Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session K59: Correlated, Interacting, and Magnetic Topological Phases in SolidState MaterialsRecordings Available

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Sponsoring Units: DCMP Chair: Jiabin Yu, University of Maryland Room: Hyatt Regency Hotel DuSable AB 
Tuesday, March 15, 2022 3:00PM  3:12PM 
K59.00001: Signaturefor an excitonic insulator phasein quasionedimensionalvan der Waals materialsTa_{2}Pb_{3}Te_{5} Yupeng Li The excitonic insulator predicted 50 years ago is a correlated electron phase of narrowgap semiconductor, and a gap spontaneously appears analogous to a BardeenCooperSchrieffer (BCS) superconductor. In this work, we find Ta_{2}Pd_{3}Te_{5} is not only a predicted quantum spin Hall insulator[1], but also an excitonic insulator candidate, similar to InAs/GaSb quantum wells[2]. Different from a power law relationship between temperature and resistivity at low temperature, which may be originated from the edge state, a metal to semiconductor transition is observed in the resistivity at nearly 350 K, suggesting the open of gap, further confirmed by specific heat jump and susceptibility measurements. The lowtemperature xray diffraction and transmission electron microscope measurements illustrate this transition is not a structure transition or chargedensitywave transition. Moreover, the gap below the transition increases with decreasing temperature, confirmed by both transport measurements and ARPES measurements. With tuning the Fermi level in nanodevice by back gate or electron/holedoping materials, the paw law behavior/correlated strength and gap can be well controlled. All these behaviors imply Ta_{2}Pd_{3}Te_{5} is an excitonic insulator. 
Tuesday, March 15, 2022 3:12PM  3:24PM 
K59.00002: GateTunable Anomalous Hall Effect in Van der Waals ferromagnetic insulator–topological insulator heterostructure Andres E Llacsahuanga Allcca, XingCheng Pan, Katsumi Tanigaki, Yong Chen The search of novel topological phases like the quantum anomalous Hall insulator has motivated different schemes to introduce magnetism in topological surface states. One way to achieve this was obtained by the growth of ferromagnetically doped topological insulators (TI), but the dopants added disorder and introduced defects. A different route is by achieving the magnetic proximity effect, in which a ferromagnet is directly in contact with a topological insulator forming a heterostructure. Previously, most of the heterostructures have been assembled via growth techniques that have materials constraints related to chemical diffusion, difference in the growth conditions and lattice mismatch. Here, we show that the magnetic proximity effect can still be obtained in heterostructures assembled via a more generallyapplicable dry transfer method, as we demonstrate for exfoliated micrometersized thin flakes of van der Waals TI and ferromagnetic materials (BiSbTeSe_{2}/Cr_{2}Ge_{2}Te_{6}) and evidenced in the observation of an anomalous Hall effect (AHE). We further show that the AHE can be modulated by electrostatic gating as the carrier density in the heterostructure changes. 
Tuesday, March 15, 2022 3:24PM  3:36PM 
K59.00003: Magnetic excitons in a topological Kondo insulator YbB12 Yi Luo, Jonathan Gaudet, Lucas A Pressley, Aleksandra Krajewska, Helen C Walker, Tyrel M McQueen, Predrag Nikolic, Collin L Broholm We report a timeofflight inelastic neutron scattering experiment on a topological Kondo insulator (TKI) candidate. The spectra reveal dispersion and the Qdependent oscillator strength of two sharp modes and a continuum throughout the Brillouin zone. The absence of intensity at the Γ point shows the excitations are not simply transitions between crystal electric field levels. Instead, they appear to be collective ingap states of the df hybridized bands. The Qdependence of the modes at \hbar ω~ 40 meV and \hbar ω~ 15 meV correspond to AFM correlations between nearest and nextnearest Yb neighbors, respectively. Starting from a phenomenological tightbinding band structure, we use the Anderson lattice model to simulate the magnetic exciton modes and determine the topological class of the TKI candidate. 
