Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session S59: Transport, Probes, and Structure of Topological Insulators and Topological Crystalline InsulatorsRecordings Available

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Sponsoring Units: DCMP Chair: Dipanjan Chaudhuri, University of Illinois at UrbanaChampai Room: Hyatt Regency Hotel DuSable AB 
Thursday, March 17, 2022 8:00AM  8:12AM 
S59.00001: Can the Boltzmanntryanny be overcome via topological field effect switching?  insignts into mechanisms and fundamental limits Sagnik Banerjee, Koustav Jana, Anirban Basak, Bhaskaran Muralidharan Electric fielddriven topological phase transitions in quantum spin Hall (QSH) materials with buckled honeycomb lattice in the presence of Rashba spinorbit interactions are considered to be an interesting playground for implementing Topological Quantum FieldEffect Transistors (TQFETs). However, defying the wisdom in conventional transistors being ideally limited to 60mV/decade, we demonstrate that for a topological transistor the thermionic limit of subthreshold slope is actually twice as much. Also, by employing the nonequilibrium Green's function (NEGF) approach, we demonstrate that the mere introduction of Rashba interaction [1] does not help to reduce the subthreshold swing. As a probable solution, we propose that the introduction of antiferromagnetic exchange interaction, enabling a transition between the quantum spin valley Hall (QSVH) phase and the quantum anomalous Hall (QAH) phase, can help us go past not only the 120mV/decade limit but also the 60mV/decade limit for an appropriately chosen set of parameters. 
Thursday, March 17, 2022 8:12AM  8:24AM 
S59.00002: Role of dephasing on electric field induced topological transition in 2D Xenes Anirban Basak, Pratik Brahma, Bhaskaran Muralidharan We analyze the electric field driven topological field effect transition on 2Dxene materials with the addition of momentum relaxation effects, in order to account for dephasing processes. The topological field effect transition between the quantum spin Hall phase and the quantum valley Hall phase is analyzed in detail using the Keldysh nonequilibrium Green's function technique with the inclusion of momentum and phase relaxation, within the selfconsistent Born approximation. Details of the transition with applied electric field are elucidated for the ONOFF characteristics with emphasis on the transport properties along with the tomography of the current carrying edge states. We note that for moderate momentum relaxation, the current carrying quantum spin Hall edge states are still pristine and show moderate decay with propagation. To facilitate our analysis, we introduce two metrics in our calculations, the coherent transmission and the effective transmission. In elucidating the physics clearly, we show that the effective transmission, which is derived rigorously from the quantum mechanical current operator is indeed the right quantity to analyze topological stability against dephasing. Exploring further, we show that the insulating quantum valley Hall phase, as a result of dephasing carries bandtails which potentially activates parasitic OFF currents, thereby degrading the ONOFF ratios. Our analysis sets the stage for realistic modeling of topological field effect devices for various applications, with the inclusion of scattering effects and analyzing their role in the optimization of the device performance. 
Thursday, March 17, 2022 8:24AM  8:36AM 
S59.00003: Twistresilient and robust ferroelectric quantum spin Hall insulators driven by van der Waals interactions Marco Gibertini, Antimo Marrazzo Quantum spin Hall insulators have been proposed to power a number of applications, many of which rely on the possibility to switch on and off the nontrivial topology. Typically this control is achieved through strain or external electric fields, which require energy consumption to be maintained. On the contrary, a nonvolatile mechanism would be highly beneficial and could be realized through ferroelectricity if opposite polarizations are associated with different topological phases. While this is not possible in a single ferroelectric material owing to symmetry constraints, the necessary asymmetry could be introduced by combining a ferroelectric layer with another 2D trivial insulator. Here, by means of firstprinciples simulations, not only we propose that this is a promising strategy to engineer nonvolatile ferroelectric control of topological order in 2D heterostructures, but also that the effect is robust and can survive up to room temperature. Remarkably, the topological band gap is mediated by the interlayer hybridization and allows to maximise the effect of onsite spinorbit coupling, promoting a robust ferroelectric topological phase that could not exist in monolayer materials and is resilient against relative orientation and lattice matching between the layers. 
