in 2D Materials 2 Chair: Michael Martin 7:00 PM Alexander S. McLeod Revealing nano-plasmonics in 2D materials and correlated oxides at variable temperatures 7:30 PM Liang Wu Direct imaging of the N\'eel vector switching in the monolayer antiferromagnet MnPSe$_3$ with strain-controlled Ising order 7:50 PM Ji-Hee Kim Carrier multiplication for next-generation solar cells 8:20 PM Erik Henriksen Cyclotron resonance spectroscopy in graphene and SmB6 8:40 PM Conference Concludes

14 Abstracts - Monday, June 28

Evan Constable

Vibronic processes in the quantum spin ice candidate Tb2Ti2O7

Authors: E.Constable, Y. Alexanian, K. Amelin, T. Room, U. Nagel, Z. Wang, L. Bergen, R. Ballou, J. Robert, C. Decorse, B. Langerome, M. Verseils, J.-B. Brubach, P. Roy, E. Lhotel, V. Simonet, A. Pimenov, S. Petit and S. deBrion

Abstract: The rare-earth pyrochlores (A2B2O7: A = rare earth element, B = group IV transition metal) express a diversity of exotic magnetism as a result of their frustrated lattice composed of corner-sharing tetrahedra. Notable examples include the spin-ice states of Ho2Ti2O7 and Dy2Ti2O7 among many other novel and interesting phases. Within this family, one of the most studied yet still poorly understood cases is Tb2Ti2O7 (TTO). Despite favourable long-range magnetic correlations, TTO exhibits no conventional spin-ice nor long-range magnetic order even down to very low temperatures (~ 50 mK). One novelty of TTO is its low-energy crystal electric-field (CEF) spectrum that defines the magnetism. A growing body of evidence suggests the peculiar magnetic behaviour of TTO is the result of entanglement between these CEF spin degrees of freedom and the dynamics of the frustrated lattice. The possibility that quantum fluctuations driven by spin-lattice effects could melt the magnetic order of TTO, forming a quantum spin-liquid phase, is an intriguing and highly debated subject. As I will discuss, this possibility is supported by a growing body of optical spectroscopic work including static THz and magneto-optical measurements. These results showcase a hybridization between two magnetic Tb3+ CEF transitions and neighbouring phonon modes. Their interpretation suggests this vibronic coupling provides the crucial path for normally forbidden quantum spin-flip fluctuations to occur across the CEF levels, thus promoting spin-liquid behaviour. ....

Rolando Valdes Aguilar

Magnetic dynamics of honeycomb antiferromagnets

Authors: Evan V. Jasper, Timothy DeLazzer, Kate Ross, Rolando Valdes Aguilar

Abstract: I will present a comprehensive study of the terahertz dynamics of the honeycomb antiferromagnet CoTiO3. This material has been found to host Dirac at the K-points similar to the electronic bands of graphene. We studied the bulk and surface response in this material using a new experimental technique of attenuated total reflection at terahertz frequencies. I will show results of these measurements on CoTiO3 and in other honeycomb antiferromagnets. ....

Natalia Drichko

Time reversal and lattice symmetry breaking in Nd2Ir2O7 observed by Raman scattering spectroscopy

Authors: Y. Xu, J. Teyssier, T. Ohtsuki, S. Nakatsuji, D. van der Marel, N. Drichko

Abstract: Nd2Ir2O7 is a semimetal compound, where a competition between spin-orbit coupling and electronic correlations leads to exotic physics. A Weyl semimetal state was suggested for this material in a narrow temperature range below a transition into the insulating state at 33 K. Using polarized Raman scattering spectroscopy to study this material, we can distinguish between magnetic excitations, and transitions over the electronic gap opened in the insulating regime. Following the evolution of these features on cooling, we can identify two temperature regimes in the insulating state. Between 33 K and approximately 15 K we observe an electronic gap and excitations related to the al-in-all-out (AIAO) order of iridium spins. Below 15 K, where Nd spins undergo AIAO ordering, the spectrum of magnetic excitations changes, and the phonon changes reveal the lattice deformation.

Acknowledgement: This work was supported as part of the Institute for Quantum Matter, an Energy Frontier Research Center

15 funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0019331 ....

Itzik Kapon

Magnetic field tuning of the valley population in the Weyl semi-metal phase of Nd$_2$Ir$_2$O$_7$

Authors: Itzik Kapon, Willem Rischau, Satoru Nakatsuji, Dirk van der Marel

Abstract: We report low-energy magneto-optical spectroscopy on the pyrochlore iridate Nd$_2$Ir$_2$O$_7$ [1]. Comparing our data together with that of published angle-resolved photo-emission [2], we deduce a factor four mass renormalization of the Ir d-electrons bands owing to strong correlation effects. We observe spectral changes induced by magnetic field below $T_ N^{Nd}\sim 13K$, {\em i.e.} for temperatures below which the Nd 4f electrons spin-orbital moments freeze. We accompany the experimental data with calculations of the optical conductivity using a band structure model of the Ir 5d electrons, which takes into account the coupling to the Nd magnetic moments and the polarization of those moments in an external magnetic field. We conclude that at least for fields higher than 2 Tesla the correlation gap at $E_F$ is closed and the material is a Weyl semi-metal with pairs of Weyl nodes along high symmetry directions of the cubic lattice. The magnetic field creates charge compensating pockets of holes and electrons in different regions of momentum space, thus introducing a valley population shift that can be tuned with a magnetic field.

[1] These experiments build on the work described in K. Wang, B. Xu, C. W. Rischau, N. Bachar, B. Michon, J. Teyssier, Y. Qiu, T. Ohtsuki, Bing Cheng, N. P. Armitage, S. Nakatsuji and D. van der Marel; Nature Physics 16, 1194 (2020).

[2] M. Nakayama et al., PRL 117, 056403 (2016). ....

Nimi Bachar

Unconventional free charge in the correlated Weyl semimetal Nd2Ir2O7

Authors: K. Wang, B. Xu, C. W. Rischau, N. Bachar, B. Michon, J. Teyssier, Y. Qiu, T. Ohtsuki, B. Cheng, N. P. Armitage, S. Nakatsuji & D. van der Marel

Nd2Ir2O7 is a correlated semimetal with the pyrochlore structure, in which competing spin–orbit coupling and electron–electron interactions are believed to induce a time-reversal symmetry-broken Weyl semimetal phase characterized by pairs of topologically protected Dirac points at the . However, the emergent properties in these materials are far from clear, and exotic new states of matter have been conjectured. Here, we demonstrate optically that, at low temperatures, the free carrier spectral weight is proportional to T^2, where T is the temperature, as expected for massless Dirac electrons. However, we do not observe the corresponding T^3 term in the specific heat. That the system is not in a Fermi liquid state is further corroborated by the charge carrier scattering rate approaching critical damping and the progressive opening of a correlation-induced gap at low temperatures. I will discuss the implications of these observations in regards to the electronic band structure of pyrochlore iridates. ....

Berend Zwartsenberg

Spin-orbit coupling effects in Sr2IrO4

Authors: B. Zwartsenberg, R. P. Day, M.X. Na, E. Razzoli, M. Michiardi, N. Xu, M. Shi, J. D. Denlinger, G. Cao, S. Calder, K. Ueda, J. Bertinshaw, H. Takagi, B. J. Kim, I. S. Elfimov & A. Damascelli Sr2IrO4 hosts an insulating state that is unexpected in the context of transition metal oxides. It has been proposed that spin-orbit coupling (SOC) is the key responsible property, as it entangles the t2g states, which enables a small Coulomb interaction to open a gap. This proposed mechanism would extend filling and bandwidth, the canonical control parameters for insulating behavior, to the relativistic domain. Naturally the question arises whether in this case, SOC can in fact drive the metal-insulator transition.

16 Abstract: In order to address this question, we have studied the behaviour of Sr2IrO4 when substituting Ir for Ru or Rh[1]. Both of these elements change the electronic structure and drive the system into a metallic state. A careful analysis of filling, bandwidth, and SOC, demonstrates that only \ac{SOC} can satisfactorily explain the transition. This establishes the importance of SOC in the description of metal-insulator transitions and stabilizing the insulating state in Sr2IrO4.

It has furthermore been proposed that the Sr2IrO4 realizes an effective pseudo-spin 1/2 model in due to entanglement of the t2g states into effective j=1/2 states. We test this hypothesis directly by measuring the spin-orbital entanglement using circularly polarized spin-ARPES[2]. Our results indicate that there is a drastic change in the spin-orbital entanglement throughout the Brillouin zone, implying that Sr2IrO4 can not simply be described as a pseudo-spin 1/2 insulator, casting doubt on previous work directly comparing to the cuprate superconductors.

We thus find that SOC is a crucial ingredient in the ground of Sr2IrO4, however, SOC is not strong enough to fully entangle into a jeff=1/2 state, requiring that Sr2IrO4 is described as a multi-orbital relativistic .

[1] Zwartsenberg et al., Nat. Phys (2020) [2] Zwartsenberg et al., submitted (2021) ....

Kirill Amelin, National Institute of Chemical Physics and Biophysics, Tallinn, Estonia

Experimental observation of E8 particles in Ising-chain compounds

Authors: K. Amelin, T. Rõõm, U. Nagel, J. Viirok, J. Engelmayer, Z. Zhang, X. Wang, H. Zou, J. Yang, T. Dey, A. A. Nugroho, T. Lorenz, J. Wu, Z. Wang

Abstract: Quantum criticality is accompanied by the emergence of exotic states of matter, which we expect to feature unconventional dynamic properties. It is often characterized by many-body fluctuations and complex symmetry. Describing a system near its is therefore an exceptionally difficult task, which makes exactly solvable models rare and especially valuable. One example of such a model is the one-dimensional transverse-field Ising chain, which, when perturbed by a small longitudinal field near the quantum critical point, is predicted to feature a spectrum consisting of eight particles with specific mass ratios. The spectrum is described by the quantum integrable field theory with the symmetry of the E8 Lie algebra. Two physical representations of the ferromagnetic and the antiferromagnetic quasi-one-dimensional Ising chains are, respectively, CoNb2O6 and BaCo2V2O8. We measured THz absorption spectra of both compounds below their ordering temperatures using a liquid--bath cryostat for BaCo2V2O8 and a dilution refrigerator for CoNb2O6. The necessary critical transverse field was generated using superconducting magnets, while the perturbative longitudinal field is provided by the interchain coupling intrinsically present in these materials. As the result, we were able to observe most of the eight particles, as well as their two- particle excitations, matching the mass ratios of the theoretical predictions. Additionally, we confirm numerical predictions that the two-particle excitations must manifest as sharp peaks rather than overwhelming continua, which obscure the high-energy single E8 particles. ....

Shiming Lei, Rice University

Magnetic skyrmions in non-Gd based centrosymmetric magnet

Author: Shiming Lei

Abstract: Magnetic skyrmions are particle-like spin textures of topological origin, which exists in the real space of materials. The intensive research activity on skyrmions has been driven not only by their fundamental interest in physics, but also by their potential applications to next-generation memory, logic, and neuromorphic computing devices. The magnetic materials that host skyrmions can be classified into two categories, non-centrosymmetric and centrosymmetric. So far, the majority of experimentally identified skyrmion hosting materials are non-centrosymmetric; only three centrosymmetric materials are experimentally verified to

17 host skyrmion lattice, and they have been limited to the Gd-based compounds, including: Gd2PdSi3, Gd3Ru4Al12, and Gd2Ru2Si2. In this talk, I will introduce our recent efforts towards the discovery of novel non-Gd-based centrosymmetric materials that host magnetic skyrmions. ....

Yuanyuan Xu, Johns Hopkins University

Importance of dynamic lattice effects for crystal field excitations in quantum spin ice candidate Pr2Zr2O7

Authors: Yuanyuan Xu, Huiyuan Man, Nan Tang, Santu Baidya, Hongbin Zhang, Satoru Nakatsuji, David Vanderbilt, Natalia Drichko

Abstract: Pr2Zr2O7 is a pyrochlore quantum spin-ice candidate. Using Raman scattering spectroscopy we probe crystal electric field excitations of Pr3+, and demonstrate the importance of their interactions with the lattice. We identify a vibronic interaction with a phonon that leads to a splitting of a doublet crystal field excitation at around 55 meV. We also probe a splitting of the non-Kramers ground state doublet of Pr3+ by observing a double line of the excitations to the first excited singlet state E0g→A1g. We show that the splitting has a strong temperature dependence, with the doublet structure most prominent between 50 K and 100 K, and the weight of one of the components strongly decreases on cooling. We suggest a static or dynamic deviation of Pr3+ from the position in the ideal can be the origin of the effect, with the deviation strongly decreasing at low temperatures. ....

Carina Belvin, Massachusetts Institute of Technology

Revealing the soft electronic modes involved in magnetite’s Verwey transition

Authors: Carina Belvin, Edoardo Baldini, Martin Rodriguez-Vega, Ilkem Ozge Ozel, Dominik Legut, Andrzej Kozłowski, Andrzej M. Oleś, Krzysztof Parlinski, Przemysław Piekarz, José Lorenzana, Gregory A. Fiete, Nuh Gedik

Abstract: The Verwey transition in magnetite (Fe3O4) is one of the most enigmatic phase transformations found in strongly correlated systems. It is the first metal-insulator transition ever observed experimentally and also involves a simultaneous charge ordering, orbital ordering, and a structural rearrangement. Due to the intricate interplay among these various degrees of freedom, a complete description of the microscopic mechanism of this phase transition is still lacking. Recently, the low-temperature charge- ordered structure was established as a network of three-site small called trimerons. However, the dynamics of the Verwey transition from an electronic point of view remains not understood since no collective excitations of this trimeron order have been observed to date. In this talk, I will present our discovery of the low-energy electronic collective modes of the trimeron network using terahertz spectroscopy. By exciting these modes coherently with an ultrashort near-infrared laser pulse, we unveil their critical softening towards the transition temperature, demonstrating their direct involvement in the Verwey transition. These findings represent the first observation of collective modes of any sort displaying a critical softening in magnetite and thus shed new light on the long-sought cooperative mechanism responsible for this transition to the exotic ground state of magnetite. ....

Valentina Brosco, Institute for Complex Systems, Consiglio Nazionale delle Ricerche, Rome, Italy

Rashba-metal to Mott-insulator transition - spectral signatures

Authors: Valentina Brosco and Massimo Capone

Abstract: The recent discovery of materials featuring strong Rashba spin-orbit coupling (RSOC) and strong raises questions about the interplay of Mott and Rashba physics. In this talk, we show that RSOC strongly favors metallic phases and it competes with Mott localization, affecting the physics of the metallic and Mott insulating phases and, consequently, of the Mott transition connecting the Rashba metal and Mott insulator. The crucial features of the metallic state and the mechanism behind the metallization are analyzed by different approaches providing a clear physical picture and identifying the spectral signatures of the Rashba-Mott interplay. Our results suggest the possibility of exploiting the tunability of Rashba spin-orbit coupling to control transport in strongly correlated materials and they

18 may be extremely relevant to account for the transport properties of correlated materials oxides heterostructures, surface alloys, and polar semiconductors. ....

Wibke Bronsch, Elettra Sincrotrone Trieste

Time- and Angle-Resolved Photoemission Study on Bulk VSe2

Authors: Wibke Bronsch, Manuel Tuniz, Denny Puntel, Alessandro Gianmarino, Fulvio Parmigiani and Federico Cilento

Abstract: By means of time- and angle-resolved photoemission spectroscopy (tr-ARPES), we investigate the effect of the phase transition into the charge density wave (CDW) phase on the equilibrium and out-of-equilibrium electronic band structure of the transition metal dichalcogenide VSe2. The electronic band structure of VSe2 has recently been subject of investigation ranging from the bulk to the monolayer regime, in search for the manifestation of the opening of the band gap in its CDW phase [1,2]. At present, few time-resolved studies on the effect of optical excitation are available on the ultrafast timescales [1,3]. In our contribution we present a study on the bulk material. By changing the polarization of the probe pulses tr-ARPES allow us to disentangle the d-like and p-like states from the V and Se valence bands respectively, and selectively probe their dynamics. We observe faster initial relaxation dynamics when probing the d-states with p-polarized light. When moving across the critical temperature for the CDW phase transition, our tr-ARPES data show indication for a change in the relative population of the bands close to the Fermi level which lasts for a few picoseconds after the photoexcitation.

[1] Biswas et al., Nano Letters 21, 1968-1975 (2021). [2] Umemoto et al., Nano Research 12, 165-169 (2019). [3] Majchrzak et al., arXiv:2011.06358v1 (2020). ....

Martin Dressel, Universität Stuttgart, 1. Physikalisches Institut

Dielectric Catastrophe at the Mott Transition

Authors: M. Dressel, R. Rösslhuber, Y. Saito, A. Pustogow, E. Uykur, Y. Tan and V. Dobosavljevic

Abstract: A Mott insulator becomes metallic when the effect of correlations decreases. In the case of full frustration, a first-order phase transition is expected up to some critical endpoint. Above Tcrit there might be some crossover and quantum critical behavior. But what happens below? We start with the k-(BEDT-TTF)2Cu2(CN)3 and tune the electronic bandwidth either by external pressure or by chemical substitution. DC resistivity confirms the insulator-metal transition (and even superconductivity in between). Optical spectroscopy reveals the development of the Drude-like contribution given evidence for coherent transport. Most surprising, however, is the divergency of the dielectric permittivity at the Mott insulator-to-metal transition at temperatures T < Tcrit = 20 K with eps1 = 10^5 and more. We show that the dielectric catastrophe is a fingerprint of the first-order phase transition where metallic and insulating regions coexist spatially. The percolative nature of the Mott transition dominates the low-frequency behavior compared to the closing of the Mott-Hubbard gap seen in the infrared range. All experimental trends are captured by dynamical mean-field theory of the single-band Hubbard model supplemented by percolation theory.

Ref.: A. Pustogow et al., npj Quantum Materials 6, 9 (2021); R. Rösslhuber et al., Phys. Rev. B 103, 125111 (2021); Y. Saito et al., arXiv:1911.06766 ....

19 Giorgio Sangiovanni, University of Wuerzburg / Institute for theoretical physics and astrophysics

Design and realization of topological Dirac fermions on a triangular lattice

Authors: Maximilian Bauernfeind, Jonas Erhardt, Philipp Eck, Pardeep K. Thakur, Judith Gabel, Tien-Lin Lee, Joerg Schaefer, Simon Moser, Domenico Di Sante, Ralph Claessen and Giorgio Sangiovanni

Abstract: The key to engineer large-gap quantum spin Hall insulators is a strong spin-orbit interaction. In Kane and Mele’s suggestion for honeycomb layers, SOC is promoted via a relatively weak second neighbor hopping process. Bismuthene however has impressively proven the superiority of local/atomic SOC of the in-plane p-orbitals. A new possibility arises by halving the bismuthene honeycomb lattice but enriching the orbital subspace, i. e. considering a full p-basis on a triangular lattice. Here, we conceive and realize for the first time a triangular QSHI, ‘’indenene’’, a triangular monolayer of indium on SiC(0001) exhibiting non-trivial valley physics as a consequence of strong local spin-orbit coupling. By means of tunneling microscopy of the 2D bulk we identify the quantum spin Hall phase of this triangular lattice, unveiling how a hidden honeycomb connectivity emerges from interference effects of Bloch $p_x\pm i p_y$-derived wave functions. Severino Adler, University of Würzburg / TU Wien From Mott to Dirac Fermions via Van der Waals Stacking José M. Pizarro, Severino Adler, Karim Zantout, Thomas Mertz, Paolo Barone, Roser Valentí, Giorgio Sangiovanni, and Tim O. Wehling Modelling Dirac systems via stacking of monolayers has captured great attention over the last decade. The standard approach is to stack almost uncorrelated materials to obtain topological non-trivial, sometimes moderatly correlated states. Here we take the opposite approach, stacking two mono-layers of 1T-TaSe2, which features a Mott-insoluting star-of-david (SOD) charge density wave, to obtain a topologically non-trivial and moderately correlated system. If stacked in a fashion such that the center of the SODs form a honeycomb lattice the bilayer can be well described via a Kane-Mele model, featuring sizeable spin-orbit coupling and local interaction strength. The system turns out to be close to a between a quantum spin Hall and an antiferromagnetic insulator and can be tuned into the a topologically trivial state with a perpendicular electric field. Further, we analyse the effect of an 180 degrees twisting angle on the topology, via considering a generalised version of the Kane-Mele model. ....

Jie Shan

Electrons in 2D semiconductor moiré superlattices

Author: Jie Shan

Abstract: When two van der Waals materials of slightly different orientations or lattice constants are overlaid, a moiré pattern emerges. The moiré pattern introduces a new length scale, many times the lattice constant of the original materials, for Bragg scattering of Bloch electrons in each layer. This gives rise to moiré minibands and rich emergent phenomena. In this talk I will discuss recent experiments on angle-aligned semiconductor heterobilayers, which exhibit remarkable correlated insulating states. ....

Yiping Wang

Modulation Doping via a 2D Crystalline Acceptor

Author: Yiping Wang

Abstract: Two-dimensional (2d) nano-electronics, plasmonics, and emergent phases require clean and local charge control, calling for layered, crystalline acceptors or donors. Here, we demonstrate that the large work function narrow-band Mott insulator RuCl3 enables modulation doping of exfoliated single and bilayer graphene. Our Raman measurements show high hole densities up to 1014cm-2 in RuCl3/mlg/RuCl3 heterostructures, 3*1013cm-2 in mlg RuCl3 heterostructures and 6*1013cm-2 in bilayer graphene (blg) RuCl3 heterostructures. In addition, our study suggest photovoltage devices, charge transfer control via twist angle, and

20 charge transfer through hexagonal boron nitride (hBN). ....

Masahiro Sato

Nonlinear Optical Responses in Quantum Spin Liquids

Authors: Minoru Kanega, Tatsuhiko N. Ikeda and Masahiro Sato

Abstract: Laser science has continuously stimulated the research for laser-driven phenomena and nonequilibrium physics in matters. Among phases in matters, quantum spin liquids (QSLs) are one of the most attractive many-body states and researchers have explored various experimental ways of hunting their nature. Most of thermodynamic (static) quantities of QSLs are featureless and dynamical observables can be often useful to detect their nature. The research of QSLs with laser is thereby an attractive research direction. In fact, the recent development of THz-laser science enables us to directly control magnetic excitations since THz photon energy is comparable to that of magnetic excitations. Under this background, recently we have theoretically tried to build a bridge between QSLs and THz laser. In this conference, I will focus on our theory for THz-laser driven high-harmonic generation (HHG) in QSLs. HHG is a simple nonlinear optical effect, and both its theory and experiment have been rapidly developed in the recent years. So far researchers have concentrated on the HHG in conducting electron systems because the light-charge coupling is strongest among light-matter interactions. However, HHG through the light-spin coupling is expected to be realized if applied THz laser is strong enough. It has been indeed observed in some experiments. We have developed the microscopic theory for THz-laser driven HHG in 1D QSLs and 2D Kiteav spin liquids. We show that some features of QSLs are revealed from the HHG spectra. In the conference, I will report our result of HHG by focusing on its essential points. ....

