DOCSLIB.ORG

Momentum Shift Current at Terahertz Frequencies in Twisted Bilayer Graphene

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Momentum Shift Current at Terahertz Frequencies in Twisted Bilayer Graphene Momentum shift current at terahertz frequencies in twisted bilayer graphene Daniel Kaplan, Tobias Holder, and Binghai Yan∗ Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 7610001, Israel The detection of terahertz (THz) radiation promises intriguing applications in biology, telecommu- nication, and astronomy but remains a challenging task so far. For example, semiconductor infrared detectors (e.g., HgCdTe) utilize photo-excited electrons across the bandgap and hardly reach the far-infrared terahertz regime because they are vulnerable to thermally excited carriers. In this work, we propose the THz sensing by the bulk photovoltaic effect (BPVE) in the twisted bilayer graphene (TBG). The BPVE converts light into a coherent DC at zero bias or an open-circuit voltage. As a quantum response from the wave function's geometry (different from the p-n junction), BPVE is more robust against temperature excitation and disorders. We predict that the TBG (bandgap of several meV) exhibits a sizeable BPVE response in a range of 0.2 { 1 THz. Beyond the ordinary shift current scenario, BPVE in TBG comes from a momentum-space shift of flat bands. Our work provides a pathway to design twisted photonics for resonant terahertz detection. INTRODUCTION voltaic efficiency for energy harvesting, and have further applications in medical imaging, single-photon circuits and novel electronic devices [27, 28]. The bulk photovaltaic effect (BPVE) refers to the gen- eration of a dc current from a homogeneous solid upon Twisted bilayer graphene (TBG) exhibits flat bands irradiation with light. The so-called shift current [1{4] and narrow band gaps of several meV, and thus pro- is one of the most important mechanisms for the BPVE vides an ideal platform to investigate the THz BPVE. which has been appreciated for a long time for its po- It has attracted an immense amount of attention fol- tential in photovoltaic and photonic applications. So lowing the recent discovery of correlated insulating and far, the BVPE has been studied predominantly for near- superconducting phases at small twist angle (magic angle) infrared and optical frequencies, but it may prove useful θ ∼ 1◦ [29, 30]. While the root cause for this exciting be- to overcome the so-called 'terahertz gap' [5] in the field havior is widely accepted to be tied to a highly quenched of terahertz photonics [6]. This gap is the perceived band structure [31{33], opinions differ about the mecha- lack of robust, tunable and broad-frequency materials for nisms associated with the various correlated phases which terahertz detection. For instance, usual semiconductor have since been documented in the system [34{46]. An infrared detectors [7] harvest the photon-excited conduc- important stepping stone towards the understanding of tion electrons, which are competing with the thermal these phases is the characterization of the quantum geo- excitation across the energy gap, and thus cannot work in metric and dispersive features of the flat bands. Because the THz range. Recent developments attempt to detect of inversion symmetry breaking, TBG and similar twisted plasmons [8{10] stimulated by the THz radiation. bilayers exhibit intriguing nonlinear phenomena [47{53] such as the bulk photovaltaic effect (BPVE) [1, 54] and We point out that the shift current is quite robust nonlinear anomalous Hall effect [55, 56]. These nonlinear against thermally excited carriers, because it is a coherent probes are ideally suited for the investigation of quasipar- bulk effect driven by the quantum geometry [11{18] and ticle properties in flat bands as they are not suppressed works even in metallic systems such as Weyl semimet- by a vanishing Fermi velocity [18]. als [11, 19{26]. Most importantly, the created current does not require any DC bias or an interface to become With regards to semiconductors, the shift current had accessible to measurement. Therefore, the thermal occu- originally been interpreted as a real-space shift of wave pation of the conduction band at finite temperatures does function centers upon excitation. It has gained renewed not present a source of background current, meaning that interest in recent studies of Weyl semimetals [57, 58], be- the shift current does not compete with thermal currents, cause the shift is believed to get enhanced by the Berry save for thermal noise. While the shift current is indeed curvature near Weyl points [11, 19{21, 23{26]. Very re- arXiv:2101.07539v1 [cond-mat.mes-hall] 19 Jan 2021 diminished by occupying the conduction band, this effect cently, we proposed to instead view the shift current as a is only moderate in the terahertz regime. For example, result of the anomalous quasiparticle acceleration, which even at room temperature