Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E08: Twodimensional Topological Insulators: Transport (I) 
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Sponsoring Units: DCMP Room: LACC 153C 
Tuesday, March 6, 2018 8:00AM  8:12AM 
E08.00001: Observing Topological State Residing at Step Edge of bulk WTe_{2} Yingshuang Fu Topological states emerge at the boundary of solids as a consequence of the nontrivial topology of the bulk. In two dimensional topological insulators, the topological edge states support quantum spin Hall effect, where the electrons at the edge of the system possess different spins when propagating along opposite directions. Spectroscopic imaging scanning tunneling microscopy (SISTM) can access the electronic states of the system with high energy and spatial resolution, and thus offers an idea probe to the topological edge states. Recently, theory predicts a topological edge state on single layer transition metal dichalcogenides with 1T’ structure. However, its existence still lacks experimental proof. Here, we report the direct observations of the topological states at the step edge of WTe_{2} by SISTM. A onedimensional electronic state residing at the step edge of WTe_{2} is observed, which exhibits remarkable robustness against edge imperfections. First principles calculations rigorously verify the edge state has a topological origin, and its topological nature is unaffected by the presence of the substrate. Our study supports the existence of topological edge states in 1T’WTe_{2}, which may envision indepth study of its topological physics and device applications. 
Tuesday, March 6, 2018 8:12AM  8:24AM 
E08.00002: Transport Properties of Thin Film αSn Owen Vail, Patrick Taylor, Barbara Nichols, George de Coster, Charles Rong, Andrew Hewitt, Patrick Folkes The Army Research Lab is growing αSn via molecular beam epitaxy and investigating its topological nature for use in mesoscopic devices. The reported observation of nontrivial topology in αSn indicates its candidacy for transport studies in which the protected edge state plays a vital role. By measuring the magnetoresistance and temperaturedependent conductance of fabricated transport devices like hall bars, we characterize the electronic topology and band structure properties of αSn. Such transport experiments are necessary to evaluate the potential for highly efficient quantum and spintronic devices using αSn. 
Tuesday, March 6, 2018 8:24AM  8:36AM 
E08.00003: Anomalous Edge Plasmon in magnetically doped topological insulator Massoud Ramezani Masir, Allan MacDonald Magnetically doped topological insulators (MTIs) with ferromagnetic dopants such as chromium support a Quantum Anomalous Hall Effect state. We show that the quantum anomalous effect necessarily leads to a chiral one dimensional edge plasmon collective mode with approximately linear dispersion, which we refer to as anomalous edge plasmons (AEPs). AEPs are closely related to the socalled Edge Magneto Plasmon (EMP) excitations of twodimensional electron gases on quantum Hall plateaus in a strong magnetic field, and typically have frequencies in Microwave or Radio frequency ranges, depending on samples size. Recently, researchers in Stanford University used Microwave Impedance Microscopy (MIM) to image quantum anomalous Hall states, and identified response that is localized near Hall bar edges. We interpret these observations in terms of the properties of AEPs, addressing in particular the dependence of the degree of edge localization on the MIM imaging frequency. We also address the influence of magnetic domains on the MIM signal, and discuss the drastic difference between the MIM signal of the quantum anomalous Hall state and the axion insulator state which forms when the magnetization has opposite signs near the top and bottom surfaces of MTI thin films. 
Tuesday, March 6, 2018 8:36AM  8:48AM 
E08.00004: Topological states on grain boundary of 1T’MoTe_{2} SeoungHun Kang, HyunJung Kim, Kisung Chae, Wonhee Ko, Suyeon Cho, Heejun Yang, Sung Wng Kim, Seongjun Park, Sungwoo Hwang, YoungKyun Kwon, YoungWoo Son, Hyo Won Kim We show that on a transition metal dichalcogenides 1T’ MoTe_{2} two types of grain boundaries can exist and that electronic states associated with themselves strongly depend on their crystal symmetries as well as topological index. Using firstprinciples computational methods and model Hamiltonian analysis, we investigate their structural stabilities and electronic structures. We clearly demonstrate that onedimensional metallic state along one of the grain boundaries is a topologically protected state. The spatial symmetries created by grain boundaries is also shown to play an important role in characterizing the protected metallic states. 
