Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B51: Topological Materials: Graphene and Thin filmsFocus Live

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Sponsoring Units: DMP Chair: Peter Armitage, Johns Hopkins University 
Monday, March 15, 2021 11:30AM  11:42AM Live 
B51.00001: Z_{2} Topology and Edge States of Twisted Bilayer Graphene Qiyue Wang, Chao Ma, Fengnian Xia, Fan Zhang Recently twisted bilayer graphene(tBLG) emerges as a new strongly correlated physical platform near a magic twist angle, which hosts many exciting phenomena such as the Mottlike insulating phases, unconventional superconducting behavior and emergent ferromagnetism. Besides the apparent significance of band flatness, band topology may be another critical element in strongly correlated twistronics yet receives much less attention. While an unusual symmetry of tBLG trivializes Berry curvature, we elucidate that two highdimensional Z_{2} invariants in the TeoKane AltlandZirnbauer table characterize the topology of the moiré Dirac bands, supported by a systematic nonlocal transport study. The moiré band topology of tBLG manifests itself as two pronounced nonlocal responses in the electron and hole superlattice gaps. Moreover, the nonlocal responses are robust to the interlayer electric field, twist angle, and edge termination, exhibiting a universal scaling law. 
Monday, March 15, 2021 11:42AM  11:54AM Live 
B51.00002: Higher Chern Numbers and Quadratic Band Crossing Lines in Bilayer Lieb Lattice Avadh Saxena, Saikat Banerjee We consider a bilayer Lieb lattice which undergoes an unusual topological transition in the presence of intralayer spinorbit coupling (SOC). The specific configuration induces an effective nonsymmorphic 2D lattice structure, even though the constituent monolayer Lieb lattice is characterized by a symmorphic space group. This emergent nonsymmorphicity leads to multiple doublydegenerate bands extending over the edge of the Brillouin zone, i.e. Quadratic Band Crossing Lines. In the presence of intralayer SOC, these doublydegenerate bands typically form three 2band subspaces, mutually separated by two band gaps. We analyze the topological properties of these multiband subspaces, using specially devised Wilson loop operators to compute nonabelian Berry phases, in order to show that they carry a higher Chern number, 2. 
Monday, March 15, 2021 11:54AM  12:06PM Live 
B51.00003: NonHermitian topology and skin effect in Cographene nanoribbons Jingwei Jiang, Zhenglu Li, Jiawei Ruan, Steven G Louie Recently, nonHermitian effects on the topology of quantum materials have attracted much interest due to their unusual properties compared to Hermitian topology. A distinct character of nonHermitian topology is that it breaks the limitation of the requirement of a real band gap. The system may not have a gap in the real spectrum but could still be topological as long as a point gap in the complex spectrum exists. However, the way of realizing nonHermitian topology in a realistic 1D condensed matter system is still missing. In this work, we study a particular type of graphene nanoribbon with cobalt adatoms (CoGNR). Due to manyelectron interactions, the Hamiltonian characterizing quasiparticle excitations becomes nonHermitian. The strong spinorbit coupling brought by the cobalt atoms induces nontrivial nonHermitian topology in the GW quasiparticle band structure. We extract effective tightbinding parameters with a Wannier function basis and unveil the nonHermitian skin effect in CoGNR. 
Monday, March 15, 2021 12:06PM  12:18PM Live 
B51.00004: Observation of intrinsic timereversal invariant Z_{2} topological phase in bilayer graphene Priya Tiwari, SAURABH SRIVASTAV, Sujay Ray, Tanmoy Das, Aveek Bid Topological insulators, along with Chern insulators and Quantum Hall insulator phases, are considered as paradigms for symmetry protected topological phases of matter. In this letter, we report the experimental realization of the timereversal invariant Z_{2} topological phase [Phys. Rev. Lett. 95, 146802 (2005)] in bilayer graphenebased heterostructures  a phase generally considered as a precursor to the field of generic topological insulators. Our observation of this elusive phase depended crucially on our ability to create mesoscopic devices comprising of both a moir\'e superlattice potential and strong spinorbit coupling; this resulted in materials whose electronic band structure could be tuned from trivial to topological by an external displacement field. We find that the topological phase is characterized by a bulk bandgap and helical edge modes with electrical conductance quantized exactly to 2e^{2}/h in zero external magnetic field. We also present results of a theoretical study of a realistic model of the Z_{2} topological insulator which, in addition to replicating our experimental results, explains the origin of the topological insulating bulk and helical edge modes. 