the thermal occupation across is determined by the quantum geometric properties [18]. a 4meV gap will lead to a shift current signal with a mag- In the latter interpretation, the small effective mass in a nitude of still 5% of its zero temperature value. Thus, Weyl semimetal leads to strong acceleration in the field of the shift current can be useful for higher temperature light and hence generates large photocurrent, consistent terahertz sensing based on noncentrosymmetric materials with the shift current interpretation [11]. Flat bands, as that exhibit resonant BPVE response in the meV window. for example in TBG, represent a kind of opposite limit The shift current may substantially improve detection (large quasiparticle mass) compared to Weyl bands (nearly capabilities for terahertz radiation, increase the photo- massless quasiparticles). Using the language of a quasipar- 2 Distinct from the nonlinear anomalous Hall effect, the shift current of TBG originates from the properties of the quantum geometry of the band structure that is un- related to the Berry curvature dipole. The magnitude of the obtained current, tunability of the Hall response, the broadness of the resonances, and the terahertz regime in which they are all observed hold promise of the uti- lization of TBG devices in terahertz technologies, such photodetection and photovoltaic DC current generation. RESULTS A. Shift current theory In the clean limit, the Bloch states produce an intrinsic Figure 1. A schematic comparison of the wavefunction in dc-current response to linearly polarized light [2]. This real-space and momentum-space of electronic states at the nonlinear conductivity σaa;c is commonly formulated [2, 3] Fermi surface for three types of gapless materials. For a (s) as Weyl semimetal, states are well localized at few points in the momentum space but quite extended in the real space. In aa;c contrast, for a flat band all momenta are equally occupied, σ(s) (0; !; −!) = leading to sharply localized peaks of the wavefunction in real 3 Z πe X c a 2 space. The ordinary metal with a large Fermi surface repre- 2 fmnSmnjrmnj δ(! ± "mn); (1) sents the intermediate region between the Weyl semimetal and ~ k mn the flat-band system. If the shift current is viewed as a result of the anomalous acceleration, the Weyl semimetal exhibits where m; n are the band indices, "mn = "m − "n the a large real-space shift while the flat band displays a major band energy difference, fmn = f("m) − f("n) the Fermi a a momentum-space shift. distribution function difference, and rmn ≡ hmjr jni the dipole transition matrix element. Here, a; c represents the light field and current directions, respectively. Smn ticle shift, it is therefore less obvious to deduce the shift is the so-called shift vector, current for a flat-band system. In contrast, in the semi- c c c c classical acceleration picture, a state can be displaced (i.e. Smn = (rmm − rnn) + @kc arg rmn; (2) accelerated) in either real-space and momentum-space, c c as illustrated in Fig.1. Although the real-space shift is where (rmm −rnn) is the shift of the wave function centers, indeed small in real-space due to the large quasiparticle which is gauge dependent. mass, for flat bands the acceleration (i.e. the displacement The second term in Eq.2 is the phase derivative of c in momentum space) can still be very large. Therefore, rmn, which ensures gauge-invariance for the shift vector. we are motivated to investigate the shift current of TBG Smn has then been interpreted as the real-space shift of from the acceleration point of view. the mass center of a quasiparticle upon excitation from A previous study has reported a non-linear conductiv- band m to n [2]. As we will show now, the phase term in ity above 30meV due to the orbital magnetization at 3/4 (2) gives rise to a shift in momentum space. Therefore, filling [47]. In this work, we study shift current generation the shift current is actually the result of a shift of the below 10meV in magic angle TBG across the single parti- quasiparticle excitation in both real space and momentum cle gaps. In a numerical analysis, find a large photocurrent space, which is naturally captured using the language of response for light in the low terahertz regime. This is the anomalous quasiparticle acceleration [18]. consistent with the acceleration picture outlined above. To this end, we express the nonlinear conductivity in To further support such a connection, we formally express the form of geometric properties of the band structure as the response in terms of the real-space shift current and follows, momentum-space shift current. Due to the flat-band dis- 3 Z persion, the dominant contribution is constituted by the aa;c πe aac aac σ(s) (0; !; −!) = − 2 Rshift + Kshift (3) momentum-space shift, which we explicitly show to be ~ k dissimilar to an ordinary dispersive material (e.g., MoS2). 3 (b) 15 (a) + 10 5 ] 0 u. a. [ (c) - 5 - 10 0 - 15 Figure 2.
