Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B01: Defects in Topological Materials 
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Sponsoring Units: DCMP Chair: Jennifer Cano Room: BCEC 106 
Monday, March 4, 2019 11:15AM  11:27AM 
B01.00001: Backscattering induced by vacancies in the topological insulator phase of graphene Caio Lewenkopf, Leandro Lima Theory predicts that the spin orbit interaction (SOI) can turn graphene into a topological insulator. The SOI induces hopping processes between distant carbon atoms giving origin to the quantum spin Hall (QSH) effect, where the bulk sample is insulating and the electronic current that flows only along the system edges is robust against nonmagnetic disorder. Although vacancies are usually treated as a nonmagnetic disorder source, theoretical studies and experimental evidence show evidence of magnetic defects induced by vacancies in graphene. Thus, vacancies are likely to destroy the conductance quantization in the QSH phase by acting as magnetic defects and backscattering the helical edge states. We investigate the electronic transport of graphene in the QSH phase in the presence of those magnetic vacancies. We use an unrestricted Hubbard mean field Hamiltonian to model the electronelectron interaction, which enables both outofplane and inplane magnetization. The last is responsible for spinflip processes. We use a selfconsistent recursive Green's functions technique to calculate the transport quantities and to understand the role of magnetization due to vacancies in the electronic propagation. 
Monday, March 4, 2019 11:27AM  11:39AM 
B01.00002: Manipulation and Characterization of the ValleyPolarized Topological Kink States in GrapheneBased Interferometers Shuguang Cheng, Haiwen Liu, Hua Jiang, QingFeng Sun, Xincheng Xie Valley polarized topological kink states, existing broadly in the domain wall of hexagonal lattice systems, are identified in experiments. Using an AharanovBohm interferometer composed of domain walls in graphene systems, we study the periodical modulation of a pure valley current in a large range by tuning the magnetic field or the Fermi level. For a monolayer graphene device, there exists one topological kink state, and the oscillation of the transmission coefficients has a single period. The π Berry phase and the linear dispersion relation of kink states can be extracted from the transmission data. For a bilayer graphene device, there are two topological kink states with two oscillation periods. Our proposal provides an experimentally feasible route to manipulate and characterize the valleypolarized topological kink states in classical wave and electronic graphenetype crystalline systems. The realization of the proposal in photonic graphene is also discussed. 
Monday, March 4, 2019 11:39AM  11:51AM 
B01.00003: Partial lattice defects in higher order topological insulators Raquel Queiroz, Ion Cosma Fulga, Nurit Avraham, Haim Beidenkopf, Jennifer Cano Nonzero weak topological indices are thought to be a necessary condition to bind a single helical mode on lattice dislocations. In this work we show that higherorder topological insulators (HOTIs) can, in fact, host a single helical mode along screw or edge dislocations (including step edges) in the absence of weak topological indices. This helical mode is necessarily bound to a dislocation characterized by a fractional Burgers vector, locally detected by the existence of a stacking fault. The robustness of a helical mode on a partial defect is demonstrated by an adiabatic transformation that restores translation symmetry in the stacking fault. We present two examples of HOTIs, one intrinsic and one extrinsic, that show helical modes at partial dislocations.Since partial defects and stacking faults are commonplace in bulk crystals, the existence of such helical modes can in principle significantly affect the expected conductivity in these materials. 
Monday, March 4, 2019 11:51AM  12:03PM 
B01.00004: Honey, I Shrunk the Transport Geometry, or Carving MicroCrystals of Topological Insulators for Transport Experiments Dmitri Mihaliov, Alexa Rakoski, Shriya Sinha, Cagliyan Kurdak, Priscila Rosa, Zachary Fisk, Boyoun Kang, Myungsuk Song, Beongki Cho There exist a large number of new quantum materials that are difficult to characterize using standard electrical transport methods due to the inability to grow macroscopic crystalline samples. However, using standard TEM sample preparation methods, it is possible to carve 10s of microns size crystalline pieces and place them on an insulating substrate for transport characterization. We have demonstrated this method by carving a 2μm x 5μm x 10μm piece of SmB_{6} and placing it on a Si substrate. The Hall bar was subsequently wired using Pt contacts deposited via FIB. We are also developing more demanding transport structures, such as doublesided Corbino rings with the goal of studying transport through individual threading dislocations. We will present challenges associated with sample preparation at the micro crystalline scale. 
Monday, March 4, 2019 12:03PM  12:15PM 
B01.00005: Topological gravity, defects, and topological phases protected by both spatial and internal symmetries Bo Han, Huajia Wang, Peng Ye A substantial attention of quantum gravity has been paid in different areas of physics, from high energy physics, condensed matter physics to quantum information science. In this paper, we explore the topological quantum gravity from the perspective of symmetryprotected topological (SPT) phases. To be specific, we study the gravitational effect on the classification and response of 3+1d topological phases protected simultaneously by both spatial and internal symmetries from the topological response theory. The latter describes how geometric defect (e.g. disclination) and internal symmetry defect (e.g. domain wall) are topologically entangled. New topological terms involving nontrivial geometries are proposed, which are extensions of purely internal topological theories discussed before. Physical observables derived from these response theories are discussed and compared with their analogs in 2+1d systems, some of which have been wellstudied in quantum Hall (QH) systems, like the WenZee (WZ) term. Generalizations to symmetryenriched topological (SET) phases are also discussed (arXiv:1807.10844). 