Monday, March 15, 2021 12:18PM  12:30PM Live 
B51.00005: A New Phase of TwoDimensional Tin on Hexagonal Boron Nitride Monolayer on Metal Xi Dong, Lizhi Zhang, Mina Yoon, Pengpeng Zhang Monolayer tin (Sn) in the stanene form is expected to be a large bandgap twodimensional (2D) topological insulator. However, electronic properties and growth morphology of Sn thin films highly depend on the supporting substrate. We compare the growth of Sn on a metal substrate with and without the hexagonal boron nitride (hBN) monolayer, which binds weakly to the substrate. Distinct growth morphologies, arising from the different interaction to substrate, are revealed by scanning tunneling microscopy. Most intriguingly, a new Sn phase corresponding to the √7 × √7 superlattice of the underlying metal surface is identified on the hBN/metal substrate, suggesting the influence of metal on Sn through the decoupling hBN monolayer, as our firstprinciples calculations support. Our study demonstrates that substrates provide a platform to significantly change the electronic and topological properties of 2D material. 
Monday, March 15, 2021 12:30PM  12:42PM Live 
B51.00006: Electrical Transport Study in αSn Films Grown by Molecular Beam Epitaxy Yuanfeng Ding, Jinshan Yao, Junwei Huang, Ziyuan Yuan, Chen Li, Hongtao Yuan, Hong Lu, YanFeng Chen Alpha tin (αSn) is an allotrope of tin with a diamond crystal structure, and has been predicted to become topologically nontrivial by applying uniaxial strain. While the topological band structures have been confirmed by angleresolved photoemission spectroscopy (ARPES), however, the transport properties of αSn still need thorough investigations. 
Monday, March 15, 2021 12:42PM  12:54PM Live 
B51.00007: Measuring nonabelian Berry curvature in strained gallium arsenide using electronhole collisions Joseph Costello, Seamus O'Hara, Qile Wu, Loren Pfeiffer, Ken W. West, Mark Stephen Sherwin Berry curvature is a key quantity in characterizing the band topology of materials [1]. However, measuring Berry curvature locally in the Brillouin zone (BZ) remains challenging due to decoherence from rapid scattering processes. Highorder sideband generation (HSG) avoids this difficulty by using a strong terahertz field to drive electronhole pairs to collision before scattering. Polarimetry of sidebands has been shown to be sensitive to the local Berry curvature and the Bloch wavefunctions of semiconductors [2,3]. In strained gallium arsenide (GaAs), there is a nonabelian Berry curvature in the valence bands near the BZ center. Here we present a measurement of this nonabelian Berry curvature in GaAs using HSG. We also discuss future applications of this method to characterizing topological materials. 
Monday, March 15, 2021 12:54PM  1:30PM Live 
B51.00008: Symmetry, Topology, Exact ManyBody Ground States, and Exact Manybody Excitations in Twisted Bilayer Graphene. Invited Speaker: Zhida Song We have performed a thorough study of the manybody physics in twisted bilayer graphene (TBG) through a series of six works. First, we found useful approximations for the Hamiltonian and explained why the bands are flat over the whole Brillouin zone (not only around the Dirac points). Second, we found that, with an approximate particlehole symmetry, the continuous model of TBG is anomalous, which means that it cannot be realized on lattices. These two properties lead to a projected Coulomb Hamiltonian that can only be written in momentum space. Then we found that the projected Coulomb Hamiltonian hosts a U(4)xU(4) symmetry in the chiral limit and a U(4) symmetry otherwise, and explicitly derived the symmetry generators. Forth, with the help of symmetries, we can write down exact eigenstates of the projected Coulomb Hamiltonian at integer fillings, which are also ground states if the socalled “flatmetriccondition” is satisfied. These ground states are parent states of the valleypolarized states, valleycoherent states, and Chern states. Fifth, given the manybody symmetries and exact ground states, we can also derive the exact charge 1, 2, 0 excitations, with Goldstone modes included. Remarkably, through charge 2 excitations, we prove the absence of Cooper pairs in the project Coulomb Hamiltonian around integer fillings. Finally, we confirmed the analytical results using numerical exact diagonalization, which also suggests phase transitions to other ground states when U(4) or U(4)xU(4) is significantly broken. 
