Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H03: Phase Transitions in Topological Systems |
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Sponsoring Units: DCMP Room: BCEC 107B |
Tuesday, March 5, 2019 2:30PM - 2:42PM |
H03.00001: Surfaces of axion insulators Nicodemos Varnava, David Vanderbilt Axion insulators (AXIs) are magnetic topological insulators in which the non-trivial Ζ_{2} index is protected by inversion instead of time-reversal (TR) symmetry. The naturally gapped surfaces of AXIs give rise to a half-quantized surface anomalous Hall conductivity (SAHC), but its sign cannot be determined from topological arguments. To be explicit, we construct a minimal tight-binding model on the pyrochlore lattice and investigate the all-in-all-out and ferromagnetic spin configurations. We show that the Wilson loop eigenvalues are not required to wind for an AXI in contrast with their TR-invariant cousin. We also implement a method that lets us calculate the SAHC directly, which allows us to explore how the interplay between surface termination and magnetic ordering determines the sign of the half-quantized SAHC. We find that it is possible to construct a topological state with no protected metallic states on boundaries of any dimension, although chiral hinge modes do occur for many surface configurations. Finally, we find that rotation of the magnetization by an external field offers promising means of control of chiral hinge modes, which can also appear on surface steps or where bulk domain walls emerge at the surface. |
Tuesday, March 5, 2019 2:42PM - 2:54PM |
H03.00002: Failure of Nielsen-Ninomiya theorem and fragile topology in two-dimensional systems with space-time inversion symmetry: application to twisted bilayer graphene at magic angle Junyeong Ahn, Sungjoon Park, Bohm-Jung Yang We examine the band topology of two dimensional real fermions in systems with space-time inversion (STI) symmetry. We show that a two-band system with a nonzero Euler class cannot have STI symmetric Wannier representation. Moreover, a two-band system with the Euler class e_{2} has band crossing points whose total winding number is equal to 2e_{2}. Thus the conventional Nielsen-Ninomiya theorem fails in systems with a nonzero Euler class. We propose that the topological phase transition between insulators carrying different Euler classes can be described by pair creation and annihilation of vortices across Dirac strings. |
Tuesday, March 5, 2019 2:54PM - 3:06PM |
H03.00003: Topological phase transitions induced by tunable magnetization direction in Chern insulators Rui-An Chang, Ching-Ray Chang Quantum anomalous Hall effect (QAHE), the quantum Hall effect without Landau levels, has been widely studied in 2D electron systems for its nontrivial topology and promising applications in future technology. Magnetic topological insulators are very typical systems exhibiting QAH phase [1]. |
Tuesday, March 5, 2019 3:06PM - 3:18PM |
H03.00004: Topological Superconductivity and Density Waves on the surface of Topological Kondo Insulator Yi Luo, Predrag Nikolic We study the weak-coupling instabilities in the particle-particle and particle-hole channels on the surface of a prototype Kondo topological insulator. We draw inspiration from samarium hexaboride (SmB6), and the possible observation of its correlation physics through magnetoresistance hysteresis[1]. We work on the 100-surface, which has square C4v symmetry and contains three Dirac cones (electron or hole pockets) at Γ and two X points in momentum space. For superconducting order, we find phases supporting non-zero magnetization and spin current in the ground state, with certain symmetries broken by the condensates of inter-pocket order parameters. The intra-pocket pairings in general lead to a non-zero Chern number in Bogoliubov band, hinting topological superconductivity and the presence of Majorana zero modes bounded to defects. For exciton pairings, similar symmetry breaking phases are discovered. |
Tuesday, March 5, 2019 3:18PM - 3:30PM |
H03.00005: Higher-Order Topology of Three-Dimensional Strong Stiefel-Whitney Insulators Junyeong Ahn, Bohm-Jung Yang The Stiefel-Whitney insulator (SWI) is a two-dimensional topological phase protected by the symmetry under the combination of time reversal $T$ and a two-fold rotation $C_2$. This phase has got attention because it shows new aspects of topological crystalline insulators such as fragile topology and higher-order topology. In this talk, we study the three-dimensional generalization of the Stiefel-Whitney insulator. We show that a $C_2T$-symmetric insulator in 3D can have a stable topological invariant, contrary to its two-dimensional counterpart having fragile band topology. To characterize the bulk band topology further, we develop a new method based on the homotopy class of the symmetry representation for $C_{2z}T$ in a smooth gauge, instead of examining the obstruction to constructing smooth wavefunctions compatible with the reality condition. By using the new method, we show that the 3D topological insulator, dubbed 3D strong SWI, is characterized by the quantized magnetoelectric polarizability, which induces anomalous chiral hinge states along the edges parallel to the $C_2$ rotation axis and massless Dirac fermions on the surfaces normal to the $C_2$ axis. This establishes that a 3D strong SWI is a second-order topological insulator. |
Tuesday, March 5, 2019 3:30PM - 3:42PM |