Monday, March 4, 2019 12:15PM  12:27PM 
B01.00006: Kondo Signatures of a Quantum Magnetic Impurity in Topological Superconductor Xiaoqun Wang, Rui Wang, Wei Su, JianXin Zhu, ChinSen Ting, Hai Li, Changfeng Chen, Baigeng Wang We study the Kondo physics of a quantum magnetic impurity in twodimensional topological 
Monday, March 4, 2019 12:27PM  12:39PM 
B01.00007: Impurityinduced bound states as a signature of chiral pwave superconductivity in Pb_{1x}Sn_{x}Te Sarbajaya Kundu, Vikram Tripathi The recent observation of surface superconductivity on the (001) surface of the topological crystalline insulator Pb_{1x}Sn_{x}Te is intriguing, but the nature of the superconducting order is yet to be ascertained. In this context, we have recently proposed that a chiral pwave superconducting order can be realized on the (001) surface of Pb_{1x}Sn_{x}Te, in the presence of weak electronic correlations. In this talk, I will discuss impurityinduced subgap bound states as a possible experimental signature of the chiral pwave order. In particular, I will introduce two parameter regimes: the normal band gap regime where the subgap bound states are smoothly connected to impurity states in semiconductors, and the inverted band gap regime,where they nontrivially depend on the existence of chiral pwave superconductivity. I will further discuss properties of the analytically obtained bound state wavefunctions that are peculiar to the chiral pwave order. For a point defect, these include the quasilocalized nature of the boundstate wavefunctions, and an internal SU(2) rotational symmetry of the zeroenergy bound states that enables their use as a possible quantum qubit. 
Monday, March 4, 2019 12:39PM  12:51PM 
B01.00008: Fractional Chern insulator edges and layerresolved lattice contacts Christina Knapp, Eric Spanton, Andrea Young, Chetan Nayak, Michael Zaletel Fractional Chern insulators (FCIs) realized in fractional quantum Hall systems subject to a periodic potential are topological phases of matter for which space group symmetries play an important role. In particular, lattice dislocations in an FCI can host topologyaltering nonAbelian topological defects, known as genons. Genons are of particular interest for their potential application to topological quantum computing. In this work, we study FCI edges and how they can be used to detect genons. We find that translation symmetry can impose a quantized momentum difference between the edge electrons of a partiallyfilled Chern band. We propose layerresolved lattice contacts, which utilize this momentum difference to selectively contact a particular FCI edge electron. The relative current between FCI edge electrons can then be used to detect the presence of genons in the bulk FCI. Recent experiments have demonstrated graphene is a viable platform to study FCI physics. We describe how the lattice contacts proposed here could be implemented in graphene subject to an artificial lattice, thereby outlining a path forward for experimental dectection of nonAbelian topological defects. 
Monday, March 4, 2019 12:51PM  1:03PM 
B01.00009: Quasiparticle interference and nonsymmorphic effect on a floating band surface state of ZrSiSe Zhen Zhu, Fengqi Song, Hsin Lin, Wei Ku, Hao Zheng, Jinfeng Jia Nonsymmorphic crystals are generating great interest as they are commonly found in quantum materials, like ironbased superconductors, heavyfermion compounds, and topological semimetals. A new type of surface state, a floating band, was recently discovered in the nodalline semimetal ZrSiSe, but also exists in many nonsymmorphic crystals. Little is known about its physical properties. Here, we employ scanning tunneling microscopy to measure the quasiparticle interference of the floating band state on ZrSiSe (001) surface and discover rotational symmetry breaking interference, healing effect and anomalous halfmissing Umklapp scattering. Using simulation and theoretical analysis we establish that the phenomena are characteristic properties of a floating band surface state. Moreover, we uncover that the halfmissing Umklapp process is derived from the glide mirror symmetry, thus identify a nonsymmorphic effect on quasiparticle interferences. Our results may pave a way towards potential new applications of nanoelectronics. 
Monday, March 4, 2019 1:03PM  1:15PM 
B01.00010: Topological 0D defect states in 3D insulators Frank Schindler, Stepan Tsirkin, Titus Neupert, Andrei B Bernevig, Benjamin Wieder There has been intense interest in relating the electronic states bound to crystal defects to the bulk electronic structure of pristine crystals. By mapping the momentumspace Hamiltonians of Brillouin zone surfaces to the realspace surfaces between crystal defects, we develop a general formulation of topological (crystalline) defect states. We introduce topological invariants for these states using (nested) Wilson loops and Topological Quantum Chemistry. Our framework captures all previous results, including fractional charges bound to point defects in inversionsymmetric Chern insulators and helical modes bound to screw dislocations in weak topological insulators (TIs). However, we also discover new examples. In particular, we show that screw dislocations and edge disclinations in 3D higherorder TIs (HOTIs) can bind anomalous 0D higherorder "end states," which are equivalent to the fractionally charged corner modes of 2D "fragile" TIs and obstructed atomic limits, and persist under the relaxation of particlehole symmetry, which is not present in real materials. Using density functional theory and tightbinding calculations, we demonstrate the presence of higherorder defect end states in the HOTI and topological crystalline insulator SnTe. 
