Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W7: Focus Session: Novel Topological Phases: Theory II |
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Sponsoring Units: DMP DCMP Chair: Jens Bardarson, Max Planck Institute, Dresden Room: 006B |
Thursday, March 5, 2015 2:30PM - 2:42PM |
W7.00001: disorder effect on quantum transport properties of ultra thin Fe film Xiaotian Zhang, Kohji Nakamura, Ryuichi Shindou Ferromagnetic ultrathin films are experimentally known to often exhibit perpendicular magnetic anisotropy, when being placed on certain substrates. Based on reported ab-initio band calculations of free-standing Fe-monolayer and that on MgO substrate, we will introduce an effective tight-binding model, which capture a part of an electronic structure near Fermi level for both cases. We will show that the model supports electronic bands with non-zero Chern number and chiral edge modes which cross a direct band gap on the order of 50meV. Unluckily, however, the direct band gap is also masked by another dispersive bands which have non-zero Berry's curvature in the k-space. To demonstrate how disorder kills conducting characters of the latter bulk bands while leave intact those of the chiral edge modes, we will clarify behaviors of localization length and conductance in the effective model with on-site disorders. [Preview Abstract] |
Thursday, March 5, 2015 2:42PM - 2:54PM |
W7.00002: Complex Band Structure of the Topological Insulator Bi$_{2}$Se$_{3}$ Shijie Li, Jesuan Betancourt, J.D. Burton, Julian P. Velev, Evgeny Y. Tsymbal Recently there is a surge of interest in using topological insulators for electronic and spintronic applications. For applications it is important to understand the complex band structure (CBS) of the topological insulator, which determines the decay rate of the protected surface states into the bulk of the material. The Bi$_{2}$Se$_{3}$ family of three-dimensional topological insulators is the most studied and best understood. In this work we investigate the CBS of Bi$_{2}$Se$_{3}$ using first-principles density-functional calculations. We determine the decay rates and the symmetry of the evanescent states and we follow their evolution from those of the band insulator. We complement these results with Bi$_{2}$Se$_{3}$ (0001) slab calculations to explore the penetration depth, oscillatory behavior and spin texture of the surface states. The CBS provides a new insight into the topologically protected states and could be used for the search of new topological insulators and device concepts. [Preview Abstract] |
Thursday, March 5, 2015 2:54PM - 3:06PM |
W7.00003: Prediction of Large-Gap Two-Dimensional Topological Insulators Consisting of Hydrogenated Bilayers of Group III Elements with Bi Christian P. Crisostomo, Liang-Zi Yao, Zhi-Quan Huang, Chia-Hsiu Hsu, Feng-Chuan Chuang, Hsin Lin, Marvin A. Albao, Arun Bansil We use first-principles electronic structure calculations to predict a new class of two-dimensional (2D) topological insulators (TIs) in hydrogenated binary compositions of group III elements (B, Al, Ga, In, and Tl) and bismuth (Bi). We identify band inversions in unhydrogenated pristine GaBi, InBi, and TlBi bilayers, with gaps as large as 556 meV for the TlBi case, making these materials suitable for room-temperature applications. Double-sided hydrogenation in which hydrogen was added on opposite sides also exhibited band inversions in the case of GaBi, InBi, and TlBi just as in the unhydrogenated pristine ones. Furthermore, we report the gap to be 885 meV for the hydrogenated TlBi case. Hydrogenation enhace the band gap without changing the band topology. Moreover, our study also aim to demonstrate the possibility of strain engineering in that the topological phase transition in systems whose phase was nontrivial could be driven by suitable strain. Finally, the effect of placing hydrogen to topological edges was also demonstrated. Our findings suggest that the buckled honeycomb structure is a versatile platform for hosting nontrivial topological states and spin-polarized Dirac fermions with the flexibility of chemical and mechanical tunability. The robustness of III-Bi upon hydrogenation shows that these materials are possible to synthesize by growing on substrates. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:42PM |
W7.00004: Topological Crystalline Insulators Invited Speaker: Timothy Hsieh Topological crystalline insulators (TCI) are new phases of matter in which nontrivial band topology and crystal symmetry unite to protect metallic states on the boundary. Remarkably, TCIs have been predicted and observed in the conveniently simple rocksalt SnTe class of IV-VI semiconductors. Despite the simple crystal structure, the interplay between topology and crystal symmetry in these materials have led to a rich variety of new phenomena, including the coexistence of massless and massive Dirac fermions arising from ferroelectric distortion and strain-induced flat band superconductivity. These new physical mechanisms are not only of intrinsic interest but may also find application in new transistor devices. After discussing the topological nature and potential uses of IV-VI family TCIs, I will present recent predictions of TCIs in several anti-perovskite materials. The origin of TCI in this new class of materials is strikingly different and involves the band inversion of two J $=$ 3/2 quartets of Dirac fermions, which together form a ``Dirac octet.'' As interactions play a significant role in many anti-perovskites, this prediction serves as first step toward realizing TCIs in strongly correlated systems. