Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session M59: Disorder and Defects in Topological MaterialsRecordings Available

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Sponsoring Units: DCMP Chair: Suyang Xu, Harvard University Room: Hyatt Regency Hotel DuSable AB 
Wednesday, March 16, 2022 8:00AM  8:12AM 
M59.00001: Perfect Transmission of Topological Surface States Through Monolayer Step Edges on Sb(111) Kaiyuan Gu, Jianfeng Ge, Aditya Mahadevan, Haimei Zhang, Jiaxuan Guo, Hao Tang, Jenny E Hoffman Topological surface states are unconventional twodimensional electronic states that have chiral spin texture and are protected from backscattering. Previous experiments show that topological surface states have an enhanced transmission and a reduced reflection at the common bilayer step edges on the (111) surface of topological semimetal antimony (Sb). Using scanning tunneling microscopy and spectroscopy, we study successive monolayer step edges as partial dislocations on the (111) surface of Sb. We demonstrate from the quasiparticle interference pattern that the topological surface states are backscattered within two adjacent monolayer steps as a whole bilayer step. Remarkably, we find that the topological surface states transmit perfectly through the transitional monolayer step edge. 
Wednesday, March 16, 2022 8:12AM  8:24AM 
M59.00002: Symmetryprotected, zeroenergy disclination modes and their observation in an acoustic lattice Yuanchen Deng, Wladimir A Benalcazar, Zeguo Chen, Mourad Oudich, Guancong Ma, Yun Jing Different from the bulkboundary correspondence, bulkdefect correspondence gives rise to new wave behavior in topological systems. Such crystalline topological defects can induce a curvature singularity in a lattice, thus trapping modes within the bulk of the synthetic lattice. These trapped modes are useful in photonic and sonic crystals because the bulk surrounding a defect generates a bandgap that isolates the mode from the gapless free space. 
Wednesday, March 16, 2022 8:24AM  8:36AM 
M59.00003: A firstprinciples exploration of the effect of Se vacancies on the topological surface states of Bi_{2}Se_{3} Sharmila Shirodkar, Pratibha Dev In this work, we elucidate the effects of the most common intrinsic defect, Se vacancies, on the topological surface states of Bi_{2}Se_{3}. We explore in detail how the position of the defect affects the surface states. In addition to finding perfectly preserved topologically protected surface states in the presence of neutral Se vacancies, we see that the Dirac cone shifts deep into the valence band. Furthermore, we report for the firsttime the emergence of a band pinned near the Fermi level that exhibits signs of nonlinear dispersion. We discuss the possible origins of this departure from linearity. Our results bring us a step closer to understanding the exotic physics emerging from defects in Bi_{2}Se_{3} that had remained unexplored in prior studies. 
Wednesday, March 16, 2022 8:36AM  8:48AM 
M59.00004: Manipulation of quasi1D states bound to a line defect on topological insulator surface Eklavya Thareja, Ilya Vekhter We consider a line defect on the surface of a topological insulator (TI). It is known that purely potential scattering on such a defect results in quasi1D bound states, and we clarify the origin of these states. We further include magnetic scattering at the line defect (for example, due to localized moments). We show that magnetic scattering, by itself or in combination with potential scattering, allows for quasi1D bound states to form. The dispersion and spin structure of these states depend on the relative strengths of magnetic and potential scattering, and the direction of localized moments at the defect. This potentially allows for the manipulation of these states using a magnetic field. We also discuss the spin textures and currents generated by these states, and their role in proximityinduced superconductivity. 
Wednesday, March 16, 2022 8:48AM  9:00AM 
M59.00005: Embedded NonHermitian Topology and Skin Effect Saavanth Velury, Taylor L Hughes NonHermitian (NH) systems differ from their Hermitian counterparts and can exhibit a skin effect, a phenomenon in which an extensive number of eigenstates accumulate at an open boundary, which are dubbed `skin modes.' These skin modes are guaranteed by a nonvanishing winding number of the bulk pointgap on the complex energy plane. In this work, we consider NH models in D dimensions composed of coupled multilayers of ddimensional NH models (d 
Wednesday, March 16, 2022 9:00AM  9:12AM 
M59.00006: Elastic gapless helical edge states on the translationdislocation interface between identical 2D phononic patterns with Kekule distortion TingWei Liu, Fabio Semperlotti We present a theoretical and experimental study of the elasticwave topological edge states that propagate along the dislocation interface of two pieces of material with identical pattern only up to a rigidbody translation. This seems to violate our knowledge in topological materials about phase transition. It is due to that the elasticwave analogue to the quantum spin Hall material is based on a mechanism distinct from the quantum mechanical counterpart. Quantum electronic topological phases of materials are classified by associated topological invariants. Examples include Chern numbers for quantum (anomalous) Hall insulators, valley Chern numbers for quantum valley Hall insulators, and Z2 invariants for quantum spin Hall topological insulators. Specifically, the Z2 invariant is connected to the unique property of fermionic systems whose wavefunction acquires a negative sign upon two consecutive applications of the timereversal operator. This property leads to Kramers degeneracy and results in fully decoupled counter propagating edge states. However, this property is not acquired by the classicalwave systems. Instead, in our proposed system, a combination of spatial and temporal symmetry is utilized in replacing the timereversal operator to synthesize the Kramers degeneracy. Correspondingly, a local topological charge is defined based on the difference of integrated pseudospinresolved Berry curvature as an alternative to the Z2 invariant. Its value depends on the position of the reference frame, hence the same phononic structure can be in different topological phases upon shifting and allows edge states to exist on the dislocation interface. Our experiment result confirms the gapless spectrum and the robustness against sharp bends along the path, of the edge states. 