Monday, March 15, 2021 1:30PM  1:42PM Live 
B51.00009: Magic Manifold and Stable Symmetry Anomaly in Twisted Bilayer Graphene Zhida Song, Biao Lian, Nicolas Regnault, Andrei B Bernevig We investigate the singleparticle Hamiltonian of Twisted Bilayer Graphene (TBG) model. We provide an analytical perturbative understanding of why the TBG bands are flat over the whole Brillouin zone at the first magic angle, despite it is defined only by vanishing Dirac velocity. We derive a connected "magic manifold": w1=2 \sqrt{1+w0^2} – \sqrt{2+3w0^2}, on which the bands remain extremely flat. We also show that the entire continuous model of twisted bilayer graphene (TBG) (and not just the two active bands) with particlehole symmetry is anomalous. The fragile topology of the two flat bands is enhanced to a particleholesymmetryprotected stable topology. This stable topology implies 4n+2 Dirac points between the middle two bands. Remarkably, this table topology, as well as the corresponding 4n+2 Dirac points, cannot be realized in lattice models that preserve both C2T and particlehole symmetries. In other words, the continuous model of TBG is anomalous. 
Monday, March 15, 2021 1:42PM  1:54PM Live 
B51.00010: Twodimensional Tin on Substrates Lizhi Zhang, Xi Dong, Pengpeng Zhang, Mina Yoon We investigate 2D Sn on substrates and its electronic, magnetic, and quantum topological properties. Substrates can drastically change the Sn properties. For example, 2D Sn transforms into a 2D Weyl semimetal when large compressive strain is applied by substrates, such as 2D Sn, metals, or insulator substrates. In contrast to the buckled structure of freestanding stanene, the inversion symmetry of tin disappears with large compressive strain, which transforms a 2D topological insulator into a Weyl semimetal. We discuss realistic substrates that can accommodate desirable electronic and topological properties, and even magnetic properties to 2D Sn. Our findings provide a new avenue to experimentally realize new 2D quantum materials. 
Monday, March 15, 2021 1:54PM  2:06PM Live 
B51.00011: Topological insulators in the 1T’ phase of MX_{2} (M=Mo,W; X=S,Se,Te) revisited Prasun Boyal, Poonam Kumari, Priya Mahadevan Monolayers of Mo and W based transition metal dichalcogenides (TMD) in the 1T’ phase are known to be topological insulators [1]. This has been understood within the KaneMele model and is associated with the level inversion between the anion p states and the transition metal d states. However, what is surprising is that this happens across the entire series varying the anion from S to Se to Te for both Mo and W based systems. Examining various families of topological insulators, one finds that while one member of the series would be close to the point of inversion, it is not expected for every member to be close to the point of inversion and therefore become a topological insulator. In this work, we study the 1H , 1T and 1T’ phases of the Mo and W based TMD extensively. We carry out abinitio electronic structure calculations for each member of the TMD family. To quantify the electronic structure, we map the abinitio band structure onto a tight binding model. We show that while the level inversion is not specific to the 1T’ phase, and is also present in the other phases, the reason for 
Monday, March 15, 2021 2:06PM  2:18PM Live 
B51.00012: Effect of Sn Doping on Surface States of Bi_{2}Se_{3} Thin Films Gregory Stephen, Ivan Naumov, Siddharth Tyagi, Owen Vail, Jennifer DeMell, Michael Dreyer, Robert E Butera, Aubrey T. Hanbicki, Patrick Taylor, Isaak Mayergoyz, Pratibha Dev, Adam L Friedman

Monday, March 15, 2021 2:18PM  2:30PM Live 
B51.00013: Tunability and thickness dependence of inplane magnetoresistance and Hall effect in thin films of elemental Bismuth Eugene Ark, Deshun Hong, Terence BretzSullivan, Changjiang Liu, Steven S.L. Zhang, Leena Aggarwal, Vidya Madhavan, Anand Bhattacharya Observations of negative longitudinal magnetoresistance (NLMR), anisotropic magnetoresistance (AMR), and the planar Hall effect (PHE) under an inplane magnetic field are often used as indications of nontrivial topology in nonmagnetic material systems. We show NLMR, AMR, and PHE in crystalline epitaxial thin films (< 50 nm) of elemental Bismuth (111) grown by molecular beam epitaxy on intrinsic GaAs (111) substrates. Films were characterized insitu using scanning tunneling microscopy to confirm topography and density of states. Furthermore, the thickness and temperature dependence of these phenomena are investigated, demonstrating a high degree of tunability. These effects appear robust to contact geometry and addition of a Germanium buffer layer. 
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