Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session P03: Surfaces of Topological SystemsFocus

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Sponsoring Units: DCMP Room: BCEC 107B 
Wednesday, March 6, 2019 2:30PM  2:42PM 
P03.00001: Interaction of the Topological Surface States with Spin Excitons in SmB_{6} – Dependency on the Crystallinity Wan Kyu Park, Jennifer Sittler, Laura H Greene, DaeJeong Kim, Zachary Fisk, Wesley T Fuhrman, Juan Chamorro, Seyed Koohpayeh, William Phelan, Tyrel McQueen The true nature of the topological surface states (TSS) in SmB_{6} still remains to be unveiled, despite intensive research [1]. Our previous tunneling study [2] not only found evidence for surface Dirac cones but also unraveled their intriguing topological nature due to the inherent interaction [3] with spin excitons [4]. Extended to Smdeficient single crystals, it now reveals how their evolution is influenced delicately by such interaction depending on the crystallinity [5]. Seemingly contradictory to the anticipated topological protection, this can be understood by considering the interplay between strong correlations and topological effects. We will present how the temperature evolution of tunneling conductance differs and discuss how spin excitons influence the emergence of coherent TSS. [1] M. Dzero et al., Annu. Rev. Condens. Matter Phys. 7, 249 (2016). [2] W. K. Park et al., PNAS 113, 6599 (2016); L. Sun et al., PRB 95, 195129 (2017). [3] W. T. Fuhrman et al., PRL 114, 036401 (2015). [4] G. A. Kapilevich et al., PRB 92, 085113 (2015). [5] M. E. Valentine et al., arXiv:1712.01325v1. 
Wednesday, March 6, 2019 2:42PM  2:54PM 
P03.00002: Preparation of HighQuality Junctions on SmB_{6} for Planar Tunneling Spectroscopy Jennifer Sittler, Laura H Greene, DaeJeong Kim, Zachary Fisk, Wesley T Fuhrman, Juan Chamorro, Seyed Koohpayeh, William Phelan, Tyrel McQueen, Wan Kyu Park Previous works have utilized planar tunneling spectroscopy to investigate the possible topological surface states in SmB_{6} [1,2]. When superconducting Pb is used as a counterelectrode, the tunneling conductance spectra revealed, coherence peaks with an asymmetrical temperature evolution as well as an additional peak at ~ 5 mV. These features were attributed to inelastic tunneling processes involving the emission and absorption of spin excitons [3,4]. The tunnel barrier in these reproducible junctions was formed by oxidizing the top layer of the SmB_{6 }[1,2]. Improving the junction quality even further, would allow for measurements of the second harmonic, which may reveal additional details about the spin excitons. The preparation of highquality junctions on SmB_{6} single crystals and thin films will be discussed. [1] W. K. Park et al., PNAS, 113, 6599 (2016). [2] L. Sun et al., Phys. Rev. B. 95, 195129 (2017). [3] W. T. Fuhrman et al., PRL 114, 036401 (2015). [4] G. A. Kapilevich et al., PRB 92, 085113 (2015). 
Wednesday, March 6, 2019 2:54PM  3:06PM 
P03.00003: Magnetize Topological Surface States of Bi_{2}Se_{3} with a CrI_{3} Monolayer YUSHENG HOU, Jeongwoo Kim, Ruqian Wu To magnetize surfaces of topological insulators without damaging their topological feature is a crucial step for the realization of the quantum anomalous Hall effect (QAHE), and still remains as a challenging task. Through density functional calculations, we found that adsorption of a semiconducting twodimensional van der Waals (2DvdW) ferromagnetic CrI_{3} monolayer can create a sizable spin splitting at the Dirac point of the topological surface states of Bi_{2}Se_{3} films. Furthermore, general rules that connect different quantum and topological parameters are established through model analyses. This work provides a useful guideline for the realization of QAHE at high temperature in heterostructures of 2DvdW magnetic monolayers and topological insulators. 
Wednesday, March 6, 2019 3:06PM  3:18PM 
P03.00004: Topological invariant of S4 protected highorder topological insulator. Zhida Song The point group symmetry S4 protects a highorder topological state that is gapped on all its 2D surfaces but gapless on a 1D hinge. In this talk, I present a bulk topological invariant of this S4 topology in terms of Wilson loop of Wilson loop. The nonlocal Wilson loop operator provides an effective Hamiltonian on the surface, the Wilson loop of which provides an intuitive understanding of the hinge state. Based on this picture, the breaking of Kirchhoff's law on surfaces are discussed. 