Thursday, March 17, 2022 8:36AM  8:48AM 
S59.00004: Copper migration in intercalated topological insulator Cu_{x}Bi_{2}Se_{3} Adam Gross, Lorenz Falling, Kristie J Koski, Slavomir Nemsak, Inna Vishik Cu_{x}Bi_{2}Se_{3} is a topological insulator (TI) material in which Cu intercalation between the Bi_{2}Se_{3} quintuple layers can yield superconductivity. Using ambientpressure Xray photoelectron spectroscopy (APXPS), we report copper migration from the bulk towards the surface in this material in controlled oxidizing environments as well as ambient conditions. The migration occurs on a timescale of hours to weeks after initial cleaving, and proceeds along with the oxidation of the sample surface. These results demonstrate dynamic surface chemistry, relevant for tailoring the topological surface states for utilization in ambient environments. 
Thursday, March 17, 2022 8:48AM  9:00AM 
S59.00005: Magnetooptical Third Harmonic Generation in Topological Insulator Films Gaurav Gupta, WangKong Tse, Mahmoud M Asmar The discovery of materials where interactions with spin, lattice, and orbital degrees of freedom are linked with the electronic kinetic energy has led to a heightened interest in their nonlinear optical (NLO) response. Examples of these materials in three dimensions are topological insulators (TIs), which have a gapped bulk spectrum while hosting metallic helical states on surfaces. They exhibit strong magnetooptical responses due to the topologically nontrivial bulk and surface states, making them attractive candidates for probing enhanced NLO responses in a magnetic field. In this talk, we present our theory for magnetooptical thirdharmonic generation (THG) in topological insulator films in a quantizing magnetic field. We obtain the THG NLO conductivity arising from one, two, and threephoton processes and investigate the effects of changing magnetic field, chemical potential, and film thickness. Comparing to the zerofield case, the THG nonlinear conductivity exhibits strong enhancements due to multiphoton cyclotron resonances and bulk states contribution. We address the key differences of these enhancements in TI from the THG in graphene. 
Thursday, March 17, 2022 9:00AM  9:12AM 
S59.00006: Probing the density of states in monolayer WTe_{2} with time domain capacitance spectroscopy Evan A ZalysGeller, Sergio de la Barrera, Xirui Wang, Kenji Yasuda, Pablo JarilloHerrero, Raymond C Ashoori Time domain capacitance spectroscopy (TDCS) probes the density of states in 2D systems without electrical contact. The technique generates a purely outofplane tunneling current and thus eliminates the effects of inplane transport, maintaining sensitivity over a wide range of compressibility, from metallic to highly insulating states. We use TDCS to measure the density of states in electrostatically gated WTe_{2} embedded in a vertical tunnel structure and observed an asymmetric and “hard” (with very low density of states out to the gap edge) gap pinned to the Fermi level. The gap occurs over a range of electron densities and thus does not appear related to WTe2 band structure but instead may arise from electronelectron interactions within the WTe_{2} layer. 
Thursday, March 17, 2022 9:12AM  9:24AM 
S59.00007: Doping dependence and Fermi surface studies of the topological crystalline insulators Sn_{x}Pb_{1x}Te/Se Duncan A Miertschin, Raman Sankar, Thinh Nguyen, Liangzi Deng, Bernd Lorenz, Paul C. W Chu, Keshav Shrestha This work presents Fermi surface studies of the topological crystalline insulator Sn_{x}Pb_{1x}Te/Se. Magnetic torque measured at higher fields up to 35 T and temperatures down to 0.32 K shows clear de Haasvan Alphen (dHvA) oscillations. The dHvA oscillations are welldefined and consist of two major frequencies (F_{1 }~ 46 T, F_{2} ~ 230 T) for the x = 0.35 sample. To understand the Fermi surface properties, we have measured dHvA oscillations at different tilt angles with respect to the applied field and at different temperatures up to 60 K. The temperature dependent data was analyzed using the LifshitzKosevich (LK) formula, and we estimated several parameters characterizing the Fermi surface. 