Monday, March 4, 2019 1:15PM  1:27PM 
B01.00011: Complete description of symmetryprotected topological properties of Sn_{x}Pb_{1x}Te_{y}Se_{1y} topological crystalline insulators with a step edge Wojciech Brzezicki, Marcin Wysokinski, Timo Hyart Surfaces of multilayer semiconductors typically have regions of atomically flat terraces separated by atomhigh steps. Here we investigate the properties of the lowenergy states appearing at the surface atomic steps in Sn_{x}Pb_{1x}Te_{y}Se_{1y}. We identify the important approximate symmetries and use them to construct relevant topological invariants. We calculate the dependence of mirror and spinresolved Chern numbers on the number of layers and show that the step states appear when these invariants are different on the two sides of the step. Moreover, we find that a particlehole symmetry can protect onedimensional Weyl points at the steps. Since the local density of states is large at the step the system is susceptible to different types of instabilities, and we consider an easyaxis magnetization as one realistic possibility. We show that magnetic domain walls support a pair of zeroenergy states because the regions with opposite magnetization are topologically distinct in the presence of nonsymmorphic chiral and mirror symmetries, providing a possible explanation for the zerobias conductance peak observed in the recent experiment [Mazur et al., arXiv:1709.04000]. 
Monday, March 4, 2019 1:27PM  1:39PM 
B01.00012: Dynamically Induced Topology in the Fermi Sea Daniel Dahan, Eytan Grosfeld, Babak Seradjeh We study the dynamics of topological bound states following their coupling to the Fermi sea of a topologically trivial, gapless lead. Specifically, we study the quench dynamics of solitons in the SuSchriefferHeeger model and of Majorana zero modes of Kitaev model. Remarkably, we find bound states propagate through the lead preserving their topological nature, such as fractional charge and exchange statistics. We explain this phenomenon as a manifestation of dynamically induced topology in the Fermi sea arising from the entanglement between the topological and gapless systems. We obtain, both analytically and numerically, topological features of propagating bound states, including their fractional charge, charge fluctuations, entanglement entropy, and fractional statistics. This allows us to characterize the coherence time over which these topological features relax. We also study the effects of interactions and disorder in the lead on the integrity of the topological bound states and the coherence time of dynamically induced topology. 
Monday, March 4, 2019 1:39PM  1:51PM 
B01.00013: The influence of 1D topological states on the thermoelectric properties of Bi_{2}Te_{3} from the perspective of coupled TomonagaLuttinger liquids (TLLs). Piotr Chudzinski

Monday, March 4, 2019 1:51PM  2:03PM 
B01.00014: Probing the Topological Classification of Bismuth with Topological Defects Nurit Avraham, Abhay K Nayak, Jonathan Reiner, Raquel Queiroz, Huixia Fu, Chandra Shekhar, Claudia Felser, Binghai Yan, Haim Beidenkopf The growing diversity of topological classes may lead to ambiguity in the classification of materials. Such is the case of Bismuth. While most theoretical models indicate that Bismuth possesses a trivial topological order, other theoretical and experimental studies suggest other classifications such as a strong, a weak, and a higher order topological insulator (TI), all of which host helical modes on their boundaries. We use scanning tunneling microscopy to investigate the response of the topological edge mode, in Bismuth, to a topological defect in the form of a screw dislocation. We find that the edge mode extends over a wider energy range than previously thought, and withstands crystallographic irregularities, without showing any signs of backscattering and gapping. Moreover, it seems to bind to the bulk screw dislocation, as expected for a TI with nonvanishing weak indices. These observations disfavor its recent identification as a hinge mode of a high order TI. We argue that the small scale of the bulk $L$ gap positions Bismuth within the critical region of a topological phase transition to a strong TI with nonvanishing weak indices. Consequently, the observed boundary modes are approximately helical already on the trivial side of the topological phase transition. 
Monday, March 4, 2019 2:03PM  2:15PM 
B01.00015: Static analogues of driven phases of matter Ian Mondragon, Brahyam Rios, Boris Rodriguez, Meng Cheng Driven phases of matter are often said to arise only in nonequilibrium conditions. We here show that analogues of driven phases can be created as timeindependent eigenstates of matter interacting with quantum light. These phases are characterized by a hidden geometric phase, referred to here as optical Berry phase, which is related to the average photon number of the system. We illustrate our results by studying an analogue of a driven topological state, the so called Anomalous Floquet Anderson Insulator. We show that the optical Berry phase manifests itself as a topological pumping of photons at the boundary equal to the bulk invariant. Finally, we discuss how the tools of cavity QED systems can be brought to bear in the study of these phases of light and matter. 
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