Wednesday, March 6, 2019 3:18PM  3:30PM 
P03.00005: Bound State Signatures in a Mesoscopic TI Josephson Junction Konstantin Yavilberg, Eran Ginossar, Eytan Grosfeld The engineering of hybrid Josephson junctions has increasingly become a key source in the experimental access to topological superconductivity, as these allow a high degree of robustness and control of their characteristic excitations. Here we consider a device where a topological insulator nanowire of type Bi2Se3 is proximitized to an isolated mesoscopic Josephson junction. Threaded with magnetic flux, the nanowire nucleates two pairs of topologically protected Majorana fermions as well as Andreev bound states in the weaklink. We investigate processes of coherent charge transfer mediated by these lowenergy bound states in a mesoscopic setting, and show that they have a notable effect on the fermionic parity of the superconductors, as is evident in the response of the system to electromagnetic radiation. 
Wednesday, March 6, 2019 3:30PM  3:42PM 
P03.00006: Topological States in Restrictive Geometries Liam O'Brien, Karunya Shirali, Daniel E Sheehy, Ilya Vekhter Topological Insulators (TIs) are a novel class of materials possessing symmetryprotected surface states that have promising applications in spintronics and quantum computing. The properties and robustness of TI surface states are well understood for macrosopic samples with flat boundaries, but it is not clearly established what features are retained for curved boundaries  especially when the radius of curvature and/or the sample size are decreased and become comparable to the decay length of the surface states. Consequently, here we investigate the nature of the topological surface states for finite samples with curved interfaces. In particular, we study a model of a 2dimensional TI with circular boundary of radius R, described by a 4 × 4 Diraclike Hamiltonian. We present exact solutions for the surface states of this system, derive the effective surface Hamiltonian, and discuss the deviations of the dispersion and spin texture from the flat surface case. 
Wednesday, March 6, 2019 3:42PM  3:54PM 
P03.00007: Observation of quadrupole topological insulators Marc Serra Garcia, Valerio Peri, Sebastian D Huber Many topological phenomena can be understood as a symmetryinduced quantization of the Berry phase, which describes the charge polarization inside the unit cell. This theory relating topology and polarization has been recently extended to higherorder multipolar moments. In this talk, we will discuss experimental observations of a multipolar (quadrupole) topological insulator and quadrupole transitions in phononic and LCcircuit platforms. Our measurements confirm the theoretical predictions and provide a route towards protected wave guides in three dimensional materials. 
Wednesday, March 6, 2019 3:54PM  4:06PM 
P03.00008: Characterization of Electron Beam Damage to (Bi_{x}Sb_{1x})_{2}Te_{3}Based Topological Insulators Linsey Rodenbach, Ilan Rosen, Eli J Fox, Lei Pan, Peng Zhang, Kang L. Wang, David GoldhaberGordon Models for topological superconductivity based on proximitized superconductivity in quantum Hall systems have proven difficult to realize, in part, because high magnetic fields destroy Cooper pairing. The discovery of the quantum anomalous Hall (QAH) effect in Cr and Vdoped (Bi_{x}Sb_{1x})_{2}Te_{3} has attracted attention because it provides a platform to demonstrate the same models of topological superconductivity without high magnetic fields. A remaining barrier is fabrication of the desired structures. Photolithographic patterning has been employed with QAH materials to great effect but lacks the spatial resolution needed to define, for example, Josephson junctions and quantum point contacts. Electron beam lithography has largely been avoided for fear of damaging or doping the QAH material, whose Fermi level must be very precisely positioned to observe the QAH effect. We discuss our efforts to characterize electron beam damage to (Bi_{x}Sb_{1x})_{2}Te_{3}based topological insulators. We also propose possible approaches for submicron patterning involving no electrons hitting the substrate. 
Wednesday, March 6, 2019 4:06PM  4:18PM 
P03.00009: QuantumHall to insulator transition in ultraclean topological insulator films Maryam Salehi, Hassan Shapourian, Ilan Rosen, MyungGeun Han, Jisoo Moon, Pavel P Shibayev, Deepti Jain, David GoldhaberGordon, Seongshik Oh Under a magnetic field, many two dimensional electronic systems including semiconductor 2DEGs, graphene, and topological insulators (TIs) exhibit quantum Hall effect (QHE), where R_{xy} is quantized at h/(νe^{2}) while R_{xx}→0. In the high field limit, the QH state gives way to an insulating state in semiconductor 2DEGs as the Fermi level falls into localized states at the tail of the lowest Landau level. However, for a Dirac system such as TIs, the presence of the zeroth Landau level makes it questionable whether such a QHtoinsulatortransition (QIT) should occur at all. Here, in newlydeveloped ultraclean TI films, we report the first observation of QIT in TIs, with a welldefined scaling behavior. Surprisingly, the scaling analysis revealed the first example of an unconventional QIT behavior distinct from those in 2DEGs. This observation raises new questions on the origin of the insulating phase near the center of the zeroth Landau level in TIs and other Dirac materials as well. 
