ARPES observation of competing orders/fluctuations in high-temperature superconductors

Atsushi Fujimori Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan E-mail: [email protected]

Competition between magnetism, charge and orbital order, and superconductivity observed has been the subject of extensive research in various high-temperature superconductors discovered so far. Through the observation of band folding signature caused by such orders or fluctuations, ARPES yields deep insights into the nature of competing orders whose fluctuations may disturb or enhance the superconductivity. In this talk, two dramatic observations are presented: (i) In electron-doped cuprates after complete removal of apical oxygen, the band folding due to short- range antiferromagnetic order is suppressed while the signature of charge fluctuations remains as strong as the hole-doped cuprates [1]. (ii) In a parent compound BaFe2As2 of iron-based superconductors, band folding signature of the antiferromagnetic order persists well above the Neel temperature, i.e., into the “nematic” orbital-ordered phase, which we attribute to the existence of antiferro-orbital component in the orbital-ordered state.

This work has been done in collaboration with M. Horio, K. Koshiishi, L. Liu, K. Okazaki, S. Ideta, T. Yoshida, T. Mizokawa, M. Hashimoto, D. Lu，Z.-X. Shen, A. Ino, H. Kumigashira, K. Ono, T. Kobayashi, S. Miyasaka, S. Tajima, S. Kasahara, T. Terashima, S. Ishida, M. Nakajima, Y. Tomioka, T. Itoh, K. Kiho, C.-H. Lee, A. Iyo, H. Eisaki, S. Uchida, S. Shin, H. Anzai, M. Arita, H. Namatame, M. Taniguchi.

References: [1] M. Hiroi et al., arXiv:1502.03395, to appear in Nat. Commun.

Mott transition in the pyrochlores

Pinaki Majumdar Harish-Chandra Research Institute - Chhatnag Road, Jhusi, Allahabad 211019, India E-mail: [email protected]

We study the interplay of electronic correlation and geometric frustration in the context of the Mott transition on the pyrochlore lattice. We formulate the single band Hubbard model on this lattice in terms of electrons coupled to auxiliary magnetic moments, and treat the resulting `fermion-spin' problem through a Monte Carlo technique. While the ground state we obtain is equivalent to unrestricted Hartree-Fock, the presence of the crucial low energy fluctuations in our approach, and their coupling to the electrons, allow us to establish the temperature dependence of transport and spectral features across the Mott transition.

We find a first order metal to insulator transition in the ground state and observe a finite residual resistivity over an interaction window near the Mott transition. There is no magnetic long-range order in the Mott phase, only a power-law decay of correlations.

To make correspondence with real materials (iridates and molybdates) we include additional interactions. In the iridates, spin-orbit coupling is crucial while in the molybdates double-exchange and super-exchange interaction play a role. I will discuss the phase diagrams that emerge in the presence of these interactions and compare them to experiments.

Unified correlation properties and materials genome of complex materials

Tanmoy Das Department of Physics, Indian Institute of Science, Bangalore – 560012, India Email: [email protected]

Weakly and strongly correlated materials are relatively better understood in terms of Fermi liquid, and Mott insulator paradigms, respectively. However, there is a growing class of materials which reside in the intermediate coupling regime (where the interaction strength is of the order of the bandwidth), which cannot be modelled properly by any of these models. Over the last few years, we have been working on developing a momentum resolved density fluctuation (MRDF) theory appropriate for this problem.[13] Our intermediate coupling model is based calculating momentum dependent self-energy due to various density-density fluctuations. In this talk, I will present results for several representative correlated materials including copper oxide (cuprate) high Tc superconductors, actinide compounds, and various complex oxides. A universal feature of intermediate coupling scenario is that the self-energy splits the electronic structure into low energy coherent states (emergent Fermi liquid like state), and high energy localized state (near the Mott state), yielding a coexistence of itinerant (wave like) and localized (atom like) states. The resulting electronic fingerprint reveals a universal `waterfall’ shape in the dispersion, and a peak dip hump feature in the density of states. The results suggest a generic route for formulating the correlated electronic states for a larger energy span from which a unified theory of high Tc superconductivity, magnetism may emerge.

References [1] Das, Zhu, Graf, Phys. Rev. Lett. 108, 017001 (2012). [2] Das, Zhou, Graf, J. Mat. Res. 28, 659672 (2013). [3] Das, Markiewicz, Bansil, Adv. Phys 63, 151266 (2014).

Two-dimensional Antiferromagnetism in Cr monolayer film

Krishnakumar S. R. Menon Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata-700064, India E-mail: [email protected]

Magnetism in low dimensions is interesting from the basic science as well as applications point of view. Even though ferromagnetism in low dimensions is well known, study of two-dimensional antiferromagnetism is rare in the literature. Here we discuss the case of flat Cr monolayer film on Ag(001) substrate exhibiting c(2x2) antiferromagnetic configuration, confirmed by our Low Energy Electron Diffraction (LEED) and Angle- resolved Photoemission Spectroscopy (ARPES) studies. Our experiments as well as ab- initio DFT calculations confirm the presence of a Ag overlayer on Cr monolayer, forming a Ag/Cr/Ag(001) sandwich structure. The buried Cr monolayer is found to retain its 2D character with enhanced Cr 3d magnetic moments, due to weak hybridization between Cr 3d and Ag 4d electronic states.