Tuesday, March 6, 2018 8:48AM  9:00AM 
E08.00005: Hightemperature quantum anomalous Hall effect on posttransitionmetaldecorated graphene. Lizhi Zhang, Changwon Park, Mina Yoon Quantum anomalous Hall (QAH) insulators are a highly promising class of materials for spintronic devices and quantum computations because of their precise quantization nature, robust properties against defects, and relatively low energy consumption for operation. To realize the QAH effect quantum spin Hall (QSH) insulators must be utilized, which requires transition metal doping or surface functionality control. Here, we propose a new way to introduce ferromagnetism to largegap QSH insulators: we release the onsite magnetic momentum by increasing the lattice constants of stanene and germanene. If the lattice constant is increased to 9.5 Å, ab initio band structure calculations show that their spin–orbit coupling gaps are about 0.25 and 0.05 eV, respectively. Furthermore, the Curie temperatures, calculated by the Monte Carlo method, are 780 and 420 K. Both results indicate that the roomtemperature QAH effect can be realized on these systems. We also provide a possible experimental realization of this system on the 2√3×2√3 graphene substrate. Our calculations predict the first roomtemperature QAH insulator in the realistic materials system. 
Tuesday, March 6, 2018 9:00AM  9:12AM 
E08.00006: Counterpropagating Topological Interface States in Nano Punctured Graphene Toshikaze Kariyado, Yongcheng Jiang, Xiao Hu It has been known that superstructures such as a regular array of holes (antidot lattice) on a graphene sheet can induce a gapped state. We analyze the band structure of graphene with a hole array in more detail, and reveal that different arrangements of holes, triangular or honeycomb, can lead to topologically distinct states. The topological nontriviality is shown by explicitly calculating interface states between two regions with triangular and honeycomb hole arrangements, where the interface state appears as crossshape bands mimicking helical edge states in a quantum spin Hall state. The topological character is also discussed in terms of the parity of the wave function at the high symmetry point in the Brillouin zone, in relation to the stateofart argument on topological crystalline insulators. 
Tuesday, March 6, 2018 9:12AM  9:24AM 
E08.00007: Topologically Protected Metallic States Induced by a OneDimensional Extended Defect in the Bulk of a 2D Topological Insulator Ricardo Nunes, Erika Lima, Tome Schmidt

Tuesday, March 6, 2018 9:24AM  9:36AM 
E08.00008: Onedimensional physics in the edge states of the hightemperature quantum spin Hall system bismuthene on SiC(0001) Ralph Claessen, Raul Stuehler, Felix Reis, Joerg Schaefer Bismuthene (a monoatomic honeycomb lattice of Biatoms) chemisorbed on a SiC(0001) substrate has recently been synthesized and shown to be a promising candidate for the realization of a roomtemperature Quantum Spin Hall (QSH) effect which is based on a novel QSH mechanism [1]. Experiments with angleresolved photoelectron spectroscopy (ARPES) and scanning tunneling microscopy (STM) found excellent agreement with the calculated topological band structure. In particular, while the bismuthene film displays a large bulk band gap of ~0.8 eV, conducting edge states are observed at the boundaries of the honeycomb layer, e.g. at terrace steps of the substrate, as expected for a twodimensional topological insulator. Here we demonstrate, by a detailed analysis of tunneling spectra, that these edge states are indeed onedimensional (1D) and correlated in nature. The spectra display power law behavior with energy and temperature as well as universal scaling, consistent with the expectations for tunneling into a TomonagaLuttinger liquid and in excellent agreement with experimental observations in other 1D systems. 
Tuesday, March 6, 2018 9:36AM  9:48AM 
E08.00009: MBE growth and electronic properties of 2D topological insulators on Bi_{2}Te_{3 } Chenhui Yan, Mingxing Chen, Michael Weinert, Lian Li Twodimensional topological insulators (TIs) are distinguished by their Dirac edge states, and a topological phase transition between trivial and nontrivial states that can be tuned by composition, strain, or film thickness. In this work, we demonstrate the molecular beam epitaxial growth of Bi and Sb films on 3D TI Bi_{2}Te_{3}, and investigate their electronic properties using scanning tunneling microscopy/spectroscopy and density functional theory. We find that while Bi grows layerbylayer, the growth mode for Sb is thickness dependent: below three bilayers (BLs) is step flow, and above layerbylayer. This facilities a systematic study of their thicknessdependent electronic properties by scanning tunneling spectroscopy. We show that while few BL Bi films are 2D TIs, Sb film is a trivial insulator at below three BLs, which evolves into a 2D TI above three BLs, confirming earlier theoretical predictions. 
Tuesday, March 6, 2018 9:48AM  10:00AM 
E08.00010: A topological quantum optics interface Sabyasachi Barik, Aziz Karasahin, Chris Flower, Tao Cai, Hirokazu Miyake, Wade DeGottardi, Mohammad Hafezi, Edo Waks Topological photonics have opened up a multitude of new avenues in designing photonic devices. While exotic physics has been explored with topological photonic states in classical domain, strong light matter interaction with topological photonic states in the quantum regime remains largely unexplored. Towards this goal, we fabricate a topological photonic crystal to mediate coupling between single quantum emitter and topological photonic states. Developed on a thin slab of Gallium Arsenide membrane with electron beam lithography, such a device supports two robust counterpropagating edge states at the boundary of two distinct topological photonic crystals at nearIR wavelength. We show chiral coupling of circularly polarized lights emitted from a single Indium Arsenide quantum dot under strong magnetic field into these topological edge modes and demonstrate their robustness against sharp bends. Our technique could open up opportunities to explore manybody interaction, faulttolerant photonic circuits and unconventional quantum states of light. 