Shuqiu Wang

Scattering Interference Signature of a Pair Density Wave State in the Cuprate Pseudogap Phase

Authors: Shuqiu Wang, Peayush Choubey, Yi Xue Chong, Weijiong Chen, Wangping Ren, H. Eisaki, S. Uchida, P.J. Hirschfeld and J.C. Séamus Davis

Abstract: An unidentified quantum fluid designated the pseudogap (PG) phase is produced by electron-density depletion in the CuO2 antiferromagnetic insulator. Current theories suggest that the PG phase may be a pair density wave (PDW) state characterized by a spatially modulating density of electron pairs. Such a state should exhibit a periodically modulating energy gap Δ_P (r) in real- space, and a characteristic quasiparticle scattering interference (QPI) signature Λ_P (q) in wavevector space. By studying strongly underdoped Bi2Sr2CaDyCu2O8 at hole-density ~0.08 in the superconductive phase, we detect the 8a0-periodic Δ_1 (r) modulations signifying a PDW coexisting with superconductivity. Then, by visualizing the temperature dependence of electronic structure from the superconducting into the pseudogap phase, we find evolution of the scattering interference signature Λ_P (q) as predicted specifically for an 8a0-periodic PDW. These observations from the real- and wavevector-space are all consistent with a transition from a PDW state coexisting with d-wave superconductivity to a pure PDW state in the Bi2Sr2CaDyCu2O8 pseudogap phase.

Funding: Y.X.C. and J.C.S.D. acknowledge support from the Moore Foundation’s EPiQS Initiative through Grant GBMF9457. J.C.S.D. acknowledges from Science Foundation Ireland under Award SFI 17/RP/5445. W.C. and J.C.S.D. acknowledges support from the Royal Society through Award R64897. S.W., W.R. and J.C.S.D. acknowledge support from the European Research Council (ERC) under Award DLV-788932. ....

21 Tuesday, June 29

Ingrid Barcelos

Probing Polaritons in 2D Materials with Synchrotron Infrared Nanospectroscopy

Authors: Ingrid D. Barcelos, Hans A. Bechtel, Christiano J. S. de Matos, Dario A. Bahamon, Bernd Kaestner, Francisco C. B. Maia and Raul O. Freitas

Abstract: Polaritons, which are composed of a photon coupled to an electric or magnetic dipole, are a major focus in nanophotonic research of van der Waals (vdW) crystals and their derived 2D materials. For the variety of existing vdW materials, polaritons can be active in a broad range of the electromagnetic spectrum (meVs to eVs) and exhibit momenta much higher than the corresponding free-space radiation. Hence, the use of high momentum broadband sources or probes is imperative to excite those quasiparticles and measure the frequency-momentum dispersion relations, which provide insights into polariton dynamics. Synchrotron infrared nanospectroscopy (SINS) is a technique that combines the nanoscale spatial resolution of scattering-type scanning near-field optical microscopy with ultrabroadband synchrotron infrared radiation, making it highly suitable to probe and characterize a variety of vdW polaritons. Here, we present the advances enabled by SINS on the study of key photonic attributes of far- and mid-infrared plasmon- and phonon-polaritons in vdW and 2D crystals. ....

Hanan Herzig Sheinfux

Bound in the continuum modes in indirectly-patterned hyperbolic media

Authors: Hanan Herzig Sheinfux, Lorenzo Orsini, Minwoo Jung, Iacopo Torre, Matteo Ceccanti, Rinu Maniyara, David Barcons Ruiz, Sebastian Castilla, Niels C.H. Hesp, Eli Janzen, Valerio Pruneri, James H. Edgar, Gennady Shvets, Frank H.L. Koppens

Abstract: The general premise of nanophotonics involves shrinking light to the subwavelength nanometric scale. By compressing light into the volume of a nanocavity, the interaction of light with matter is dramatically enhanced. Recent innovations in nanocavity design have pushed the volume of such cavities deep into the nanoscale, but shrinking light typically comes at a cost – absorption losses, which plague all existing nanocavity designs and the typical quality factor of all such cavities is on the order of 10. A possible route to strong nanoscale confinement lies with hyperbolic Phonon polaritons and in particular using indirectly defined cavities. However, doing so requires a different approach to confinement, inspired by bound in continuum mode physics, and bound modes in the continuum modes have never been observed in nanophotonics. Here, we bring the BIC concept to the nanoscale and use it to achieve record breaking levels of optical confinement. We introduce and demonstrate a novel multimodal reflection mechanism of the ray-like optical excitations in hyperbolic materials. Using near-field microscopy, we observe mid-IR confinement in BIC- based nanocavities with volumes down to 23x23x3 and quality factors above 100 – a dramatic improvement in several metrics of confinement. This alliance of HyM with BICs yields a radically novel way to confine light and is expected to have far reaching consequences wherever strong optical confinement is utilized, from ultra-strong light-matter interactions, to mid-IR nonlinear optics and a range of sensing applications. ....

Jeremy Levy

Gate-Tunable Optical Nonlinearities and Extinction in Graphene/LaAlO3/SrTiO3 Nanostructures

Authors: Erin Sheridan, Lu Chen, Jianan Li, Qing Guo, Shan Hao, Muqing Yu, Ki-Tae Eom, Hyungwoo Lee, Jung-Woo Lee, Chang-Beom Eom, Patrick Irvin, Jeremy Levy

Abstract: We explore the ultrafast optical response of graphene subjected to intense (~106 V/cm) local (~10 nm) electric fields. Nanoscale gating of graphene is achieved using a voltage-biased, SrTiO3-based conductive nanowire junction “written” directly

22 under the graphene and isolated from it by an insulating ultrathin (<2 nm) LaAlO3 barrier. Upon illumination with ultrafast visible- to-near-infrared (VIS-NIR) light pulses, the local field from the nanojunction creates a strong gate-tunable second-order nonlinearity in the graphene and produces a substantial difference-frequency (DFG) and sum-frequency generation (SFG) response detected by the nanojunction. Spectrally sharp, gate-tunable extinction features (>99.9%) are observed in the VIS-NIR and SFG spectral ranges, in parameter regimes that are positively correlated with the enhanced nonlinear response. The observed graphene-light interaction and nonlinear response are of fundamental interest and open the way for future exploitation in graphene-based optical devices such as phase shifters, modulators, and nanoscale THz sources.

Reference: https://pubs.acs.org/doi/10.1021/acs.nanolett.0c01379 ....

Alexey Kuzmenko

Infrared nanoscopy of polaritons in functional oxides and interfaces

Authors: Y. Zhou, A. Bercher, A. Waelchli, W. Luo, A. M. Boselli, J.-M. Poumirol, I. Ardizzone, J. Teyssier, D. van der Marel, S. Gariglio, J.-M. Triscone and A.B. Kuzmenko

Abstract: FunctionalqQuantum oxides exhibit a remarkable variety of ordering phenomena that can be useful for applications, such as (multi-)ferroicity, superconductivity and metal-insulator transitions. Typically, near the ordering temperature, different phases coexist at the nanoscale. Furthermore, some of the important effects appear at the nanometer-thick interfaces between the different oxide layers, exemplified by the famous LAO/STO system ([1-3]). Infrared spectroscopy probes charge, spin and lattice dynamics and therefore is extremely useful to elucidate the physical origin of various ordered phases. However, conventional infrared measurements are diffraction limited and do not provide nanoscale resolution. On the other hand, the technique of scattering-type scanning near-field optical microscopy (s-SNOM) [4], based on the near-field interaction between an atomic-force microscope (AFM) tip and the sample, overcomes the diffraction limit, while keeping all the advantages of conventional spectroscopy. Recently, it has been extended to broad-temperature range operation down to 5 K, opening the opportunity to optically examine the mentioned phenomena with nanoscale resolution.

In this contribution, we will present SNOM mapping of LAO/STO systems and nanostructures [5,6], as a function of temperature, the laser wavelength and the gate voltage. We find that the s-SNOM signal and polariton propgation are highly sensitive to the density and the mobility of the charge carriers in the 2DEG, which is explained by the formation of coupled plasmon-phonon polariton modes at the interface. We also demonstrate that SNOM can be used to map spatially inhomogeneous structures, such as AFM-written conducting wires, lithographically made nanostructures and domain walls in the materials potentially useful in oxide electronics.

References: [1] A. Ohtomo and H.Y. Hwang, Nature 427, 423 (2004). [2] S. Thiel, G. Hammerl, A. Schmehl, C.W. Schneider and J. Mannhart, Science 313, 1942 (2006). [3] A.D. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart and J.M. Triscone, Nature 456, 624 (2008). [4] F. Keilmann and R. Hillenbrand, Phi. Trans. R. Soc. Lond. A 362, 787 (2004). [5] W. Luo, M. Boselli, J.-M. Poumirol, I. Ardizzone, J. Teyssier, D. van der Marel, S. Gariglio, J.-M. Triscone and A.B. Kuzmenko, Nature Communications 10, 2774 (2019). [6] M. Boselli, G. Scheerer, M. Filippone, W. Luo, A. Waelchli, A. B. Kuzmenko, S. Gariglio, T. Giamarchi and J.-M. Triscone, Phys. Rev. B, in press (2021), arXiv:2009.07867. ....

23 Amalia Coldea

Anomalous magnetotransport of the nematic FeSe and related chalcogenides

Author: Amalia Coldea

Abstract: A nematic electronic state that breaks the rotational symmetry of the lattice can potentially promote unique conditions for anomalous scattering relevant for superconductivity. Here, I present magnetotransport behaviour of the superconducting FeSe S across a nematic phase transition probed in magnetic fields up to 38T. Inside the nematic phase, the transverse magnetoresistance is very large and anomalous and the Hall coefficient changes sign both as a function of temperature and substitution, beyond the expectation for a two-band model system. A region of linear resistivity is detected inside the nematic phase, where spin fluctuations are present. Near the nematic end point, the resistivity displays a T dependence [1,2,3] and the divergent critical fluctuations are suppressed by a finite nematoelastic coupling [2]. We discuss the role played by the high mobility bands and the scattering processes with spin fluctuations, phonos and impurities on the overall magnetotransport behaviour of FeSe S [3]. I will also compare the behaviour of bulk single crystals as compared to FeSe thin flakes [4].

[1] M. Bristow et al., PRR 2,013309 (2020); [2] P. Reiss et al., Nat. Phys. 16,89 (2020); [3] A.I. Coldea, arXiv:2009.05523 (2020); https://doi.org/10.3389/fphy.2020.594500 [4] L. Farrar et al., in preparation (2021); ....

Roberta Citro

Topological superconductivity by orbital confinement in oxide nanowires

Authors: J. Settino , C. A. Perroni, V. Cataudella, M. Cuoco and R. Citro

Abstract: We determine the conditions to achieve topological superconductivity in an oxide nanowire in the presence of an in- plane external magnetic field. We show that lateral confinement introduces a splitting of the d orbitals that alters the orbital energy hierarchy and significantly affects the topological phase diagram. The increase of the nanowire thickness leads to dense distribution of topologically nontrivial domains when orbital population inversion takes place. We also demonstrate that the electronic structure exhibits an orbital-dependent magnetic anisotropy which affects the character of theMajorana bound states (MBSs) and gives rise to a rich behavior of spin-orbital polarization. These findings highlight the strong interplay of spin-orbital degrees of freedom due to the competition of the orbital Rashba interaction, atomic spin-orbit coupling, and structural distortions in oxides. ....

Marco Marciani

Resistivity anisotropy in nematic FeSe from multiorbital Boltzmann equation

Authors: Marco Marciani and Lara Benfatto

Abstract: Iron pnictides have drawn the attention of the high-Tc superconductor community due to the peculiar interplay between magnetism, superconductivity and electronic nematicity. To better single out such physics many studies have focused on the normal phase of these materials. A still debated issue is the nature of the structural transition in FeSe. Indeed, the marked anisotropy of the transport properties and the reconstruction of the Fermi surface at Ts=90 K clearly point towards an electronic nature of the order. Nonetheless, the exact form of the electronic order parameter is still quite controversial. Here I will show how bulk transport properties can be used to discriminate among different models for the nematic order[1]. To this end, I will present first the analytical solution of a generic multiorbital Boltzmann equation including scattering from impurities. Secondly I will discuss its implications within different scenarios for the nematic phase. Interestingly, a recent proposal[2] invoking d_xy orbital order along with the well-established xz/yz orbital splitting turns out to provide an excellent description of the measured resistivity anisotropy in experiments[3].

24 [1] M. Marciani and L. Benfatto, preprint 2021 [2] L. C. Rhodes, J. Böker, M. A. Müller, M. Eschrig, I. M. Eremin, arXiv:2009.00507 (2020). [3] M. A. Tanatar, A. E. Böhmer, E. I. Timmons, M. Schütt, G. Drachuck, V. Taufour, K. Kothapalli, A. Kreyssig, S. L. Bud’ko, P. C. Canfield, R. M. Fernandes, and R. Prozorov, PRL 117, 127001 (2016). ....

Jean-Côme Philippe

Orbital dichotomy of Fermi liquid properties in Sr2RuO4 probed by Raman spectroscopy

Authors: Jean-Côme Philippe, Benoît Baptiste, Chanchal Sow, Yoshiteru Maeno, Anne Forget, Dorothée Colson, Maximilien Cazayous, Alain Sacuto & Yann Gallais

Abstract: We report a polarization-resolved Raman spectroscopy study of the orbital dependence of the quasiparticles properties in the prototypical multi-band Fermi liquid Sr2RuO4. We show that the quasiparticle scattering rate displays omega^2 dependence as expected for a Fermi liquid. Besides, we observe a clear polarization-dependence in the energy and temperature dependence of the quasiparticle scattering rate and mass, with the dxz/dyz orbital derived quasiparticles showing significantly more robust Fermi liquid properties, than the dxy orbital derived ones. The observed orbital dichotomy of the quasiparticles is consistent with the picture of Sr2RuO4 as a Hund's metal. Our study establishes Raman scattering as a powerful probe of Fermi liquid properties in correlated metals. ....

Jonathan B. Curtis

Spectroscopic signatures of time-reversal symmetry breaking superconductivity

Authors: Nicholas R. Poniatowski, Jonathan B. Curtis, Amir Yacoby, Prineha Narang

Abstract: The collective mode spectrum of a symmetry-breaking state, such as a superconductor, provides crucial insight into the nature of the order parameter. In this context, we present a microscopic weak-coupling theory for the collective modes of a generic multi-component time-reversal symmetry breaking superconductor, and show that fluctuations in the relative amplitude and phase of the two order parameter components are well-defined underdamped collective modes, even in the presence of nodal quasiparticles. We then demonstrate that these "generalized clapping modes" can be detected using a number of experimental techniques including ac electronic measurements, ultrafast THz spectroscopy, and microwave power absorption. Finally, we discuss the implications of our work as a new form of "collective mode spectroscopy" that drastically expands the number of experimental probes capable of detecting time-reversal symmetry breaking in unconventional superconductors such as Sr2RuO4, UTe2, and moire heterostructures. ....

Ivan Fotev

Ultrafast Pump-Probe Spectroscopy of BaFe2As2 under High Pressures

Authors: Ivan Fotev, Saicharan Aswartham, Bernd Büchner, Harald Schneider, Manfred Helm, and Alexej Pashkin

Abstract: An important member of the iron-based high-temperature superconductor family, BaFe2As2 undergoes a transition to a spin-density wave (SDW) state on cooling below Tsdw=137 K. Under application of external pressure Tsdw gradually decreases, and the SDW phase gets completely suppressed above 3 GPa, enabling the onset of superconductivity. Such phenomenon is often referred to as quantum phase transition. Optical pump-probe spectroscopy has been used previously to investigate the dynamics of the SDW and the superconducting order at various temperatures and different doping levels. However, a direct study of the quantum phase transition induced by external high pressure has not been carried out until now. In our study, pump and probe pulses with the central wavelength of 800 nm were focused onto the sample inside the diamond anvil

25 cell, mounted inside a cryostat. The reflected probe signal was collected using a confocal microscopy scheme. From the measured pump-probe traces the quasiparticle lifetimes and the condensation energy of the SDW state were obtained at various pressures up to 4.4 GPa and the fixed temperature of 8 K. The SDW condensation energy decreases with pressure, and already below 4.4 GPa the SDW state is completely suppressed. At the same time, the decrease of the condensation energy is accompanied by the increase of the quasiparticle lifetimes. Since the lifetimes should be inversely proportional to the SDW gap energy, this critical slowing down of the quasiparticle relaxation dynamics confirms the vanishing of the SDW gap at the quantum phase transition. ....

Daniele Nicoletti

Radiating Stripes

Authors: Daniele Nicoletti, Michele Buzzi, Pavel Dolgirev, Marios Michael, Eugene Demler, Andrea Cavalleri

Abstract: In superconductors, terahertz waves can be generated upon illumination with femtosecond optical pulses, as a consequence of the ultrafast modulation of supercurrents. This phenomenon has been typically observed in presence of an external magnetic field or by applying a current bias. Here, we report the detection of a clear THz emission signal in the superconducting state of photoexcited single-layer cuprates, in total absence of magnetic fields or external biases. This emitted THz field is polarized along the out-of-plane crystallographic axis and its quasi-monochromatic spectrum is peaked at the Josephson Plasma frequency. Importantly, a sizeable THz emission is only observed in cuprates with a robust stripe phase which coexists with superconductivity, as La2-xBaxCuO4, while no such signal was found in “homogeneous” superconductors with no stripes (e.g., optimally-doped La2- xSrxCuO4). The intimate relation between THz emission and stripes is confirmed by the observation that the lifetime of the emitted plasma oscillations scales with the intensity and correlation length of the charge order modulation, reaching its maximum coherence in compounds close to 1/8 doping. In order to explain this newly discovered phenomenon, we propose a scenario in which Josephson plasmons are excited impulsively by our femtosecond optical pulses, and THz emission at the plasma frequency is enabled through the symmetry breaking originated by the stripe phase. Our results highlight a further peculiarity in the out-of-equilibrium optical response of striped superconductors, for which an anomalous THz third harmonic generation, as well as optically-enhanced superconductivity had already been reported. ....

Michele Buzzi

Photo-molecular high temperature superconductivity

Authors: M. Buzzi, D. Nicoletti, M. Fechner, N. Tancogne-Dejean, M. A. Sentef, A. Georges, T. Biesner, E. Uykur, M. Dressel, A. Henderson, T. Siegrist, J. A. Schlueter, K. Miyagawa, K. Kanoda, M.-S. Nam, A. Ardavan, J. Coulthard, J. Tindall, F. Schlawin, D. Jaksch, and A. Cavalleri

Abstract: The non-equilibrium control of emergent phenomena in solids is an important research frontier, encompassing effects such as the optical enhancement of superconductivity. In high-Tc cuprates and alkali doped fullerides, resonant driving of specific phonon modes with mid-infrared pulses has been shown to induce transient optical properties reminiscent of superconductivity for temperatures far above the equilibrium Tc. Here, we discuss an analogous effect in the quasi two-dimensional organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br. At equilibrium, this material bears similarities to high-Tc cuprates, showing unconventional superconductivity that emerges from a correlated metallic state with a Tc of 11 K, in the vicinity of a Mott insulating phase. In this experiment, we resonantly excited local vibrational modes of the BEDT-TTF molecule using mid-infrared pulses. Starting from the equilibrium metallic state at T >> TC, we observed a photo-induced response compatible with that of a transient superconductor, which was strongly reduced when the pump wavelength was tuned away from the vibrational resonances. Our results suggest that optically-enhanced superconductivity might be a far broader phenomenon than previously envisaged, thus opening new opportunities and challenges for both theoretical and experimental investigations. ....

26 Min-Cheol Lee

Ultrafast strain modulation of superconductivity in cuprate heterostructures

Authors: Min-Cheol Lee

Abstract: Modulating the lattice structure provides a unique pathway for controlling superconductivity, especially on ultrafast timescales. For instance, it has been recently revealed that photoexcitation resonant to specific c-axis phonon modes yields a transient superconducting-like state above the equilibrium critical temperature. In this presentation, we report on ultrafast optical experiments using femtosecond near-infrared pulses to excite a c-axis strain wave in heterostructures, composed of epitaxial thin films of metallic SrRuO3 (SRO) and c-axis oriented YBa2Cu3O7 (YBCO) superconducting layers. By pumping the sample with ultrashort optical pulses, a coherent strain wave in SRO is launched and propagates into YBCO, modulating the lattice and electronic properties of the superconducting film. Time-resolved x-ray diffraction (TR-XRD) experiments captured the relevant c-axis lattice modulations in the YBCO layer. Time-resolved terahertz transmittance measurements probed the optical conductivity at different times and were compared to our TR-XRD results, demonstrating that the conductivity is modified by optically driven strain waves. Our results reveal that c-axis lattice modulation is critical for controlling superconductivity in a cuprate. ....

Claudio Giannetti

Non-thermal metallic phase emerging from nanoscale complexity in a photo-excited Mott material

Authors: Andrea Ronchi, Paolo Franceschini, Andrea De Poli, Pia Homm, Ann Fitzpatrick, Francesco Maccherozzi, Gabriele Ferrini, Francesco Banfi, Sarnjeet S. Dhesi, Mariela Menghini, Michele Fabrizio, Jean-Pierre Locquet and Claudio Giannetti

Abstract: The interplay between electronic and lattice degrees of freedom in correlated materials often leads to spontaneous nanoscale architectures, which can favour and stabilize photoinduced emergent states with no counterpart at equilibrium. State- of-the-art light excitation protocols offer space-integrated information, which are insufficient to link and control the temporal and real-space dynamics of non-equilibrium states. Here, we report time-resolved photoemission microscopy experiments on a V₂O₃ thin film, which at equilibrium undergoes a transition at T≈140 K from intrinsically nanotextured monoclinic insulator to homogeneous corundum metal. We demonstrate that the excitation with infrared light pulses turns the low-T insulating phase into a non-thermal metallic state that retains the monoclinic in-plane shear strain. Mean-field modelling shows that the topology of the monoclinic nanotexture is key for stabilizing the emergent photo-induced metal state. Engineering the nanotexture of insulating strained domains may thus constitute a new tool to control non-thermal phases in correlated materials. ....

Johnpierre Paglione

Exotic superconductivity in nearly ferromagnetic UTe2

Authors: Johnpierre Paglione

Abstract: Topological superconductivity has attracted great interest in because of its potential applications in quantum computing. Spin-triplet superconductors are one promising class that can host the topological excitations of interest, but experimental realizations are few and far between. Here we report the discovery and properties of superconductivity in UTe2, a material closely related to known ferromagnetic superconductors such as UGe2, URhGe, and UCoGe, but lacking long-range magnetic order. Several experimentally measured properties feature telltale indications of an unconventional energy gap and a spin- triplet pairing state that is consistent with the presence of strong magnetic fluctuations due to an incipient quantum critical point. Furthermore, the superconductivity in UTe2 is remarkably robust to extremely high magnetic fields, showing re-entrant pairing up to at least 65 Tesla. I will review basic properties and our detailed investigations of the gap structure, relation to incipient magnetic order and Kondo coherence, as well as indications of an anomalous normal state fluid that suggest many surprises await for this exotic material. ....

27 Peter D. Johnson

Time Reversal Symmetry Breaking in the FeTe1-xSex family of high Tc superconductors

Authors: Peter D. Johnson

Abstract: Laser-based ARPES with variable light polarization offers a powerful probe of the electronic structure near the center of the Brillouin zone. Here the technique is used to examine the Fe-based superconductor family, FeTe1-xSex. At the zone center we observe the presence of Dirac cones with helical spin structure as expected for topological surface states and as previously reported in the related FeTe0.55Se0.45.[1,2] At the bulk superconducting transition a gaps are observed to open both at the and at the Dirac point in the topological surface state. With a single Dirac cone at the center of the zone, the associated mass acquisition at the Dirac point is indicative of time reversal symmetry breaking which in turn suggests the potential formation of in the surface layer. Such intrinsic ferromagnetism combined with strong spin-orbit coupling provides an ideal platform for the Quantum Anomalous Hall effect. Doping with Ni to obtain Fe1-xNixTe0.55Se0.45 allows us to modify the magnetic and topological properties. The observations are discussed within the context of phase models previously used in the analysis of Shockley surface states.