Understanding the thermoelectric properties of Heusler alloys by using density functional theory

Sudhir K. Pandey School of Engineering, Indian Institute of Technology Mandi, Kamand-175005, India E-mail: [email protected]

In this talk we investigate the thermoelectric properties of some Heusler alloys by using ab initio electronic structure calculations in combination with the Boltzmann transport theory. Most of the Heusler alloys studied are found to exhibit flat conduction band along Γ – X direction, which gives rise to large value of Seebeck coefficient. The Boltzmann transport theory within relaxation time approximation provides good explanations for the temperature dependent behavior of Seebeck coefficient, electrical conductivity and electronic thermal conductivity of the compounds. The two currents model is found to explain the thermoelectric behavior these compounds in better way. The present methodology is also applied to predict good thermoelectric materials which are expected to be used in wide temperature range.

References: 1. S. Sharma and S. K. Pandey, J. Phys.: Condens. Matter 26, 215501 (2014). 2. S. Sharma and S. K. Pandey, J. Phys. D: Appl. Phys. 47, 445303 (2014) 3. S. Sharma and S. K. Pandey, Phys. Lett. A 379, 2357 (2015)

Insulator to metal transition in compressed molecular solid XeF2

G. Vaitheeswaran Advanced Centre of Research in High Energy Materials (ACRHEM), University of Hyderabad, Hyderabad-500046, India. E-mail: [email protected]

Understanding insulator to metal transitions in materials under pressure have been a subject of interest for both experimentalist and theoreticians. In this talk, results of ab initio calculations for molecular solid XeF2 under pressure is presented in order to examine the structural stability and possible metallization. Experimental high pressure study of solid XeF2 reported a series of phase transitions (nearly four phases) up to 100 GPa and possible metallization at 70 GPa. Contrary to experiment our total energy calculations show that the parent I4/mmm phase of XeF2 is stable up to 110 GPa and inclusion of van der Waals forces are quite important in order to obtain an agreement between the calculated and the experimental structural parameters at ambient conditions. Besides total energies, the computed phonon dispersion curves at ambient as well as different pressures unambiguously confirm the dynamical stability of tetragonal-I4/mmm phase of XeF2. At pressures beyond 110 GPa the orthorhombic structure with Pnma symmetry is energetically favourable than the tetragonal structure. The electronic band structure is calculated by means of the Quasiparticle Self-consistent GW (QSGW) approximation. The calculated electronic structure exhibits a gap of 7.5 (6.0) eV without (with) inclusion of spin-orbit interaction at ambient conditions, and the I4/mmm phase of XeF2 remains semiconductor up to very high pressures, with a possibility to metallize beyond 150 GPa.

Charge dynamics at semiconductor surfaces investigated by time resolved Scanning Tunneling Microscopy

Philipp Kloth, Katharina Kaiser, Judith von der Haar, Ole Bunjes, Terence Thias and Martin Wenderoth 4th Physical Institute - Solids and Nanostructures, Georg-August-University of Göttingen, 37077 Göttingen, Germany E-mail: [email protected]

The combination of Scanning Tunneling Microscopy (STM) and optical excitation merges two of the most successful experimental techniques in solid-state physics. The combination of optical Pump-Probe techniques with Scanning Tunneling Microscopy (STM) enables us to get atomic resolution of an STM with time resolution on the ns time scale, i.e. beyond the bandwidth of the current amplifier. This approach provides the prospect to resolve surface dynamics on the atomic scale.

A serious challenge of optical excitation in STM is controlling the thermal load at the tunnel junction. We present a very compact and versatile laser setup that addresses various requirements of this experimental technique. First of all, the laser source must provide a very low-noise and stable output power. Next, in order to find the spot of maximum excitation in a standardized manner, we implemented a sub-micrometer precise stage that allows the scanning of the focus point of the laser beam along the tip-surface junction – even during tunnel conditions. At last, standard pump-probe pulses must be transformed into complex laser pulse patterns. Using an optical modulator with a bandwidth in the gigahertz range and a high frequency function generator, we process the continuous wave laser beam into nanosecond pulses implementing a Shaken-Pulse-Pair-Excitation technique [1].

More specific, optical excitation and Scanning Tunneling Microscopy (STM) for studying the carrier dynamics at the GaAs(110) surface is discussed. By illuminating the tunnel contact between a tip and an n- doped GaAs crystal we generate electron-hole pairs, which will be separated in the tip-induced space charge region (SCR). A detailed spectroscopic analysis [2] has shown that photo-excited charge carriers, trapped in a very local region beneath the STM tip, contribute to the tunneling current. By adjusting the current in a controlled manner we are able to actively access different screening conditions of the electric potential at the surface. Studying the time evolution of the photo-induced tunnel current gives access to the charge dynamics. Surprisingly, we have found a dependency of the temporal response on the pulse duration of excitation as well as on the tunnel current. We discuss three main processes determining the relaxation characteristic of the excited system. One process is the filling of the photo-generated holes trapped at the surface, the two others are the charging and discharging of dopants changing the local SCR beneath the STM tip. Depending on the dominant tunneling channel, pump-probe excitation can resolve different recombination processes of charge in the nanoscaled SCR.

By using the lateral resolution of the STM, the influence of single dopants on the relaxation dynamics of the system is investigated. We discuss the impact of these charged and surface-sided defects in terms of their varying binding energy [3] in comparison to conventional bulk-positioned donors.