Tuesday, March 15, 2022 3:36PM  3:48PM 
K59.00004: Quantum Oscillations in the Magnetization and Density of States of Insulators Animesh Panda, Sumilan Banerjee, Mohit Randeria The observation of quantum oscillations in Kondo insulators SmB_{6} and YbB_{12} challenge the conventional wisdom that these must arise from Fermi surfaces in metals. We revisit recently proposed theories, focusing on a minimal model with two oppositeparity hybridized bands, one light mass and one heavy mass in an inverted band structure, along with disorder induced ingap states. We present a detailed comparison of the frequency, phase and amplitude of the 1/Bperiodic oscillations in the magnetization (dHvA) and the low energy density of states (LEDOS, a proxy for SdH). While the LEDOS probes thermal excitations near the chemical potential, the magnetization takes into account all occupied Landau levels. We find that at the lowest nonzero temperature the two observables have distinctly different frequencies, but above a certain crossover both oscillations have the same frequency as the unhybridized case. The temperature dependence of the amplitudes in the two observables are qualitatively different with none having a LifshitzKosevich form. Our conclusions are based on a combination of numerical calculations and analytical results based on the semiclassical approximation. We also compute transport to ensure that we are in a regime with insulating upturns in the dc resistivity. 
Tuesday, March 15, 2022 3:48PM  4:00PM 
K59.00005: Atomically defined topological edge modes in functionalized xenes: an abinitio study Mark R Hirsbrunner, Jennifer Coulter, Oleg Dubinkin, Boris Kozinsky, Taylor L Hughes The xene family of twodimensional topological insulators plays a key role in many proposals for realizing topological electronic, spintronic, and valleytronic devices. These proposals use either the application of electric fields, proximity magnetism and superconductivity, or chemical functionalization to create topological edge modes in xenes. A key drawback of these techniques is their lack of control over the geometry of interfaces between topological regions, a critical aspect of engineering topological devices. Motivated by recent advances in adatom deposition technology, we propose atomically precise adatom decoration as a novel method for engineering topological edge modes in xenes. We show through firstprinciples calculations that decorating stanene with zinc adatoms exclusively on one of two sublattice sites induces a topological phase transition from the quantum spin Hall (QSH) to quantum valley Hall (QVH) phase. We confirm the existence of valley and spinvalley polarized edge modes propagating at QVH/QVH and QVH/QSH interfaces through further firstprinciples simulations. We conclude by discussing technological applications of these edge modes that are enabled by the atomic precision of the technique. 
Tuesday, March 15, 2022 4:00PM  4:12PM 
K59.00006: Magnetooptical Landau level spectroscopy of Pb_{1x}Sn_{x}Se/EuSe heterostructures Jiashu Wang, Xinyu Liu, Hoai Trinh, Mykhaylo Ozerov, Maksym Zhukovskyi, Tatyana Orlova, Dmitry Smirnov, Badih A Assaf Magnetic topological insulators have been greatly studied in the past decades to achieve and enhance the critical temperature of the Quantum anomalous Hall. One way to induce magnetization into TIs is by using the magnetic proximity effect (MPE). It will introduce a magnetic exchange gap which has never been directly measured. Pb_{1x}Sn_{x}Se (0.161xSn_{x}Se/EuSe heterostructures with x=0.10.3 and probe the exchange induced gap at the interface using magnetooptical spectroscopy. Landau level transitions can be extracted as of 4T and are fitted using a k·p quantum well model. The high mobility and low Fermi energy of these heterostructures will allow to directly probe the energy gap induced by the MPE. 
Tuesday, March 15, 2022 4:12PM  4:24PM 
K59.00007: Effective field theories of topological crystalline insulators and topological crystals ShengJie Huang, ChangTse Hsieh, Jiabin Yu In this talk, I will present a general approach to obtain effective field theories for topological crystalline insulators whose lowenergy theories are described by massive Dirac fermions. We show that these phases are characterized by the responses to spatially dependent mass parameters with interfaces. These mass interfaces implement the dimensional reduction procedure such that the state of interest is smoothly deformed into a network of defects (dubbed topological crystal), where each defect supports a shortranged entangled state. Effective field theories are obtained by integrating out the massive Dirac fermions, and various quantized topological terms are uncovered. I will describe how to apply this strategy through a few simple examples and comment on the relation to the topological elasticity theory. 
