Bulletin of the American Physical Society
APS March Meeting 2023
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session W36: Nonequilibrium and equilibrium current distributions in Chern insulatorsInvited

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Sponsoring Units: DCMP Chair: Morteza Kayyalha, Pennsylvania State University Room: Room 236 
Thursday, March 9, 2023 3:00PM  3:36PM 
W36.00001: Exchangebiasdriven and Interactiondriven Quantum Anomalous Hall Effect in Topological Insulator Heterostructures Invited Speaker: Chaoxing Liu Quantum anomalous Hall (QAH) state is a magnetic topological state with zero longitudinal resistance and quantized Hall resistance, occurring in the absence of external magnetic fields due to spontaneously broken timereversal symmetry. The QAH effect has now been realized in several different classes of twodimensional materials, including magnetically (Cr or V) doped (Bi,Sb)_{2}Te_{3}, MnBi_{2}Te_{4}, and Moire materials of graphene and transition metal dichalcogenides. Despite of these impressive progress, the temperature at which the QAH effect is observed is still limited to a few Kelvins. Two general approaches are expected to enhance this critical temperature [1]: (1) highquality heterostructures between topological insulators and high Curie temperature ferromagnetic insulator; and (2) new Moire systems. In this talk, I will describe our theoretical efforts along these directions. (1) The strong exchange bias between MnBi2Te4 and CrI3 can give rise to the electrically tunable QAH effect in MnBi2Te4/CrI3 heterostructure [2]. (2) Isolated topologically nontrivial Moire minibands can be formed for a topological insulator film under a Moire superlattice and Coulomb interaction can drive this system into the QAH state when the nontrivial Moire miniband is half filled [3]. 
Thursday, March 9, 2023 3:36PM  4:12PM 
W36.00002: When we drive a current through a quantum anomalous Hall insulator, where does it flow? A view informed by measuring a spatial variation of potential in the presence of weak dissipation Invited Speaker: David GoldhaberGordon Ideally, quantum anomalous Hall systems should display zero longitudinal resistance. Yet in experimental practice elevated temperature can make the longitudinal resistance finite, indicating dissipative flow of electrons. Here, we show that this can give insights into how and where the current flows, even when the dissipation is very weak ($ ho_{ m xx}sim 10^{2} e^2/h$.) We find that the measured potentials at multiple locations within a (Cr,Bi,Sb)$_2$Te$_3$ Hall bar are welldescribed by solution of Laplace's equation, assuming spatiallyuniform conductivity. This suggests that nonequilibrium current flows through the twodimensional bulk. Extrapolation suggests that at even lower temperatures current may still flow primarily through the twodimensional bulk rather than, as had been assumed, through edge modes. An argument for bulk current flow previously applied to quantum Hall systems supports this picture. 
Thursday, March 9, 2023 4:12PM  4:48PM 
W36.00003: Electrical control of magnetism in magnetic Chern insulators Invited Speaker: Charles L Tschirhart A variety of intrinsic magnetic Chern insulators have been discovered in moiré superlattice systems, including in both graphene and transition metal dichalcogenide heterostructures. Unlike in Chern insulators fabricated by adding magnetic dopants to thin films of topological insulators, these systems have magnetism supported entirely by electronic interactions intrinsic to the topological bands. This fact helps limit disorder in these systems by removing the need for magnetic dopants, but it also intimately ties the magnetic order to electronic properties of the system, and thus facilitates electronic control of magnetism. I will discuss electronic switching of magnetization in Chern insulators through two different mechanisms: topological contributions to magnetization and intrinsic spinorbit torques. I will explain how these mechanisms work in practice in twisted bilayer graphene, twisted monolayer/bilayer graphene, and ABWSe2/MoTe2 using transport measurements and magnetic imaging performed with our nanoSQUID microscopes. 
Thursday, March 9, 2023 4:48PM  5:24PM 
W36.00004: Nanoscale imaging of equilibrium quantum Hall edge currents in graphene Invited Speaker: Aviram Uri The quantum Hall effect has served as a model system to understand topology in condensed matter physics. Although edge states play an important role in explaining the quantization of the Hall conductance, the actual edge currents have evaded direct measurement. Using a scanning nano SQUIDontip, we image the equilibrium currents of individual quantum Hall edge states in monolayer graphene. We reveal that the edge states, which are commonly assumed to carry only a chiral downstream current, in fact, carry a pair of counterpropagating currents, in which the topological downstream current in the incompressible region is counterbalanced by a nontopological upstream current flowing in the adjacent compressible region. 
Thursday, March 9, 2023 5:24PM  6:00PM 
W36.00005: Direct visualization of electronic transport in a quantum anomalous Hall insulator Invited Speaker: Katja C Nowack A quantum anomalous Hall (QAH) insulator is characterized by quantized Hall and vanishing longitudinal resistances at zero magnetic field that are protected against local perturbations and independent of sample details. This insensitivity makes the microscopic details of the local current distribution inaccessible to global transport measurements. Accordingly, the current distributions that give rise to the transport quantization are unknown. In this talk, I will show how we use magnetic imaging to directly visualize the transport current in the QAH regime. As we tune through the QAH plateau by electrostatic gating, we clearly identify a regime in which the sample transports current primarily in the bulk rather than along the edges. Images close to the source and drain of the device show that the current enters and leaves through hotspots in the corners of the contacts. Furthermore, we image the local response of the equilibrium magnetization to electrostatic gating. Combined, these measurements suggest that the current flows through incompressible regions throughout the QAH regime. Identifying the appropriate microscopic picture of electronic transport in QAH insulators and other topologically nontrivial states of matter is a crucial step toward realizing their potential in nextgeneration quantum devices. 
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