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W7.00005: The nontrivial electronic structure of Bi/Sb honeycombs on SiC(0001) Feng-Chuan Chuang, Chia-Hsiu Hsu, Zhi-Quan Huang, Chien-Cheng Kuo, Yu-Tzu Liu, Hsin Lin, Arun Bansil We discuss two-dimensional (2D) topological insulators (TIs) based on planar Bi/Sb honeycombs on a SiC(0001) substrate using first-principles computations. The Bi/Sb planar honeycombs on SiC(0001) are shown to support a nontrivial band gap as large as 0.56 eV, which harbors a Dirac cone lying within the band gap. Effects of hydrogen atoms placed on either just one side or on both sides of the planar honeycombs are examined. The hydrogenated honeycombs are found to exhibit topologically protected edge states for zigzag as well as armchair edges, with a wide band gap of 1.03 eV and 0.41 eV in bismuth and antimony films, respectively. Our findings pave the way for using planar bismuth and antimony honeycombs as potential new 2D-TI platforms for room-temperature applications. [Preview Abstract] |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W7.00006: Hydrogenated ultra-thin tin films predicted as two-dimensional topological insulators Zhi-Quan Huang, Bo-Hung Chou, Chia-Hsiu Hsu, Feng-Chuan Chuang, Yu-Tzu Liu, Hsin Lin, Arun Bansil Using thickness-dependent first-principles electronic structure calculations, we predict that hydrogenated ultra-thin films of tin harbor a new class of two-dimensional (2D) topological insulators (TIs). A single bilayer (BL) tin film assumes a 2D-TI phase, but it transforms into a trivial insulator after hydrogenation. In contrast, tin films with 2 and 3 BLs are found to be trivial insulators, but hydrogenation of 2 to 4 BL films results in a non-trivial TI phase. For 1 to 3 BLs, H-passivation converts the films from being metallic to insulating. Moreover, we examined iodine-terminated tin films up to 3 BLs, and found these to be non-trivial, with the films becoming semi-metallic beyond 1 BL. In particular, the large band gap of 340 meV in an iodine-terminated tin bilayer is not sustained in the iodine-terminated 2BL and 3BL tin films. [Preview Abstract] |
Thursday, March 5, 2015 4:06PM - 4:18PM |
W7.00007: Real-structure influence on topological states of HgTe quantum wells: Ab initio studies Sebastian Kuefner, Friedhelm Bechstedt The electronic properties of HgTe quantum well structures are studied by means of $ab$-$initio$ calculations including spin-orbit interaction and quasiparticle effects. In agreement with earlier $\mathbf k\cdot \mathbf p$ calculations and experiments we find a topological transition from the trivial insulator into the quantum-spin Hall (QSH) state with increasing QW thickness. The QSH state is characterized by the existence of spin-polarized helical edge states bridging the fundamental gap giving rise to intrinsic spin currents. The occurrence and localization of the edge states are independent of the interface orientation and barrier material. Together with their spin polarization this indicates that they are topologically protected. The nonexistence of inversion symmetry, the atomic geometry, and the real QW barriers do not completely destroy the predictions within toy models but cause significant deviations. The deviations concern the critical thickness, the number and localization of edge states, and the possibility to find QW subbands between edge states. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W7.00008: Edge state transport in 2D topological insulators without inversion symmetry Yang-Zhi Chou, Matthew Foster We investigate finite temperature transport within one and between two edges of a 2D $Z_2$ topological insulator. For experimentally relevant systems (e.g., HgTe and GaSb/InAs quantum wells), inelastic spin flip backscattering can occur in the absence of inversion symmetry. We use bosonization and the effective action formalism to compute the dc conductivity of helical Luttinger liquid edge states in the absence of inversion symmetry, due to interactions and disorder. These perturbations manifest as irrelevant operators that control the temperature dependence of the conductivity in a single edge. With respect to two edges, the importance of the inelastic mechanism and applications to Coulomb drag will be discussed. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W7.00009: Twisting a topological phase on a lattice Meng Cheng, Yi-Zhuang You When a two-dimensional topological phase inhabits a torus, it possesses a ground state degeneracy robust to any local perturbations. These degenerate ground states can transform nontrivially under modular transformations of the torus, generated by Dehn twists. Representation of Dehn twists on the ground states characterizes the topological order. We propose that the Dehn twists can be obtained as a non-Abelian Berry phase of an adiabatic deformation of the lattice model. We apply this method to the example of a $p_x+ip_y$ superconductor and provide a TQFT interpretation of the numerical results. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W7.00010: Quantum criticality of 1D topological Anderson insulators Alex Kamenev, Dmitry Bagrets, Alex Altland We present an analytic theory, based on exact transfer-matrix solutions of super-symmtetric nonlinear sigma-models, of quantum criticality in quasi one-dimensional topological Anderson insulators. We describe these systems in terms of two parameters (g, $\chi )$ representing localization and topological properties, respectively. Certain critical values of $\chi $ (half-integer for Z classes, or zero for Z2 classes) define phase boundaries between distinct topological sectors. Upon increasing system size, the two parameters exhibit flow similar to the celebrated two parameter flow of the integer quantum Hall insulator. However, unlike the quantum Hall system, an exact analytical description of the entire phase diagram can be given. We check the quantitative validity of our theory by comparison to numerical transfer matrix computations. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W7.00011: Berry Curvature and Chiral Plasmons in Massive Dirac Materials Justin Song, Mark Rudner In the semiclassical model of carrier dynamics, quasiparticles are described as nearly free electrons with modified characteristics modified characteristics such as effective masses which may differ significantly from those of an electron in vacuum. In addition to being influenced by external electric and magnetic fields, the trajectories of electrons in topological materials are also affected by the presence of an interesting quantum mechanical field - the Berry curvature - which is responsible for a number of anomalous transport phenomena recently observed in Dirac materials including G/hBN, and MoS2. Here we discuss how Berry curvature can affect the collective behavior of electrons in these systems. In particular, we show that the collective electronic excitations in metallic massive Dirac materials can feature a chirality even in the absence of an applied magnetic field. The chirality of these plasmons arises from the Berry curvature of the massive Dirac bands. The corresponding dispersion is split between left- and right-handed modes. We also discuss experimental manifestations. [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W7.00012: First-Principles Study on Dirac Cones in a Single-Component Molecular Crystal Under High Pressure Takao Tsumuraya, Heng-Bo Cui, Hiori Kino, Tsuyoshi Miyazaki, Reizo Kato Most single-component molecular crystals show insulating or semiconducting properties at ambient pressure. Recently, metal dithiolene complexes have attracted much attention ever since a metallic state was realized in Ni(tmdt)$_2$ at ambient pressure. Even if the system is insulating at ambient pressure, it possibly turns into a metallic or superconducting state by application of pressure. In this study, we have found anisotropic linear (tilted Dirac cone) dispersions near the Fermi level in a single-component molecular crystal, Pd(dddt)$_2$ at 8 GPa by first-principles density functional theory calculations. Recent electrical resistivity at 12.6 GPa shows temperature independent behavior as is observed in the massless Dirac fermion system, $\alpha$-(BEDT-TTF)$_2$I$_3$. Our analysis of the electronic structure indicates that the band structure at ambient pressure has quasi-one-dimensional character, which corresponds to the stacking of Pd(dddt)$_2$ molecules along the $b$-axis, and the dimensionality of the band structure near the Fermi level is changed under the pressure of 8~GPa, where intermolecular hybridization increases due to the reduced intermolecular distances. We also discuss anisotropy of the Dirac cones and their possible origin in the multi-orbital system. [Preview Abstract] |
Thursday, March 5, 2015 5:18PM - 5:30PM |
W7.00013: Unified Topological Field Theory for Gapped and Gapless Systems Daniel Bulmash, Pavan Hosur, Shou-Cheng Zhang, Xiao-Liang Qi We present a scheme for systematically enumerating the responses of gapped as well as gapless systems of free fermions to electromagnetic and strain fields starting from a common parent theory. Using the fact that position operators in the lowest Landau level of a quantum Hall state are canonically conjugate, we consider a massive Dirac fermion in $2n$ spatial dimensions under $n$ mutually orthogonal magnetic fields and reinterpret physical space in the resulting zeroth Landau level as phase space in $n$ spatial dimensions. The bulk topological responses of the parent Dirac fermion, given by a Chern-Simons theory, translate into quantized insulator responses, while its edge anomalies characterize the response of gapless systems. Moreover, various physically different responses are seen to be related by the interchange of position and momentum variables. We derive many well-known responses, and demonstrate the utility of our theory by predicting spectral flow along dislocations in Weyl semimetals. [Preview Abstract] |
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