[1] P. Zhang et al., Science 360, 182 (2018) [2] J. Rameau et al., Phys. Rev. B., 99, 205117 (2019) ....

Ryan Day

The Three-Dimensional Electronic Structure of LiFeAs: Strong-coupling Superconductivity and Topology in the Iron Pnictides

Authors: RP Day, MX Na, M Zingl, B Zwartsenberg, S Zhdanovich, T Pederson, S Gorovikov, M Michiardi, M Schneider, D Wong, P Dosanjh, G Levy, S Chi, R Liang, WN Hardy, DA Bonn, IS Elfimov, A Damascelli

Abstract: In the present discourse surrounding Fe-based superconductors, two broadly distinct research programs have evolved. While a considerable effort continues to be applied towards understanding the fundamentals of bulk superconductivity in this expansive family of materials, a nearly orthogonal avenue is directed towards the topic of topology. As a high purity material devoid of complications associated with disorder, magnetism and structural distortions, LiFeAs plays a central role in connection to both research programs.

However, the bulk electronic structure of LiFeAs reported in the literature is highly contentious. In this work, we combine polarization- and photon-energy dependent photoemission measurements with realistic numerical simulations thereof, to unravel the convoluted experimental signatures which have been interpreted previously towards conflicting descriptions of this material. Doing so, we identify an electronic structure which cannot easily be reconciled with the prevailing description of the bulk superconductivity in LiFeAs as a weak-coupling Fermi surface instability. At the same time, these results provide support and justification to intimations of a topological surface state in LiFeAs. Although tunnelling spectroscopy has indicated that any proximitized superconductivity on the surface state is topologically non-trivial, we provide a mechanism by which such topological superconductivity may be stabilized in LiFeAs. ....

Masamichi Nakajima

Effects of electronic correlations and nematicity in FeSe1-xTex studied by optical spectroscopy

Authors: M. Nakajima, K. Yanase, M. Kawai, D. Asami, T. Ishikawa, F. Nabeshima, Y. Imai, A. Maeda, and S. Tajima

Abstract: Electronic correlations and nematicity is of fundamental importance to iron-based superconductors. In the present study, we systematically measured in-plane optical conductivity of FeSe1-xTex thin films fabricated on CaF2 substrates for x = 0, 0.1, 0.2, and 0.4. This system shows a large enhancement of superconducting transition temperature Tc at x ~ 0.2 and a gentle decrease in Tc with further increasing x. The low-energy optical conductivity spectrum is described by the sum of narrow and broad Drude

28 components, associated with coherent and incoherent charge dynamics, respectively. With increasing Te content, the spectral weight of the narrow Drude component decreases, while the total weight of the two Drude components increases. As a consequence, the fraction of the narrow Drude weight significantly decreases, indicating that Te substitution leads to stronger electronic correlations. Below the nematic transition temperature, the narrow Drude weight decreases with decreasing temperature. This indicates the reduction of the coherent carrier density, resulting from the Fermi-surface modification induced by the development of orbital ordering. The reduction of the narrow Drude weight with temperature stopped at x ~ 0.2, corresponding to the disappearance of the nematic transition. Our result suggests that the increase in the coherent carrier density induced by the suppression of the nematic transition gives rise to the enhancement of Tc. The decrease in Tc with further Te substitution likely arises from too strong electronic correlations, which are not favorable for superconductivity. ....

29 Wednesday, June 30

Tommaso Cea

Plasmons, phonons and superconductivity in twisted bilayer graphene

Authors: Tommaso Cea and Francisco Guinea

Abstract: We analyze the polarizability of twisted bilayer graphene, due to the combined effect of electron-hole pairs, plasmons, and acoustic phonons. The screened Coulomb interaction allows for the formation of Cooper pairs and superconductivity in a significant range of twist angles and fillings. The tendency toward superconductivity is enhanced by the coupling between longitudinal phonons and electron-hole pairs. Scattering processes involving large momentum transfers, Umklapp processes, play a crucial role in the formation of Cooper pairs. The magnitude of the superconducting gap changes among the different pockets of the Fermi surface. ....

Milan Orlita

Suppressed Auger scattering and tunable light emission of Landau-quantized massless Kane electrons

Authors: D. B. But, M. Mittendorff, C. Consejo, F. Teppe, N. N. Mikhailov, S. A. Dvoretskii, C. Faugeras, S. Winnerl, M. Helm, W. Knap, M. Potemski, and M. Orlita

Abstract: The Landau level laser was proposed long ago as a unique source of monochromatic radiation that would be widely tunable in the THz and infrared spectral ranges using a magnetic field. However, despite many efforts, this appealing concept never progressed to the design of a reliable device. This is because of the efficient Auger scattering of Landau-quantized electrons, an intrinsic non-radiative recombination channel that eventually gains over cyclotron emission in all materials studied so far (conventional semiconductors with parabolic bands, but also in graphene with massless electrons). Auger processes are favored in these systems because the Landau levels (or their subsets) are equally spaced in energy. Here, we show that this scheme does not apply to massless Kane electrons in gapless HgCdTe, where undesirable Auger scattering is strongly suppressed and sizeable cyclotron emission is observed. The gapless HgCdTe thus appears as a material of choice for future Landau level lasers.

But et al., Nature Photon. 13, 783 (2019) ....

Prineha Narang

Predicting Correlated Light-Matter Interactions

Authors: Prineha Narang

Abstract: In this talk, I will present work from my research group on describing, from first principles, the microscopic dynamics, decoherence and optically-excited collective phenomena in quantum matter at finite temperature to quantitatively link predictions with 3D atomic-scale imaging, quantum spectroscopy, and macroscopic behavior. Capturing these dynamics poses unique theoretical and computational challenges. The simultaneous contribution of processes that occur on many time and length-scales have remained elusive for state-of-the-art calculations and model Hamiltonian approaches alike, necessitating the development of new methods in computational physics. I will show selected examples of our predictions of linear and nonlinear dynamics and transport in Weyl semimetals. I will discuss the anomalous landscape for electron hydrodynamics in systems beyond graphene, highlighting that previously-thought exotic fluid phenomena can exist in both two-dimensional and anisotropic three-dimensional materials. Our work identifies phonon-mediated electron-electron interactions as critical in a microscopic understanding of hydrodynamics. Non-diffusive electron flow, and in particular electron hydrodynamics, has far-reaching implications in quantum materials science, as I will show in this talk. Finally, I will present an outlook on driving topological quantum materials far out-of-

30 equilibrium to control the coupled degrees-of-freedom. ....

Méasson Marie-Aude Collective mode of the Hidden Order State in URu2Si2: Degeneracy and Symmetry

Authors: Jonathan Buhot, Gregory Setnikar, Femke Bangma, Mikhail Prosnikov, Gérard Lapertot, Dai Aoki, Nigel Hussey, and Marie- Aude Méasson

Abstract: Thirty years after the discovery of the Hidden Order in the heavy fermions compound URu2Si2, the nature of this mysterious electronic state remains a major puzzle of condensed matter research [1]. Raman scattering spectroscopy recently brought important insights on the nature of this state [2,3]. Clear signatures of the Hidden order state were measured, a peak at 14 cm-1 and a gap below ~55cm-1, both in a peculiar chiral A2g symmetry. The nature of the former mode particularly attracts attention; it has been shown to match the resonance measured at Q0=(001)[2], one of the main fingerprint of the Hidden Order and it has been suggested that its symmetry transforms into the Ag one due to the new local point group on the Uranium site in the Hidden Order [3]. Nevertheless crucial information is still missing: not all Raman active symmetries of the mode have been explored, i.e. Eg is unexplored, and its degeneracy is still unknown.

Here, thanks to a newly developed Raman spectroscopy set-up under very high magnetic field, we report low-energy Raman spectroscopic measurements in all symmetries, including the Eg one and under high magnetic field up to 25 T. We discuss the Raman activity of the Hidden Order mode in all symmetries. A single mode is shown to harden under magnetic field up to 25 T. In the light of this observation, we discuss the degeneracy of the mode and comparison with the magnetic field dependence of the neutron resonance.

1) J. A. Mydosh and P. M. Oppeneer, Reviews of Modern Physics 83, 1301 (2011). 2) J. Buhot, et al., Phys. Rev. Lett. 113, 266405 (2014). 3) H.-H. Kung et al., Science 347, 1339 (2015). ....

Changmin Lee

Observation of a phase transition within the domain walls of the magnetic Weyl semimetal Co3Sn2S2

Authors: Changmin Lee, Praveen Vir, Kaustuv Manna, Chandra Shekhar, Claudia Felser, and Joseph Orenstein

Abstract: In magnetic Weyl semimetals, spatial and temporal fluctuations in the local magnetization generate axial gauge fields that couple to the chiral charge of the emergent Weyl fermions. Recent theoretical studies have focused on propagating domain walls (DWs), where the associated time-dependent magnetization generates axial electric and magnetic fields that are predicted to drive novel phenomena such as a current of “real” charge. Here we report a key step in testing these predictions: detection of driven DWs in the Weyl semimetal Co3Sn2S2 using scanning magneto-optic Kerr microscopy. We observe an unexpected deep minimum in the temperature dependence of the DW mobility, indicating a crossover between two regimes of propagation. The measurements provide direct evidence for a phase transition in the topology of the DW well below the Curie temperature, in which the magnetization texture changes from continuous rotation (elliptical wall) to a linear wall whose unidirectional magnetization passes through zero at the wall center. ....

Yue Sun

Mapping Domain Wall Topology in the Magnetic Weyl Semimetal CeAlSi

Authors: Yue Sun, Changmin Lee, Hung-Yu Yang, Darius H. Torchinsky, Fazel Tafti, Joseph Orenstein

Abstract: We report full vector mapping of local magnetization in the magnetic Weyl semimetal CeAlSi, revealing unanticipated features both within domains and at their boundaries. Within the domains, we observe that the previously reported set of four easy

31 axes aligned along the in-plane diagonals of the tetragonal structure actually split to form an octet with decreasing temperature below the magnetic transition. Boundaries between domains form two kinds of walls with different topology and can be expected to generate different emergent fields. Remarkably, walls aligned along the tetragonal axes, e.g. (100), are found to exhibit emergent chirality forbidden by the bulk space group, while the diagonal walls are non-chiral. The above phenomena are ultimately traced to the noncollinear magnetic structure of CeAlSi. ....

Kazuya Shinjo

Effect of phase string on single-hole dynamics in the two-leg Hubbard ladder

Authors: Kazuya Shinjo, Shigetoshi Sota, and Takami Tohyama

Abstract: Optical measurements in doped Mott insulators have discovered the emergence of spectral weights at mid-infrared (MIR) upon chemical doping and photodoping. MIR weights may have a relation to string-type excitation of spins, which is induced by a doped hole generating misarranged spins with respect to their sublattice. There are two types of string effects: one is an Sz string that is reparable by quantum spin flips and the other is a phase string irreparable by the spin flips. We investigate the effect of Sz and phase strings on MIR weights. Calculating the optical conductivity of the single-hole Hubbard model in the strong-coupling regime and the t−J model on two-leg ladders by using time-dependent Lanczos and density-matrix renormalization group, we find that phase strings make a crucial effect on the emergence of MIR weights as compared with Sz strings. Our findings indicate that a mutual Chern-Simons gauge field acting between spin and charge degrees of freedom, which is the origin of phase strings, is significant for obtaining MIR weights. Conversely, if we remove this gauge field, no phase is picked up by a doped hole. As a result, a spin accompanied by a local spin distortion emerges and a quasiparticle with a cosine-like energy dispersion is formed in single-particle spectral function. Furthermore, we suggest a Floquet engineering to examine the phase-string effect in cold atoms. ....

Ran Jing

Terahertz response of monolayer and few-layer WTe_2 at the nanoscale

Authors: Ran Jing, Yinming Shao, Zaiyao Fei, Chiu Fan Bowen Lo, Rocco A. Vitalone, Francesco L. Ruta, John Staunton, William J.-C Zheng, Alexander S. Mcleod, Zhiyuan Sun, Bor-yuan Jiang, Xinzhong Chen, Michael M. Fogler, Mengkun Liu, David H. Cobden, Xiaodong Xu and D. N. Basov

Abstract: ditelluride (WTe_2) is an atomically layered transition metal dichalcogenide whose physical properties change systematically from monolayer to bilayer and few-layer versions. In this presentation, we use apertureless scattering-type near- field optical microscopy operating at Terahertz (THz) frequencies and cryogenic temperatures to study the distinct THz range electromagnetic responses of mono-, bi- and trilayer WTe2 in the same multi-terraced micro-crystal. THz nano-images of monolayer terraces uncovered weakly insulating behavior that is consistent with transport measurements. The near-field signal on bilayer regions shows moderate metallicity with negligible temperature dependence. Subdiffractional THz imaging data together with theoretical calculations involving thermally activated carriers favor the semimetal scenario with Δ≈-10 meV over the semiconductor scenario for bilayer WTe2. Also, we observed clear metallic behavior of the near-field signal on trilayer regions. Our data are consistent with the existence of polaritons in the THz range confined to trilayer terraces in our specimens. Finally, data for microcrystals up to 12 layers thick reveal how the response of a few-layer WTe2 asymptotically approaches the bulk limit. ....

Takahiro Ito

Angle-resolved photoemission study of MAX phase compound Ti2SnC

Authors: Takahiro Ito, Masashi Ikemoto, Damir Pinek, Youngsoo Kim, Masashi Nakatake, Shin-ichiro Ideta, Kiyohisa Tanaka, Patrick Le Fèvre, Francois Bertran, Thierry Ouisse

Abstract: MAX phase compounds, i.e., Mn+1AXn where M is a transition metal, A belongs to groups 13-16 and X is the C or N

32 element, have recently been attracted much attention due to their possible application for new class of two-dimensional systems called MXenes by removing A atoms [1]. On the other hand, the bulk electronic structure of MAX phase has been studied mostly by calculations, mainly because of lack of well-established single crystalline samples. To understand the relation between its thermodynamic properties and the electronic structure of MAX phases, we have performed angle-resolved photoemission spectroscopy on MAX phase compound Ti2SnC, which is known as promising material applied for a reinforcement for and metals due to its high electrical conductivity [2]. As results, we have successfully observed the band structure and Fermi surfaces of Ti2SnC, which is well reproduced by the DFT calculation. Around the KH line, we have found the Dirac-cone-like dispersive features that show opposite sign of circular dichroism (CD) against the ΓM line as well as the crossing-point with using circular-polarized light, while the features show the same sign of linear dichroism with using linear-polarized one. The observed polarization dependence might suggest the change of chirality of orbital angular momentum on the Dirac-cone-like features on Ti2SnC. [1] M. Basoum, MAX phases (Wiley, Weinheim 2013). [2] J. Y. Wu, Y. C. Zhou, J. Y. Wang, Mat. Sci. Eng. A 422 (2006) 266. ....

Jae Hoon Kim

Terahertz Electrodynamics of Superconducting Nb Films in External Magnetic Field

Authors: Ji Eun Lee, Joonyoung Choi, Kyung Ik Sim, Jae Ha Kim, Younjung Jo, and Jae Hoon Kim

Abstract: We obtained the optical conductivity of type-II superconductor niobium (Nb) in the terahertz region under an in-plane magnetic field at 1.5 K. The superfluid density and the London penetration depth of superconducting Nb films were extracted from the imaginary part of conductivity. The superfluid density decreases monotonously with external magnetic field. As the magnetic field approaches the upper critical field, the London penetration exhibits anomalous features related to the nonlinear Meissner effect and the gapless behavior. Some issues related to the symmetry of the order parameter will be addressed as well. ....

Gael Grissonnanche

T-linear resistivity from an isotropic Planckian scattering rate

Authors: Gael Grissonnanche, Yawen Fang, Anaelle Legros, Simon Verret, Francis Laliberte, Clément Collignon, Jianshi Zhou, David Graf, Paul Goddard, Louis Taillefer, and B. J. Ramshaw

Abstract: Perfectly T-linear resistivity is observed in a variety of strongly correlated metals close to a quantum critical point [1] and has been attributed to a scattering rate 1/τ of charge carriers that reaches the Planckian limit [2,3], with ℏ/𝜏 = α 𝜏 𝜏 where α is of order unity. While this relationship is often inferred from simple estimates, a T-linear scattering rate has yet to be measured. To directly access the Planckian scattering rate, we measured the angle-dependent magnetoresistance (ADMR) of Nd-LSCO at p = 0.24: a cuprate that demonstrates T-linear resistivity over a wide temperature range at the pseudogap critical point p* [4]. The ADMR reveals a well-defined Fermi surface that precisely agrees with ARPES [5]. In addition, we extract a T-linear scattering rate that has the Planckian value, namely α = 1.2 ± 0.4. Remarkably, this inelastic scattering rate is isotropic.

Our findings suggest that T-linear resistivity in strange metals emerges from a generic isotropic, momentum-independent inelastic scattering rate that reaches the Planckian limit.

[1] J. Zaanen, SciPost Phys. 6, 061 (2019). [2] J. A. N. Bruin et al., Science 339, 804 (2013) [3] A. Legros et al., Nat. Phys. 15, 142 (2019) [4] R. Daou et al., Nat. Phys. 5, 31 (2009). [5] C. Matt et al., Phys. Rev. B 92, 134524 (2015) ....

33 Yinming Shao

Nonlinear Nano-electrodynamics of a Weyl semimetal

Author: Yinming Shao

Abstract: Weyl semimetals feature chiral Weyl fermions with linear energy-momentum dispersion accompanied with singularities of Berry curvature in the momentum space. Discovered through the energy dispersion, direct experimental evidences for the enhanced Berry curvature remain elusive for type-II Weyl fermions, where the Weyl cones overtilt. Second-order nonlinear effects are ideal probes for these momentum-space monopoles since the requirement of inversion-symmetry breaking happen to be one ingredient to realize Weyl fermions. However, the additional in-plane mirror symmetries in existing type-II Weyl semimetals (WTe2 family) forbid the generation of second-order response purely in the ab-plane, whereas smooth ac-planes are not obtainable. Here, we demonstrate direct coupling to the c-axis of a type-II Weyl semimetal thin flake using near-field optical microscope and concurrent nano-photocurrent imaging. Real-space photocurrent map shows intriguing direction-switching pattern near gold electrical contacts that reveals the intrinsic momentum-space anisotropies near the Weyl points. ....

Ilya Sochnikov

Microscopy of tunable magnetic domains in noncentrosymmetric ferromagnetic Weyl semimetal

Authors: B. Xu, J. Franklin, A. Jayacody, H.-Y. Yang, F. Tafti, and I. Sochnikov

Abstract: Textures and dynamics of magnetic domain walls (DW) in Weyl semimetals are predicted to have their electromagnetic behavior strongly altered by emergent gauge fields. Therefore, knowing the detailed domain physics in these materials is important for the experimental realization of such theoretical proposals. In this work, we imaged the spontaneous magnetization and magnetic susceptibility of a ferromagnetic (FM) Weyl semimetal: CeAlSi [1,2]. We utilized a scanning SQUID susceptometer microscopy, which can be considered a type of low-frequency and low-energy spectroscopic imaging technique [3,4]. We observed large ferromagnetic DWs lined-up with the main crystallographic directions and discovered the coexistence of stable and metastable domain phases, which likely arise due to the magnetoelastic and magnetostriction effects and are potentially highly tunable with small strains. We found that the pattern of the FM domains is strongly correlated with both the amplitude and the orientation of an external in-plane magnetic field. The average area of the stable domains increases strongly even with very small fields. The length of the domain walls is maximized when the in-plane field is directed along the main crystallographic axes and the area of the domains is maximized when the field is at 45 degrees to the crystallographic axes. Our results show how these domains and heterogeneous phases can be fine-tuned and, therefore, this work provides guidance for future studies on the fundamental interplay between magnetism and electronic properties in Weyl systems and how to utilize these systems in realizing new devices.

[1] H.-Y. Yang et al., ArXiv:2006.07943 to appear in Phys. Rev. B (2021). [2] B. Xu et al., Adv. Quantum Technol., Cover Article, 2000101 (2021). [3] Z.-C. Wang et al., Advanced Materials, 2005755 (2021). [4] C. Herrera et al., Phys. Rev. B, Editor's Choice Article, 103, 024528 (2021). ....

Shingo Toyoda

Nonreciprocal second harmonic generation in a magnetoelectric CuB2O4

Authors: Shingo Toyoda, Manfred Fiebig, Taka-hisa Arima, Yoshinori Tokura, and Naoki Ogawa

Abstract: Symmetry breaking is closely related to the emergence of intriguing optical phenomena. Specifically, breaking of spatial inversion is essential for optical second harmonic generation (SHG), and time reversal symmetry is generally associated with nonreciprocal phenomena. The simultaneous breaking of these two symmetries leads to an unusual difference between forward and backward propagation of SHG. However, this nonreciprocal response is usually a small effect. In this presentation, we demonstrate that CuB2O4 exhibits the transmission SHG changing by almost 100% upon reversal of a magnetic field of just ± 10 mT.

34 This nonreciprocal SHG originates from an interference between the magnetic-dipole- and electric-dipole-type nonlinear optical processes. The nonreciprocity enhances when these transitions interfere at the same amplitude and phase. However, the former is inherently weaker than the latter, which has hampered the manifestation of large nonreciprocal SHG. In CuB2O4, we found that the magnetic dipole SHG is resonantly enhanced at 1.4 eV which corresponds to the intratomic transition of Cu2+ holes. The resonantly enhanced magnetic-dipole SHG and non-resonant electric-dipole SHG now realize maximum nonreciprocity. We observed a Fano- resonance-like SHG spectra with the large nonreciprocal signal reaching to 97 % in the vicinity of the resonance energy. With the broken space-time-inversion-symmetry, multiferroics and magnetoelectrics are an obvious materials platform to materialize and explore the nonreciprocal and nonlinear-optical functionalities. ....

Changyoung Kim

Sign-tunable anomalous Hall effect induced by two-dimensional symmetry-protected nodal structures in ferromagnetic perovskite oxide thin films

Authors: Byungmin Sohn, Eunwoo Lee, Se Young Park, Wonshik Kyung, Jinwoong Hwang, Jonathan D. Denlinger, Minsoo Kim, Donghan Kim, Bongju Kim, Hanyoung Ryu, Soonsang Huh, Ji Seop Oh, Jong Keun Jung, Dongjin Oh, Younsik Kim, Moonsup Han, Tae Won Noh, Bohm-Jung Yang, and Changyoung Kim

Abstract: Magnetism and spin-orbit coupling (SOC) are two quintessential ingredients underlying novel topological transport phenomena in itinerant ferromagnets. When spin-polarized bands support nodal points/lines with band degeneracy that can be lifted by SOC, the nodal structures become a source of Berry curvature; this leads to a large anomalous Hall effect (AHE). Contrary to three-dimensional systems that naturally host nodal points/lines, two-dimensional (2D) systems can possess stable nodal structures only when proper crystalline symmetry exists. We show that 2D spin-polarized band structures of perovskite oxides generally support symmetry-protected nodal lines and points that govern both the sign and the magnitude of the AHE. To demonstrate this, we performed angle-resolved photoemission studies of ultrathin films of SrRuO3, a representative metallic ferromagnet with SOC. We show that the sign-changing AHE upon variation in the film thickness, magnetization, and chemical potential can be well explained by theoretical models. This result is the first to directly characterize the topological band structure of 2D spin-polarized bands and the corresponding AHE. ....