References: [1] Terada et al., Nature Photonics, 4(12), 2010. [2] Kloth et al., Nat. Comm. (2015) [3] Teichmann et al., PRL (2009)

The dilute magnetic semiconductors: A playground for the Zaanen-Sawatzky-Allen phase diagram

Basudeb Mandal, Hirak Kumar Chandra and Priya Mahadevan S. N. Bose National Centre for Basic Sciences Block-JD, Sec-III, Salt Lake, Kolkata-700098 E-mail: [email protected]

The electronic properties of transition metal compounds have been classified in terms of the Zaanen-Sawatzky-Allen (ZSA) phase diagram and its variants1,2. In this talk I will present our results on placing two well studied dilute magnetic semiconductors (DMS) in this phase diagram and try to understand their ensuing magnetic properties depending on where they belong. We find Mn doped GaAs has a negative charge transfer energy which places it in the p-d metal regime of the phase diagram. This explains why Coulomb interactions on the Mn site do not localize the hole associated with Mn doping and drive the system insulating. Unlike in a bulk solid, at the dilute doping limit a good approximation to the levels with which a Mn atom interacts with are the dangling bond states associated with the Ga vacancy that Mn substitutes3,4. These follow the valence band maximum of the host semiconductor. Solving for the electronic and magnetic properties within a multiband Hubbard model at the mean field limit, we show that the ferromagnetic stability increases with quantum confinement. This opens up a new route to increase the ferromagnetic transition temperature in Mn doped GaAs. Our results are consistent with recent experimental observations by Kanski et al5 who find spin polarised bands even at room temperature in Mn doped GaAs where bulk magnetization measurements find a ferromagnetic transition temperature much smaller. Considering the example of Mn doping in GaN we find that the system has a negative charge transfer energy, with the transition metal d bands lying inside the N p bands. Inspite of this the system is insulating and this can be explained by the fact that covalency effects drive the system insulating. The system can be identified as a covalent insulator of the modified ZSA phase diagram2.

References: [1] J. Zaanen, G. A. Sawatzky and J. W. Allen, Phys. Rev. Lett. 55, 418 (1985). [2] S. Nimkar, D. D. Sarma, H. R. Krishnamurthy and S. Ramasesha, Phys. Rev. B 48, 7355 (1993). [3] P. Mahadevan and A. Zunger, Phys. Rev. B 69, 115211 (2004). [4] P. Mahadevan, A. Zunger and D. D. Sarma, Phys. Rev. Lett. 93, 177201 (2004). [5] J. Kanski, L. Ilver, K. Karlsson, M. Leandersson, I. Ulfat and J. Sadowski, arXiv:1410.8842 (2014).

High resolution photoemission study of some electron doped manganites

Manas Kumar Dalai CSIR – National Physical Laboratory, New Delhi – 110012, India E-mail: [email protected]

One of the most interesting and established system in the field of condensed matter Physics is “Mixed Valence Manganites” having general formula R1−xAxMnO3 (where R and A are trivalent- rare earth and divalent-alkaline earth ions, respectively). The tuning of the physical properties of such system is basically governed either chemically by changing the concentration and nature of the R and A cations or physically by applying external stimuli viz. pressure, magnetic field, electric field, etc. Both physical and chemical parameters can dramatically influence the internal structure such asMn3+(d4)/Mn4+(d3) ratio, lattice distortion, and spin state. The electronic occupation at the Mn site and the lattice distortion controls, respectively the band filling and eg bandwidth, thereby greatly influence the electronic properties of manganites. A large number of scientific literature available so far is based on hole doped manganites because of their alluring colossal magnetoresistive effect. Relatively less attention is paid to the electron doped system, which possesses somewhat similar as well as significantly different properties from its hole doped counterparts. Since the charge transport and metallic magnetism is influenced greatly by the electronic states close to the Fermi level, it would be ideal to understand how the near EF electronic structure evolves when a system undergoes phase transition. In this talk, I will be discussing about the near EF electronic structure of some electron doped manganites using high resolution photoemission spectroscopy.

Non-Fermi-liquid scattering rates, anomalous band dispersions, and Hund’s metal behaviour in iron pnictides and iron chalcogenides - an ARPES study

Jörg Fink Leibniz Institute for Solid State and Materials Research Dresden, Germany E-mail: [email protected]

Unconventional/high temperature superconductivity (SC) is observed in heavy fermion systems, cuprates, molecular crystals, and ferropnictides close to a point in the phase diagram where, as a function of a control parameter such as pressure, chemical pressure, or doping, the antiferromagnetic order is suppressed. A widespread view is that at this point, which is called a quantum critical point, strong antiferromagnetic fluctuations are a candidate for the glue mediating superconductivity and that these fluctuations would also account for the strange normal state non- Fermi-liquid behavior as is visible in transport and thermal properties. Using angle-resolved photoemission spectroscopy (ARPES) we have studied the scattering rates and band dispersion of various iron pnictides and iron chalcogenides as a function of the control parameter. The detected scattering rates of all electron and hole pockets do not diverge at optimal doping, i.e., at the expected quantum critical point. This result is at variance with the above described scenario for quantum critical behavior. The scattering rates strongly differ for pockets having different orbital character, and are linear in energy, indicating marginal Fermi liquid behavior. The scattering rates for hole doped compounds are considerably larger than those of the electron doped systems, indicating a dependence on the Fe 3d count leading for a 3d5 configuration to a strongly correlated Hund’s metal. Near optimal doping the measurements also indicate a crossing of the top of hole or electron pockets, through the Fermi level which is related to Lifshitz transitions. Based on these experimental results together with calculations, we establish the following scenario which is different from the traditional view related to strong fluctuations at the quantum critical point: a co- action between a highly correlated electron liquid and a Lifshitz transition causes an anomalous band dispersion at the Fermi level which leads to a strong mass enhancement in the normal state, detected in the transport and thermal properties and to a small effective Fermi energy favoring a Bardeen-Cooper-Schrieffer - Bose-Einstein crossover state in the superconducting phase. The results can be generalized to other unconventional superconductors.