Thursday, March 17, 2022 9:24AM  9:36AM 
S59.00008: Lowenergy effective theory and anomalous Hall effect in monolayer WTe_{2} Snehasish Nandy, Dmytro Pesin Monolayer WTe_{2} in its 1T’ phase has been realized as a quantum spin Hall insulator with various intriguing properties such as superconductivity and nonlinear Hall effect. We develop a symmetrybased effective lowenergy theory for monolayer WTe_{2} in its 1T’ phase. We find that to the leading order, the spinorbit coupling in the k.ptheory near the gammapoint in the Brillouin zone is independent of the quasimomentum, and describes four fully spinpolarized bands, which are represented by two Kramerspartner pairs. Using the lowenergy model, we study the spin susceptibility and timedependent anomalous Hall effect induced by injected or equilibrium spin polarization in this system. We show that these measurements can help to determine the orientation of the conserved spin projection, the dimensionless tilt of band dispersion and the strength of the spinorbit coupling of the system. 
Thursday, March 17, 2022 9:36AM  9:48AM 
S59.00009: Observation of a Smoothly Tunable Dirac Point in Ge(Bi_{x}Sb_{1x})_{2}Te_{4} Arjun Raghavan, Sean T Howard, Arjun Raghavan, Davide Iaia, Caizhi Xu, David Flototto, ManHong Wong, SungKwan Mo, Bahadur Singh, Raman Sankar, Hsin Lin, TaiChang Chiang, Vidya Madhavan Nextgeneration topological devices use topologically protected surface states to drive electronic transport. In this study, we examine Ge(Bi_{x}Sb_{1x})_{2}Te_{4}, a tunable topological system, for a range of x values from 0 to 1 using a combination of FourierTransform Scanning Tunneling Spectroscopy (FTSTS) and AngleResolved Photoemission Spectroscopy (ARPES). We show that the Dirac point changes linearly with x, crossing the Fermi energy at x = 0.7. This novel observation of a smoothly tunable, isolated Dirac point traversing the topological transport regime and having strong linear dependence with substitution can be critical in designing new topological spintronics applications. 
Thursday, March 17, 2022 9:48AM  10:00AM 
S59.00010: Chiral Hall effect in the kink states in topological insulators with magnetic domain walls Martyna Sedlmayr, Nicholas Sedlmayr, Józef Barnaś, Vitalii Dugaev We consider the chiral Hall effect due to topologically protected kink states formed in topological insulators at boundaries between domains with differing topological invariants. Such systems include the surfaces of threedimensional topological insulators magnetically doped or in proximity to ferromagnets, as well as certain twodimensional topological insulators. We analyze the equilibrium charge current along the domain wall and show that it is equal to the sum of counterpropagating equilibrium currents flowing along the external boundaries of the domains. In addition, we also calculate the current along the domain wall when an external voltage is applied perpendicularly to the wall. 
Thursday, March 17, 2022 10:00AM  10:12AM 
S59.00011: Quantum phase transitions in the disordered chalcogenide topological insulators (Bi_{1x}Sb_{x})_{2}Y_{3 }and X_{2}(Te_{y}Se_{1y})_{3}, (X = Bi/Sb, Y = Te/Se) Karunya Shailesh Shirali, Duane D Johnson, Prashant Singh, William A Shelton, Ilya Vekhter Topological systems with various Bi/Sb and Te/Se concentrations have been studied with a view towards tuning topological phase transitions, and we try to clarify this by varying the lattice parameters in this family of materials and driving electronic topologicaltotrivial phase transitions. We first investigate disorder on each site and perform firstprinciples calculations on (Bi_{1x}Sb_{x})_{2}Y_{3 }and X_{2}(Te_{y}Se_{1y})_{3} (X=Bi/Sb, Y=Te/Se) in which we systematically vary the c/a ratio of lattice constants to study the topological phase transition, before studying alloying on all sites. To model substitutional disorder, we have constructed supercells, alloying on one sublattice, where the atomic pair correlations are zero up to the third nearest neighbor shell, and we vary the impurity concentration across the phase diagram with the goal of identifying the influence of each atomic species on the transition. We determine the bulk electronic structure and pay special attention to the band inversion near the topological transition. 