Artem Strashko

Crescent states in charge-imbalanced polariton condensates

Authors: Artem Strashko, Francesca M. Marchetti, Allan H. MacDonald, and Jonathan Keeling

Abstract: Polariton condensation is a well-established phenomenon featuring all the signatures of an ordinary condensate. However, in the context of polariton condensation, almost exclusively balanced systems, with equal densities of electrons and holes, have been studied. This misses a whole class of potential exotic imbalanced condensed states like an FFLO or a breached-pair (BP) state.

Inspired by pioneering works on imbalanced electron-hole systems in TMDC monolayers strongly coupled to a cavity photon, using variational mean-field theory, we explore whether a combination of strong matter-light coupling and electric eld biasing promotes novel condensed states, which do not exist otherwise.

At small charge imbalance we find novel imbalanced polaritonic states with spontaneously broken time-reversal and rotational symmetry, and unpaired carriers occupying an anisotropic crescent-shaped sliver of momentum space --- crescent states (CS). These states arise due to combination of strong matter-light coupling and long-range Coulomb potential. Moreover, akin to a usual balanced polariton condensate, these new states are stable at high temperatures (within a mean-eld approximation). At larger imbalance we recover a breached-pair state with unpaired carriers filling a ring-shaped region, which is replaced by an FFLO state as imbalance rises further.

Therefore, we predict new imbalanced polariton states, which should be possible to observe in an existing experimental platform of electrically biased TMDC monolayers.

35 Thursday, July 1

Bastien Michon

The spectral weight of hole doped cuprates across the pseudogap critical point.

Authors: Bastien Michon, A. Kuzmenko, M. K. Tran, C. W. Rischau, A. Georges and D. van der Marel, University of Geneva, Switzerland L. Taillefer, University of Sherbrooke, Canada S. Uchida, S. Komiya, S. Pyon, T. Takayama and H. Takagi, University of Tokyo, Japan S. Ono, CRIEPI-Tokyo, Japan B. Buechner, IFW-Dresden, Germany

Abstract: The mysterious phase diagram of hole doped cuprates is a big puzzle for researchers in condensed matter physics. This phase complexity in cuprates encloses the highest Tc at ambient pressure (Tc ~ 100K). Recent experiments have highlighted the interplay of the pseudogap phase and superconductivity: the pseudogap vanishes at a critical carrier concentration p*, constituting a quantum critical point around which the superconducting Tc forms a dome. At this critical doping p*, two important breakthroughs were exhibited: (i) A drop in carrier density n from 1+p to p as the system enters the pseudogap phase observed by Hall effect [1,2] indicating a sudden Fermi surface reconstruction. (ii) A dramatic effective mass enhancement at p* observed by specific heat [3], believed to result from enhanced fluctuations of the pseudogap at the quantum critical point.

In our study, we used infrared optics to measure the free carrier spectral weight. This quantity, expressed in appropriate units rep- resents the kinetic energy of the charge carriers per primitive cell. The spectra exhibit a narrow “coherent” zero energy mode and a broad “incoherent” mid-infrared band. The total (coherent + incoherent) spectral weight represents the unrenormalized kinetic energy K of the charge carriers per primitive cell, and the spectral weight of the narrow zero energy mode is the renormalized kinetic energy K* [4].

We demonstrate that in the normal state of LSCO and Eu-LSCO cuprates K and K* are significantly influenced by the behavior of n and m*, as their values remain extremely small (3 to 4 times smaller) compared to the band calculation. If we compare K* with the super- conducting penetration depth 1/ λ^2, we show that superconductivity could be a BEC (Bose-Einstein condensate) in the underdoped region. In addition, the spectral weight enclosed inside the MIR band KMIR=K-K* tracks the superconducting critical temperature Tc, suggesting a candidate for the pairing mechanism. Finally the analysis of the optical spectra of LSCO and Eu-LSCO reveals quantum critical behavior similar to optimally doped Bi2212 [5]: Power law frequency dependence of the modulus of the optical conductivity |σ(w)|, constant phase, and linear temperature and frequency/temperature scaling of the scattering rate 1/τ. We observe this behavior in LSCO and Eu-LSCO for a higher carrier concen- tration than for Bi2212.

[1] S. Badoux et al., Nature 531, 210 (2016). [2] C. Collignon et al., Phys. Rev. B 95, 224517 (2017). [3] B. Michon et al., Nature 567, 218-222 (2019). [4] S. I. Mirzaei et al, PNAS 110, 5774 (2013). [5] D. van der Marel et al, Nature 425, 271 (2003). ....

Erik van Heumen

Disentangling carrier density and momentum relaxation in cuprate superconductors

Authors: Erik van Heumen, Xuanbo Feng, Maarten Berben, Silvia Cassanelli, Lennart de Jager, Linda Neubrand, Steef Smit, Nigel Hus- sey, Takeshi Kondo, Tsunehiro Takeuchi, Jan Zaanen

Abstract: One of the key mysteries in the cuprate phase diagram is the strange metallic phase, which features a variety of anomalous electronic properties. In recent years, various (magneto-) transport experiments have reported a singular behavior of the carrier den-

36 sity near a critical doping and these results have been interpreted as essential features of the strange metal response. I will review these results in light of the doping evolution of the optical conductivity across the phase diagram for the single layer material Bi2201. Whereas transport experiments only probe a combination of momentum relaxation and carrier density, the opti- cal response enables us to disentangle the two. I will show that the doping and temperature evolution of the resistivity can be fully understood from changes in the momentum relaxation rate. At the same time, the carrier density displays a continuous and gradual evolution n 𝜏 p across the phase diagram.

I will explain how these results provide new insight in the dynamics of the strange metal phase and will conclude with an alternative explanation for the singular doping dependence of the magneto-transport experiments. ....

Fabio Boschini

Non-monotonic electron interactions in the copper oxide plane

Authors: F. Boschini, M. Minola, R. Sutarto, E. Schierle, M. Bluschke, S. Das, Y. Yang, M. Michiardi, Y. C. Shao, X. Feng, S. Ono, R. D. Zhong, J. A. Schneeloch, G. D. Gu, E. Weschke, F. He, Y. D. Chuang, B. Keimer, A. Damascelli, A. Frano, and E. H. da Silva Neto

Abstract: In strongly correlated systems the strength of Coulomb interactions between electrons plays a central role in determining their emergent quantum mechanical phases. Specifically, while the Coulomb interaction energy of electrons in free space decays monotonically with distance, in solids, where the electrons move in a polarizable crystal, the effective Coulomb interaction may be non-monotonic, with a minimum at a finite length. This general concept of how electrons in solids interact may be key to understand- ing the emerging electronic phases of strongly correlated quantum materials. For instance, a non-monotonic behavior may result in an attractive interaction that could lead to superconductivity or density wave order. We performed resonant x-ray scattering on Bi2Sr2CaCuO8+x, a prototypical cuprate superconductor, to probe electronic correlations within the CuO2 plane. We discovered a dy- namic quasi-circular pattern in the x-y scattering plane with a wave vector radius that is determined by the minimum of the Coulomb potential [1]. Furthermore, this radius exactly matches the wave vector magnitude of the well-known static charge order patterns. Along with a comprehensive set of doping- and temperature-dependent measurements, our experiments reveal a picture of charge order competing with superconductivity where short-range domains along x and y are allowed to dynamically rotate into any other in-plane direction [1]. These findings may have immediate consequences to our understanding of rotational and translational- sym metry breaking in the cuprates.

[1] Boschini et al. Nature Communications 12, 597 (2021) ....

Francesco Barantani

Temperature dependence of d-d excitons in hole-doped cuprates

Authors: F. Barantani, M. K. Tran, I. Madan, I. Kapon, N. Bachar, T. Asmara, E. Paris, Y. Tseng, W. Zhang, E. Giannini, G. Gu, X. Huang, C. Berthod, T. Schmitt, A. Georges, T. P. Devereaux, F. Carbone, D. van der Marel

Abstract: The mechanism of superconductivity constitutes the key question of the physics of high-Tc cuprates. Electron-phonon inter- action, while manifested in many experimental probes, appears to be too weak in the cuprates to be compatible with the observed Tc reaching 160 K in certain cases. The Hubbard model, while promising, is not fully conclusive lacking an analytical solution in 2 and 3 dimensions. Little attention has been paid to the role of intra Cu 3d-shell excitons which are clearly observable in resonant inelastic x-ray scattering (RIXS). We measured the intensity of the d-d excitons in Bi2Sr2CaCu2O8+δ with various hole-doping levels in a broad temperature range around the superconducting transition temperature Tc, using a dense temperature sampling. We clearly observe a variation with temperature of the d-d excitons, and a significant change of temperature dependence at Tc. The overall temperature dependence and the change at Tc depend strongly on the hole concentration: the superconductivity-in- duced effect shows opposite sign for underdoped and overdoped samples. The observed behaviour is similar along the nodal and anti-nodal directions. These observations re-open a discussion started long ago by W. Little about superconducting pairing mediated by excitons. Our data

37 clearly demonstrate that the conduction bands are coupled to the d-d excitons, and that they are affected when entering into the superconducting phase. ....

Marco Grilli

Ubiquitous suppression of the nodal coherent spectral weight in Bi-based cuprates

Authors: M. Zonno, F. Boschini, E. Razzoli, S. Y. Dufresne, M. Michiardi, M. X. Na, A. K. Mills, S. Zhdanovich, G. Levy, D. J. Jones, A. Damascelli.

Abstract: High-temperature superconducting cuprates exhibit an intriguing phenomenology for the low-energy elementary exci- tations. In particular, an unconventional temperature dependence of the coherent spectral weight (CSW) has been observed in the superconducting phase by angle-resolved photoemission spectroscopy (ARPES), both at the antinode where the d-wave paring gap is maximum, as well as along the gapless nodal direction. Here, we combine equilibrium and time-resolved ARPES to track the tempera- ture dependent meltdown of the nodal CSW in Bi-based cuprates with unprecedented sensitivity [1]. We find the nodal suppression of CSW upon increasing temperature to be ubiquitous across single- and bi-layer Bi cuprates, and uncorrelated to superconducting and pseudogap onset temperatures. We quantitatively model both the lineshape of the nodal spectral features and the anomalous suppression of CSW within the Fermi-Liquid framework, establishing the key role played by the normal state electrodynamics in the description of nodal quasiparticles in superconducting cuprates.

[1] M. Zonno, F. Boschini et al., Accepted in PRB ; arXiv:2009.05058 (2020) ....

Petr Adamus

Pseudogap in the c-axis (interplane) conductivity of a pair of t-J planes with a single doped hole

Authors: Petr Adamus, Dominik Munzar

Abstract: We report on results of our study of the c-axis (interplane) optical conductivity of a pair of weakly coupled copper oxygen planes with a single doped hole. The planes are described using the t-J model and approximations of the well known spin polaron approach [1] are employed. The polaron problem is solved using a simple variational technique, on the subspace where the hole is dressed by a single spin excitation. The calculated dispersion relation is roughly consistent with that of the selfconsistent Born approximation. The variational ansatz allows us to analyze the optical transitions, to evaluate the corresponding matrix elements and to estimate the temperature dependence of the conductivity. The conductivity diplays a pseudogap opening with decreasing temperature associated with a spectral weight shift from low to a band arround ~ 2J. Our results provide a qualitative explanation for the trends in the data of underdoped cuprates [2].

[1] G. Martinez, P. Horsch, Phys. Rev. B 44, 317 (1991). [2] Li Yu et al., Phys. Rev. Lett 100, 177004 (2008). ....

Adrian Gozar

Surface Josephson plasma waves in LaSrCuO superconductor

Authors: Qianbo Lu, A.T. Bollinger, X He, R. Sundling, I. Bozovic and A. Gozar

Abstract: Electron density oscillations with acoustic dispersions and sustained at boundaries between different media provide infor- mation about surface and interface properties of heterostructures. In ultrathin metallic films these plasmonic excitations are heavily damped. Superconductivity is predicted to reduce dissipation allowing detection of these resonances. Emerging low-loss interface

38 Cooper-pair waves have been studied before but the observation of surface-confined Cooper-pair plasmons has remained elusive. IN this presentation I will discuss on generation and coupling to these excitations in a thin film of copper-oxide superconductor La1.85Sr0.15CuO4. The crystal becomes brighter than Au below the critical temperature when probed with sub-gap THz photons. We show that the enhanced signal in the superconducting state originates from near-field coupling to surface Josephson plasmons. The results open a path towards non-invasive investigation of superconductivity at interfaces in artificial multilayers and non-linear phenomena in Josephson devices.

Reference: Lu et al., npj | Quantum Materials 5, 69 2020 ....

Florence Levy-Bertrand

Observation of sub-gap superconducting modes with Kinetic Inductance Detectors

Authors: F. Levy-Bertrand, T. Klein, T. Grenet, O. Dupré, A. Benoît, A. Bideaud, O. Bourrion, M. Calvo, A. Catalano, A. Gomez, J. Goupy, L. Grünhaupt, U. v. Luepke, N. Maleeva, F. Valenti, I. M. Pop and A. Monfardini

Abstract: Kinetic Inductance Detectors (KIDs) are state-of-the-art detectors used for millimeter wave observations in astrophysics. They are planar resonant circuits made of superconductors deposited on an insulating substrate. The photon detection principe con- sists of monitoring the resonance frequency shift that is proportional to the incident power [1].

In principle, no optical absorptions are expected below twice the superconducting gap. In this work, we provide direct evidence for well resolved sub-gap absorptions in superconducting granular aluminum (grAl) thanks to a spectroscopy technique based on KID [2]. We explore the electrodynamics of grAl at 100 mK from 0 GHz up to 300 GHz with a resolution of 1GHz. The combination of this orignal technique to an extensive exploration of the phase diagram of granular aluminum evidence two different types of sub-gap superconducting modes and allow to compare their variation in energy with the variation of the superconducting gap, the critical temperature Tc, the phase stiffness J and the Coulomb repulsion. We suggest that these superconducting sub-gap modes might be useful for photon detection [3,4].

[1] P. K. Day et al, Nature, vol. 425, 817 (2003). [2] F. Levy-Bertrand et al, Phys. Rev. B 99, 094506 (2019). [3] O. Dupré et al, Superconductor Science and Technology, 30, 045007 (2017). [4] F. Levy-Bertrand et al, Physical Review Applied 15, 044002 (2021). ....

Francesco Gabriele

Coupled plasma waves in layered cuprates

Authors: Francesco Gabriele, Lara Benfatto, Claudio Castellani

Abstract: Plasma excitations play a primary role in cuprates: indeed, they are responsible of the optical properties and also thought to be linked to the origin of high-temperature superconductivity in such systems[1].

Due to their anisotropic structure, these systems can sustain two kind of plasma oscillations, the high-energy in-plane plasmon and the soft out-of-plane Josephson Plasma Mode (JPM). So far, TeraHertz (THz) radiation has been successfully used to resonantly excite the JPM [2][3]. Also, the in-plane plasmon has been shown, despite of his high energy, to play a key role in mediating the THz non-lin- ear optical response in such systems[4].

Due to the breakdown of the longitudinal-transverse decomposition occurring in anisotropic systems, plasma waves cannot propa- gate along arbitrary directions in layered cuprates: one would observe, in general, mixed longitudinal-transverse excitations, which are linked to a finite coupling between the in-plane and the out-of-plane plasmons. A full quantum treatment of these collective anisotropic modes has not been yet discussed in the literature. Here we fill this knowledge gap and discuss their relevance for recent experiments[5].

39 Firstly, a brief review of the theoretical framework used to investigate the spectral properties and the dynamics of these coupled plasma waves is provided. Then, I will address how such excitation respond to electromagnetic fields with an arbitrary polarization direction, focusing on non-linear optical phenomena such as Third-Harmonic Generation and Pump-Probe Oscillations. A bridge- be tween these anisotropic bulk excitations and surface plasmon-polaritons in such systems is also discussed.

[1]Anderson, P. W. "The Theory of Superconductivity in the High-Tc Cuprate Superconductors". Princeton University Press (1997). [2]Savel'ev, Sergey, et al. "Terahertz em plasma waves in layered superconductors: spectrum, generation, nonlinear and quantum phenomena." Reports on Progress in Physics 73.2 (2010): 026501. [3]Rajasekaran, Srivats, et al. "Probing optically silent superfluid stripes in cuprates." Science 359.6375 (2018): 575-579. [4]Gabriele, Francesco, et al. "Non-linear Terahertz driving of plasma waves in layered cuprates." Nature communications 12.1 (2021): 1-8. [5]F. Gabriele, L. Benfatto, C. Castellani, preprint 2021 ....

Xiao-Xiao Zhang

Ultrafast spin dynamics in 2D antiferromagnet

Author: Xiao-Xiao Zhang

Abstract: The recently discovered atomically-thin magnetic crystals provide a unique playground to develop new approaches to manipulate magnetism, and rapid progresses have been made that demonstrate the potentials of utilizing 2D magnets to construct novel spintronics devices. However, their spin dynamics, which are crucial for microscopic understanding and determine the funda- mental limit of spin manipulation, remain elusive due to the difficulty to characterize these micron-sized samples with conventional microwave techniques. In this talk, I will show how we can optically access and probe the collective spin excitations in atomically-thin van der Waals antiferromagnets with ultrafast pump-probe spectroscopy. The first half of the talk will focus on the coherent spin waves (magnon) that correspond to the antiferromagnetic resonance in bilayer CrI3. Both the transverse and longitudinal magnon branches are identified, allowing us to extract magnetic anisotropy and exchange energies in these atomically-thin magnetic materi- als. In the second half of the talk, I will discuss the diverging spin dynamics in few-layer FePS3, which can be attributed to the critical behaviors near the Neel transition temperature. ....

Md Mofazzel Hosen

Anomalous Raman response of a Charge Density Wave in a high mobility, 2D antiferromagnet

Authors: Md Mofazzel Hosen, Yiping Wang, and Kenneth S. Burch

Abstract: Combining magnetism with charge density waves (CDW) offers a unique regime for strong correlations. Here we study a 2D material with high mobility, despite the presence of antiferromagnetic order and charge density wave. We detail comprehensive Raman measurements that show a strong interaction between the lattice and CDW modes. These experiments also demonstrate a surprising non-reciprocal response, even well above the magnetic ordering temperature. ....

40 Xiaodong Xu

Intertwined Topological and Magnetic Orders in Atomically Thin Chern Magnet

Author: Xiaodong Xu

Abstract: The interplay between topology and broken time reversal symmetry in a solid-state system can create a Chern insulator. The non-trivial property of its band structure enables quantized Hall transport with dissipation-free chiral edge states. The recent- ly-discovered topological magnet, MnBi2Te4, provides a desirable platform to study tunable Chern insulator phase. In this talk, I will describe our recent progress in understanding this new material system by the combination of transport, magneto-optical Kerr effect, and microwave impedance microscopy measurement. These probes reveal the formation of the Chern insulator gap, image the associated chiral edge states, and visualize the band-crossing in atomically thin MnBi2Te4 devices. Our results offer insights into the topological character of the band structure in this new Chern insulator. ....

In-Sang Yang

Spin Excitation in Hexagonal LuMnO3

Authors: Seung Kim1, Jiyeon Nam1, Xueyun Wang2, Sang-Wook Cheong2, and In-Sang Yang1,* 1Department of Physics, Ewha Womans University, Seoul Korea 2Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, New Jersey, USA

All the spin-excitation peaks observed by Raman spectroscopy in LuMnO3 below the Néel temperature are explained by a spin-spin interaction Hamiltonian. Spin-excitation peaks of relatively high energy (~ 0.1 eV) have been observed in hexagonal RMnO3 (R = rare earths, Y, and Lu) system, but no microscopic explanation was given until now. In this paper, we propose a model associated with the spin rotation of the Mn ions in the symmetry of the triangular lattice with the antiferromagnetic (AFM) ordering. We found that excitation energy is quantized into multiples of 60-degree rotation of the Mn spins; the spin-excitation peaks observed at 197, 580, and 805 cm-1 are corresponding to the Mn-spin rotations by 60, 120, and 180, respectively. Our result constitutes an example of energy quantization by the symmetry allowance. Our analyses suggest a possibility of the spin excitation through an optical resonance with the Mn d-d transition in hexagonal RMnO3. Resonance Raman process itself is exciting the spin rotation-/spin flip-excitations as well as detecting the spin excitations. This would be an example of optical write/read of the spin excitations in multiferroic RMnO3.

41 Monday, July 5

Nurit Avraham

Visualization of Topological Boundary Modes Manifesting Topological Nodal-Point Superconductivity

Authors: Abhay Kumar Nayak, Aviram Steinbok, Yotam Roet, Jahyun Koo, Gilad Margalit, Irena Feldman, Avior Almoalem, Amit Kanigel, Binghai Yan, Gregory A. Fiete, Yuval Oreg, Nurit Avraham, and Haim Beidenkopf

Abstract: Much work has been carried out in the last few years towards the study of topological superconductors of different kinds and origins with the aim to induce and manipulate Majorana zero modes. While numerous realizations of 1D topological superconductors have been discovered and studied, a far smaller number of 2D topological superconductors have been proven to exist, with almost no examples for topological superconductivity in naturally occurring layered materials. In my talk I will present scanning tunneling microscopy and spectroscopy measurements which provide direct evidence for topological superconductivity on the surface of the transition metal dichalcogenide 4Hb-TaS2, in which 1H- and 1T-TaS2 layers are interleaved. We find a residual within the superconducting gap. We visualize an exponentially decaying bound mode within the superconducting gap along the boundaries of the exposed 1H layer, characteristic of a gapless Majorana edge mode. This edge mode exhibits anisotropic nature of the localization length depending on the step-edge orientation. A zero-bias conductance peak is further imaged within fairly isotropic vortex cores. All our observations are captured by a theoretical model of a two-dimensional nodal Weyl-like superconducting state, which originates from inter-orbital Cooper pairing. I will present our theoretical model and discuss it with relation to our experimental observations. ....

Andrea Cavalleri

Advances in Optically Driven Superconductivity

Author: Andrea Cavalleri

Abstract: I will discuss how coherent electromagnetic radiation at Tera-Hertz frequencies can be used to drive complex solids periodically, controlling the coupling between their collective excitations. These drives give rise to non-thermal states with unconventional types of microscopic order and new functional properties. Important examples involve the nonlinear control of the crystal lattice, used to induce non-equilibrium superconductivity at high temperatures. ....

Denitsa Baykusheva

Ultrafast renormalization of the onsite Coulomb repulsion in an underdoped cuprate

Authors: D. Baykusheva, Y. Wang, H. Jang, A. Husain, S. Lee, S. TenHuisen, P. Zhou, H.-D. Kim, M. Kim, S.-Y. Park, S. Park, H. Kim, J. Kim, B. Kim, G. D. Gu, P. Abbamonte, M. Mitrano

Abstract: Intense ultrashort electromagnetic fields are an increasingly important tool to realize and control novel emergent phases in quantum materials. Among a variety of nonthermal excitation pathways, a particularly intriguing route is represented by the direct light-engineering of effective many-body interactions, such as electron hopping amplitudes and electron-electron repulsion. Achieving a light-induced dynamical renormalization of the screened onsite Coulomb repulsion (“Hubbard U”) would have far- reaching implications for high-harmonic generation [1], attosecond spectroscopy [2] and ultrafast magnetism [3] in the solid state. However, experimental evidence for a dynamically controlled Hubbard U remains scarce [4], [5].