Theoretical Spectroscopy for Correlated Materials: Rethinking the Interface of Electronic Structure and Many-Body Theory

Silke Biermann Centre de Physique Théorique, Ecole Polytechnique, 91128 Palaiseau, France E-mail: [email protected]

Combined density functional (DFT) + dynamical mean field theory (DMFT) has established itself as a method of choice for the theoretical description of spectral properties of correlated electron materials. Nevertheless, increasingly refined applications and attempts to use the theory in a truly predictive manner have also revealed the limits of the current ways of branching many-body theory onto electronic structure calculations [1]. The most subtle points are related to 1. the determination of the effective local Hubbard interactions 2. the dynamical character of these interactions 3. the use of DFT for the one-body part of the Hamiltonian. 4. the elimination of double counting issues Here, we present recent work attempting to push these frontiers forward, by explicitly taking into account dynamically screened Hubbard interactions and a one-body Hamiltonian beyond DFT. The new “screened exchange dynamical mean field theory” is successfully tested on the transition metal pnictide BaCo2As2 [2] and the ternary oxide SrVO3 [3].

REFERENCES: 1. S. Biermann, J. Phys. Condensed Matter, 26 173202 (2014). 2. A. van Roekeghem, T. Ayral, J.M. Tomczak, M. Casula, N. Xu, H. Ding, M. Ferrero, O. Parcollet, H. Jiang, S. Biermann, Phys. Review Letters 113, 266403 (2014). 3. A. van Roekeghem and S. Biermann, Europhysics Letters 108 57003 (2014).

Surface Band Structure of Topological Insulators Studied using ARPES

H. Lohania, P. Mishraa, K. Majhib, A. Banerjeeb, P. S. Anilkumarb, and B.R. Sekhara

aInstitute of Physics, Sachivalaya Marg, Bhubaneswar, India. bDepartment of Physics, Indian Institute of Science, Bangalore, India. E-mail: [email protected]

Topological insulators (TI) have brought in an unprecedented excitement to the research in condensed matter due both to the intriguing fundamental physics involved and the tremendous possibilities in application. The strong spin-orbit coupling in these systems give rise to exotic phenomena like Dirac fermion, Majorana surface states, Wyle Fermion etc. Recent breakthrough in this field is the discovery of tetramite Bi2Te2Se, which is isostructural to Bi2Se3 but exhibits relatively large resistivity. These compounds provides an ideal platform to study the nature of topological surface states by tuning the Dirac node within the bulk energy gap by controlling the chalcogen/pnictogen stoichiometry. Angle Resolved Photoelectron Spectroscopy (ARPES) being a direct tool capable of detecting the surface states has played a vital role in understanding the physics of TIs. In this talk, I will be discussing some of our recent work on the changes in the surface band structure of some TIs under doping using ARPES in conjunction with theoretical band structure calculations.

Transport on topological insulator surfaces

Krishnendu Sengupta Department of Theoretical Physics, IACS, 2A&2B Raja S. C. Mullick Road, Kolkata - 7000032, India E-mail: [email protected]

In this talk, I am going to discuss two junction geometries involving surfaces of topological insulators, which hosts Dirac like quasiparticles. The first demonstrates the possibility of realization of a magnetic switch i.e. controlling electric current by tuning an induced magnetization, while the second leads to the possibility of realization of electrically controllable spin current. I shall point out the crucial role of the spin momentum locking of the Dirac quasiparticles for realization of both these phenomena.

Surface structure of SmB6 investigated by STM

Steffen Wirth Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Straße 40, D-01187 Dresden, Germany E-mail: [email protected]

The intermediate-valence compound SmB6 is typically considered a “Kondo insulator" albeit the concept of the Kondo effect does, in principle, not hold for an intermediate-valence material. Nonetheless, the hybridization between conduction electrons and the strongly interacting Sm f- electrons results in a gap at the Fermi energy and hence, an insulating ground state arises at temperatures below about 40 K. Recently, SmB6 has become of enormous topical interest because it is a candidate material for hosting topologically protected surface states.

Scanning tunneling microscopy (STM) has the unique capability of providing combined topographic and spectroscopic information. We conducted STM on numerous samples cleaved in situ at around 20 K [1]. Cleavage along the {001} plane of the cubic structure through breaking inter-octahedral B-B bonds gives rise to polar surfaces. In result, we found disordered chain-like as well as ordered (2 × 1) surface reconstructions. Occasionally, we also observed patches of non- reconstructed surface areas of both, Sm and B termination. On such areas, we found indications for the Kondo effect being at play.

For non-reconstructed surface areas of some ten nanometers in size the dI/dV -curves can be well described by a Fano resonance. Thus, the hybridization picture typically considered for this material could be fully confirmed. However, locally resolved tunneling spectroscopy within such areas exhibited clear evidence for electronic inhomogeneity. In addition, at temperatures as low as 0.3 K, the so called in-gap states are observed as a peak in the dI/dV –curves at -2 meV. This peak appears to be not influenced by the application of a magnetic field up to 12 T.

All types of surfaces, reconstructed and non-reconstructed, displayed a finite zero-bias conductance of considerable magnitude. This finding, in spite of different surface topologies, confirms the robustness of the metallic states and is in line with the proposal of SmB6 being a topological insulator.

References: [1] S. Roessler et al., Proc. Natl. Acad. Sci. USA 111 (2014) 4798.