Tuesday, March 15, 2022 4:24PM  4:36PM 
K59.00008: Nafunctionalized IrTe_{2} monolayer: Suppressed charge ordering and electric field tuned topological phase transition Xiaoyin Li, Feng Liu, Qian Wang Twodimensional materials with atomic thickness usually possess superior tunability by surface adsorption than their bulk counterparts, showing great promise for novel nanotechnologies. In layered transitionmetal dichalcogenide IrTe_{2} there exhibits complex structural distortions induced by charge ordering, resulting in difficulties for the applications of its corresponding monolayer material. Here, using firstprinciples calculations, we demonstrate that depositing Na on the surface of the IrTe_{2} monolayer can suppress the structural distortion to form a stable NaIrTe_{2} sheet. It naturally breaks the inversion symmetry to enable a Rashbatype spin splitting for potential spintronic applications. In addition, the introduced empty Na s band and the valence band of the IrTe_{2 }monolayer can be inverted by the application of a perpendicular electric field, achieving a quantum phase transition from normal to topological insulator. Such an electric fieldcontrolled topological phase transition is promising for the realization of topological fieldeffect transistors. These findings not only provide a feasible approach to stabilizing the IrTe_{2} monolayer, but also broaden its applications in spintronics and lowdissipation topoelectronics. 
Tuesday, March 15, 2022 4:36PM  4:48PM 
K59.00009: Observation of topological bulkboundary correspondence in a strongcoupling charge density wave state Maksim Litskevich, Md. Shafayat Hossain, Qi Zhang, Zijia Cheng, Zhiwei Wang, Satya N Guin, Nitesh Kumar, Chandra Shekhar, Yongkai Li, Ying Yang, Guoqing Chang, JiaXin Yin, Nana Shumiya, YuXiao Jiang, Guangming Cheng, Tyler A Cochran, Daniel Multer, Xian Yang, Nan Yao, Yugui Yao, Claudia Felser, Zahid M Hasan The interplay between topology and charge order is at the frontier of condensed matter research. However, there is no direct experimental evidence for a topological bulkboundary correspondence within a fully gapped chargeordered state yet. Here, using scanning tunneling microscopy, we directly visualize the topological bulkboundary correspondence in a topological charge density wave (CDW) material. Below the CDW transition temperature, tunneling spectra on an atomically resolved lattice reveals a large insulating gap (over 500 meV) whose △/k_{B}T_{c}≈20, suggesting a strong coupling nature of the CDW. Remarkably, in an atomically sharp monolayer step edge, we find an ingap gapless helical state. Both the insulating gap and the gapless edge state disappear above the CDW transition temperature. The presence of the edge state within the insulating CDW gap indicates the topological bulkboundary correspondence, pointing to the realization of a topological CDW state. 
Tuesday, March 15, 2022 4:48PM  5:00PM 
K59.00010: Exchangebiased Quantum Anormalous Hall Effect in Magnetic Topological Insulator and Antiferromagnetic Insulator Heterostructure Peng Zhang, Purnima P Balakrishnan, Chris Eckberg, Peng Deng, Tomohiro Nozaki, Alexander Grutter, KangLung Wang The integration of topological and (anti)ferromagnetic layers has attracted wide interest, to introduce ferromagnetism into nonmagnetic topological insulators (TIs) or to enhance the Curie temperature of magnetically doped topological insulators (MTIs). In addition, the exchange coupling between the layers provides an additional degree of freedom through which the topological surface states can be manipulated. Here, we report on exchange coupling between the quantum anomalous Hall (QAH) state of MTI and an antiferromagnetic insulator with a net parasitic magnetization, Aldoped Cr_{2}O_{3}. Polarized neutron reflectometry (PNR) measurements reveal a strong exchange coupling between the two magnetic layers pinning the moments at the interface to point outofplane, which leads to the observation of an exchangebiased QAH on the transport signal for the first time. 
Tuesday, March 15, 2022 5:00PM  5:12PM 
K59.00011: Interplay of Spin Current and Magnetization in a Topologicalinsulator/Magneticinsulator Bilayer Structure Xiaohang Zhang, Connie H Li, Jisoo Moon, Serhiy Leontsev, Michael R Page, Berend T Jonker, Olaf M Van T Erve The interaction between accumulated spins on the surface of a heavy metal and the magnetization of an adjacent magnet material leads to various spin phenomena, such as spin orbit torque, spin pumping, and spin Hall magnetoresistance (SHMR). However, the exploration of device applications based on these spin phenomena is often limited by the low chargetospin conversion efficiency of the heavy metal. Recent studies have suggested that topological insulators are promising candidates for device applications due to their potentially higher chargetospin conversion efficiency. In this work, we systematically studied the interplay of spin current and magnetization in a bilayer structure consisting of Bi_{2}Se_{3} and YIG. Our ferromagnetic resonance measurements demonstrate that microwave excitation can efficiently pump spins from YIG to Bi_{2}Se_{3}. The transfer of spin current from YIG to Bi_{2}Se_{3} is further confirmed by detection of an electromotive force generated in Bi_{2}Se_{3} via spintocharge conversion. Moreover, angle dependent magnetotransport measurements suggest that despite the large outofplane magnetoresistance of Bi_{2}Se_{3} itself, the interfacial spin diffusion from the magnetic insulator to the topological insulator can effectively affect the longitudinal transport in a way similar to the SHMR effect. 