Load more

Recommended publications
  • Experimental Investigations of Contact Friction and Transport Properties of Monolayer and Bilayer Graphene
    Graduate Theses, Dissertations, and Problem Reports 2021 Experimental Investigations of Contact Friction and Transport Properties of Monolayer and Bilayer Graphene Prakash Gajurel [email protected] Follow this and additional works at: https://researchrepository.wvu.edu/etd Part of the Condensed Matter Physics Commons Recommended Citation Gajurel, Prakash, "Experimental Investigations of Contact Friction and Transport Properties of Monolayer and Bilayer Graphene" (2021). Graduate Theses, Dissertations, and Problem Reports. 7994. https://researchrepository.wvu.edu/etd/7994 This Dissertation is protected by copyright and/or related rights. It has been brought to you by the The Research Repository @ WVU with permission from the rights-holder(s). You are free to use this Dissertation in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you must obtain permission from the rights-holder(s) directly, unless additional rights are indicated by a Creative Commons license in the record and/ or on the work itself. This Dissertation has been accepted for inclusion in WVU Graduate Theses, Dissertations, and Problem Reports collection by an authorized administrator of The Research Repository @ WVU. For more information, please contact [email protected]. Graduate Theses, Dissertations, and Problem Reports 2021 EXPERIMENTAL INVESTIGATIONS OF CONTACT FRICTION AND TRANSPORT PROPERTIES OF MONOLAYER AND BILAYER GRAPHENE Prakash Gajurel Follow this and additional works at: https://researchrepository.wvu.edu/etd Part of the Condensed Matter Physics Commons EXPERIMENTAL INVESTIGATIONS OF CONTACT FRICTION AND TRANSPORT PROPERTIES OF MONOLAYER AND BILAYER GRAPHENE Prakash Gajurel Dissertation submitted to the Eberly College of Arts and Sciences at West Virginia University in partial fulfillment of the requirements for the degree of Doctor of Philosophy In Physics Mikel Holcomb, Ph.D., Chair Wathiq Abdul -Razzaq, Ph.D.
    [Show full text]
  • Arxiv:2103.08851V2 [Cond-Mat.Mes-Hall] 28 Mar 2021
    Macroscopically degenerate localized zero-energy states of quasicrystalline bilayer systems in strong coupling limit 1 1, 2, 3, Hyunsoo Ha and Bohm-Jung Yang ∗ 1Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea 2Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul 08826, Korea 3Center for Theoretical Physics (CTP), Seoul National University, Seoul 08826, Korea (Dated: March 30, 2021) When two identical two-dimensional (2D) periodic lattices are stacked in parallel after rotating one layer by a certain angle relative to the other layer, the resulting bilayer system can lose lattice periodicity completely and become a 2D quasicrystal. Twisted bilayer graphene with 30◦ rotation is a representative example. We show that such quasicrystalline bilayer systems generally develop macroscopically degenerate localized zero-energy states (ZESs) in strong coupling limit where the interlayer couplings are overwhelmingly larger than the intralayer couplings. The emergent chiral symmetry in strong coupling limit and aperiodicity of bilayer quasicrystals guarantee the existence of the ZESs. The macroscopically degenerate ZESs are analogous to the flat bands of periodic systems, in that both are composed of localized eigenstates, which give divergent density of states. For monolayers, we consider the triangular, square, and honeycomb lattices, comprised of homogenous tiling of three possible planar regular polygons: the equilateral triangle, square, and regular hexagon. We construct a compact theoretical framework, which we call the quasiband model, that describes the low energy properties of bilayer quasicrystals and counts the number of ZESs using a subset of Bloch states of monolayers. We also propose a simple geometric scheme in real space which can show the spatial localization of ZESs and count their number.
    [Show full text]
  • What Drives Superconductivity in Twisted Bilayer Graphene?