Electronic structure of quasicrystals

S. R. Barman UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452001, India

Quasicrystals have fascinated scientists from the time of their discovery. Quasicrystals have forbidden symmetry and exhibit remarkable physical properties such as high resistivity and low thermal conductivity. On the basis of theoretical calculations, it is generally accepted that the stability of quasicrystals arises from a Brillouin zone-Fermi surface interaction that induces a pseudogap at the Fermi level. However, although soft x-ray photoemission studies have indicated the existence of a pseudogap from the shape of the spectral function near EF, always a metallic Fermi edge was unambiguously observed even at low temperature in high resolution studies.1 However, these studies were always conducted under surface-sensitive conditions and it might be argued that a metallic surface electronic structure hides the true bulk electronic structure.2

In the present work, using hard x-ray photoelectron spectroscopy we establish existence of a pseudogap at the Fermi level in a range of quasicrystalline solids such as icosahedral (i)-Al-Pd- Mn, i-Al-Cu-Fe,3 and Zn-Mg-Dy, Zn-Mg-Y4 which explains the mechanism of the formation of their aperiodic structure. By using a detailed curve fitting scheme we find that, compared to i-Al- Pd-Mn, the pseudogap is fully formed in i-Al-Cu-Fe. This is in agreement with the transport studies that have shown that i-Al-Cu-Fe is close to a metal-insulator phase boundary. Any significant recoil effect in the states near EF is ruled out on the basis of core-level spectra recorded with hard and soft x-ray photons.

We have also studied the unoccupied region of the electronic structure of (i) Al-Pd-Mn 5 quasicrystal. A feature that exhibits parabolic dispersion with an effective mass of (1.15 ± 0.1)me and tracks the change in the work function is assigned to an image potential resonance because our density functional calculation shows an absence of band gap in the respective energy region. The image potential resonance appears much weaker in the spectrum from the related crystalline Al- Pd-Mn surface, demonstrating that its strength is related to the compatibility of the quasiperiodicwave functions in i-Al-Pd-Mn with the free-electron like image potential states. Our investigation of the energy region immediately above EF provides unambiguous evidence for the presence of a pseudogap, in agreement with our density functional theory calculations.

References: 1Z. M. Stadnik, D. Purdie, M. Garnier, Y. Baer, A.-P. Tsai, A. Inoue, K. Edagawa, S. Takeuchi, and K. H. J. Buschow, Phys. Rev. B 55, 10938 (1997). 2G. Neuhold, S. R. Barman, K. Horn, W. Theis, P. Ebert and K. Urban, Phys. Rev. B 58, 734 (1998). 3J. Nayak, M. Maniraj, A. Rai, S. Singh, P. Rajput, A. Gloskovskii, J. Zegenhagen, D. L. Schlagel, T. A. Lograsso, K. Horn, and S. R. Barman, Phys. Rev. Lett. 109, 216403 (2012). 4J. Nayak, M. Maniraj, A. Gloskovskii, M. Krajci, S. Sebastian, I. R. Fisher, K. Horn, and S. R. Barman, Phys. Rev. B 91, 235116 (2015). 5M. Maniraj, A. Rai, S. R. Barman, M. Krajci, D. L. Schlagel, T. A. Lograsso, K. Horn, Phys. Rev. B 90, 115407 (2014).

Electronic and bosonic excitations in high temperature superconductors analyzed by time-resolved ARPES

Uwe Bovensiepen University Duisburg-Essen, Faculty of Physics, Lotharstr.1, 47057 Duisburg, Germany E-mail: [email protected]

Analysis of excitations in materials is of wide spread interest due to the coupling of electronic and bosonic degrees of freedom, in particular for high temperature superconductors. Typically the spectrum and dispersion of excitations is investigated by e. g. inelastic scattering and angle- resolved photoemission spectroscopy (ARPES). Here we report on femtosecond time-resolved ARPES results on the cuprates and the Fe-pnictides which were obtained by 1.5 eV pump and 6 eV probe photon energies with typically 100 fs time resolution. We discuss how such excitations are probed in tr-ARPES. On the cuprates we have identified a weakening of the well known kink in the electronic structure E(k) near 70 meV below the Fermi level EF, which represents a pump- induced reduction of the electron-boson coupling strength [1]. Coupling of electrons to that mode is also evident at an energy of 70 meV above EF from a step in the energy-dependent electron E E) with increasing pump fluence reflects that weakening of coupling also above EF. Experiments on Fe-pnictides exhibit a similar step in the energy dependent electron relaxation times, although the effect is weaker and occurs at higher energies in agreement with e. g. inelastic neutron scattering experiments. Furthermore, electron redistribution upon pump laser excitation modifies the Fermi momentum kF [1], which allows (a) to transiently change the effective doping level and (b) suggests a new way to probe the dynamic response of the Fermi surface of complex materials.

This work was conducted in collaboration with I. Avigo, S. Freutel, M. Ligges, L. Rettig, M. Sandhofer, J. D. Rameau, P. D. Johnson, P. Zhou, G. D. Gu, H. Eisaki, T. Wolf, P. Gegenwart, H. S. Jeevan, A. F. Kemper, and M. Sentef.

Funding by the priority program SPP 1458 of DFG, by the Mercator Research Center Ruhr, and the EU within the FP 7 under GO FAST is gratefully acknowledged.

References: [1] J. D. Rameau et al., Phys. Rev. B 89, 115115 (2014).

The Electron Spectro-Microscopy beamline at National Synchrotron Light Source II: a wide photon energy range, micro-focusing beamline for photoelectron spectro-microscopies

Elio Vescovo Brookhaven National Laboratory, National Synchrotron Light Source, Upton, United States E-mail: [email protected]

A comprehensive optical design for a high-resolution, high-flux, wide-energy range, micro- focused beamline working in the vacuum ultraviolet and soft x-ray photon energy range will be illustrated. The beamline is to provide monochromatic radiation to two photoelectron microscopes: a full-field x-ray photoelectron emission microscope (XPEEM) and a scanning instrument dedicated to angle resolved photoemission spectroscopy (μ-ARPES). Micro-focusing is achieved with state of the art elliptical cylinders, obtaining a spot size of 1 μm for ARPES. A detailed optical analysis, using geometrical as well as physical optics, quantitatively evaluates the beamline performances.