Here, we employ time-resolved x-ray absorption spectroscopy (trXAS) to demonstrate the ultrafast renormalization of the Hubbard U parameter in the underdoped cuprate La1.905Ba0.095CuO4 (LBCO, x=9.5%). Our element-specific measurements reveal that intense femtosecond optical pulses (1.55 eV, 50 fs) induce a dramatic shift of the x-ray absorption maxima associated with transitions to

42 the upper Hubbard bands (UHB), while the transition energy into Zhang-Rice singlet states near the Fermi level remains unaffected. Based on exact-diagonalization calculations of the time-dependent spectrum within the single- as well as the three-band models of cuprate superconductors, we determine a pump-induced suppression of the Hubbard U up to 10% of its equilibrium value on the Cu sites.

Our results represent a first precision measurement of dynamically-renormalized Hubbard U in strongly correlated oxides and have significant implications for the on-demand engineering of their magnetic interaction spectrum.

References [1] N. Tancogne-Dejean, M. A. Sentef, and A. Rubio, Phys. Rev. Lett., vol. 121, no. 9, p. 97402, 2018. [2] N. Tancogne-Dejean, M. A. Sentef, and A. Rubio, Phys. Rev. B, vol. 102, no. 11, p. 115106, 2020. [3] M. Claassen, H. C. Jiang, B. Moritz, and T. P. Devereaux, Nat. Commun., vol. 8, no. 1, pp. 1–8, 2017. [4] R. Singla et al., Phys. Rev. Lett., vol. 115, no. 18, p. 187401, 2015. [5] S. Beaulieu et al., arXiv, p. arXiv:2003.04059v1, 2020. ....

Stuart Brown

Even parity superconducting state of the strongly correlated Fermi Liquid Sr2RuO4

Author: Stuart Brown

Abstract: Unambiguous identification of the superconducting state of the strongly correlated Fermi Liquid Sr2RuO4 has remained elusive for more than a quarter century. While a chiral p-wave ground state analogue to superfluid 3He-A was ruled out only very recently [1], other proposed p-wave scenarios have been considered viable. We study the field-dependence of the 17O Knight shift at very low temperature, and by way of comparing to corresponding specific heat measurements, conclude that the shift results can be accounted for by the expected field-induced quasiparticle response only; the condensate response is limited to <10%, relative to the normal state response [2]. The results are inconsistent with odd-parity candidate states. Further clues follow from recent ultrasound measurements (Ghosh, et al. (2021), S. Benhabib, et al. (2021)), indicating a two-component order parameter, a finding supported by the recent examination of the onset of time reversal symmetry breaking under strained conditions (Grinenko, et al. (2021)). Implications are discussed in relation to theoretical proposals characterizing the order parameter components as either degenerate by symmetry, or accidentally degenerate.

[1] A. Pustogow, et al., Nature 574, 72–75 (2019). [2] A. Chronister, et al., arXiv:2007.13730. ....

Grégory Setnikar

Structural instability and superconducting state of V3Si: a Raman study

Authors: Grégory Setnikar, Amit Pawbake, Jerôme Debray, Frank Weber, Matteo D’Astuto, and Marie-Aude Méasson

Abstract: V3Si is a material of the A15 family, which is the first "High Tc" superconductor family discovered in 1953 [1]. Extensively studied during the following years, it has been shown [2, 3] that this compound also presents a so-called "martensitic transition", a structural instability. Questions about the origin, and the nature of this transformation are still under debate. Many theoretical studies have been done but two of them stand out: on one hand the work of Labbé and Friedel [4], describing the transition as a second order Jahn-Teller Instability and on the other hand the work of Gorkov [5] assigning this transition as a pseudo Peierls charge density wave in one-dimensional chain. Underlying the complexity of these compounds, it seems that there is no experimental consensus on this question nor on the interaction with other electronic order.

Moreover, in very recent studies using Raman spectroscopy in the dichalcogenides family, which also present an interplay between two electronic orders (well known here as charge density wave and superconductivity), the collective amplitude mode of

43 superconductivity also known as the Higgs mode as been observed twice [6, 7]. In the past, many Raman spectroscopy studies have been done on V3Si [8] showing Raman signature of a superconducting excitation. With that in mind, we propose to study again V3Si with more recent technique of Raman spectroscopy, allowing us to work under various extreme conditions (high pressure, low temperature, low energy resolution). The aim is to investigate the structural instability, the link with an possible electronic order, and the interplay with superconductivity, leading maybe to a new interpretation of the superconducting excitation as a Higgs Mode.

During the presentation, I will introduce our Raman spectroscopy technique and the main advantages of such technique for the study of structural instability, electronic order, and possible interplay between these. I will also present some preliminary results and perspectives of this study.

[1] G. F. Hardy and J. K. Hulm, Physical Review 89, 884 (1953). [2] B. Batterman and C. Barrett, Physical Review 145, 296 (1966). [3] M. Goringe and U. Valdre, Physical Review Letters 14, 823 (1965). [4] J. Labbé and J. Friedel, Journal de Physique 27, 708 (1966). [5] L. Gorkov, Zh. Eksp. Teor. Fiz. Pis’ma Red. 17, 525 (1973). [6] M.-A. Méasson, Y. Gallais, M. Cazayous, B. Clair, P. Rodière, L. Cario, and A. Sacuto, Phys. Rev. B 89, 060503 (2014). [7] R. Grasset, Y. Gallais, A. Sacuto, M. Cazayous, S. Mañas Valero, E. Coronado, and M.-A. Méasson, Phys. Rev. Lett. 122, 127001 (2019). [8] R. Hackl and R. Kaiser, Journal of Physics C: Solid State Physics 21, L453 (1988). [9] R. Hackl, R. Kaiser, and S. Schicktanz, Journal of Physics C: Solid State Physics 16, 1729 (1983). ....

Götz Seibold

Third harmonics generation from collective modes in disordered superconductors

Authors: G. Seibold, M. Udina, C. Castellani, L. Benfatto

Abstract: Recent experiments with strong THz fields in both conventional and unconventional superconductors have clearly evidenced a marked third-harmonic generation below the superconducting temperature Tc. Its interpretation challenged substantial theoretical work aimed at establishing the relative efficiency of quasiparticle excitations and collective modes in triggering such a resonant response. Here we compute the non-linear current by implementing a time-dependent Bogoljubov de-Gennes approach, with the twofold aim to account non-perturbatively for the effect of local disorder, and to include the contribution of all collective modes, i.e. charge, superconducting amplitude (Higgs) and phase fluctuations. We show that, in agreement with previous work, already at small disorder the quasiparticle response is dominated by paramagnetic effects. However, we further demonstrate that paramagnetic processes mediate also the response of all collective modes, with a substantial contribution of charge/phase fluctuations. These processes, which have been overlooked so far, turn out to dominate the third-order current at strong disorder. In addition, we show that disorder strongly influences the polarization dependence of the non-linear response, with a marked difference between the homogenous and the disordered case. Our results are particularly relevant for recent experiments in cuprates, whose band structure is in a first approximation reproduced by our lattice model. ....

Federico Cilento

Decoupling electronic and lattice contributions to the unconventional charge-density-wave transition of NbSe2 by time- resolved optical spectroscopy

Authors: D. T. Payne, P. Barone, L. Benfatto, F. Parmigiani, F. Cilento

Abstract: The complex Fermi surfaces of transition-metal dichalcogenides challenge the standard Peierls-instability-driven charge- density-wave (CDW) formation. Recently, evidence has been accumulating of a prominent role of ionic thermal fluctuations, which frozen out below TCDW inducing a periodic lattice distortion (PLD). We focus on 2H-NbSe2, displaying am incommensurate CDW

44 below TCDW≈33 K, and use time-resolved optical spectroscopy to detect and disentangle the electronic and lattice degrees of freedom. We reveal a fingerprint of the ordered phase at hν𝜏 1 eV: at TCDW, a divergent relaxation timescale and a sign-change of the differential reflectivity indicate that CDW gap opening and PLD formation occur at the same temperature. Remarkably, we show that these effects can be decoupled under moderate photoexcitation, forming a long-lived state in which the electronic order is destroyed, but the lattice distortion is not. Our results and computations suggest an unconventional CDW mechanims in 2H-NbSe2, highlighting the dominant role of the lattice in driving the ordered-phase formation. ....

Riccardo Arpaia

Doping evolution of charge density fluctuations across the entire dome of high-Tc superconductors

Authors: R. Arpaia, L. Martinelli, M. Moretti Sala, S. Caprara, P. Camisa, Q. Li, A. Nag, M. Garcia-Fernandez, K.-J. Zhou, E. Schierle, Y. Y. Peng, T. Bauch, F. Lombardi, N. B. Brookes, C. Di Castro, L. Braicovich, M. Grilli, G. Ghiringhelli

Abstract: Charge order, i.e. the incommensurate modulation of electron density in the CuO2 planes, although quite elusive might be the key for explaining the mysterious properties of high-Tc superconductors(HTS). Indeed charge density waves (CDW) were found in all cuprates,and their interplay with superconductivity is proved by many experimental evidences. However,this phenomenology is present in underdoped samples and below the pseudogap temperature only,casting doubts about their actual role in shaping the transport properties of cuprates either in the normal or superconducting states. By taking advantage of the superior sensitivity of modern Resonant Inelastic X-ray Scattering (RIXS),in the Y123 family we have discovered another form of charge modulation,named charge density fluctuations (CDF) [Science365,906(2019)]. They are very short-ranged -can be viewed as the precursors of CDW- and pervade a large area of the phase diagram, being present at least up to room temperature and in a broad doping range. Notably,these modulations are dynamical,being characterized by small energies in the meV range. Because of their properties, theoretical proposals indicate that they are likely the long-sought excitations underlying the anomalous strange metal behavior of HTS [Commun.Phys.4,7(2021)].

We have recently investigated with RIXS several YBCO and Bi2212 samples,covering the entire superconducting dome, including doping levels where CDF can be studied alone, i.e. without any contribution of CDW. We therefore studied with unprecedented accuracy the doping and temperature evolution of CDF correlation length, intensity and energy. Our results pave the way for a better understanding of the role charge order has in shaping the ground state of HTS. ....

Jan Gospodaric

Energy spectrum of semi-metallic HgTe quantum wells

Authors: J. Gospodaric, A. Shuvaev, N. N. Mikhailov, Z. D. Kvon, E. G. Novik, and A. Pimenov

Abstract: Extensive research on quantum wells based on mercury (HgTe) thin films revealed many exotic physical forms such as 2D Dirac semimetals or a 3D . These layer-thickness-dependent properties result from the inverted band structure of bulk HgTe. The energy dispersion relations of HgTe cannot be studied experimentally with the use of the established technique of angle-resolved photoemission spectroscopy due to the multilayer structure of the present samples. Here, we report an experimental and theoretical study of two HgTe quantum wells in which semi-metallic states with coexisting electrons and holes are realized. Magneto-optical spectroscopy at sub-THz frequencies allowed to obtain properties of the electrons and holes in the systems at all relevant Fermi-level positions. The outcome is also supported by a Shubnikov-de Haas analysis of capacitance measurements. Two accesses properties, the cyclotron mass and the charge density, can be used to construct the electron and hole dispersion relations. Comparison between the experimental band structures and the theoretical models gives a very good agreement, revealing subtle properties such as the band splitting, the overlap between the first valence and the first conduction band, and even the second conduction band. Our study demonstrated that the cyclotron resonance experiments can be used to directly obtain the band structure of 2D materials. ....

45 Alexander Boris

Approaching 2D Superconductivity in Ultrathin DyBa2Cu3O7-δ Films

Authors: R.D. Dawson, K.S. Rabinovich, D. Putzky, G. Christiani, G. Logvenov, B. Keimer, and A.V. Boris Understanding the effect of reduced dimensionality on superconductivity in the layered copper oxide high-Tc superconductors (HTSCs) is key to elucidating the mechanism of HTSC. Despite many efforts, observation of Berezinskii-Kosterlitz-Thouless (BKT) transition in thin film HTSCs remains challenging. BKT physics is very sensitive to the level of disorder, which is highly inherent in thin films and cuprates in general. Based on our systematic study of the superfluid density in a series of ultrathin MBE-grown high quality DyBa2Cu3O7-δ superconducting films with a combination of broadband from submillimeter to UV spectroscopic tools, we address this important issue [1]. Our finding is that the temperature evolution of the superfluid density follows a universal dependence for all films 10 u.c. and thicker. However, at thicknesses below a critical threshold of 7 u.c. the superfluid density deviates from this universal temperature dependence. We argue that this behavior reveals signatures of the BKT vortex unbinding transition near Tc. Our analysis also provides evidence for a sharply defined 4 u.c. non-SC interfacial layer, leaving a quasi-2D SC layer on top. We propose that the SC state in this interfacial layer is suppressed by competing charge order.

[1] R. D. Dawson et al., Phys. Rev. Lett. 125, 237001 (2020). ....

Fabian Mooshammer

Ultrafast low-energy dynamics of excitons in twisted van der Waals bilayers

Authors: F. Mooshammer, P. Merkl, M. Plankl, M. Zizlsperger, T. Siday, K.-Q. Lin, M. Liebich, C.-K. Yong, J. Maultzsch, T. L. Cocker, J. M. Lupton, E. Malic, R. Huber

Abstract: In van der Waals stacks of atomically thin crystals, the twist angle has emerged as a powerful tuning knob for tailoring novel phase transitions[1] – a paradigm shift for condensed-matter physics. Yet, a precise understanding of the underlying Coulomb correlations has remained challenging. Here, we introduce a direct, ultrafast access to Coulomb correlations in van der Waals bilayer structures out of equilibrium[2,3]. Phase-locked mid-infrared pulses interrogate Lyman-like 1s–2p transitions of optically bright and dark excitonic species on subcycle timescales. These transitions manifest as novel resonances in the extracted, complex-valued dielectric response and reveal how the twist angle alone affects the hybridization between the layers thereby renormalizing the exciton binding energy, their lifetime, and the exciton-exciton interaction. Our work significantly advances the concept of tailoring quasiparticles in search of new phases of matter. The recent combination of sub-cycle time-domain spectroscopy with near-field microscopy[4] also paves the way for resolving ultrafast charge carrier dynamics in van der Waals heterostructures on the nanoscale.

1. Balents, L., Dean, C. R., Efetov, D. K. & Young, A. F. Superconductivity and strong correlations in moiré flat bands. Nat. Phys. 16, 725–733 (2020). 2. Merkl, P. et al. Ultrafast transition between exciton phases in van der Waals heterostructures. Nat. Mater. 18, 691–696 (2019). 3. Merkl, P. et al. Twist-tailoring Coulomb correlations in van der Waals homobilayers. Nat. Commun. 11, 2167 (2020). 4. Plankl, M. et al. Subcycle contact-free nanoscopy of ultafast interlayer transport in atomically thin heterostructures. Nat. Photonics, in press (2021). ....

Rocco Vitalone

Cryogenic THz Nano Imaging and Spectroscopy Graphene/RuCl3 Heterostructures

Authors: Rocco Vitalone, Daniel Rizzo, Bjarke Jessen, Ran Jing, Alex Mcleod, Brian Kim, Dmitri Basov

Abstract: A number of recent studies have shown that graphene can be doped to high carrier concentrations when it shares an interface with a number of different materials via a charge transfer process. One such material is α-RuCl3, which has been shown

46 to dope graphene to a fermi energy around 0.6 eV. One question of interest is what effect this charge transfer process has on α-RuCl3. Namely, is α-RuCl3 doped into a metallic state. To investigate this claim, we probe graphene/RuCl3 heterostructures in the low-energy THz regime to analyze the Drude components of both the graphene and the α-RuCl3. Using our home built cryogenic microscope, we both image a graphene/RuCl3 stack using THz light and extract local spectra from 0.5 - 1.5 THz with nanoscale resolution. First, in our imaging measurements we observe clear contrast between the three different regions of our heterostructure stack: graphene/RuCl3, graphene, and α-RuCl3 (listed in order of decreasing contrast). Further, there are clear qualitative differences in the extracted spectra from the three distinct regions. We are in the process of modelling our nano-imaging and spectroscopy data in order to better understand our observations and their underlying physics. ....

Stefano Dal Conte

Interlayer charge transfer and spin/valley dynamics in TMD heterostructures

Authors: Stefano Dal Conte, Veronica Policht, Giulio Cerullo

Abstract: Heterostructures (HS) of semiconducting monolayer transition metal dichalcogenides (TMDs) are an emerging class of nanomaterials. Thanks to the weak interlayer van der Waals interactions, TMD HS are not subject to any lattice mismatch limitations and provide the opportunity for novel materials with tailored electronic and optical properties. In certain TMD HS the staggered band alignment favours ultrafast interlayer charge transfer (ICT) and the formation of long-lived interlayer excitons. A clear picture of ICT in TMD HS is impeded by spectral congestion, caused by multiple excitonic transitions lying close in energy. Ultrafast transient absorption spectroscopy suffers from a trade-off between temporal and spectral resolution. Two-dimensional electronic spectroscopy (2DES) overcomes these limitations and allows one to simultaneously maximize temporal and spectral resolution, making it the ideal technique for the study of systems with a high degree of spectral congestion. Here we apply broadband 2DES with sub-20-fs temporal resolution, to time-resolve ICT processes in a large area MoS2/WS2 HS, which has three close lying excitonic transitions. We unambiguously disentangle and resolve both the interlayer hole transfer and electron transfer processes on sub-100 fs timescale. By using circularly polarized pulses, we furthermore study the spin/valley directional interlayer charge transfer process and we find that spin/valley polarization is efficiently transferred from one layer to another resulting in a valley-dependent circular dichroism signal characterized by a longer lifetime than that of exciton valley polarization signal of isolated TMD layers. ....

Miguel-Ángel Sánchez-Martínez

Optical signatures of multifold fermions in the chiral topological semimetals RhSi and CoSi

Authors: Miguel Ángel Sánchez Martínez, Bing Xu , Zhenyao Fang, Zhuoliang Ni, Jorn W. Venderbos, Fernando De Juan, Eugene J. Mele, Andrew M. Rappe, Adolfo G. Grushin, Liang Wu

Abstract: The chiral topological semimetals RhSi and CoSi exhibit band degeneracies near the Fermi level enforced by the crystal symmetries. The low-energy quasiparticles emerging near these band degeneracies, referred to as multifold fermions, have no counterpart as elementary fermionic particles. We calculate the linear optical conductivity of all chiral multifold fermions[1] and show that it provides an experimental fingerprint for each type of multifold . We use a tight-binding model for space group 198, where RhSi and CoSi crystallize, and obtain the parameters for both materials from first-principles calculations. The results reveal that the location of the chemical potential is crucial to understand the optical response seen in experiments[2,3], determined at low energies by the threefold fermion at the Γ point in both materials, and providing signatures of the existence of a spin-3/2 fourfold fermion in CoSi.

[1] M.-Á S.-M. et al., Phys. Rev. B, 99 155145 (2019). [2] B. Xu et al, PNAS Nov 2020, 117 (44) 27104-27110 [3] Z. Ni et al., arXiv:2005.13473 (2020).

This work is supported by the ANR, grant ANR-18-CE30-0001-01 (TOPODRIVE), and the European Union grant agreements Nº829044 (SCHINES) and Nº 754303 (MSCA GreQue Cofund). ....

47 MengXing Na

Time-and Angle-Resolved Photoemission studies of electron-phonon coupling in graphite

Authors: MengXing Na, Arthur K. Mills, Fabio Boschini, Matteo Michiardi, Benjamin Nosarzewski, Ryan P. Day, Elia Razzoli, Sergey Zhdanovich, Michael Schneider, Doug Wong, Giorgio Levy, Thomas P. Devereaux, Alexander F. Kemper, David J. Jones, Andrea Damascelli.

Abstract: Pump-probe spectroscopies have extended many well-established equilibrium techniques into the time domain. Among them, time-resolved ARPES is especially exciting, as it provides direct access to the electronic structure and many-body interactions on an ultrafast timescale. In the past, the applicability of TR-ARPES has been curtailed by the limited photon energy, energy resolution, and repetition rate of available laser sources, which generally confine experiments to the low-momentum, high-fluence regime. However, the continued maturity of high-harmonic sources now enables detailed, low-fluence TR-ARPES studies over the full Brillouin zone of quantum materials. We apply such a source to the study of electron-phonon coupling on graphite. While many experimental techniques are sensitive to electron-phonon coupling, the measured quantity is usually averaged over electron degrees of freedom or bosonic degrees of freedom. Using a high energy resolution, high repetition rate cavity-based high-harmonic source [1], we observe quantized energy- loss processes that correspond to the emission of strongly coupled optical phonons, which allow for the quantitative extraction of the mode-projected electron-phonon matrix element, for specific initial and final electron states [2]. The features that we observe come from the non-thermal occupation of electrons. In high-fluence experiments, electron dynamics are often described by the evolution of the electronic temperature, which masks the underlying microscopic scattering processes that are rich with information. Using the study of graphite as a benchmark, we use numerical simulations to elucidate non-thermal regimes at high and flow fluences, and further highlight the need for detailed perturbative experiments to exploit non-equilibrium electron properties in materials [3].

[1] Rev. Sci. Instrum. 90, 083001 (2019) [2] Science 366, 1231 (2019) [3] PRB 102, 184307 (2020) ....

Yaxian Wang

Ab initio signatures of phonon-mediated hydrodynamic transport in semimetals

Authors: Yaxian Wang, Georgios Varnavides, Prineha Narang

Abstract: Hydrodynamic electron flow in condensed matters has been one of the most active research areas recently. While progress from both theory and experimental techniques are made, open questions regarding the underlying mechanisms still remain. We utilize ab initio techniques to treat the electron scattering events explicitly, and show in combination with the Boltzmann transport equation a more applicable metric of hydrodynamic transport taking into account temperature, channel width, and impurity length, which can be directly verified by various experimental techniques. By investigating different electron scattering mean free paths in WTe2, PdCoO2, and other metallic phases, we show that that phonon mediated electron-electron interaction could lead to much shorter momentum conserving mean free path (lmc) than momentum relaxing lmr, facilitating hydrodynamic behavior in systems where the direct Coulomb interaction is largely screened. We further discuss the dynamics of phonons through the lifetime hierarchy from interactions with electrons and the lattice, and confirm that phonons do play an important role in the electronic transport phenomena. Finally, we propose a few key ingredients including high carrier mobility, large electron-phonon matrix elements, and slow phonon-phonon scattering. The plethora of low symmetry crystals whose Fermi surfaces are composed of d orbitals from the transition metal and p orbitals from the metalloids provides a much larger pool for further study. This work provides ab initio signatures of material-specifics to explore hydrodynamic electron flow in a much larger family of condensed matter systems, and thus offers insights into further study of electron interactions through transport phenomena. ....