Tuesday, March 6, 2018 10:00AM  10:12AM 
E08.00011: Thickness Dependence of the Energy Band Structure and Topological Property of Topological Crystalline Insulator SnTe Films Yan Gong, Zuzhang Lin, Kejing Zhu, Qikun Xue, Yong Xu, Ke He A topological crystalline insulator (TCI) is characterized by gapless surface states that are protected by the crystalline symmetry. For an enough thin film of threedimensional TCI, the top and bottom surface states hybridize, opening up an energy gap at the Dirac surface states. According to theoretical prediction, the quantum spin Hall (QSH) phase may occur in such a film of certain thickness, and the topological phase diagram is dependent on the electrical field perpendicular to the film plane. Here we present a systematic angleresolved photoemission spectroscopy (ARPES) study on molecular beam epitaxygrown TCI SnTe (111) films with different thicknesses and on different substrates. A thicknessdependent oscillation of the surface state gap was observed in the films and was found to strongly depend on the substrate used. The origin of the oscillation and its relationship with topological phase transitions were discussed. 
Tuesday, March 6, 2018 10:12AM  10:24AM 
E08.00012: Solution to the holedoping problem and tunable quantum Hall effect in Bi_{2}Se_{3} thin films Jisoo Moon, Nikesh Koirala, Maryam Salehi, Wenhan Zhang, Weida Wu, Seongshik Oh Bi_{2}Se_{3}, one of the most widely studied topological insulators (TIs), is naturally electrondoped due to ntype native defects. However, many years of efforts to achieve ptype Bi_{2}Se_{3} thin films have failed so far. In this presentation, we provide a solution to this longstanding problem, showing that the main culprit has been the high density of interfacial defects. By suppressing these defects through an interfacial engineering scheme, we have successfully implemented ptype Bi_{2}Se_{3} thin films down to the thinnest topological regime. On this platform, we present the first tunable quantum Hall effect (QHE) study in Bi_{2}Se_{3} thin films, and reveal not only significantly asymmetric QHE signatures across the Dirac point but also the presence of competing anomalous states near the zeroth Landau level. 
Tuesday, March 6, 2018 10:24AM  10:36AM 
E08.00013: Quantum Anomalous Hall Effect in Twodimensional Organic Mn_{2}L_{3} Lattice X. Ni, Wei Jiang, Huaqing Huang, KyungHwan Jin, Feng Liu Using firstprinciples calculations, we predict the existence of nontrivial topological states in a monolayer metalorganic framework Mn_{2}L_{3} (L = C_{6}O_{4}Cl_{2}), which has been experimentally synthesized. A band gap of 7.8 meV at the Dirac point near the Fermi level is opened by spinorbital coupling, with the attributes of C and O porbitals mediated by Mn dorbitals. We further construct a tightbinding model to characterize the nonzero Chern number and edge states within the Dirac gap, confirming its nontrivial topological properties. Our results suggest that Mn_{2}L_{3} could provide an organic platform for developing lowenergyconsumption spintronics devices based on the quantum anomalous Hall effect. 
Tuesday, March 6, 2018 10:36AM  10:48AM 
E08.00014: STM study of Stanene Sara Mueller, Denise McKellick, Meron Dibia, Joshua Schulter, Jay Gupta The tin analog to graphene, stanene, is predicted to be a Quantum Spin Hall Insulator whereas, stanane is anticipated to behave as a trivial insulator. This suggests that tuning the physical properties of stanene could be achieved by direct modification of the surface. However, experimentally realizing stanene has been challenging as the parent crystal cannot be exfoliated and growing the material directly requires molecular beam epitaxy. Here we present Scanning Tunneling Microscopy study of stanene and its surface alloy on Cu(111). By thermal evaporation we deposited submonolayer coverage of tin on the surface. We observe a triangular lattice with 4.6Å spacing, consistent with the upward sites of the buckled honeycomb structure of stanene as calculated by DFT. Coadsorbed on the surface a 5.2Å lattice, consistent with a 2x2 reconstruction of the Cu(111) surface, indicates a CuSn alloy. Tunneling spectroscopy on stanene do not clearly show the anticipated spinorbit gap, and we see contributions of the underlying Cu(111) states, similar to what we previously reported for graphene. Consequently, we show progress towards isolating stanene with an insitu growth of a hexagonal boron nitride buffer layer. 
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