PREVAC - your worldwide partner in surface science

Slawomir Frysztak PREVAC sp. z o.o., PL-44362 Rogow, ul. Raciborska 61, Poland E-mail:

PREVAC is a world leading manufacturer of UHV scientific research instruments and systems for the investigation of chemical and physical properties of solid state surfaces, thin films and nanomaterials. We specialize in delivering custom deposition and analysis systems to clients who find that standard, off the shelf “solutions” simply do not meet the expectations demanded by the very latest cutting edge experimental investigations.

Time-of-flight momentum microscopy

Felix Leyssner1, Thorsten U. Kampen1,2, Andreas Oelsner3, C. Tusche4, Gerd Schönhense5 1SPECS Surface Nano Analysis GmbH, Voltastrasse 5, 13355 Berlin, Germany 2Institu für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany 3Surface Concept GmbH, Am Sägewerk 23a, 55124 Mainz, Germany 4Max Planck Institute for Microstructure Physics, Weinberg 2, 06120 Halle, Germany 5Institut für Physik, Johannes Gutenberg-Universität, Staudingerweg 7, 55099 Mainz, Germany E-mail: [email protected]

Our newly developed time-of-flight momentum uses an optimized lens design which provides simultaneously highest energy, angular and lateral resolution. The lens provides a full 2π solid acceptance angle with highest angular resolution. In contrast to standard ARPES measurements with a conventional hemispherical analyzer, electronic structure data from and beyond the 1st Brillouin zone is recorded without any sample movement. In addition the lens of such an instrument can work in a lateral imaging mode for microscopy as well. This enables navigation on the sample and reduces the size of the area under investigation in ARPES down to a few micrometers in diameter. The combination of large acceptance angle, high angular resolution and small acceptance area, makes this instrument the ideal tool for electronic structure studies on small samples or sample areas. The design is compact with a straight optical axis. Operation modes are (kx,ky,Ek) data aquisition by operation in energy filtered k-space imaging, ToF-PEEM mode, energy-filtered real space imaging and micro-spectroscopy mode.

The 3D (kx, ky, Ek) data recording is done with a 2-dimensional delayline detector, with a time resolution of 150 ps and count rates up to 8 Mcps. It uses channelplates with 40 μm spatial resolution. While the x,y position of an incoming electron is converted into a kx,ky wave vector, the kinetic energy Ek is determined from the flight time t. Spin-resolved imaging is achieved by electron reflection at a W(100) spin-filter crystal prior to the 2-dimensional delayline detector. Electrons are reflected in the [010] azimuth at 45° reflection angle. Varying the scattering energy one can choose positive, negative, or vanishing reflection asymmetry.

We will present data taken on different materials like Mo(110) and ferroelectric α-GeTe(111) films.

Electronic structure of layered RRh2Si2 (R = Coe, Emu, God, Ho, by) compounds: A high-resolution ARPES study

C. Laubschat1, D. Vyalikh1, S. Danzenbächer1, M. Höppner1, A. Chikina1, M. Güttler1, S. Patil1, K. Kummer2, C. Geibel3, S. Seiro3, Yu. Kucherenko4, and V. Chulkov5 1Technische Universität Dresden, Dresden, Germany 2European Synchrotron Radiation Facility, Grenoble, France. 3Max-Planck Institute for Chemical Physics, Dresden, Germany 4Institute for Metal Physics, National Academy of Sciences, Kiev, Ukraine 5Donostia Science Center, San Sebastian, Spain E-mail: [email protected]

Rare-earth transition-metal compounds have attracted much interest due to their unusual electronic properties which arise from an interplay of strongly localized 4f and itinerant valence-band states. The RRh2Si2 (R = rare earth element) series of compounds crystallizes in the layered tetragonal ThCr2Si2 structure where layers of rare-earth atoms are separated by tightly bond Si-Rh-Si trilayers. Eu. Gd, and Ho compounds reveal antiferromagnetic order whereby the rare-earth derived moments order ferromagnetically within the planes while neighbouring R-layers are coupled antiferromagnetically to each other. Ce and Yb-based compounds are well-known heavy-Fermion systems. High-quality single- crystalline samples cleave easily along the R-planes and yield atomically flat surfaces terminated either by R or Si atoms. Different terminations can easily discriminated spectroscopically by monitoring characteristic surface signals.

Si-terminated surfaces are characterized by Shockley-like surface-states in huge gaps of the projected bulk band-structure around the M-points and a surface resonance at the -point in form of a Dirac-cone. The Shockley-states reveals a strong Stoner-like splitting due to exchange coupling to the ferromagnetically ordered R-moments in the fourth layer below the surface, which remains stable up to the bulk Neel- temperature and in case of the Emu compound even above1). The Dirac cone reveals a weaker splitting at lower temperatures due to finite overlap with the second subsurface layer of R-atoms which couples antiferromagnetically to the first R subsurface layer. Both states contribute to a ferromagnetic polarization of the outermost Si-Rh-Si trilayer. The phenomena are well described in the framework of slab calculations for the paramagnetic and antiferromagnetic phases based on density functional theory (DFT).