Thursday, March 17, 2022 10:12AM  10:24AM 
S59.00012: TimeDomain THz studies of topological insulator waveguides Zhenisbek Tagay, Xiong Yao, Seongshik Oh, Norman P Armitage Topological insulators (TI) subject to timereversal symmetry breaking perturbations are well known to exhibit quantized magnetoelecric effect. In conventional transmission experiments, light being transmitted through such TI's will experience Faraday and Kerr rotations of polarization. However, these effects are on the order of fine structure constant α which makes it challenging to observe experimentally. In this work, we utilize TI's at the interface between guiding and cladding layers of a parallelplate waveguide to induce coupling between guided TE and TM modes. It was previously predicted that this coupling is dependent on the number of filled Landau levels in the TI surface plus an axionic term that is measure of the topological invariant. Here we present the working principle of the waveguide device and provide experimental results of timedomain terahertz (THz) spectroscopy studies on it. 
Thursday, March 17, 2022 10:24AM  10:36AM Withdrawn 
S59.00013: Tunable interscattering between counterpropagating quantum Hall edge channels in a topological insulator HgTe Lixian Wang Recent advances reveal that the surfaces of threedimensional (3D) topological insulators can host massless (Dirac) and massive surface states. The surface of a 3D topological insulator is a closed 2D manifold. Therefore, the interplay of top and bottom surfaces is expected to be prominent via sides, especially as the edge states are predominant in quantum Hall regime. Here we demonstrate the formation of counterpropagating edge states, originating from opposite surfaces. Additionally, our measurements show that the counterpropagating edge states are intensively scattered into each other, evidenced by a deviation from the noninterscatering LandauerBüttiker formalism. In this case, a scattering model helps to quantify the interscattering strength among counterpropagating edge states, which is later shown to be tuable by gate voltages and temperature. The modelling for muliple edge states suggests a selection rule of interscattering among edge states, which only allows for prominent interscattering between paried edge states with the same Landau index N. Our research provides a platform to further investigate the microscopic mechanism of interaction of edge channels in quantum Hall regime. 
Thursday, March 17, 2022 10:36AM  10:48AM 
S59.00014: Controlling Current in Topological Insulators with Magnetic Impurities Dejuan Winters Topological insulators are theoretical materials that allow for an ideal conductivity of electricity with minimal loss of energy. This proves to be of great use in the field of electronics, as it greatly increases the energy efficiency of electronic components. Local conductive properties of the material are dependent on the location and physical properties of impurities placed within the material, and this allows for control of direction and strength of the current traveling through this material. We implemented a discrete lattice model of a topological insulator known as the BHZ model and constructed its Hamiltonian matrix to predict the behavior of electrons in this lattice. We numerically explored the relationship between conductance and the location and energy of magnetic impurities in the BHZ model. We found that conductance is largely dependent on the dimensions of the lattice used in the model as well as the configuration of the coupling between the impurity and the lattice. This conductance was indirectly quantified by the transmission coefficient of incoming electrons. We observed a significant decrease of the transmission coefficient after altering the impurity coupling, yielding a greater control of the current. 
Thursday, March 17, 2022 10:48AM  11:00AM 
S59.00015: Highthroughput firstprinciples exploration of 2Dtin structures and their topological properties Sinchul Yeom, Mina Yoon Stanene (2D Sn) is a promising material for nanoelectronics or quantum computing because it is a proposed topological insulator (TI) at the room temperature. Because electrons can travel on the edges of these TIs without heat dissipation, this could greatly increase the efficiency and performance of electronic devices. We explored the (meta)stable structures of 2D Sn using our automated highthroughput workflow that combines firstprinciples calculations and particle swarm optimization and identified new structures of 2D Sn with different electronic and topological properties. We also found that substrates play a critical role in the stability of 2D Sn structures with versatile properties. Our findings should be instrumental for the experimental development of new 2D Sn with desirable properties. 
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