48 Bohm Jung Yang

Wave function geometry and anomalous Landau levels of flat bands

Authors: Jun Won Rhim, Kyoo Kim, Yoonseok Hwang, Junseo Jung, Bohm-Jung Yang

Abstract: Semiclassical quantization of electronic states under magnetic field describes not only the Landau level spectrum but also the geometric responses of metals under a magnetic field. However, it is unclear whether this semiclassical idea is valid in dispersionless flat-band systems, in which an infinite number of degenerate semiclassical orbits are allowed. Here we show that the semiclassical quantization rule breaks down for a class of flat bands, including singular flat bands and isolated flat bands. The Landau levels of such a flat band develop in the empty region in which no electronic states exist in the absence of a magnetic field, and exhibit an unusual dependence on the Landau level index n, which results in anomalous orbital magnetic susceptibility. The total energy spread of the Landau levels of flat bands is determined by the quantum geometry of the relevant Bloch states, which is characterized by their Hilbert–Schmidt quantum distance and fidelity tensors. The results indicate that the anomalous Landau level spectrum of flat bands is promising for the direct measurement of the quantum geometry of wavefunctions in condensed matter. ....

Sandeep Joy

Transparent mirror effect in twist-angle-disordered bilayer graphene

Authors: Sandeep Joy, Saad Khalid, and Brian Skinner

Abstract: Motivated by recent observations of spatial fluctuations of twist angle in twisted bilayer graphene (TBG) samples, we investigate the effect of such twist angle fluctuations on the transport of Dirac electrons. Variations in twist angle produce spatial modulation of the Fermi velocity as well as a random gauge field. We consider a quasi-one-dimensional model of disorder, which has a direct analogy with the propagation of light in a medium with random refractive index. In such situations Anderson localization of light leads to an exponential decay of transmitted intensity, known as the "transparent mirror effect." In the case of Dirac electrons, however, we show that the localization length depends strongly on the angle of incidence and diverges at a nonzero "Brewster angle". This divergence leads to a power-law decay of the transmission when averaged over incidence angles. Our results have direct implications for the conductivity and Fano factor of TBG samples. They also suggest a mechanism for disorder-induced collimation, valley filtration, and energy filtration of Dirac electron beams, so that TBG offers a promising new platform for Dirac fermion optics.

49 Tuesday, July 6

Prachi Sharma

Optical conductivity of Dirac Fermi liquid

Authors: Prachi Sharma, Alessandro Principi, Dmitrii Maslov

Abstract: A Dirac-Fermi liquid (DFL) —a doped system with Dirac spectrum—is a special and important subclass of non-Galilean- invariant Fermi liquids (FLs) which includes, e.g., graphene and the surface state of a three-dimensional topological insulator. We study the effect of electron-electron interactions on the optical conductivity of a DFL. It is shown that the effective current relaxation rate behaves as $1/\tau_J\sim \left(3\omega^4 + 20 \pi^2\omega^2 T^2+ 32 \pi^4 T^4\right)/\mu^3$ for $\max\{\ omega,T\}\ll \mu$, where $\mu$ is the chemical potential. The quartic term in $1/\tau_J\propto$ competes with a small FL-like term, $\left(\omega^2+4\pi^2 T^2\right)/\mu$, due to weak trigonal warping of graphene dispersion. In the presence of weak disorder, the optical conductivity is described by the sum of two Drude-like terms, with widths given by the electron-electron and electron-impurity scattering rates, respectively. The $dc$ resistivity varies non-monotonically with temperature, approaching the identical values given by the residual resistivity in the limits of both low and high $T$, with a maximum in between. We also calculated the dynamic charge susceptibility, $\chi_c(\bq,\omega)$, outside the particle-hole continua and to one-loop order in the dynamically screened Coulomb interaction. We find the dissipative part of $\chi_c(\bq,\omega)$ scales as $q^2\omega$. ....

Fahad Mahmood

Observation of a marginal Fermi glass using THz 2D coherent spectroscopy

Authors: Fahad Mahmood, Dipanjan Chaudhuri, Sarang Gopalakrishnan, Rahul Nandkishore, N. P. Armitage

Abstract: A long-standing open problem in condensed-matter physics is whether or not a strongly disordered interacting insulator can be mapped to a system of effectively non-interacting localized excitations. Using terahertz two-dimensional coherent spectroscopy, we investigate this issue in phosphorus-doped silicon, a classic example of a correlated disordered electron system in three dimensions. Despite the intrinsically disordered nature of these materials, we observe coherent excitations and strong photon echoes that provide us with a powerful method for the study of their decay processes. We extract the energy relaxation and decoherence rates close to the metal–insulator transition. We observe that both rates are linear in excitation frequency with a slope close to unity. The energy relaxation timescale counterintuitively increases with increasing temperature, and the coherence relaxation timescale has little temperature dependence below 25 K, but increases as the material is doped towards the metal–insulator transition. Here we argue that these features imply that the system behaves as a well-isolated electronic system on the timescales of interest, and relaxation is controlled by electron–electron interactions. Our observations constitute a distinct phenomenology, driven by the interplay of strong disorder and strong electron–electron interactions, which we dub the marginal Fermi glass. ....

Elena Bascones

Correlated states in graphene based moiré systems

Authors: M.J. Calderón, A. Camjayi and E. Bascones

Abstract: The discovery of superconducting and insulating states in magic angle twisted bilayer graphene (MATBG) in 2018 opened a new area in the study of strongly correlated systems, the correlations in flat band moiré systems. The different moiré systems showing insulating states is growing fast. The correlated states in these systems are highly tunable with gate voltages, magnetic fields or via the alignment with the substrate. However the driving force of these correlated states is still under discussion. In the talk I will discuss several aspects of the interactions and correlated states in two graphene based moiré systems: MATBG and trilayer ABC-graphene on boron nitride (ABC-TLG/hBN).

50 ....

Sharareh Sayyad

Pairing and non-Fermi liquid behavior in partially flat-band systems

Author: Sharareh Sayyad

Abstract: There is an increasing fascination with superconductivity that involves flat bands [1]. Usually one considers multi-band systems that consist of a flat band along with dispersive one, where virtual pair scatterings between the flat and dispersive bands can enhance superconductivity. Now, the case of one-band systems whose band dispersion contains a flat portion should also be interesting because pair scatterings between the flat and dispersive parts may also enhance superconductivity. The large density of states in the flat region also makes the correlation physics interesting, where one can question whether non-Fermi liquid behavior can arise. Motivated by this, here we propose a simple one-band model for a partially flat-band system with an on-site Hubbard repulsion. The DMFT+FLEX formalism is used to obtain the electronic structure and pairing symmetry [2]. The results for the pair correlation at various doping concentrations exhibit that singlet pairing is dominant close to the half-filling. The structure of the gap function in the momentum and real spaces shows versatile pairing symmetries, where a pair can be significantly extended in real space, which is a consequence of the partially flat band. We have also observed a non-Fermi liquid behavior, as seen from the low-frequency behavior of the self-energy and the momentum-dependent distribution function in moderate (U

[1] See, e.g., K. Kobayashi, et al, Phys. Rev. B 94, 214501 (2016). [2] Sh. Sayyad, et al, Phys. Rev. B 101, 014501 (2020). ....

Leslie M. Schoop

From chemical bonds to topology

Author: Leslie M. Schoop

Abstract: As chemists, we are familiar with guidelines and heuristics that help us to predict how chemical reactions will proceed. My group is interested to expand these heuristics to understand if we can predict topological materials, which is a class of quantum matter. In this talk, I will show how delocalized chemical bonds in certain structural networks allow us to define chemical descriptors that predict band inversions. Using these descriptors, we found a layered, antiferromagnetic van der Waals material with very high mobility. This is the first time that these properties are combined in one material, which is promising for applications in novel types of data storage or computing devices. We further implemented our heuristics to discover novel complex topological phases, including magnetic ones, and phases that are in competition with complex structural distortions. I will show how structural distortions can have a positive effect on topological band structures.

If time allows, I will also briefly discuss the concept of chemical exfoliation. With this method, we can exfoliate materials for which the scotch tape method fails. I will show how we were able to synthesize a new chromium chalcogenide this way, which might be a new 2D magnetic material. ....

Ming Yi

Room-Temperature Topological Phase Transition in Quasi-1D Bi4I4

Authors: Jianwei Huang, Sheng Li, Chiho Yoon, Ji Seop Oh, Han Wu, Xiaoyuan Liu, Nikhil Dhale, Yan-Feng Zhou, Yucheng Guo, Yichen Zhang, Makoto Hashimoto, Donghui Lu, Jonathan Denlinger, Xiqu Wang, Chun Ning Lau, Robert J. Birgeneau, Fan Zhang, Bing Lv, and Ming Yi

Abstract: Quasi-one-dimensional (1D) materials provide a superior platform for characterizing and tuning topological phases for two

51 reasons: i) existence for multiple cleavable surfaces along the crystal chain direction that enables better experimental identification of topological classification, and ii) stronger response to perturbations such as strain for tuning topological phases. In this work, we present experimental evidence for a room-temperature topological phase transition in the quasi-1D material Bi4I4. In particular, combining angle-resolved photoemission spectroscopy on multiple naturally cleaved surfaces, transport, and x-ray diffraction, we report a room-temperature topological phase transition between a weak topological insulator phase and a higher order topological insulator phase via a first-order structural transition that modifies the stacking of the weakly coupled chains. ....

Ece Uykur

Optical fingerprints of unconventional carriers in kagome metals

Author: Ece Uykur

Abstract: Kagome metals are the newly emerging class of quantum materials, where the peculiar kagome structure along with the itinerant character of the electrons give rise to a non-trivial combination of entangled magnetic states, electronic correlations, and topological orders. Two dissimilar features of massive, localized carriers at dispersionless flat bands and the massless Dirac fermions at the linearly dispersing bands coexist in their electronic structure, giving rise to various exotic phenomena.

Optical spectroscopy is a useful method that shows excellent sensitivity to both linear and flat bands. The opportunity to combine with other external stimuli such as magnetic field and pressure offers a viable route to disentangle different contributions. In this talk, I will highlight the optical findings in these unique materials. ....

Graham Baker

Non-local microwave electrodynamics in ultrapure PdCoO2

Authors: Graham Baker, James Day, Seunghyun Khim, Mohamed Oudah, Roderich Moessner, Jörg Schmalian, Andrew Mackenzie, D.A. Bonn

Abstract: We present microwave spectroscopy measurements of ultrapure PdCoO2. Measurements for two different crystal orientations reveal anisotropy that would be symmetry-forbidden in the context of local electrodynamics. In the first orientation, we observe a crossover from the local classical skin effect (CSE) to the non-local anomalous skin effect (ASE). In the second orientation, we also observe a high-frequency deviation from the CSE, but with a frequency dependence that is inconsistent with the standard theory of the ASE [1]. We suggest that this discrepancy arises as a result of the strongly-faceted, nearly-hexagonal Fermi surface of PdCoO2 [2]. We explore the close connection of our measurements to those of the DC resistivity of mesoscopic channels [3], showing that both can be described with the same Boltzmann model. Finally, we discuss in general how ballistics, hydrodynamics, and anisotropic Fermi surfaces can be probed via AC electromagnetic measurements.

[1] A. Pippard, Proceedings of the Royal Society A 224, 273 (1954) [2] A. Mackenzie, Reports on Progress in Physics 80, 32501 (2017) [2] arXiv:2103.01332 ....

Yasunori Toda

Systematic study of photoinduced quasiparticle dynamics in Bi-based cuprates with out-of-plane disorder

Authors: Y. Toda, S. Tsuchiya, S. Katsumata, M. Oda, T. Mertelj, D. Mihailovic

Abstract: We performed optical time-resolved pump-probe spectroscopy on the optimally-doped (OPD) Bi2+xSr2-xCaCu2O8+d (Bi2212) with out-of-plane disorder induced by Bi-Sr substitution. Four samples with different composition ratios (x = 0.1, 0.15, 0.2, 0.25) optimized for OPD were measured by a combination of near-ultraviolet (pump) and near-infrared (probe) short laser

52 pulses. The temperature dependences of the pseudogap (PG) quasiparticle dynamics show an increase in T* with Bi-Sr substitution, supporting an increase in disorder. The development of the PG with the substitution is also confirmed by the dramatic increase in the photodestruction threshold for the PG, which is in contrast to the nearly constant change in that for the superconducting quasiparticles. The quasiparticle dynamics of the sample with x=0.25 ear almost comparable to those of the underdoped sample. The systematic substitution dependence of the PG quasiparticle dynamics shows that the relaxation time increases as T approaches Tc from above, and that this increase becomes more gradual with the development of the PG. The result implies that the interaction between the PG and the superconducting quasiparticles is moderated by the disorder. ....

Alfred Zong

Disentangling fluctuations from long-range order in a light-induced phase transition

Authors: Alfred Zong, Pavel E. Dolgirev, Anshul Kogar, Yifan Su, Xiaozhe Shen, Joshua A. W. Straquadine, Xirui Wang, Duan Luo, Michael E. Kozina, Alexander H. Reid, Renkai Li, Jie Yang, Stephen P. Weathersby, Suji Park, Edbert J. Sie, Pablo Jarillo-Herrero, Ian R. Fisher, Xijie Wang, Eugene Demler, Nuh Gedik

Abstract: Engineering novel states of matter with light is at the forefront of materials research. Central to our understanding of photoinduced states is an accurate description of two key concepts involved in a phase transition: long-range order and order parameter fluctuations. While long-range order is often shown to be suppressed by photoexcitation, experimental information on the fluctuation dynamics has so far remained out of reach. Taking advantage of two competing charge density waves (CDWs) in the rare-earth tritelluride family, we gain independent access to the long-range order and fluctuations through the dominant and subdominant CDW orders, respectively. We demonstrate that order parameter fluctuations are prominently enhanced by a light pulse, giving rise to an excited state that closely resembles the equilibrium critical regime near the CDW transition temperature. Our finding fills the gap in our understanding of fluctuation dynamics during a photoinduced phase transition. It further suggests that materials with strong fluctuations in equilibrium are promising platforms to host "hidden'" orders after photoexcitation. ....

Kota Katsumi

Photoexcited nonequilibrium state of underdoped YBa2Cu3Oy studied by the terahertz nonlinear optical responses

Authors: Kota Katsumi, Morihiko Nishida, Shigeki Miyasaka, Setsuko Tajima and Ryo Shimano

Abstract: The observation of superconducting-like 1/ω-response in the c-axis optical conductivity that emerges far above the superconducting critical temperature Tc in underdoped YBa2Cu3Oy (YBCO) under intense laser pulse irradiation has caused excitement among the broad field of condensed matter science [W. Hu et al., Nat. Mater. 13, 705 (2014)]. Since then, various theoretical and experimental studies have been devoted to elucidating its microscopic origin. One remaining ambiguity is that one cannot distinguish the superconducting response and the Drude response of the quasiparticle excitation with an extremely low scattering rate from the measured optical conductivity in the terahertz (THz) frequency range. The experimental difficulty is that, since the 1/ω-like transient response relaxes in a few picoseconds after the photoexcitation, an ultrafast probe for superconducting order parameter with picosecond time resolution is required to elucidate the photo-induced nonequilibrium state. To this aim, we investigate the photo-induced nonequilibrium state in YBCO using the THz nonlinear optical responses arising from the superconducting collective excitations: the Higgs mode and the ac-driven Josephson current. We have indeed observed the photo-induced 1/ω-like increase in the imaginary part of the c-axis optical conductivity above Tc, consistent with the previous studies. In such a photoexcited state, neither the THz nonlinear response of the Higgs mode nor that of the Josephson current are observed, indicating that the photo-induced state exhibiting the 1/ω-like response is distinct from the superconductivity in equilibrium. The absence of the superconducting collective excitations after the photoexcitation provides an essential clue to understanding the photo-induced nonequilibrium state. ....

53 Pavel Volkov

Electronic phase diagram of the excitonic insulator candidates Ta2Ni(Se1-xSx)5 probed by Raman scattering

Authors: P. A. Volkov, M. Ye, H. Lohani, I. Feldman, A. Kanigel, M. Kim, and G. Blumberg

Abstract: Excitonic insulator (EI) is a phase driven by Coulomb attraction between electrons and holes leading to a proliferation of particle-hole pairs. EIs break the lattice symmetries, raising the question of whether a particular transition is excitonic or structural. Recently, the transition origin in a candidate material Ta2NiSe5 has become a subject of interest, with both excitonic and lattice mechanisms proposed.

I will report the results of the study of collective excitations in Ta2NiSe5 with polarization-resolved Raman scattering that allows to selectively probe the quadrupolar ones with the symmetry of the order parameter. We observe an overdamped electronic mode, consistent with excitonic fluctuations in a semimetal, softening at the transition temperature, indicating a strong electronic contribution to ordering. At the same time, the optical phonons do not soften, leaving the interplay of the excitonic mode and acoustic strain as the transition origin. On cooling, we demonstrate the gradual emergence of coherent superpositions of band states at the gap edge, with strong departures from mean-field theory predictions. Extending our measurements to the Ta2Ni(Se1-xSx)5 family, we find a strong suppression of the electronic contribution to ordering with x. In Ta2NiS5, we observe a sharp in-gap exciton that does not soften, instead of an overdamped mode, consistent with a suppression of the EI instability by the band gap. At the same time, we detect broken symmetry to be present for all x, indicating that the transition becomes fully elastically driven for large S content. ....

Yang Yang

Raman scattering in spin-orbit coupled Mott insulators: application to β-Li2IrO3

Authors: Yang Yang, Mengqun Li, Ioannis Rousochatzakis, and Natalia B. Perkins

Abstract: We present a theoretical study of the magnetic Raman response in the Mott insulator systems with strong spin-orbit coupling. We show that in these systems both one-magnon and two-magnon Raman scattering can originate from the same exchange mechanism. We argue that the dominant contribution to the one-magnon response comes from the microscopic processes beyond the Loudon-Fleury approach. We apply our theory to the Kitaev magnet β-Li2IrO3. ....

Abhishek Kumar

Zero-field electron spin resonance in graphene with proximity-induced spin-orbit coupling

Authors: Abhishek Kumar, Saurabh Maiti, and Dmitrii Maslov

Abstract: We investigate the spectrum of spin excitations in graphene with proximity-induced spin-orbit couplings, both of the Rashba and valley-Zeeman types. We show that such a system exhibit both a zero-field electron spin resonance (ESR) and zero-field electron dipole spin resonance (EDSR). We analyze the effect of electron-electron interaction within a two-valley Fermi-liquid theory and show that the spin-exchange interaction splits the resonance peak into two. This effect can be used to extract the parameters of many-body interactions that are not accessible by other methods. ....

Calvin Pozderac

Magnetoresistance from Guiding Center Drift of Two-Dimensional Electrons in a Moiré Potential

Authors: Calvin Pozderac, Brian Skinner

54 Abstract: In moiré materials, the moiré pattern leads to a periodic electric potential throughout the material. Here we examine the low-density, two-dimensional, semiclassical magnetoresistance in such a moiré potential. In the presence of a sufficiently large magnetic field, electron trajectories experience a drift of their guiding centers along equipotential contours, in addition to the rapid cyclotron motion. Scattering from impurities or phonons allows electrons to hop from one equipotential contour to another, and at large enough magnetic field this process dominates their diffusion. The guiding center motion together with scattering leads to a modified random walk in which the electrons move along the moiré equipotential lines punctuated with random kicks of the cyclotron radius. We study the resulting electron diffusion constant, which determines the electrical resistance. Using scaling arguments and numerical simulations, we find regimes in which the magnetoresistance scales as B^0, B^(1/2), and B. We further apply our method to a two-dimensional electron system with a smooth disorder potential and obtain regimes in which the magnetoresistance scales as B^0, B^(10/13), B^(10/7), and B^(12/7). ....

Daniel J. Rizzo

Charge-Transfer Plasmon Polaritons at Graphene/α-RuCl3 Interfaces

Authors: Daniel J. Rizzo, Bjarke S. Jessen, Zhiyuan Sun, Francesco L. Ruta, Jin Zhang, Jia-Qiang Yan, Lede Xian, Alexander S. McLeod, Michael E. Berkowitz, Kenji Watanabe, Takashi Taniguchi, Stephen E. Nagler, David G. Mandrus, Angel Rubio, Michael M. Fogler, Andrew J. Millis, James C. Hone, Cory R. Dean, D.N. Basov

Abstract: The fundamental opto-electronic properties of two-dimensional (2D) materials can be tailored based on their nanoscale charge environment. While electrostatic doping offers a means of wholesale tuning of 2D charge densities, the minimum size of charge features is limited by fields fringing through relatively thick gate insulators. Conversely, charge transfer at the interface of two atomically-thin layers with different work functions should not be subject to such limitations. Specifically, the large work function of α-RuCl3 (6.1 eV) makes it an ideal 2D electron acceptor. In our study, we exploit this behavior to generate charge-transfer plasmon polaritons (CPPs) in graphene/α-RuCl3 heterostructures. Using infrared near-field optical microscopy we measure the CPP dispersion, yielding a quantitative measure of the graphene Fermi energy (~0.6 eV) and thus the charge exchanged between 𝜏 -RuCl3 and graphene (~2.7x10^13 cm^–2). Concurrently, we observe dispersive edge modes and internal “circular” CPPs which reveal sharp (< 50 nm) changes in the graphene optical conductivity that correspond to nanoscale modulations in the graphene doping level. Further analysis of the CPP losses implies the presence of emergent optical conductivity in the doped interfacial layer of α-RuCl3 and suggests that it no longer possesses a Mott insulating ground state. Our results demonstrate that using high work function materials such as α-RuCl3 in Van der Waals heterostructures presents new opportunities for controlling the local charge carrier density of graphene and other 2D materials on nanometer length scales in excess of what can be achieved with an external gate. ....

David Barbalas

Deviations from Matthiessen’s rule in PdCoO2 Thin Films

Authors: Barbalas, David; Legros, Anaëlle; Rimal, Gaurab; Seongshik Oh; Armitage, Peter

Abstract: The quasi-2D delafossite material PdCoO2 is a strongly correlated metallic system that forms a natural analogue of a heterostructure due to its layered crystal structure. The metallic conducting layers of Pd atoms are separated by insulating CoO2 octahedral and gives rise to large anisotropic transport and results in the extremely low residual resistivity, even lower than that of noble metals (Cu, Ag, Au). Recent material developments have allowed for clean MBE thin films of PdCoO2 to be synthesized and we have used time-domain THz spectroscopy to enable a study of the low-frequency electrodynamics. The observed complex conductivity shows narrow Drude-like features and can be fit with a single Drude term. Analysis of the optical scattering rate demonstrates T-linear resistivity at high temperatures as expected for a simple single band metallic system. However, we observe deviations from Matthiessen’s rule as a function of film thickness and find evidence for an extended low temperature Fermi liquid phase characterized by the T^2 dependence of the scattering rate. As this is not observed in dc transport, this suggests that at low temperatures the optical scattering rate has a dominant contribution from electron-electron umklapp scattering arising from the hexagonal Fermi surface in these materials. ....

55 Dominik M. Juraschek

Highly confined phonon polaritons in monolayers of oxide perovskites

Authors: Dominik M. Juraschek, Prineha Narang

Abstract: The confinement of light hybridized with collective excitations in solids (polaritons) promises to enable new ways of dynamical control of matter, such as radiative electronic transitions [1] and redistribution of vibrational energy [2]. Two-dimensional (2D) materials have become a prominent platform for polaritonic applications, where hexagonal boron nitride (hBN) is the gold standard for 2D phonon-polaritons that confine light to volumes several orders of magnitude smaller than in free space [1,2]. Recently, freestanding monolayers of oxide perovskites have been synthesized, which possess highly infrared-active phonon modes and a complex interplay of competing interactions [3].