Tuesday, March 15, 2022 5:12PM  5:24PM 
K59.00012: Quantum Anomalous Hall Effect in SubMicrometerSize Devices Lingjie Zhou, Yifan Zhao, Ruobing Mei, Ruoxi Zhang, Deyi Zhuo, Zijie Yan, Morteza Kayyalha, Moses H Chan, Chaoxing Liu, CuiZu Chang The quantum anomalous Hall (QAH) effect is the zero magnetic field manifestation of the integer quantum Hall effect. The QAH effect is usually realized in mechanically scratched millimetersize magneticallydoped topological insulator (TI) devices. However, to access the potential applications of the QAH phenomenon, the nanofabrication of small QAH devices within quantum coherent length is essential. In this talk, we will introduce our recent progress on submicrometersize QAH devices. Based on our macroscopic molecular beam epitaxy grown magnetically doped TI films, we used electronbeam lithography to fabricate the Hall bar devices within a width range from 10μm to 100nm. We found the QAH effect still survives in the Hall bar devices with a width of ~100 nm. Moreover, we found that the Hall resistance plateau which usually corresponds to the magnetic exchange gap size becomes narrower with reducing the Hall bar width. By combining theoretical calculations, we demonstrate that the narrower Hall resistance plateau is because of the interaction effect between two chiral edge channels in submicrometersize Hall bar devices. Our results open a new route to study the spatial distribution of the QAH chiral edge states and pave the way for the potential applications of the QAH phenomenon. 
Tuesday, March 15, 2022 5:24PM  5:36PM 
K59.00013: Confinement induced topological phase transition in thin film rareearth pnictides QuocDai Ho, Ruiqi Hu, D. Quang To, Garnett W Bryant, Anderson Janotti The metalinsulator phase transition is a classical phenomenon induced by various effects. The transition caused by sizequantization in going from a bulk material to the very thin film limit is wellknown. By using first principles simulation methods, we investigate the phase transition when a 3D bulk semimetallic rareearth pnictide becomes as thin as the 2D material. Due to the unique electronic structure of the rareearth pnictides, different electron pockets in the Brillouin zone of the material shift differently upon the sizequantization with certain pockets shifting only weakly due to confinement to 2D. This small shift prevents a trivial gap opening in thin film rareearth pnictides that are thin in one direction. More interestingly, there is eventually a topological phase transition from a semimetal to a 2D spin Hall insulator/Chern insulator with a sizeable 2D bulk gap due to spinorbit coupling interaction. This finding introduces a new member to the 2D spin Hall insulator/Chern insulator family and offers a new platform to realize the 2D spin Hall insulator/Chern insulator experimentally. 
Tuesday, March 15, 2022 5:36PM  5:48PM 
K59.00014: Observation of Quantum Oscillations in the Low Temperature Specific Heat of SmB_{6} Nathanael A Fortune, Patrick LaBarre, Andreas Rydh, Joyce E PalmerFortune, Julia A Frothingham, Scott Hannahs, Arthur P Ramirez We report measurements of the lowtemperature specific heat of Alfluxgrown samples of SmB_{6} in magnetic fields up to 32T. Quantum oscillations periodic in 1/H are observed between 8 and 32 T at selected angles between [001] and [111]. The observed frequencies and their angular dependence are consistent with previous magnetic torque measurements of SmB_{6} but the effective masses (inferred from LifshitzKosevich theory) are significantly larger than those inferred from magnetic torque. The magnetic field dependence of the oscillation amplitude is inconsistent with that expected to arise from aluminum inclusions. Instead, our results suggest a bulk density of states origin for the quantum oscillations. 
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