In Ce, Eu, and Yb-compounds the Dirac-cone undergoes strong hybridization with the 4fstates characterized by avoid-crossing behaviour and admixture of 4f-charakter to the Rh-4d derived Dirac-cones. In case of the two heavy-Fermion compounds, this interaction causes strong dispersion of the crystal- electric field (CEF) split 4f-states close to the Fermi energy leading to Fermi-level crossings of 4f-derived quasi-particle bands and k-dependent changes in the sequence of the individual CEF levels2). The phenomena are well reproduced by results of a simple hybridization model that considers interactions of atomic-like 4fn final state multiplets with the valence-bands described by the above mentioned DFT calculations. For CeRh2Si2 coexistence of antiferromagnetic order and Kondo-effect is reflected by strong dispersion of CEF-levels in the neighbourhood of the -point while a lack of dispersion in other regions of k-space points to a rather localized behaviour of the 4f-states. In YbRh2Si2, admixture of 4f-character to the valence-bands leads to a “large” Fermi-surface below the Kondo-temperature TK = 26 K. A predicted transition to a “small” Fermi surface for temperatures above TK as expected for a stabilization of the trivalent configuration is not observed in-spite of a weak linear increase of the valence3).

1) M. Höppner et al., Nature Communications, 4, 1646 (2013) 2) D. V. Vyalikh et al., Phys. Rev. Lett. 105, 237601 (2010) 3) K. Kummer et al., Phys. Rev. X 5, 011028 (2015)

Spectral properties of correlated electron systems from Quantum Monte Carlo simulations

Fakher Assad Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany E-mail: [email protected]

Quantum Monte Carlo simulations offer the possibility to compute spectral properties, in the single and particle-hole channels, for a number of models of correlated electron systems. The approach is approximation free and allows for a detailed temperature analysis. In this talk, I will concentrate on two subjects. This first one is the dimensional crossover from one to two dimensions. Emphasis will be placed on the confinement of fractionalized spin (spinons) excitations present in the one- dimensional limit. The second example is that of correlated topological insulators as realized in simple models of topological Kondo insulators. Here I will concentrate on the temperature evolution of the single spectral function which shows the emergence of the topological state below the so called coherence temperature.

Title –

Pratap Raychaudhuri Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India E-mail: [email protected]

Scanning Tunneling Microscopy and spectroscopy study of FeAS based crystals with magnetic and superconducting order

Anjan K. Gupta Department of Physics, Indian Institute of Technology Kanpur, Kanpur 208016, India E-mail: [email protected]

Coexisting superconducting and magnetic phases in layered pnictides have attracted much attention after the discovery of superconductivity (SC) in pnictides. We present temperature dependent scanning tunneling microscopy and spectroscopy studies of FeAs-based 122 layered crystals with magnetic and SC orders. The topographic images show atomically flat surfaces with some signatures of atomic resolution. The tunnel spectra in the parent compounds, EuFe2As2 and SrFe2As2, show a homogeneous partial-gap in the local electronic density of states (DOS) below the spin density wave (SDW) transition temperature. The SDW-gap is found to decrease at low temperatures as if giving way to SC phase. In fact in some of the SrFe2As2 crystals the bulk transport showed evidence of SC phase.

In a doped pnictide, CaFe1.96Ni0.04As2, having both SDW ordering (below 125K) and superconductivity (Tc = 15 K), we see significant inhomogeneities in superconducting phase, which slowly disappear as we go above Tc. With increasing temperature the inhomogeneities disappear and the spectra become similar to the SDW phase of the parent compounds. We discuss this result in terms of inhomogeneous electronic phases that may co-exist due to the vicinity of this composition to the phase boundary between SC and SDW phases, which could even be a quantum critical point.

Mesoscopic superconductivity and multiple Andreev reflection on the 3D Dirac semimetal Cd3As2

Goutam Sheet IISER, Mohali, India E-mail: [email protected]

Recently, we have shown that an unconventional superconducting phase can be induced at the mesoscopic point contacts formed between elemental normal metals and the 3-D Dirac semimetal Cd3As2, which does not show superconductivity otherwise. The phase is created in situ only under the point contacts. Therefore, conventional measurement techniques traditionally used for probing superconductivity in bulk materials cannot be employed for probing the nature of superconductivity in this case. We have performed point contact spectroscopy at different regimes of mesoscopic transport to investigate the physics of this phase. We found that the superconducting phase shows a high critical temperature up to 8 K, an unusually large superconducting gap of 6 meV and a robust normal state pseudogap. The Andreev reflection spectra obtained for such point contacts show a pronounced zero-bias conductance peak indicating the possibility of spin-triplet pairing and/or topological superconductivity. Using sharp tips of superconducting Nb on Cd3As2, we have observed clear signature of multiple Andreev reflection.

In this talk, first I will give a brief overview of point contact spectroscopy between a normal metal and a superconductor. After that I will explain how such measurements on Cd3As2 led to the discovery of the new superconducting phase. I will also comment on the possible origin of the new phase.

Using Unique Architectures to Create Novel Materials

J.W. Freeland Advanced Photon Source, Argonne National Laboratory, Argonne, IL USA E-mail: [email protected]

Complex oxides are a class of materials containing a variety of competing strong interactions that create a subtle balance to define the lowest energy state, which leads to a wide variety of interesting properties (e.g. superconductivity, magnetism,...). These states arise from the interaction between the charge, orbital, spin, and lattice degrees of freedom. The challenge is in understanding how to rationally control these different degrees of freedom in the search for new materials. Here I will present recent results on Nickelates in layered architectures ranging from bulk to artificially made ultra-thin heterostructures. In order to harness these materials for the future, one of the grand challenges is to understand how to map the non-equilibrium phase space both to seek conditions where new states emerge, but also as a basis for the design of materials. However, to understand the resulting properties requires a detailed knowledge of the structural, chemical, and magnetic properties and the use of forefront X-ray tools are helping us to scratch the surface of this problem.

Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.