Here, we show that phonon polaritons in these oxide-perovskite monolayers can keep up with the benchmarks set by hBN and confine electromagnetic radiation to extremely small volumes in the technologically important low mid-infrared and terahertz spectral region. Specifically, we use a combination of first-principles calculations and phenomenological modeling to compute central polaritonic figures of merit, such as confinement, propagation quality, and deceleration factors, for monolayers of SrTiO3, KTaO3, and LiNbO3 and show that they are comparable to those found in hBN. Our results suggest that monolayers of oxide perovskites may become an emergent platform for 2D polaritonics in the near future that enable possibilities to control complex phases of matter through strongly enhanced electromagnetic fields.

[1] Siyuan et al., Adv. Mater. 1806603 (2019) [2] Rivera, Christensen, and Narang, Nano Lett. 19, 2653 (2019) [3] Dianxiang et al., Nature 570, 87 (2019) [4] Juraschek and Narang, arXiv:2103.06949 (2021) ....

56 Wednesday, July 7

Alain Sacuto

Energy scales in cuprate superconductors revealed by electronic Raman spectroscopy

Author: Alain Sacuto

Abstract: High-Tc cuprate superconductors are one of the iconic quantum materials. Although discovered more than 35 years ago, the complexity of their physics remains misunderstood. It calls for new concepts where the quantum electronic orders of matter are no longer independent of each other as in traditional materials but they are intertwined [1]. In order to understand the physics of cuprates, it is important to identify the energy scales that underlie their (T-p) phase diagram [2]. Here, carrying out electronic Raman scattering measurements, we have succeeded in probing these distinct energy scales. In particular, we reveal the charge density wave energy scale and discussed its relationship to the more exhaustively studied pseudogap and superconducting gap energy scales. Our findings [3-3] provide important clues on the connections between the different quantum electronic orders in cuprates.

[1] B. Keimer et al. “From quantum matter to high-temperature superconductivity in copper oxides”, Nature 518, 179 (2015). [2] T and p are respectively the temperature and the hole carrier’s concentration. [3] B. Loret et al. Intimate link between Charge Density Wave, Pseudogap and Superconducting Energy Scales in Cuprates, Nature Physics 15, 771 (2019). [4] B. Loret et al . “Universal relationship between the energy scales of the pseudogap phase, the superconducting state and the charge density wave order in copper oxide superconductors”, Phys. Rev. B 101, 214520 (2020) [5] N. Auvray et al., Nematic Fluctuations in the Cuprate Superconductor Bi2Sr2CaCu2O8+δ, Nature Comm. 10, 5209 (2019) ....

Sean Hartnoll

Planckian electrons and phonons in strange metals

Author: Sean Hartnoll

Abstract: The room temperature thermal diffusivity of strange metals is often dominated by phonons (unlike in conventional metals). This allows the scattering of phonons by electrons to be discerned. I will argue that the measured strength of this scattering suggests a converse Planckian scattering of electrons by phonons across the room temperature phase diagram of these materials. I will discuss various aspects of the interplay between electronic and electron-phonon interactions in strange metals. ....

Avraham Klein

A critical theory of quantum ferroelectric metals

Authors: Rafael Fernandes, Avraham Klein, Vladislav Kozii, Jonathan Ruhman

Abstract: According to standard lore, metallicity and should be mutually exclusive phenomena, and if they were to coexist, they should be very weakly coupled. In sharp contrast with this view, experiments have identified dozens of compounds, typically doped semimetals or semiconductors, that can undergo a quantum phase transition to a ferroelectric state with broken inversion symmetry and signatures of strong correlations. I will discuss a theory of the quantum critical behavior of these systems, based on a model of soft transverse phonons interacting with electrons via spin-orbital or inter-orbital couplings. The system evinces a rich phase diagram including non- and marginal- Fermi liquid behavior, enhanced pairing, and quantum order-by-disorder. It is also highly tunable via strain, both homogeneous and inhomogeneous, making it a prime candidate for quantum device applications. ....

57 Assa Auerbach

Hall anomalies in strongly correlated metals and superconductors Author: Assa Auerbach

Abstract: Fermi-liquid with Boltzmann transport theory fails to explain ‘’Hall anomalies near Mott phases of strongly correlated metals and flux flowing superconductors. Recent advances of our group include new exact formulas for Hall and Thermal Hall coefficients which allow us to explain enigmatic density dependences and Hall sign reversals in the 2D Hubbard and t-J models and Weyl semimetals. A theory of moving vortex charge in superconductors can quantitatively explains Hall sign reversals in superconductors.

1.Hall Number of Strongly Correlated Metals, A. Auerbach, Phys. Rev. Lett. 121, 066601 (2018). 2. Equilibrium formulae for transverse magnetotransport of strongly correlated metals, A. Auerbach, Phys. Rev. B 99, 115115 (2019) 3. Hall Coefficient of Semimetals, A. Samanta, D. P. Arovas, and A. Auerbach, Phys. Rev. Lett. 126, 076603 (2021). ....

Hector Pablo Ojeda Collado

Emergent parametric resonances and time-crystal phases in driven BCS systems

Authors: H. P. Ojeda Collado, Gonzalo Usaj, C. A. Balseiro, Damian H. Zanette, and Jose' Lorenzana

Abstract: Parametric oscillators are the cornerstone of many emergent technologies with a broad range of applications.They are used in the fabrication of sensors, to amplify small electromagnetic signals for dispersive qubit readout, to create squeezed light and entangled photons and represent a fundamental building block to quantum information processing. In this work, we demonstrate that periodically driven BCS systems with electron-hole symmetry show non-trivial parametric resonances that emerge exclusively because the collective interactions between quasiparticles. We identify the relevant order parameter encoding the resonances and dynamical phase transitions (DPTs). The phase diagram shows multiple Arnold tongues that appear when the perturbation frequency matches 2∆0/n with n a natural number and ∆0 the equilibrium superconducting gap. Outside these tongues, due to synchronization, the system exhibits a collective Higgs-like mode which can be launched even using small perturbations.On the another hand, deep inside the Arnold tongues, we observe the emergence of many-body time-translational symmetry breaking phases with period- doubling oscillations of the superconducting order parameter. Our results are directly relevant to cold-atom experiments and provide a benchmark in the quest for the precise quantum control of non-equilibrium superconducting systems for technological applications. ....

Mark D. Thomson

Towards a complete, broadband picture of collective modes in incommensurate charge-density-wave systems

Authors: Mark. D. Thomson, Konstantin Warawa, Fanqi Meng, Damir Dominko, Jure Demsar, Hartmut. G. Roskos

Abstract: The collective phase modes in incommensurate charge-density-wave (CDW) materials are a sensitive probe of the symmetry-broken state, as these modes (occurring in the THz range) typically only become IR-active due to coupling between Raman-active phonons and the spatial electronic modulation. Hence, as per the Goldstone “phason” mode (close to zero frequency), they are much more closely tied to CDW current motion and impurity pinning, compared to the Raman-active amplitude modes. Here we present temperature-dependent experiments on two well-established quasi-1D CDW systems, blue bronze (K0.3MoO3) and the Weyl semimetal (TaSe4)2·I, using ultrabroadband THz time-domain-spectroscopy, and resolve phase modes up to ~10 THz. We analyze the temperature dependence of the full spectrum of modes, combined with the complementary amplitude modes from impulsive optical pump-probe spectroscopy, and assess the applicability of a time-dependent Ginzburg-Landau model in each case, which yields coupling strengths between the bare modes and the electronic order parameter. For the Weyl semimetal (TaSe4)2·I, where the CDW mixes carriers of different chirality, it has been shown by others groups that this sliding phason exhibits axionic magnetotransport, raising the intriguing question of in how far such effects extend to the finite- frequency phase modes.

58 ....

Stephan Bron

Imaging the insulator-to-metal transition of thin-film VO2 with sSNOM and AFM IR

Authors: Stephan Bron, Erik van Heumen

Abstract: The insulator-metal transition (IMT) in VO2 has been of much interest for its potential application in neural networks, ultrafast switches and more. Recently, scattering scanning near-field optical microscopy (sSNOM) demonstrated that electronic phase separation accompanies the first order structural phase transition in VO2 thin films, single crystals and nanobeams. Here I present such measurements taken on a VO2 thin film grown on top of Ti0.87O2 membranes, which enables transfer of the film to technologically relevant substrates. My results demonstrate that the IMT in these films shows similar electronic phase separation as observed previously. In addition, I will present the first measurements on VO2 using AFM-IR, which provides complementary nanoscale information on the optical and thermodynamic properties of these films. Direct comparison of co-localized sSNOM and AFM-IR data highlights intriguing differences. From an analysis of filling fractions I present evidence for two temperature regimes where the transition takes place. ....

Xuanbo Feng

Elucidating the role of electronic correlations in Van der Waals materials VSe2 and Se doped TaS2 with optical spectroscopy

Authors: X. Feng, J. Groefsema, J. Henke, C. Morice, C. J. Sayers, E. Da Como, J. van Wezel, E. van Heumen

Abstract: Transition metal dichalcogenides (TMDCs) display a wide variety of ordered phases due to an interplay of lattice, orbital and spin degrees of freedom. The detailed balance between various interactions can lead to surprising differences between otherwise very similar materials. Here we will present a detailed IR optical spectroscopy study of two TMDCs, VSe2 and TaS1.2Se0.8. The former features a charge density wave (CDW) transition where the transition from the normal to the CDW state is accompanied by a lowering of the resistivity. The latter features several transitions, each accompanied by an increase of the resistivity. To further elucidate the origin of this striking difference, the bulk sensitive optical response can provide new insight. The frequency dependence gives insight in CDW gap sizes, the changes in the IR active lattice vibrations and, in the case of TaS1.2Se0.8 an estimate of the Hubbard gap associated with the Mott insulating phase at low temperatures. The detailed temperature dependence and comparison between the two materials provides new insight in the evolution of the CDW phases in transition metal dichalcogenides. ....

Dániel Datz

Polariton-enhanced molecular absorption in boron nitride nanotubes: experiments and numerical calculations

Authors: Dániel Datz, Gergely Németh, Áron Pekker and Katalin Kamarás

Abstract: Chemical reactions in confined space have been an important topic of research in the past three decades, with applications ranging from catalytic reactions, through storage to biochemical applications. The detection of the outcome of these reactions is sometimes hindered by the container (nanotube) itself, especially in the IR spectrum, where the free charge carriers in carbon nanotubes effectively shield the infrared response of the encapsulated molecules. Recently, near-field spectroscopy has been extensively applied to analyze the collective optical response of materials at the nanoscale. We show that phonon-polaritons of boron nitride nanotubes (BNNT) enhance the near-field vibrational spectra of molecules in close proximity to the surface. By encapsulating C60 fullerene in BNNTs, we can reach a sensitivity level of a few hundred molecules. Furthermore, we show by the photopolymerization of C60 that products of chemical reactions inside the tubes can be identified, so long as their vibrational signatures lie in the Reststrahlen band of the BNNT.

59 To further characterize the enhancement mechanism, numerical calculations have been developed based on the Mie scattering formalism. Incident and reflected fields are represented as linear combinations of spherical harmonics. Interaction between particles and the surface are treated by expanding the scattered and the scattered-reflected field as the same spherical harmonics, and solving the Maxwell equations with the appropriate boundary conditions. The scattered fields are treated in a self-consistent way, fully describing the interactions. This multipole reflection theory is capable of describing scattering processes from surfaces to arbitrary precision. ....

Philipp Eck

Design and realization of a triangular QSHI: Indenene Authors: Philipp Eck, Maximilian Bauernfeind, Jonas Erhardt, Pardeep K. Thakur, Judith Gabel, Tien-Lin Lee, Jörg Schäfer, Simon Moser, Domenico Di Sante, Ralph Claessen, and Giorgio Sangiovanni

Abstract: The key to engineer large-gap quantum spin Hall insulators is a strong spin-orbit interaction. In Kane and Mele’s suggestion for honeycomb layers, SOC is promoted via a relatively weak second neighbor hopping process. Bismuthene however has impressively proven the superiority of local/atomic SOC of the in-plane p-orbitals. A new possibility arises by halving the bismuthene honeycomb lattice but enriching the orbital subspace, i. e. considering a full p-basis on a triangular lattice. Here, we conceive and realize for the first time a triangular QSHI, ‘’indenene’’, a triangular monolayer of indium on SiC(0001) exhibiting non-trivial valley physics as a consequence of strong local spin-orbit coupling. Via tunneling microscopy of the 2D bulk we identify the quantum spin Hall phase and unveil a hidden honeycomb charge localization emerging from interference patterns in the valley Bloch wave functions. ....

Niklas Wagner

Resistivity Exponents in 3D-Dirac Semimetals From Electron-Electron Interaction

Authors: Niklas Wagner, Sergio Ciuchi, Alessandro Toschi, Björn Trauzettel and Giorgio Sangiovanni

Abstract: We study the resistivity of three-dimensional semimetals with linear dispersion in the presence of on-site electron-electron interaction. The well-known quadratic temperature dependence of the resistivity of conventional metals is turned into an unusual T^6 -behavior. An analogous change affects the thermal transport, preserving the linearity in T of the ratio between thermal and electrical conductivities. These results hold from weak coupling up to the non-perturbative region of the Mott transition. Our findings yield a natural explanation for the hitherto not understood large exponents characterizing the temperature-dependence of transport experiments on various topological semimetals. ....

Luca Tomarchio

THz and Optical Spectroscopy Study of Magnetic Topological Materials

Authors: Luca Tomarchio, Lorenzo Mosesso, Anastasios Markou, Ke He, Stefano Lupi

Abstract: Intrinsic magnetic topological materials are of interest for possible technological applications in dissipationless quantum transport, electro-optical devices, and magneto transport. The combination of topology and magnetism permits the breaking of the time reversal symmetry, with the production of novel quasiparticles and topological phases of matter, like the magnetic topological insulators, magnetic Weyl semimetals and axion insulators. The study of these materials in the thin-film limit is recently being addressed due to the need of characterizing the interplay between the bulk and surface states, usually plagued by the presence of defects in single crystal materials, while also covering the demand of their exotic properties to produce novel electro-optical devices.

In this context, optical linear and nonlinear THz spectroscopy are key tools for the characterization of the topological features of these topological materials. In this talk, we will discuss recent experimental results for the study of two novel topological magnetic materials based on Mn atoms: Co2MnGa, a nodal line semimetal studied at different film thicknesses, ranging from 10 nm up to

60 80 nm, and MnBi2Te4, a material that has been predicted to host all the magnetic topological phases. For Co2MnGa, we have studied its optical conductivity from THz to UV highlighting the variation of its electronic band structure at various thicknesses. Moreover, we will show how high-intensity THz transmission measurements (with a THz electric field up to 2 MV/cm) indicate an induced electromagnetic transparency, with a subtle dependence on the film thickness. For MnBi2Te4, we will present temperature dependent spectroscopic measurements from THz to UV. We highlight the weight of the surface topological states to the optical conductivity and its modification below the ferromagnetic transition at 20 K. ....

Mattia Udina

THz pump-probe spectroscopy: instantaneous response and coherent oscillations

Authors: Mattia Udina, Lara Benfatto, Goetz Seibold, Francesco Gabriele, Tommaso Cea

Abstract: The latest advances in time-resolved spectroscopic techniques, based on the generation of intense ultra-short light pulses, have paved new intriguing ways for the investigation of collective phenomena in many complex systems [1]. In spite of this huge experimental progress, an exhaustive theoretical framework able to connect the measured quantity to the microscopic properties of the material under study is still lacking. With the aim to fill in part this knowledge gap, we provide a general scheme shedding light on some of the most relevant features observed in the experiments, such as the instantaneous response following the squared pump field in wide band insulators, also referred to as THz Kerr effect [3], and the appearance of time-resolved oscillations at the typical frequencies of different collective modes [2]. We will show how the first aspect can be related to the off-resonant excitation of the electronic population as well as to absorption effects due to the lattice [4], while the latter is associated with a Raman-like excitation process, explaining both the generation of coherent optical phonons and electronic collective modes, such as charge, amplitude (Higgs) and phase fluctuations in a superconductor [5-6].

[1] C. Giannetti, et al., Ultrafast optical spectroscopy of strongly correlated materials and high-temperature superconductors: A non- equilibrium approach, Adv. Phys. 65, 58 (2016). [2] M. Udina, T. Cea, and L. Benfatto, Theory of coherent-oscillations generation in terahertz pump-probe spectroscopy: From phonons to electronic collective modes, Phys. Rev. B 100, 165131 (2019). [3] M. C. Hoffmann, et al., Terahertz Kerr effect, Appl. Phys. Lett. 95, 231105 (2009). [4] M. Udina, et al., preprint 2021. [5] G. Seibold, M. Udina, C. Castellani, and L. Benfatto, Third harmonic generation from collective modes in disordered superconductors, Phys. Rev. B. 103, 014512 (2021). [6] F. Gabriele, M. Udina, and L. Benfatto, Non-linear Terahertz driving of plasma waves in layered cuprates, Nat. Commun. 12, 752 (2021). ....

Ivan Mohelsky

Landau level spectroscopy of Bi2Te3

Authors: I. Mohelsky, A. Dubroka, J. Wyzula, A. Slobodeniuk, G. Martinez, Y. Krupko, B. A. Piot, O. Caha, J. Humlicek, G. Bauer, G. Springholz, M. Orlita

Abstract: Bismuth telluride (Bi2Te3) is nowadays a widely explored material in the condensed-matter community. Intensive investigations of Bi2Te3 started more than fifty years ago and they were, to a great extent, driven by its remarkable thermoelectric properties. More recently, Bi2Te3 appeared among the very first experimentally verified three-dimensional topological insulators which host a relativistic-type conical band on the surface. Such surface states appear in Bi2Te3 due to band inversion at the center of the Brillouin zone where spin-orbit interaction reverses the order of p-states as compared to isolated atoms of tellurium and bismuth. Despite considerable experimental effort, the electronic band structure of Bi2Te3 is nowadays only partly understood. The current consensus implies that Bi2Te3 is a narrow-gap semiconductor. Nevertheless, the number and positions of extrema in the lowest-lying conduction and top-most valence bands, still remain under debate. Hence, it is still not clear whether the energy band gap is direct – with the extrema in the conduction and valence band aligned in the momentum space – or indirect.

61 Here we report on Landau level spectroscopy in magnetic fields up to 34 T performed on a thin film of Bi2Te3 epitaxially grown on a BaF2 substrate. The observed response is consistent with the picture of a direct-gap semiconductor in which charge carriers closely resemble massive Dirac particles. The fundamental band gap reaches Eg = (175±5) meV at low temperatures and it is not located on the trigonal axis, thus displaying either six or twelvefold valley degeneracy. Notably, our magneto-optical data do not indicate any band inversion. This suggests that the fundamental band gap is relatively distant from the Γ point where profound inversion exists and gives rise to relativistic-like surface states of Bi2Te3. ....

Jan Wyzula

Magneto optical spectroscopy of Dirac nodal line semimetal NbAs2

Authors: J. Wyzula, X. Lu, D. Santos-Cottin, Y. Krupko, I. Mohelsky, D. Mukherjee, F. L. Mardelé, J. Novak, O. Caha, R. Sankar, M. Novak, B. Piot, W-L Lee, A. Akrap, M. O. Goerbig, M. Orlita

Abstract: Transition metal di-pnictides (TMdP) family consist of six materials with formula XPn2 (X = Ta, Nb, Pn = P, As, Sb) crystallizing in the monoclinic system with the centrosymmetric space group C12/m1. According to first-principle calculations, all TMdP materials posses a nodal line region in their band structure, which is gapped, if the spin-orbital coupling is considered. NbAs2 has been so far explored in a few experiments only. Magneto-resistance studies revealed the presence of several Fermi surfaces, as well as extremely large and strongly anisotropic magnetoresistance. Optical measurements performed at zero magnetic field identified the high anisotropic response in the mid-infrared region depending on the polarization of incident light with respect to crystallographic orientation. Nearly vanishing optical conductivity deduced for light polarized parallel with the b-axis of NbAs2 was interpreted as a signature of a nodal line extend indefinitely through multiple Brillouin zones. At present, the magneto-optical response of NbAs2 has been barely explored, let alone understood, which calls for further investigations. Here we report on magneto-reflectivity studies of NbAs2 (up to 16T @ 4.2K), performed on a set of samples (and their facets with various crystallographic orientation) in order to map the dispersion of the nodal line. We conclude that the observed response can be always understood in terms of two series of electric-dipole-active inter-Landau level excitations. A simplified theoretical approach with relativistic-like corrections is suggested in order to describe the observed magneto-optical response as a function of the angle between the nodal line direction and the wave vector. ....

Qiong Ma

Geometry and Topology in Quantum Materials

Authors: Qiong Ma

Abstract: Close to large masses, the geometry of space-time warps, bending the light trajectory and leading to dramatic phenomena like black holes and gravitational waves. The quantum world of electrons likewise has a geometrical structure, constructed from electronic quantum wave functions. Such a quantum geometrical structure, studied as quantum metric and Berry curvature, can reshape an electron’s behaviors in nontrivial ways. Quantum geometry also has a deep relation with topological phases of matter, which have taken center stage in condensed matter over the past decade. Although many topological phases have been experimentally discovered in quantum materials, the understanding of how bulk quantum geometry is manifest in material properties has remained limited. In this talk, I will show a few examples in 2D layered materials how quantum geometry can strongly modify the electronic response to external electromagnetic waves and give rise to a wide range of previously unexplored nonlinear responses. ....

Bolin Liao

Time-resolved Imaging of Photocarrier Dynamics with Scanning Ultrafast Electron Microscope

Authors: Taeyong Kim, Usama Choudhry, Bolin Liao

62 Abstract: Probing surface photocarrier dynamics with high spatial and temporal resolution is essential for understanding photovoltaic and optoelectronic materials, as well as topological materials with nontrivial surface electronics states. Existing methods based on ultrafast optical spectroscopy can provide sufficient time resolution, while the spatial resolution is limited by optical diffraction. In this talk, I will discuss our development of a scanning ultrafast electron microscope that combines the temporal resolution of femtosecond lasers and the spatial resolution of an SEM. I will discuss benchmarking of the instrument performance, as well as preliminary test data imaging photoexcited charge carrier transport in a variety of systems, including conventional semiconductors, 2D materials and their heterostructures, and conducting polymers. ....

Darius Torchinsky

Probing Weyl Semimetal Physics with Nonlinear Photocurrents

Author: Darius Torchinsky

Abstract: At the heart of the Weyl semimetal are massless, chiral quasiparticles that derive from electronic band-crossings split by either spatial inversion or time-reversal symmetry breaking. The resulting nodal points in the bulk band structure serve as sources and sinks of units of Berry curvature or “topological charge” that are responsible for the phenomenology usually associated with these materials, including open Fermi arc surface states and the chiral anomaly. However, Weyl semimetals with acentric crystal structures can also support second order nonlinear optical responses that reveal their unique band structure in surprising ways. In this talk, I will discuss our recent work using laser-driven currents in an attempt to study topological charge, as well as our experiments on the Fermi arc surface states that reveal an unexpected emergent symmetry in the material response. ....

