Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session N70: Theory of Topological Materials II: Models and PropertiesRecordings Available
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Chair: Jorn Venderbos, Drexel University Room: Hyatt Regency Hotel -Jackson Park B |
Wednesday, March 16, 2022 11:30AM - 11:42AM |
N70.00001: Topological Surface States in an Ideal Weak Topological Insulator Ji Seop Oh, Tianyi Xu, Nikhil Dhale, Sheng Li, Chao Lei, Hanlin Wu, Jianwei Huang, Makoto Hashimoto, Donghui Lu, Christopher Jozwiak, Aaron Bostwick, Eli Rotenberg, Bing Lv, Fan Zhang, Robert J Birgeneau, Ming Yi The quasi-one-dimensional bismuth halides Bi4X4 (X = I, Br) constitute a recently realized material platform that harbors a number of non-trivial topological phases such as weak topological insulator (TI) and higher-order TI. In addition, superconductivity has been observed under pressure, suggesting potentially important roles played by electronic/structural instabilities. Thus, Bi4X4 is a promising platform that facilitates non-trivial topology in the presence of electronic/structural instabilities. Therefore, characterizing these compounds’ topological phases as well as finding the origin of the instabilities become essential for both a deeper understanding and future topological engineering. Among the Bi4X4 series of materials, we find a specific case of the simplest AA stacking order of the crystal structure at cryogenic temperatures, enabling us to conduct high energy resolution angle-resolved photoemission spectroscopy (ARPES) experiments. In this talk, I will present our ARPES results for identification of its topological character as a weak TI by measurements on both (100) and (001) cleaved surfaces. Furthermore, combined with an effective Hamiltonian, characterization for in-gap van Hove singularities purely derived from topologically non-trivial surface states will be presented. |
Wednesday, March 16, 2022 11:42AM - 11:54AM |
N70.00002: First-principles electronic structrure study of substitution of Sb with Sn and Te in CsV3Sb5 Farnaz Kaboudvand Kagome metals attract a lot of attention due to their fascinating electronic structures. Owing to their special geometry and depending on the degree of electron filling in their lattices, these materials are predicted to host a variety of instabilities like superconductivity, spin liquid states, charge density waves (CDW), and more. One of the recently discovered non-magnetic kagome metals is AV3Sb5 (A = K, Rb, Cs). This family of layered metals exhibits CDW, unconventional superconductivity and Z2-type band topology which believed to arise due to the proximity of saddle points in the vicinity of Fermi level. Among all the compounds, CsV3Sb5 is the heaviest member of the family which exhibits superconductivity at 2.5 K and a CDW transition at 94 K. Here, using the first-principles study, we analyzed the substitution of Sb atoms with Sn and Te. We used Density functional theory (DFT) calculations to gain insight into the electronic states and explore the impact of doping on band structure and CDW states in these compounds. |
Wednesday, March 16, 2022 11:54AM - 12:06PM |
N70.00003: First-principles feasibility assessment of a topological insulator at the InAs/GaSb interface Derek Dardzinski, Shuyang Yang, Andrea Hwang, Dmitry I Pikulin, Georg W Winkler, Noa Marom First-principles simulations are conducted to shed light on the question of whether a 2D topological insulator phase may be obtained at the interface between InAs and GaSb. To this end, the InAs/GaSb interface is compared to the HgTe/CdTe interface. Density functional theory (DFT) simulations of these interfaces are performed using a machine-learned Hubbard U correction. For the HgTe/CdTe interface, our simulations show that band crossing is achieved and an inverted gap is obtained at a critical thickness of 5.1 nm of HgTe, in agreement with experiment and previous DFT calculations. In contrast, for InAs/GaSb, the gap narrows with increasing thickness of InAs; however, the gap does not close for interfaces with up to 50 layers (15 nm) of each material. When an external electric field is applied across the InAs/GaSb interface, the GaSb-derived valence band maximum is shifted up in energy with respect to the InAs-derived conduction band minimum until the bands cross and an inverted gap opens. Our results show that it may be possible to reach the topological regime at the InAs/GaSb interface under the right conditions. However, it may be challenging to realize these conditions experimentally, which explains the difficulty of experimentally demonstrating an inverted gap in InAs/GaSb. |
Wednesday, March 16, 2022 12:06PM - 12:18PM |
N70.00004: Revealing the boundary Weyl physics of the four-dimensional Hall effect via phason engineering in metamaterials Wenting Cheng
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Wednesday, March 16, 2022 12:18PM - 12:30PM |
N70.00005: Unsupervised Learning of Symmetry Protected Topological Phase Transitions EnJui KUO We can use a diffusion map to detect the phase transition of the 1+1 d Symmetry protected topological phase. Our model works for spin half and spin 1 system and 1d fermion chain. We can create the phase diagram for all the possible phases in the 1+1 d dimensions for both short-range and long-range phases. |
Wednesday, March 16, 2022 12:30PM - 12:42PM |