H03.00006: Revisiting topological property of a (t_{2g})^{5} system with a honeycomb lattice Beom Hyun Kim, Kazuhiro Seki, Tomonori Shrakawa, Seiji Yunoki Na_{2}IrO_{3} is the transition-metal system with a honeycomb lattice in which five electrons per site are occupied in Ir t_{2g} bands. It was firstly proposed as a candidate to show the quantum spin Hall (QSH) phase because spin-orbital entangled j_{eff} = 1/2 bands across the Fermi level can be map into the Kane-Mele model. However, its topological phase has not been reported yet. Contrarily, Na_{2}IrO_{3} and its isostructural systems have turned out to be Mott insulator with antiferromagnetic order. Nevertheless, the possibility of the QSH phase in these systems is still an interesting subject. State-of-the-art structural controlling with high pressure, chemical substitution, or substrate engineering can potentially manipulate their electronic kinetics and correlation effect. |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H03.00007: Vertical quantum transport in magnetic topological insulator heterostructures Nezhat Pournaghavi, Cecilia Holmqvist, Anna Pertsova, Carlo Canali, Allan MacDonald Magnetic topological insulator (TI) thin films with time-reversal symmetry broken by proximity to magnetic layers support the quantum anomalous Hall effect (QAHE) and the topological magneto-electric (TME) effect. We investigate theoretically a heterostructure composed of a TI thin film sandwiched between two antiferromagnetic (AFM) metal layers. The magnetic structure in the AFM films consists of alternating planes with ferromagnetic order parallel and antiparallel to the surface normal direction. We show that quantum transport along the vertical direction of this heterostructure is strongly influenced by whether the exchange fields on the top and bottom surface of the TI are parallel or antiparallel. In the first case, QAHE conducting topological edge states on the sidewall surfaces contribute significantly to a leak conductance. In the case of opposite magnetization, the system is in the axion-insulator state, all the edge states are gapped, and the vertical conductance is small. By using a microscopic tight-binding model combined with a non-equilibrium Green’s function approach, we explore the dependence of the sidewall electronic structure and transport on the profile of the exchange coupling, and the TME effect in the presence of external electric and magnetic fields. |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H03.00008: Current Noise in InAs/GaInSb Corbino structures Loah Stevens, Tingxin Li, Rui-Rui Du, Douglas Natelson We report preliminary shot noise measurements in InAs/GaInSb quantum well interfaces. The band structure of these composite quantum wells is inverted, and electron and hole densities are equal at the cross point of the valence and conduction bands. The holes in the strained GaInSb layer couple strongly to the electrons in the InAs layer, opening a gap in the spectrum of the 2d bulk. At low temperatures or when gated into the appropriate regime, bulk transport is greatly suppressed as the hybridized gap is opened. Corbino disk devices consist of concentric rings, meaning no edge conduction is present. Therefore, in these structures, noise measurements examine transport through the bulk and the role played by contacting electrodes. We will compare current noise measurements of samples in the nominal Quantum Spin Hall Insulator state and in the lower-resistance, bulk transport state. |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H03.00009: Superconducting proximity effects and pairing in (Bi_{1-x}Sb_{x})_{2}Te_{3} films on niobium Joseph Hlevyack, Yang Bai, Meng-Kai Lin, Peng Chen, David Flötotto, Ro-Ya Liu, Akihiro Tsuzuki, Kozo Okazaki, Shik Shin, James N Eckstein, Tai-Chang Chiang Interfacing a topological insulator (TI) with an s-wave superconductor (SC) can favor the formation of helical-Cooper pairing in the topological surface states. However, realizing this system is often challenging, due to the impracticality of growing TIs on most SCs and possibly defects and Fermi surface/lattice mismatch at the TI/SC interface. By developing a novel cleavage-based “flip chip” approach, we have successfully fabricated single-crystalline (Bi_{1-x}Sb_{x})_{2}Te_{3} thin films of a predetermined thickness (3 – 10 quintuple layers) on top of bulk Nb film. Moreover, by carefully controlling the growth conditions of the prototypical TI Bi_{2}Te_{3 }films, we find from our angle-resolved photoemission spectroscopy (ARPES) measurements of Bi_{2}Te_{3}/Nb that the Fermi level can lie in the band gap, giving rise to a lightly n-doped TI on a SC substrate. Ongoing laser ARPES and STM measurements would clarify the mechanism of coupling between the SC and the topological surface states in these doped TI/SC thin-film heterostructures. Our study underlines methods for realizing superconductivity in TIs and for clarifying the pairing in these heterostructures for applications in topological quantum computing. |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H03.00010: Theory of topological glass transitions in amorphous topological matter Isac Sahlberg, Alex Westström, Kim Pöyhönen, Teemu Ojanen Amorphous systems have recently been identified as promising platforms for topological matter. In this work we introduce a scaling theory of amorphous topological phase transitions in two dimensional systems. We investigate the universal behaviour of a density-driven Chern and Z_{2} glass transitions by carrying out a finite-size scaling analysis of topological invariants averaged over random geometries. We find that the universal properties of continuum problems can be captured by studying random geometries generated by percolation lattices. Strikingly, our results show that even for short-range hopping the topological phase may persists down to the classical site percolation threshold. Furthermore, our theory suggests that the value of the critical exponent describing the diverging localization length near the critical density is close to that of the exponent of the correlation length of the critical percolation cluster. Our theory supports the conclusion that density-driven amorphous topological transitions have their unique properties not shared by disordered systems and occupy separate universality classes. |