Zhiyuan Sun

Light induced order parameter steering in excitonic insulators

Authors: Zhiyuan Sun, Andrew J. Millis, Tatsuya Kaneko, Denis Golež

Abstract: An emerging subject in nonequilibrium physics is “order parameter steering”: the use of electromagnetic radiation to control the order parameter of broken symmetry phases. This talk presents the theory of order parameter steering in excitonic insulators, which are phases of matter characterized by a condensate of electron-hole pairs represented by complex order parameters. In excitonic insulators with s-wave electron-hole pairing in the ground state, we show that an applied electric field pulse can induce a p-wave component to the order parameter, and further drive it to rotate in the s+ip plane. Each cycle of rotation pumps exactly two electrons per transverse charge channel across the sample, realizing a Thouless charge pump as a collective many body effect. In electron-hole bilayer realizations of excitonic insulators where the relevant conduction and valence bands are formed by atomic orbitals with differently parities, nonzero interlayer tunneling leads to a ground state that breaks parity or time reversal symmetry and exhibits a second order . Electric field pulses perpendicular to the layers can naturally steer the order parameter phase between the two degenerate ground states, leading to switching of the polarization or anomalous Hall conductivity. The steering is also applicable to the excitonic insulator candidate Ta2NiSe5.

63 Thursday, July 8

Adolfo G. Grushin

Linear and Nonlinear Optics in Chiral Topological Metals

Authors: Adolfo G. Grushin

I will discuss, from a theoretical perspective, the current challenges that optical experiments face to measure a quantized nonlinear response, determined solely by fundamental constants. I will discuss two related effects: the circular photogalvanic effect and difference frequency generation. The former is a non-linear response that is quantized in Weyl semimetals where all mirror symmetries are broken, which is proving challenging to observe experimentally. I will review the recent experimental efforts in RhSi and CoSi, and discuss how difference frequency generation can circumvent some of the challenges these experiments face. ....

Lara Benfatto

Third-Harmonic Generation from plasma waves in cuprates

Authors: Francesco Gabriele, Mattia Udina and Lara Benfatto

Abstract: The hallmark of superconductivity is the rigidity of the quantum-mechanical phase of electrons, responsible for superfluid behavior and Meissner effect. The strength of the phase stiffness is set by the Josephson coupling, which is strongly anisotropic in layered superconducting cuprates, leading to a soft c-axis plasmon, whose energy if of order of 1 THz, and to a hard in-plane plasmon, whose energy (~100 THz) exceeds the THz range. So far, THz light pulses have been efficiently used to excite resonantly the out-of-plane Josephson plasma mode, as is typically shown by enhanced third-harmonic generation (THG) and marked pump- probe oscillations. On the other end, the high-energy in-plane plasma mode has been assumed to be insensitive to THz pumping. Nonetheless, recent experiments in cuprates show several effects that cannot be easily ascribed to the BCS response or to the Higgs mode. Here I will discuss how THz driving of both low-frequency and high-frequency plasma waves is possible via a general two- plasmon excitation mechanism. The anisotropy of the Josephson couplings leads to marked differences in the thermal effects among the out-of-plane and in-plane response, consistently with the experiments. In particular our results link the observed survival of the in-plane THz non-linear driving above Tc to enhanced fluctuating effects in the phase stiffness in cuprates, paving the way to THz impulsive control of phase rigidity in unconventional superconductors. ....

Angel Rubio

Floquet and cavity QED materials engineering

Author: Angel Rubio

Abstract: We provide an overview of how well-established concepts in the fields of quantum chemistry and materials have to be adapted when the quantum nature of light becomes important. We will pursue the question whether it is possible to create these new states of materials as groundstates of the system. To this end we will show how the emerging (vaccum) dressed states resembles Floquet states in driven systems. A particular appeal of light dressing is the possibility to engineer symmetry breaking which can lead to novel properties of materials. Strong light–matter coupling in cavities provides a pathway to break fundamental materials symmetries, like time-reversal symmetry in chiral cavities. We will discuss the potential to realize non-equilibrium states of matter that have so far been only accessible in ultrafast and ultrastrong laser-driven materials. We illustrate the realisation of those ideas in molecular complexes and 2D materials and show that the combination of cavity-QED and 2D twisted van der Waals heterostructures provides a novel and unique platform for the seamless realisation of a plethora of interacting quantum phenomena, including exotic and elusive correlated and topological phases of matter. We will briefly introduce our newly developed quantum electrodynamics density-functional formalism (QEDFT) as a first principles framework to predict, characterize and control the spontaneous

64 appearance of ordered phases of strongly interacting light-matter hybrids. ....

Stefano Lupi

Plasmons in Topological Materials and Highly Conductive Oxides: Linear and Non Linear Optical Properties

Author: Stefano Lupi

Abstract: Collective electronic (plasmon) excitations characterize metals and semiconductors giving rise to bulk and surface modes. Their properties span from a strong confinement of the electromagnetic field, to a peculiar dependence on the interface properties, are used in many applications. In this talk I will discuss the unconventional plasmon excitations in topological matter and in highly conductive oxides. In particular, I will review several experiments in which plasmons of Bi2Se3 topological insulator have been studied both in the linear and nonlinear regime [1,2,3]. Moreover, I will also discuss the appearance of plasmonic excitations in PdCoO2, probably the most conductive oxide never discovered [4]. [1] P. Di Pietro et al., Nature Nanotechnology 8, 556 (2013); [2] M. Autore et al., Advanced Optical Materials 3, 1257 (2015); [3] P. Di Pietro et al., Phys. Rev. Lett. 124, 226403 (2020); [4] S. Macis et al., submitted (2021); ....

Tamaghna Hazra

Band inversion and topology of the bulk electronic structure in FeSe_{0.45}Te_{0.55}

Authors: Himanshu Lohani, Tamaghna Hazra, Amit Ribak, Yuval Nitzav, Huixia Fu, Binghai Yan, Mohit Randeria, Amit Kanigel

Abstract: Bulk FeSe_{0.45}Te_{0.55} has recently emerged as a promising candidate to host topological superconductivity on its surface, with experimental signatures for a Dirac surface state and Majorana bound states in vortex cores. However, ARPES measurements of the bulk band structure show essentially no kz dispersion, in apparent contradiction with DFT predictions for the significant kz dispersion that drives the band inversion. We reconcile the observed lack of dispersion with the predicted band inversion using a tight binding model with strongly renormalized inter-layer hopping and reasonable values of spin-orbit coupling. We use symmetry arguments to identify a sharp signature of bulk band inversion in the photon energy dependence of the ARPES matrix elements. We test our prediction for the change in orbital character of the band using ARPES data with a wide range of photon energies covering several Brillouin zones along kz. We thus provide direct evidence for the non-trivial topology of the bulk bands in Fe(Se,Te), where the band inversion occurs in a nearly flat band due to an interplay of strong correlations and topology. ....

Yinan Dong

Fizeau drag in graphene plasmonics

Authors: Y. Dong, L. Xiong, I.Y. Phinney, Z. Sun, R. Jing, A.S. McLeod, S. Zhang, S. Liu, F. L. Ruta, H. Gao, Z. Dong, R. Pan, J. H. Edgar, P. Jarillo-Herrero, L.S. Levitov, A.J. Millis, M. M. Fogler, D.A. Bandurin, D.N. Basov.

Abstract: Here we report on the Fizeau drag of surface plasmon polaritons (SPPs) by flowing electrons in graphene. The drag is visualized directly through infrared nano-imaging of propagating plasmonic waves in the presence of a high-density current. The polaritons in graphene shorten their wavelength when launched against the drifting carriers. Unlike the Fizeau effect for light, the SPP drag by electrical currents defies the simple kinematics interpretation and is linked to the nonlinear electrodynamics of the Dirac electrons in graphene. The observed plasmonic Fizeau drag enables breaking of time-reversal symmetry and reciprocity at infrared frequencies without resorting to magnetic fields or chiral optical pumping. ....

65

Chelod Paingad Vaisakh

Ultrafast plasmon thermalization in epitaxial graphene probed by time-resolved THz spectroscopy

Authors: C. P. Vaisakh, J. Kunc, I. Rychetský, and P. Kužel

Abstract: Inhomogeneities in SiC-grown epitaxial graphene due to wrinkles and substrate terraces bring about natural confinement potentials for the charge carriers. Such confinement potentials induce a localized plasmonic optical response. We use optical pump— terahertz (THz) probe spectroscopy to investigate ultrafast sheet conductivity dynamics in various epitaxially grown graphene layers representing a large variety of carbon allotropes, including H2 intercalated films. Our graphene layers display a prominent plasmonic response and pump-induced THz transparency on an ultrashort timescale. The measured dynamics are entirely controlled by the temperature of laser-excited carriers undergoing a super-collision cooling process with a picoseconds long transfer of the optically deposited heat to the lattice followed by a sub-nanosecond relaxation governed by the lattice cooling. We describe the transient spectra by a two-temperature Drude-Lorentz model revealing the ultrafast evolution of the carrier temperature and crucial material parameters such as Fermi energy, carrier mobility, carrier confinement length, and disorder mean free path. ....

Rebecca Cervasio

Probing thin films of functionalized materials by SR infrared and THz spectroscopy

Authors: R. Cervasio, M. Verseils, JB Brubach, P. Roy, P. Rojo Romeo, B Vilquin, I. Cañero Infante

Abstract: Functionalized materials for novel electronic and devices require to exist in the thin film form to be competitive for the applications. Indeed, progress in term of storage density can be brought by the extension to the nanometer scale. As a consequence it is a big challenge to perform spectroscopic measurements on a film of few nanometers. In particular it is important to determine how, the shrinking of the thickness at the nanoscale, the presence of the substrate and the presence of interfaces induce new physical phenomena. The probing of the signature of electric and structural transitions, plays a key role in determining how to control the desired properties. Exploiting the high brilliance and the stability of the SR in a wide infrared spectral range available at the AILES beamline of SOLEIL, we were able to perform infrared and Terahertz spectroscopic measurements on several thin film materials promising for applications. The recently discovered ferroelectricity in thin (below 20 nm) HfO2-based films has stimulated new research into understanding the origin of the ferroelectricity in these systems and opened a path to ferroelectrics memristors. Beyond these promising applications, many fundamental aspects related to the ferroelectricity order in HfO2-based systems remain to be unveiled e.g. ferroelectric transition temperature, role of non-polar coexisting phases, imposed electrode boundary conditions, electrical cycling effects. We report the results obtained from temperature and electric field dependence of transmission infrared and THz spectroscopy measurements. Other systems will also be described. ....

Gergely Németh

Ultrasensitive molecule detection via tip-launched graphene plasmons

Authors: Gergely Németh, Dániel Datz, Áron Pekker, Ferenc Borondics, Katalin Kamarás

Abstract: Scattering-type near-field optical microscopy (s-SNOM) is a rapidly rising technique which enables the investigation of nanoscale systems by overcoming the diffraction limit. There is an ongoing challenge to find new ways to increase the sensitivity of s-SNOM for vibrational spectroscopy towards the single molecule level.

Low-dimensional materials such as graphene possess spatially confined low-energy plasmon-polariton excitations. Subwavelength light confinement resulting in extreme concentration of electromagnetic field can result in coupling between polaritons and weak molecular vibrations. Metallic tips in s-SNOM can effectively excite propagating plasmons in graphene.

In this study we examine the possibilities of utilizing graphene as a plasmonic enhancement platform for the s-SNOM mid-IR

66 signatures of weakly absorbing molecules. We studied the possible coupling between graphene surface plasmon-polaritons (GSP) and molecular vibrations of a C60 fullerene molecule thin layer. The calculation included different arrangements, Fermi level, and layer thicknesses. Using the transfer matrix approach and finite dipole model for the near-field interaction we found that GSP excitation can be tuned even at the upper part of the fingerprint region utilizing the splitting of plasmonic modes in graphene double layer structures. Initial measurements will also be presented here that show the validity of the calculations and the strong potential of tunable graphene plasmons as a general s-SNOM enhancement platform for ultrasensitive molecule detection. ....

Dorri Halbertal Nano-optical studies of Moiré super-lattice domains in twisted bilayer graphene heterostructures

Authors: Dorri Halbertal, Sai S. Sunku, Shaowen Chen, Stephen Carr, Derick Gonzalez-Acevedo, Andrey Rikhter, Alexander S. McLeod, Michael E. Berkowitz, Chiu Fan Bowen Lo, Efthimios Kaxiras, Tobias Stauber, Cory R. Dean, James. C. Hone, Michael M. Fogler, Dante M. Kennes, Angel Rubio and Dmitri N. Basov

Abstract: Harnessing the twist angle between atomic layers of a 2D material has gained much interest allowing band-structure tuning and realizing new states of matter. For low twist angles, the system relaxes to form distinct phases separated by domain walls (DWs). In this work we studied minimally twisted bilayer graphene, using several nano-optical techniques, including scattering type SNOM and photo-current nano-imaging. This allowed us to spatially resolve the internal structure of the triangular Moiré superlattice of the twisted bilayer graphene system. We describe two conceptually different regimes: In the first, the photo-current signal is dominated by photo-current generation sources at the AA vertices of the Moiré lattice [1]. As temperature drops and doping level increases, a different regime is realized, which we coin non-local photocurrent regime, where absorption features dominate the signal [2]. From the detailed image of the relaxed moiré superlattice we perform moiré metrology [2], gaining insight into the stacking energy landscape of the system. In addition, we utilize the hyperbolic nature of phono-polaritons in hexagonal boron nitride to reach magnification of the nearfield signal originating from the domain walls [1].

References: 1. Sunku*, Halbertal* et al., Nat. Comm. 12, 1641 (2021). 2. Halbertal et al. Nature Communications 12, 242 (2021). ....

Alexey Shuvaev

Solution of MIRO polarization immunity problem

Authors: Shuvaev, M. L. Savchenko, I. A. Dmitriev, Z. D. Kvon, A. Pimenov

Abstract: Microwave Induced Resistivity Oscillations (MIRO) are typically observed in high-mobility two dimensional systems, e.g. semiconductor heterostructures. Under microwave illumination giant magneto-oscillations with multiple of the cyclotron resonance frequency are observed in resistivity. Experiments on related oscillations in terahertz transmission are challenging and reveal complicated polarization dependence. Almost all measurements up to date showed symmetric polarization response in magnetic field, even when the concomitant cyclotron resonance in circular polarization is highly asymmetric. This has puzzled the scientific community and stimulated the development of several microscopic theories to explain the effect. Recent terahertz transmission experiments in our group show that MIRO can be highly asymmetric in certain conditions. The symmetry of MIRO can be controlled by near-field polarization in the focal plane that can differ from the far-field polarization. Our results thus provide the solution to the long standing polarization immunity problem in MIRO. ....

Aaron Sternbach

Programmable hyperbolic polaritons in Van der Waals semiconductors

Authors: J. Sternbach, S. H. Chae, S. Latini, A. A. Rikhter, Y. Shao, B. Li, D. Rhodes, B. Kim, P. J. Schuck, X. Xu, X.-Y. Zhu, R. D. Averitt, J.

67 Hone, M. M. Fogler, A. Rubio, and D. N. Basov

Abstract: The layered van der Waals (vdW) materials are naturally anisotropic. Consequentially, dipole active resonances can render the principle values of the dielectric tensor of opposite sign along orthogonal crystallographic directions within these vdW materials. Chief among the resultant non-intuitive optical properties is the formation of sub-diffractional wave packets that travel as conical rays with hyperbolic dispersion throughout their bulk. Here I discuss our work on producing an on-demand hyperbolic response within the vdW semiconductor WSe2 [1]. We utilized photoexcitation to inject electron-hole pairs in WSe2 and then visualized, by transient nano-imaging, hyperbolic rays that travel along conical trajectories inside of the crystal.

[1] A.J. Sternbach et al., Science 371, 617-620 (2021) ....

Abhay Kumar Nayak

Resolving topological classification through topological defects

Authors: Jonathan Reiner, Raquel Queiroz, Huixia Fu, Chandra Shekhar, Binghai Yan, Claudia Felser, Nurit Avraham, Haim Beidenkopf

Abstract: Bulk boundary correspondence has been the cornerstone in the study of electronic topological phases of matter. It has enabled the exploration of electronic bulk properties through the investigation of topological boundary modes. However, the growing diversity and profusion of topological classes has lead to ambiguity between classes sharing similar boundary phenomenology. This is the current status of bismuth, for which recent studies have suggested nontrivial classifications like strong or higher-order TI, both of which hosts 1D helical modes on their boundaries. Here, we use a novel approach to resolve the topological classification of bismuth by spectroscopically mapping the response of its boundary modes to a topological lattice defect, in our case a screw dislocation using scanning tunneling microscopy [1]. We find a 1D edge mode, bound to the step edges of bismuth, extending to the core of the screw dislocation without gapping out. This signifies that the edge mode binds to the topological defect, characteristic of a material with nonzero weak indices. This work paves the way for the identification of novel electronic topological phases through the study of boundary modes associated with topological defects.

[1] Nayak, A. K. et al. Resolving the topological classification of bismuth with topological defects. Science Advances 5, eaax6996 (2019). ....

Adamya Prakhar Goyal

Intrinsic Optical Absorption in Dirac Metals

Authors: Adamya P. Goyal, Abhishek Kumar, Dmitrii L. Maslov

Abstract: In an ideal Dirac metal, optical absorption is absent for frequencies below the Pauli threshold (twice the Fermi energy). In real systems, however, e.g., in doped graphene, both optical absorption [1] and Raman scattering [2] find a very broad transition region around the Pauli threshold. While a number of extrinsic damping mechanisms were proposed to explain this observation in the past, we argue that the effect can be explained by an intrinsic mechanism -- Auger-like recombination of optically excited minority carriers with equilibrium majority carriers. The idea goes back to a similar mechanism proposed for doped gapped semiconductors by Gavoret et al [3]. The width of the transition region in this mechanism is comparable to the Fermi energy. We also discuss a scenario in which the Auger width is small and thus well-defined excitons below the Pauli threshold become possible.

[1] Li, Z., et al. Nature Phys. 4, 532–535 (2008) [2] E. Riccardi, et al. Phys. Rev. Lett. 116, 066805 (2016) [3] J. Gavoret, et al. Journal de Physique, 1969, 30 (11-12), pp.987-997. *National Science Foundation under Grant No. NSF DMR-1720816 ....

68 Artem Pronin

Faraday rotation due to topological Hall effect in Mn2−xPtSn

Authors: Di Liu, Peter Swekis, Anastasios Markou, Claudia Felser, Martin Dressel, and Artem Pronin

Abstract: Rotation of the light polarization in an applied magnetic field (the Faraday effect) – a high frequency analogue of the Hall effect – was studied at sub-THz frequencies in thin films of Mn2−xPtSn (x = 0, 0.2). These Heusler compounds with non-coplanar magnetic structure and non-zero scalar spin chirality have recently been reported to demonstrate the topological Hall effect (THE) related to an additional Berry phase acquired by the electrons moving in an emergent gauge field arising in such spin structures. Our sub-THz measurements reveal that a significant contribution to the total Faraday angle originates from the same source as the dc THE: this contribution is not proportional to the applied magnetic field or magnetization, but instead scales with the field and temperature in the same way as the THE angle. This observation demonstrates that the topological Hall effect can also be observed optically, i.e., that the emergent field directly affects the charge dynamics at high frequencies. ....

Alexander S. McLeod

Revealing nano-plasmonics in 2D materials and correlated oxides at variable temperatures

Authors: A.S. McLeod, G.X. Ni, L. Wang, Z. Sun, J. Zhang, X. Chen, S. Li, M.K. Liu, M. Fogler, Y. Hao, C. Dean, J. Hone, D. N. Basov

Abstract: Extended to variable and cryogenic temperatures, scanning near-field optical microscopy now offers a versatile platform for nano-imaging and spectroscopy of fundamental optical phenomena in quantum materials. We apply this technique to visualize and quantify ultra-low-loss propagation of gate-tuned plasmon polaritons in graphene. This technique of real-space investigation provides a temperature-resolved probe of the quasiparticle scattering mechanisms that govern plasmonics in this canonical van der Waals semimetal. By comparison, analogous low temperature nano-imaging reveals fundamentally different scattering mechanisms relevant for propagating phonon polaritons in polar van der Waals crystals like hexagonal boron nitride and alpha- MoO3. Meanwhile, we also show how minimally propagating plasmons in transition metal oxides like quasi-2D MoO2 and VO2 still govern the fundamental nano-scale infrared response of these materials, thus providing a new avenue to probe the frequency- and temperature-resolved optical conductivity of complex oxides through nano-imaging. ....

Liang Wu

Direct imaging of the N\'eel vector switching in the monolayer antiferromagnet MnPSe$_3$ with strain-controlled Ising order

Author: Liang Wu

Abstract: Antiferromagnets have attracted enormous interest recently in spintronics due to the absence of stray fields and their terahertz resonant frequency. Despite the great advantages of antiferromagnetic spintronics, controlling and detecting N ́eel vectors have been limited in bulk materials. In this work, we developed a sensitive second harmonic generation (SHG) microscope and detected long- range N ́eel antiferromagnetic (AFM) order and N ́eel vector switching down to the monolayer in MnPSe3. Temperature-dependent SHG measurement in repetitive thermal cooling surprisingly collapses into two curves, which correspond to the switching of an Ising type N ́eel vector reversed by the time-reversal operation, instead of a six-state clock ground state expected from the threefold rotation symmetry in the structure. We imaged the spatial distribution of the N ́eel vectors across samples and rotated them by an arbitrary angle irrespective of the lattice in the sample plane by applying strain. By studying both a Landau theory and a microscopic model that couples strain to nearest-neighbor exchange, we conclude that the phase transition of the XY model in the presence of strain falls into the Ising universality class instead of the XY one, which could explain the extreme strain tunability. Finally, we found that the 180◦ AFM domain walls are highly mobile down to the monolayer after thermal cycles, paving the way for future control of the anti- ferromagnetic domains by strain or external fields on demand for ultra-compact 2D AFM terahertz spintronics. Reference. Ni, et al. Nat. Nano. (2021) DOI:10.1038/s41565-021-00885-5 ....

69 Ji-Hee Kim

Carrier multiplication for next-generation solar cells

Author: Ji-Hee Kim

Abstract: A major challenge to improve solar energy conversion lies in the efficient use of the excess energy of photoexcited hot carriers. In general, the single photon absorption in a semiconductor produces one electron-hole pair while the photon energy in excess of the energy gap is dissipated as heat by phonon emission. This limits a maximum power conversion efficiency of a single junction solar cell of ~33%, known as Shockley-Queisser limit. To break this threshold and boost the efficiency of solar cells, carrier multiplication (CM) has been suggested as a potential approach but yet to be proven relevant to practical devices. We suggest 2D materials as a solution of this issue and believe that the observation of CM in 2D materials will be a breakthrough within the field of 2D research. In this talk, we will present carrier multiplication in 2D materials with the lowest threshold energy and the highest conversion efficiency among the published results so far. Also, we will show that our new strategy can boost the carrier population in transition metal dichalcogenides/organic semiconductor type II heterostructure, which will be a breakthrough for 2D-based solar cells. ....

Erik Henriksen

Cyclotron resonance spectroscopy in graphene and SmB6

Authors: Erik Henriksen, Yashika Kapoor, Jordan Russell

Abstract: We will present recent broadband infrared measurements of graphene and SmB6 at low temperatures (<1K) and high magnetic fields. In mono- and bilayer graphene, we map the smooth evolution of broken symmetry states as a function of the Landau level filling factor, and observe the impact of many-particle contributions to the transition energies. In SmB6, we seek for cyclotron transitions from the impurity states or more exotic neutral fermion scenarios.

70 LEES 2021 June 28 - July 8, 2021

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