N70.00006: Deep learning for disordered topological materials through entanglement spectrum Juan J Palacios, Alejandro Uría Calculation of topological invariants for crystalline systems is well understood in terms of the Wilson loop technique in reciprocal space, allowing for algorithmic evaluation of the invariants for a wide spectrum of materials. While this same approach may still be valid for disordered or complex materials, where the supercell must be big enough to capture the physics of the material, it becomes a limiting factor as the calculation is expensive, disabling high-throughput computations. On top of that, the Wilson loop technique is only well-defined in insulating materials, i.e. systems whose bands are fully occupied. There are several methods to try to overcome these difficulties, such as the local Chern marker or the Bott index, which are defined in real space, but that are only applicable without time-reversal symmetry. In our work, we present a new technique based on the entanglement spectrum of a system, which contains the topological nature as it measures charge pumping. We show that it is possible to train a neural network to distinguish between trivial and topological phases using the entanglement spectrum from crystalline phases, and use it to predict the topological phase diagram of time-reversal disordered systems, or fractal lattices such as the Bethe lattice. This approach is shown to be robust, as it does not depend on boundary conditions and can be computed without need of a gap, also providing a speed-up compared with the Wilson loop technique. |
Wednesday, March 16, 2022 12:42PM - 12:54PM |
N70.00007: Robustness of Helical Edge States Under Edge Reconstruction Bernd Rosenow, Niels John, Adrian G Del Maestro The helical edge states of time-reversal invariant two-dimensional topological insulators are protected against backscattering in idealized models. In more realistic scenarios with a shallow confining potential at the sample boundary, additional strongly interacting edge states may arise, that could interfere with the topological protection of edge conduction. We find that interaction effects within the reconstructed edges are well described by the Luttinger liquid model. While interactions between this Luttinger liquid and the helical edge states can in principle give rise to dynamical spin polarization and the breaking of time-reversal symmetry, we demonstrate that random spin-orbit coupling strongly suppresses such dynamical spin polarization, resulting in the persistence of near quantized edge conduction. |
Wednesday, March 16, 2022 12:54PM - 1:06PM |
N70.00008: Local probes of higher order topology in chiral matter: theory and experiments Marcelo Guzmán, Denis Bartolo, David Carpentier The standard description of chiral topological insulators relies on winding and sub winding numbers of eigenstates in periodic bipartite lattices. Generalised bulk-boundary correspondences then relate these topological bulk properties to a zero mode counting at the surfaces, hinges, and corners of finite samples. |
Wednesday, March 16, 2022 1:06PM - 1:18PM |
N70.00009: The reciprocal skin effect and its realization in a topolectric circuit Tobias Helbig, Tobias Hofmann, Ching Hua Lee, Martin Greiter, Titus Neupert, Ronny Thomale The non-Hermitian skin effect constitutes a new paradigm in the research and design of synthetic metamaterials. It fundamentally relies on the combined breaking of Hermiticity and reciprocity. In this talk, we present the novel approach of obtaining extensive eigenmode localization in the presence of reciprocity. In contrast to non-reciprocal implementations of the Skin effect which require an external energy supply, our approach can be implemented by exclusively passive components. We demonstrate this in a passive RLC circuit network. The reciprocal Skin effect suggests itself for realizations in a plethora of metamaterial platforms with limited availability of active components. |
Wednesday, March 16, 2022 1:18PM - 1:30PM |
N70.00010: Phonon with a large magnetic moment in topological materials Yafei Ren, Cong Xiao, Daniyar Saparov, Qian Niu The Born-Oppenheimer approximation assumes an adiabatic evolution of electronic states following ions’ motion. During the evolution, the electronic state can collect nontrivial geometrical phases. In response to a circularly polarized phonon, the geometrical phase leads to a topological magnetic moment expressed in a second Chern form [arXiv: 2103.05786]. This refines the Born effective charge to be k-resolved and reveals the essential role of the phonon modified boundary current that is related to the momentum space Berry curvature. When a Yang's monopole presents near the parameter space of interest, the second Chern form can diverge, resulting in a large phonon magnetic moment. Topological insulators/semimetals generally host phonons with a large magnetic moment. Our results agree with recent experiments. |
Wednesday, March 16, 2022 1:30PM - 1:42PM |
N70.00011: Interfacial Corner Modes in a Topolectrical Heterojunction Haydar Sahin, S M M Rafi-Ul-Islam, Zhuo Bin Siu, Ching Hua Lee, Mansoor B.A. Jalil Non-Hermitian topolectrical circuit (TE) networks consisting of capacitors, inductors, and operational amplifiers can be designed to realize higher-order topological states in which all the eigenmodes are exponentially localized at the corners of a square lattice network. Here, we utilize a TE circuit to realize a higher-order non-Hermitian (HONH) heterojunction in which all the eigenstates are localized at the interfacial nodes. The novel localization of these eigenstates, which we dub as the interfacial higher-order non-Hermitian corner modes (INHCM), arises whenever two HONH topological lattices with opposite signs of the non-Hermitian parameter are cascaded together. Furthermore, the INHCMs appear under both open boundary conditions (OBC) and periodic boundary conditions (PBC), in marked contrast to the conventional non-Hermitian skin effect which occurs only under OBC but not under PBC. We analyze the decay lengths of the interfacial corner mode localization and establish its dependence on the asymmetric intra-unit cell couplings in the heterojunction circuit. Our study establishes the conditions for the onset of HONH effects in heterojunction systems and unveils a new type of skin mode localization at the interface. |