    Journal Club for Condensed Matter Physics DOI:10.36471/JCCM_May_2020_03 https://www.condmatjclub.org What drives superconductivity in twisted bilayer graphene? 1. The interplay of insulating and superconducting orders in magic- angle graphene bilayers Authors: Petr Stepanov, Ipsita Das, Xiaobo Lu, Ali Fahimniya, Kenji Watanabe, Takashi Taniguchi, Frank H. L. Koppens, Johannes Lischner, Leonid Levitov, Dmitri K. Efetov https://arxiv.org/abs/1911.09198 2. Decoupling superconductivity and correlated insulators in twisted bilayer graphene Authors: Yu Saito, Jingyuan Ge, Kenji Watanabe, Takashi Taniguchi, and Andrea F. Young https://arxiv.org/abs/1911.13302 3. Tuning electron correlation in magic-angle twisted bilayer graphene using Coulomb screening Authors: Xiaoxue Liu, Zhi Wang, K. Watanabe, T. Taniguchi, Oskar Vafek, J. I. A. Li https://arxiv.org/abs/2003.11072 4. Nematicity and Competing Orders in Superconducting Magic-Angle Graphene Authors: Yuan Cao, Daniel Rodan-Legrain, Jeong Min Park, Fanqi Noah Yuan, Kenji Watanabe, Takashi Taniguchi, Rafael M. Fernandes, Liang Fu, Pablo Jarillo-Herrero https://arxiv.org/abs/2004.04148 Recommended with a Commentary by T. Senthil, Massachusetts Institute of Technology, USA. When two layers of graphene are stacked on top of each other with a small relative twist, a long period moire pattern forms. For the electronic states, this moire superlattice defines a new mini-Brillouin zone determined by the long real space period. Early theory[1] showed that near certain ``magic angles” these reconstructed electronic bands have very narrow bandwidths, comparable or lower than the estimated strength of Coulomb interactions between the electrons. This makes twisted bilayer graphene (TBLG) a possible platform for correlated electron physics in a tunable two dimensional material.
    [Show full text]
  • Electronic Properties of Low-Angle Twisted Bilayer Graphene
    Electronic properties of low-angle twisted bilayer graphene Viet-Hung Nguyen1 Dawid Paszko1, Benoit van Troeye2, Michael Lamparski2, Vincent Meunier2, Jean-Christophe Charlier1 1Université Catholique de Louvain (UCLouvain), Louvain-la-Neuve, Belgium 2Rensselaer Polytechnic Institute, Troy, New York 12180, USA Contact: [email protected] Abstract Twisted bilayer graphene (tBG) at and around specific magic angles [1] exhibiting correlated insulating phases and superconductivity has boosted the new field of “twistronics” [2] where strong electron- electron interactions play a dominant role in the electronic properties of the system. Below a threshold twist angle qc ~ 1°, this superlattice undergoes self-organized lattice reconstruction [3], forming a periodic domain and thus modifying strongly its electronic structure, compared to those osberved above such threshold angle. Although low-angle tBG has been intensively investigated using effecttive electronic approaches (see an example in [4]), an in-depth investigation using more accurate calculations is still highly desirable. In this work, we developed and performed atomistic calculations using the Green’s function techniques to solve tight-binding models, where the lattice reconstruction obtained by simulations as in [5] is taken into account. Indeed, it was shown that the lattice reconstruction presents very significant effects on the electronic structure of low-angle (around and below qc) tBG systems. Especially, the second magic angle around q≈0.5° predicted in [1] is no longer observed (see Fig.1), which is a direct consequence of the mentioned lattice reconstruction. In addition, the local electronic properties and the helical network in these tBG systems were systemmatically investigated. Our work thus presents valuable contributions, that could be helpful for further development of the field “twistronics”, e.g., as it helped to interpret the experimental data [6].
    [Show full text]
  • Arxiv:2006.00399V2 [Cond-Mat.Mes-Hall] 8 Oct 2020
    Twisted Trilayer Graphene: a Precisely Tunable Platform for Correlated Electrons Ziyan Zhu,1 Stephen Carr,1 Daniel Massatt,2 Mitchell Luskin,3 and Efthimios Kaxiras1, 4 1Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA 2Department of Statistics, The University of Chicago, Chicago, Illinois 60637, USA 3School of Mathematics, University of Minnesota - Twin Cities, Minneapolis, Minnesota 55455, USA 4John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA We introduce twisted trilayer graphene (tTLG) with two independent twist angles as an ideal system for the precise tuning of the electronic interlayer coupling to maximize the effect of correlated behaviors. As established by experiment and theory in the related twisted bilayer graphene system, van Hove singularities (VHS) in the density of states can be used as a proxy of the tendency for correlated behaviors. To explore the evolution of VHS in the twist-angle phase space of tTLG, we present a general low-energy electronic structure model for any pair of twist angles. We show that the basis of the model has infinite dimensions even at a finite energy cutoff and that no Brillouin zone exists even in the continuum limit. Using this model, we demonstrate that the tTLG system exhibits a wide range of magic angles at which VHS merge and the density of states has a sharp peak at the charge-neutrality point through two distinct mechanisms: the incommensurate perturbation of twisted bilayer graphene’s flatbands or the equal hybridization between two bilayer moiré superlattices. Introduction. — Electronic properties in stacked of moiré length as a function of twist angles, in which graphene layers can be tuned by a small twist angle each lobe corresponds to the region where a different that modifies the interlayer interaction strength, an ef- harmonic (m, n) dominates.