Tuesday, March 5, 2019 4:30PM - 4:42PM |
H03.00011: Anomalous scaling law in the quantum Hall plateau-plateau transitions of topological insulator Sn-Bi_{1.1}Sb_{0.9}Te_{2}S devices Faji Xie, Shuai Zhang, Fengqi Song The surfaces of three dimensional topological insulators are two dimensional electron gases of Dirac fermions without electron-phonon scattering contamination. In the quantum Hall states of the surface states system, the mechanism of transport dispassion change from thermal activation to the variable range hopping at a critical temperature 20K. The quantum Hall plateau-to-plateau transition behavior in the temperature regime below 20K gives with a universal exponent k=0.20 . It indicates that only electron-electron interaction dominates the inelastic scattering mechanism in the two dimensional topological electron system. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H03.00012: Mirror Chern number in the hybrid Wannier representation Tomáš Rauch, Thomas Olsen, David Vanderbilt, Ivo Souza We formulate the mirror Chern number (MCN) of a two-dimensional insulator with reflection symmetry M_{z }in terms of hybrid Wannier functions (the eigenfunctions of PzP, the position operator projected onto the valence bands). Because PzP and M_{z }anticommute, the spectrum of "Wannier bands" is symmetric about the mirror plane, and an excess of one mirror eigenvalue over the other in the occupied manifold leads to the appearance of flat bands on the mirror plane. (This structure is reminiscent of the energy bands of a bipartite lattice, where the Hamiltonian anticommutes with the sublattice symmetry operator.) In the absence of flat bands, pairs of dispersive bands may touch at isolated points on the mirror plane. These Dirac cones are protected by symmetry, and the MCN is given by the sum of their winding numbers. When flat bands are present the Dirac cones are no longer protected, and the MCN is related instead to the Chern number of the flat bands. In three dimensions, the present formalism reveals a simple relation between the MCNs and the quantized axion angle θ, whose expression in the hybrid Wannier representation was previously obtained. |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H03.00013: Strongly enhanced Berry-dipole at topological phase transitions in BiTeI Jorge Facio, Dmitri Efremov, Klaus Koepernik, Jhih-Shih You, Inti Sodemann, Jeroen Van den Brink Transitions between topologically distinct electronic states have been predicted in different classes of materials and observed in some. A major goal is the identification of measurable properties that directly expose the topological nature of such transitions. Here we focus on the giant-Rashba material bismuth tellurium iodine (BiTeI) which exhibits a pressure-driven phase transition between topological and trivial insulators in three-dimensions. We demonstrate that this transition, which proceeds through an intermediate Weyl semi-metallic state, is accompanied by a giant enhancement of the Berry curvature dipole which can be probed in transport and optoelectronic experiments. From first-principles calculations, we show that the Berrry-dipole --a vector along the polar axis of this material-- has opposite orientations in the trivial and topological insulating phases and peaks at the insulator-to-Weyl critical points, at which the nonlinear Hall conductivity can increase by over two orders of magnitude (arXiv:1805.02680). |
Tuesday, March 5, 2019 5:06PM - 5:18PM |
H03.00014: Contrasting the Surface Phonon Dispersion of the Topological Crystalline Insulator Pb_{0.7}Sn_{0.3}Se in its Topologically Trivial and Nontrivial Phases Samuel Kalish, Luiz Santos, Raman Sankar, Fangcheng Chou, Claudio Chamon, Maged Mansour El-Batanouny We report inelastic He atom surface-scattering measurements of the (001) surface phonons dispersion of the topological crystalline insulator Pb_{0.7}Sn_{0.3}Se. Because this material exhibits a temperature-dependent topological transition, we measure the surface dispersion curves in both the trivial and topological phases. Peculiarly, most surface phonon modes appear as resonances, rather than pure surface states. We find that a vertical shear surface resonance branch around 1.9 THz dramatically changes on going from the trivial to the topological phase. We associate this remarkable change with a strong interaction of the emergent surface Dirac fermions with the modes of this branch. We use the measured dispersion of this resonance branch to determine the corresponding mode-dependent electron-phonon coupling λ_{ν}(q). |
Tuesday, March 5, 2019 5:18PM - 5:30PM |
H03.00015: Localization-Driven Correlated States of Two Isolated Interacting Helical Edges Yang-Zhi Chou We study the localization-driven correlated states among two isolated dirty interacting helical edges as realized at the boundaries of two-dimensional $\mathbb{Z}_2$ topological insulators. We show that an interplay of time-reversal symmetric disorder and strong inter-edge interactions generically drives the entire system to a gapless localized state, preempting all other intra-edge instabilities. For weaker interactions, an anti-symmetric interlocked fluid, causing a negative perfect drag, can emerge from dirty edges with different densities. We also study the stability of the inter-edge correlated states against finite size and/or finite temperature corrections. |
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