Wednesday, March 6, 2019 4:18PM  4:30PM 
P03.00010: Electronic Properties of Bismuth Iodide (Bi_{4}I_{4}) Yulu Liu, Ruoyu Chen, Sheng Li, Xiaoyuan Liu, Takashi Taniguchi, Kenji Watanabe, Bing Lv, Fan Zhang, Chun Ning Lau In recent years, topological insulator materials have been attractive to many scientists because of their gapless conductive surface states that are protected against perturbations. Bi_{4}I_{4} is a quasionedimensional van der Waals material, and its βphase is predicted to be a topological insulator, thus offering a new platform for exploring the physics of topological phases. Here we report fabrication and measurement of Bi_{4}I_{4 }devices. Its electrical transport properties are investigated as a function of channel length, electrode materials, mobility and magnetic field. 

P03.00011: ABSTRACT WITHDRAWN

Wednesday, March 6, 2019 4:42PM  4:54PM 
P03.00012: Commutingprojector Hamiltonians for 2D timereversalinvariant fermionic topological phases and manybody topological invariants Jun Ho Son, Jason Alicea Inspired by a recently constructed commutingprojector Hamiltonian for a 2D timereversalinvariant topological superconductor [Wang et al., Phys. Rev. B 98, 094502 (2018)], we introduce a commutingprojector model that describes an interacting yet exactly solvable 2D timereversalinvariant topological insulator. We explicitly show that the edge properties of our model, both gapped and gapless, are consistent with those of bandtheoretic or weakly interacting quantum spin Hall systems. Additionally, the models for both topological insulators and superconductors can be defined on nonorientable spatial manifolds while retaining the commutingprojector Hamiltonian structure. Groundstate wavefunctions of these models on nonorientable manifolds provide intuitive pictures of manybody topological invariants of timereversalinvariant fermionic topological phases. 
Wednesday, March 6, 2019 4:54PM  5:06PM 
P03.00013: Ferromagnetism in Crdoped Bi_{2}Se_{3} thin films via surfacestate engineering and carrierselective anomalous Hall effect Jisoo Moon, Nikesh Koirala, Maryam Salehi, Seongshik Oh Anomalous Hall effect (AHE) is a nonlinear Hall effect appearing on magnetic conductors, boosted by internal magnetism beyond what is expected from ordinary Hall effect. With the recent discovery of quantized version of the AHE, the quantum anomalous Hall effect (QAHE), in Cr or Vdoped topological insulator (TI) (Sb,Bi)_{2}Te_{3} thin films, AHE in magnetic TIs has been attracting significant interest. However, one of the puzzles in this system has been that while Cr or Vdoped (Sb,Bi)_{2}Te_{3} and Vdoped Bi_{2}Se_{3} exhibit ferromagnetic (hysteretic) AHE, Crdoped Bi_{2}Se_{3} has failed to exhibit even ferromagnetic AHE, the necessary predecessor to QAHE, though it is the first material predicted to exhibit QAHE. Here, we have successfully implemented ferromagnetic AHE in Crdoped Bi_{2}Se_{3} thin films by utilizing a surface state engineering scheme. Surprisingly, all the observed ferromagnetic AHE, as in other Cr or Vdoped TIs, are always ptype even if the majority carrier type remains ntype. This can be well explained by a carrierselective anomalousHalleffect model in which the local magnetic moments interact only through ptype carriers. 
Wednesday, March 6, 2019 5:06PM  5:18PM 
P03.00014: Topological States at Interfaces and in Heterostructures Karunya Shailesh Shirali, William A. Shelton, Ilya Vekhter Engineering topological insulator (TI) devices involves making heterostructures which combine these materials with other compounds. For a model Hamiltonian, it was shown [1] that properties of the topological interface states may differ from those of the surface states. Motivated by this observation we use a combination of ab initio and model calculations to investigate the consequences of interface potentials breaking spatial symmetries (with and without time reversal symmetry breaking) on the spectral and spin properties of the interface states. We use first principles density functional theory calculations to investigate the topological states at the boundaries between Bi_{2}Se_{3} and binary semiconductors and magnetic insulators, obtain the form of the spinmomentum locking and anisotropy of energy dispersion, and compare these features with the outcomes of model analytical calculations. We also make connections with recent experiments on topological heterostructures, and discuss the importance of our results for tuning topological materials for potential device applications. 
Wednesday, March 6, 2019 5:18PM  5:30PM 
P03.00015: Dice Lattice in Ribbons and their Topological Properties Rahul Soni, Nitin Kaushal, Satoshi Okamoto, Elbio R Dagotto The existence of nearly flat bands with nonzero Chern numbers in a twodimensional dice lattice has been studied [1] in the presence of Rashba spinorbit coupling and a magnetic field. In this work, we studied the topological properties of the dice lattice defined using ribbons both with open and periodic boundary conditions. We found that the Chern numbers [2] are finite even for narrow systems such as Nx3 [3]. We also observed spin currents moving along the edges at half filling, when ribbons are in a cylindrical geometry with open conditions in the long direction. Understanding these topological properties in ribbonlike geometry is important because of computationally exact techniques like DMRG can be used to investigate the effect of Hubbard interactions on dice lattices. 
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