Wednesday, March 16, 2022 9:12AM  9:24AM 
M59.00007: Visualizing topological defects of charge density wave in Weyl semimetal (TaSe_{4})_{2}I Zengle Huang, Hemian Yi, Lujin Min, Zhiqiang Mao, CuiZu Chang, Weida Wu The chargedensitywave (CDW) compound (TaSe_{4})_{2}I is recently established as a Weyl semimetal above its CDW transition temperature (~260 K) [1]. It has been proposed that CDW nesting of Weyl nodes could lead to the correlated axionic insulator state of matter [2,3]. A recent transport study suggests that (TaSe_{4})_{2}I is a candidate of axionic CDW [4]. However, it is still under debate whether (TaSe_{4})_{2}I in the CDW phase is an axion insulator [5]. In theory, axionic CDW carries onedimensional gapless modes called axion strings at the CDW dislocations [2,3]. The ingap states could be detected by tunneling spectroscopy, providing smokinggun evidence of the axionic CDW phase. Here we present the first comprehensive realspace characterization of CDW modulation and CDW edge dislocations in (TaSe_{4})_{2}I using scanning tunneling microscopy and spectroscopy (STM/STS). Our tunneling spectroscopic results on the CDW edge dislocations shed light on the topological nature of the CDW phase. 
Wednesday, March 16, 2022 9:24AM  9:36AM 
M59.00008: Structural disorder and topology in Bi_{2}Se_{3} and BiTeI. Paul Corbae, Samuel T Ciocys, Daniel Varjas, Alessandra Lanzara, Sinead M Griffin, Frances Hellman The use of crystal symmetries and band representations have enabled a classification scheme for crystalline topological materials, leading to large scale topological materials discovery [1]. We address the role of structural disorder in topological insulators and show that amorphous topological materials, which lack of longrange order, exist in the solid state. Amorphous Bi_{2}Se_{3} shows a high but temperature independent resistance, weak antilocalization consistent with decoupled conductance channels, and ARPES data showing a dispersive spinpolarized electronic structure [2]. These experimental results are consistent with an amorphous tight binding model. Next, we show how breaking of crystalline symmetries in the trivial insulator BiTeI leads to a topological phase transition via first principles calculations. Structural disorder removes the degeneracy of the orbitals near the Fermi level and increases the crystal field pushing the bands closer together and incorporating spinorbit coupling leads to a band inversion [3]. Our work provides an understanding on how local environments produce topological phases and is a key step for predicting disordered and amorphous topological materials to be used in scalable topological devices. 
Wednesday, March 16, 2022 9:36AM  9:48AM 
M59.00009: Defect scattering on the ultraclean topological semimetal PtSn4 via STM Dong Chen, Sarah Burke, Douglas A Bonn, Ashley N Warner, Seokhwan Choi, Samikshya Sahu, Mohamed Oudah, Alannah M Hallas, Jisun Kim, Vanessa King PtSn4 is a Dirac nodal arc semimetal with high mobility and a residual resistivity ratio approaching 1000. This indicates a scarcity of scattering by defects at low temperatures. Two potential factors may be at play: PtSn4 has an extremely low defect density, and, also, some backscattering channels may be blocked by the topological band structure. 
Wednesday, March 16, 2022 9:48AM  10:00AM 
M59.00010: Disorderdominated nonsaturating linear magnetoresistance in Srdoped Bi2Se3 thin films JIAYUAN HU Sr_{x}Bi_{2}Se_{3 }thin films was known as a candidate of topological superconductor. We report that the Linear Magnetoresistance (LMR) of Sr_{x}Bi_{2}Se_{3 }films originates from mobility fluctuation induced by disorder, rather than the gapless linear energy dispersion emphasized by Abrikosov’s quantum model. We also find the transition of disorder parameters from average mobility 〈μ〉to the width of mobility disorder △μ, indicting by 1/K_{F}l where l is the mean free path and K_{F} is the fermi wave factor, from Sr_{0.025}Bi_{2}Se_{3} to Sr_{0.085}Bi_{2}Se_{3}. It demonstrates the dramatic decline in film mobility caused by slight increase in doping concentration x. The film was prepared by Molecular Beam Epitaxy (MBE) on Al_{2}O_{3}(0001) substrate with doping concentrations x from 0.025 to 0.125, and the LMR is still not saturated at 14 Tesla. For comparison, we also systematically studied Mn_{x}Bi_{2}Se_{3} films and showed similar results. 
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