Wednesday, March 16, 2022 1:42PM - 1:54PM |
N70.00012: Geometrical Rabi oscillations in non-Abelian systems Hannes Weisbrich, Gianluca Rastelli, Wolfgang Belzig Topological phases of matter became a new standard to classify quantum systems in many cases, yet key quantities like the quantum geometric tensor providing local information about topological properties are still experimentally hard to access, especially in non-Abelian systems when states are degenerate and the quantum geometric tensor has a non-Abelian form. We propose protocols to determine the quantum geometric tensor in non-Abelian quantum systems. We show theoretically that for a weak resonant driving of the local parameters the coherent Rabi oscillations and their frequencies are related to the non-Abelian quantum geometric tensor. Our schemes suggest also a way to prepare eigenstates of the quantum metric, a task that is difficult otherwise in a degenerate subspace. |
Wednesday, March 16, 2022 1:54PM - 2:06PM |
N70.00013: Unconventional Hybridization of Skin Modes in Coupled non-Hermitian Chains Haydar Sahin, S M M Rafi-Ul-Islam, Zhuo Bin Siu, Ching Hua Lee, Mansoor B.A. Jalil Non-Hermitian systems exhibit a plethora of unusual topological phenomena including the extreme eigenstate localization of eigenstates, known as the non-Hermitian skin effect (NHSE), which occurs under open boundary conditions (OBC). Here, we report on the eigenstate localization in a system of two coupled non-Hermitian chains with dissimilar inverse skin lengths in which the NHSE can be switched on or off by modulating the inter-chain coupling strength. In the limit of small inter-chain strength, the NHSE is present at both ends of the coupled system because of the weak hybridization between the eigenstates of the individual chains. The eigenspectrum under OBC exhibits a discontinuous jump known as the critical NHSE (CNHSE) as the chain length increases. However, when the inter-chain coupling strength and hence the hybridization between eigenstates becomes significant, the CNHSE vanishes and the eigenstates are no longer all localized at the same boundary. Instead, a peculiar “half-half skin localization” occurs in composite chains with opposite signs of inverse skin lengths, where half of the eigenstates are exponentially localized at one chain and the remainder of the eigenstates on the other chain. Our results provide new theoretical insights into the critical non-Hermitian phenomena in coupled systems. |
Wednesday, March 16, 2022 2:06PM - 2:18PM |
N70.00014: Fermi surface studies of YSi from dHvA oscillations and first principles calculations VIKAS SAINI, SOUVIK SASMAL, Bahadur Singh, A Thamizhavel Topologically nontrivial compounds are enormously explored for their novel symmetry protected bulk and surface states. We present our experimental results on the Fermi surface study of YSi using de Haas-van Alphen (dHvA) oscillations combined with theoretical first principles calculations. The fast Fourier transform of the dHvA oscillations has revealed three frequencies α, β, and γ for B || [010] at 36 T, 295 T, and 61T, respectively. For other two configurations of magnetic field B || [100] and B || [001], quantum oscillations are observed only for α pocket. Further quantum oscillations simulation confirms the two-dimensional character of the β and γ pockets whereas α pocket is existing along all three directions. The estimated Berry phase of α pocket is 0.52p and 1.39p for B || [100] and [001], which gives the non-trivial character of α band. From the first principles calculations, in the absence of spin-orbit coupling (SOC) nodal line crossings are observed along various high symmetry paths whereas inclusion of spin-orbit coupling opens a small gap between the bands along the generic directions and symmetry enforced robust Dirac states are obtained at time reversal invariant points. |
Wednesday, March 16, 2022 2:18PM - 2:30PM |
N70.00015: Relativistic topological molecular crystals Tonghua Yu, Ryotaro Arita, Motoaki Hirayama Topological phases are normally difficult to realize in molecular systems, because the inter-molecular interactions are in general too weak to mediate band inversion. In this talk, however, we shall present a general strategy to hunt for relativistic topological molecular crystals (RTMCs), where the band topology emerges from the band inversion between the interstitial electron states and molecular orbitals, as well as from the strong spin-orbit coupling of heavy molecular building blocks. As an example, we identify the three-dimensional molecular crystal K4Ba2[SnBi4] as a strong topological insulator in a first-principles manner. This RTMC system manifests exceptional characteristics as follows. (1) It hosts cleavable ends along all three directions, with clean pronounced topological surface states. (2) Strong response to external pressure or strain, and hence topological phase transition under relatively low pressure. (3) High-efficiency thermoelectricity, with the Seebeck coefficient S ~ 220 μV/K, and lattice thermal conductivity κL ~ 0.22 W/mK. (4) Ultralow work function (~2.3 eV) caused by the interstitial states. Several other RTMCs will be introduced as well. Our work opens an avenue of realizing topological phases in molecular systems, which could be a novel playground of topological and versatile other physics. |
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