DFT study of graphene on magnetoelectric chromia: A probable material for spinFET

Arti Kashyap School of Basic Sciences, IIT Mandi, Mandi, HP 175001, India E-mail: [email protected]

For past decade, graphene has been an intense area of research because of its potential in number of applications ranging from electronic devices to energy storage [1]. Induced polarization in graphene has some potential for narrow-channel conduction in a spin field effect transistor (spin- FET). For such a spin FET to actually work, based on the induced polarization from the gate, the device requires a dielectric gate with interface polarization in proximity to the graphene. Most promising would be a nonvolatile spin-FET, with a multiferroic or magnetoelectric gate dielectric, having high interface polarization like chromia [2] where the interface polarization can be controlled by voltage [3-5]. There have been several efforts to investigate the induced magnetism or spin polarization in graphene supported on magnetic insulators. For example, based on transport measurements, Wang et al. [6] concluded that graphene on the magnetic insulator yttrium-iron garnet becomes magnetic, without being able to determine or even estimate the moment. Here, we investigate the reason for the observed small induced moment in graphene but disproportionally strong effect on electronic transport using density functional calculations. We show that how one antiferromagnetic dielectric oxide with high boundary polarization, used as a substrate, affects the physical behavior of graphene.

References [1]. K.S. Novoselov, V.I. Falko, L. Colombo, P.R. Gellert, M.G. Schwab and K. Kim, Nature 490,192– 200, (2013). [2]. Dowben P A, Binek, C, Nikonov D E, Chapter 11 in Silicon Nanoelectronics; 2nd edition; edited by Shuni Oda and David Ferry; Taylor and Francis (CRC Press) (2015). [3]. Cao S, Zhang X, Wu N, N’Diaye A T, Chen G, Schmid A K, Chen X, Echtenkamp W, Enders A, Binek C, Dowben P A, New J. Phys. 16, 073021 (2014) [4]. He X, Wang Y, Wu N, Caruso A N, Vescovo E, Belashchenko K D, Dowben P A, Binek C, Nat. Mater. 9, 579 (2010) [5]. Street M, Echtenkamp W, Komesu T, Cao S, Dowben P A, Binek C, Appl. Phys. Lett. 104, 222402 (2014). [6]. Wang Z Y, Tang C, Sachs R, Barlas Y, Shi J, Phys. Rev. Lett. 114, 016603 (2015).

11-type Fe-chalcogenide superconductors investigated by a scanning tunneling microscope

Sahana Rößler, C. Koz, U. Schwarz, and S. Wirth MPI for Chemical Physics of Solids, Dresden E-mail: [email protected]

The structurally simplest Fe-based superconductor FeSe with a critical temperature Tc ≈ 8.5 K displays a breaking of the fourfold rotational symmetry at a temperature Ts ≈ 87 K [1,2]. No long- range magnetic order is observed down to the lowest measured temperature in FeSe. On the other hand, isostructural Fe1+yTe displays a complex interplay of magnetic and structural phase transitions in dependence on the tuning parameters such as excess amount of Fe or pressure[3-5], but it becomes a superconductor only when Te is substituted by sufficient amount of Se [6]. Here we present the electronic properties of FeSe and Fe1.11Te investigated by scanning tunneling microscopy/spectroscopy (STM/STS), complemented by structural, magnetic, and transport measurements. Results will be discussed in terms of competing ordering modes in this exotic class of materials.

References: [1] S. Rößler, C. Koz, L. Jiao, U. K. Rößler, F. Steglich, U. Schwarz, and S. Wirth, Phys. Rev. B 92, 060505(R) (2015). [2] C. Koz, M. Schmidt. H. Borrmann, U. Burkhardt, S. Rößler, W. Carrillo-Cabrera, W. Schnelle, U. Schwarz, and Y. Grin, Z. Anorg. Allg. Chem. 640 (2014) 1600-1606. [3] C. Koz, S. Rößler, A. A. Tsirlin, S. Wirth, and U. Schwarz, Phys. Rev. B 88, 094509 (2013). [4] C. Koz, S. Rößler, A. A. Tsirlin, D. Kasinathan, C. Börrnert, M. Hanfland, H. Rosner, S. Wirth, and U. Schwarz, Phys. Rev. B 86, 094505 (2012). [5] P. Materne, C. Koz, U. K. Rößler, M. Doerr, T. Goltz, H. H. Klauss, U. Schwarz, S. Wirth, and S. Rößler, Phys. Rev. Lett. 115, 177203 (2015). [6] S. Rößler, Dona Cherian, S. Harikrishnan, H. L. Bhat, Suja Elizabeth, J. A. Mydosh, L. H. Tjeng, F. Steglich, and S. Wirth. Phys. Rev. B 82, 144523 (2010).

“Simple” metals revisited: A renormalized screened exchange approach

Pascal Delange, Leonid Pourovskii, and Silke Biermann CPHT, Ecole Polytechnique, 91128 Palaiseau cedex, France E-mail: [email protected]

While Density Functional Theory (DFT) in its local density approximation (LDA) has well known shortcomings when applied to transition metals with partly filled d shells (such as Fe or Ni), the calculated bands for Cu, with its filled d-band, coincide surprisingly well with angle- resolved photoemission data. On the other hand, the energetic position of the d-states in Zn is significantly overestimated, although its electronic properties are closely related. We use a scheme combining screened exchange and the renormalization through the frequency-dependence of the effective local Hubbard interactions [1] to study the electronic properties of simple metals. We find corrections to the LDA concerning the d-band positions, and a specific heat coefficient in agreement with experiments [2]. Our work provides insights into the nature of the deficiencies of DFT-LDA for some materials and explanations for its success in some others.

References: [1] A. van Roekeghem, S. Biermann, Europhys. Lett. 108, 57003 (2014). [2] P. Delange, L. Pourovskii, and S. Biermann (in preparation).