    [Show full text]
  • Arxiv:2002.05124V5 [Cond-Mat.Mes-Hall] 4 Jun 2020 [45]
    Effective Floquet Hamiltonians for periodically-driven twisted bilayer graphene Michael Vogl∗ Department of Physics, The University of Texas at Austin, Austin, TX 78712, USA Martin Rodriguez-Vega∗ Department of Physics, The University of Texas at Austin, Austin, TX 78712, USA and Department of Physics, Northeastern University, Boston, MA 02115, USA Gregory A. Fiete Department of Physics, Northeastern University, Boston, MA 02115, USA and Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA (Dated: June 5, 2020) We derive effective Floquet Hamiltonians for twisted bilayer graphene driven by circularly polar- ized light in two different regimes beyond the weak-drive, high frequency regime. First, we consider a driving protocol relevant for experiments with frequencies smaller than the bandwidth and weak amplitudes and derive an effective Hamiltonian, which through a symmetry analysis, provides an- alytical insight into the rich effects of the drive. We find that circularly polarized light at low frequencies can selectively decrease the strength of AA-type interlayer hopping while leaving the AB-type unaffected. Then, we consider the intermediate frequency, and intermediate-strength drive regime. We provide a compact and accurate effective Hamiltonian which we compare with the Van Vleck expansion and demonstrate that it provides a significantly improved representation of the exact quasienergies. Finally, we discuss the effect of the drive on the symmetries, Fermi velocity and the gap of the Floquet flat bands. I. INTRODUCTION an exponentially-long pre-thermal time regime [59{64] in driven interacting quantum systems allows one to intro- The recent discovery of strong-correlation effects in duce the notion of effective time-independent theories.
    [Show full text]
  • Arxiv:2009.11301V2 [Cond-Mat.Mes-Hall] 27 Oct 2020
    TBG I: Matrix Elements, Approximations, Perturbation Theory and a k · p 2-Band Model for Twisted Bilayer Graphene B. Andrei Bernevig,1 Zhi-Da Song,1 Nicolas Regnault,1, 2 and Biao Lian1 1Department of Physics, Princeton University, Princeton, New Jersey 08544, USA 2Laboratoire de Physique de l'Ecole normale superieure, ENS, Universit´ePSL, CNRS, Sorbonne Universit´e, Universit´eParis-Diderot, Sorbonne Paris Cit´e,Paris, France (Dated: October 28, 2020) We investigate the Twisted Bilayer Graphene (TBG) model of Bistritzer and MacDonald (BM) [1] to obtain an analytic understanding of its energetics and wavefunctions needed for many-body calculations. We provide an approximation scheme for the wavefunctions of the BM model, which first elucidates why the BM KM -point centered original calculation containing only 4 plane-waves ◦ provides a good analytical value for the first magic angle (θM ≈ 1 ). The approximation scheme also elucidates why most of the many-body matrix elements in the Coulomb Hamiltonian projected to the active bands can be neglected. By applying our approximation scheme at the first magic angle to a ΓM -point centered model of 6 plane-waves, we analytically understand the reason for the small ΓM -point gap between the active and passive bands in the isotropic limit w0 = w1. Furthermore, we analytically calculate the group velocities of the passive bands in the isotropic limit, and show that they are almost doubly degenerate, even away from the ΓM -point, where no symmetry forces them to be. Furthermore, moving away from the ΓM and KM points, we provide an explicit analytical perturbative understanding as to why the TBG bands are flat at the first magic angle, despite the first magic angle is defined by only requiring a vanishing KM -point Dirac velocity.
    [Show full text]
  • Topological Floquet Engineering of Twisted Bilayer Graphene
    Topological Floquet Engineering of Twisted Bilayer Graphene Gabriel E. Topp,1 Gregor Jotzu,1 James W. McIver,1 Lede Xian,1 Angel Rubio,1, 2 and Michael A. Sentef1, ∗ 1Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany 2Center for Computational Quantum Physics (CCQ), The Flatiron Institute, 162 Fifth Avenue, New York NY 10010 (Dated: July 1, 2019) We investigate the topological properties of Floquet-engineered twisted bilayer graphene above the magic angle driven by circularly polarized laser pulses. Employing a full Moir´e-unit-celltight- binding Hamiltonian based on first-principles electronic structure we show that the band topology in the bilayer, at twisting angles above 1.05◦, essentially corresponds to the one of single-layer graphene. However, the ability to open topologically trivial gaps in this system by a bias voltage between the layers enables the full topological phase diagram to be explored, which is not possible in single- layer graphene. Circularly polarized light induces a transition to a topologically nontrivial Floquet band structure with the Berry curvature of a Chern insulator. Importantly, the twisting allows for tuning electronic energy scales, which implies that the electronic bandwidth can be tailored to match realistic driving frequencies in the ultraviolet or mid-infrared photon-energy regimes. This implies that Moir´esuperlattices are an ideal playground for combining twistronics, Floquet engineering, and strongly interacting regimes out of thermal equilibrium. I. INTRODUCTION used as a means to break the symmetry for transient in- termediate states, such as in the case of laser-controlled Light-matter coupled systems are emerging as an novel superconductors [12].
    [Show full text]
  • Murjmassachusetts Institute of Technology Undergraduate Research Journal
    Volume 39 Spring 2020 MURJMassachusetts Institute of Technology Undergraduate Research Journal Features p. 11 Interview: Whitehead Director Ruth Lehmann News p. 4 A New Twist on Graphene Research p. 24 Cleaning with Vibration BE in charge of your next career move Search for your new role quickly by discipline, country, salary and more on naturecareers.com A85088 Contents Introductory Letter 2 From the Editors News 4 A look at the latest MIT Science News Features 11 MURJ Spotlight: Ruth Lehmann Rachel Rock Drosophila ovary [Credit: Hogan Tang and Denise Montell, Johns Hopkins University and University of California, Santa Barbara] UROP Summaries 19 13C-2H REDOR Magic-Angle-Spinning Solid-State NMR for Characterizing Biomacromolecular Conformational Dynamics Kelly Chen, Martin Gelenter, Mei Hong Reports 23 Vibration-induced Anti-biofouling of Reverse Osmosis Membranes Maisha M. Prome, Grace Goon, Xuanhe Zhao 28 Characterizing Aperture Masking Interferometry in the Near- Infrared as an Effective Technique for Astronomical Imaging Kyle Morgenstein, Michael J. Person, Kerri Cahoy 34 Adapting TextRank for Medical Text Summarization Gloria (Yu Liang) Fang Volume 39, Spring 2020 MURJ Introductory Letter MURJ Staff Massachusetts MIT Undergraduate Research Journal Institute of Technology UNDERGRADUATE RESEARCH JOURNAL May 2020 Volume 39, Spring 2020 Dear MIT Community, Editors-in-Chief Allison Huske We are delighted to present the 39th issue of the MIT Natasha Joglekar Undergraduate Research Journal. Over the past few months, the Vaibhavi Shah COVID-19 pandemic has scattered the MIT community, but our Content Editor undergraduates continue their research. In this time of crisis, the Aman Patel ongoing efforts of scientists and the open sharing of research are absolutely necessary.
    [Show full text]
  • Spin-Polarized Superconductivity: Order Parameter Topology, Current Dissipation, and Multiple-Period Josephson Effect
    Spin-polarized superconductivity: order parameter topology, current dissipation, and multiple-period Josephson effect Eyal Cornfeld,1 Mark S. Rudner,2 and Erez Berg1 1Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 7610001, Israel 2Niels Bohr International Academy and Center for Quantum Devices, University of Copenhagen, 2100 Copenhagen, Denmark We discuss transport properties of fully spin-polarized triplet superconductors, where only elec- trons of one spin component (along a certain axis) are paired. Due to the structure of the order parameter space, wherein phase and spin rotations are intertwined, a configuration where the super- conducting phase winds by 4π in space is topologically equivalent to a configuration with no phase winding. This opens the possibility of supercurrent relaxation by a smooth deformation of the order parameter, where the order parameter remains non-zero at any point in space throughout the entire process. During the process, a spin texture is formed. We discuss the conditions for such processes to occur and their physical consequences. In particular, we show that when a voltage is applied, they lead to an unusual alternating-current Josephson effect whose period is an integer multiple of the usual Josephson period. These conclusions are substantiated in a simple time-dependent Ginzburg-Landau model for the dynamics of the order parameter. One of the potential applications of our analysis is for moir´esystems, such as twisted bilayer and double bilayer graphene, where superconductivity is found in the vicinity of ferromagnetism. I. INTRODUCTION Spin-triplet superconductors (SCs) and superfluids are predicted to exhibit rich phenomena owing to the inter- play between the spin and phase degrees of freedom of their order parameters.
    [Show full text]