Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A10: Topological Matters 
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Sponsoring Units: DCMP Chair: Jason Alicea, California Polytechnic Institute Room: 007A 
Monday, March 2, 2015 8:00AM  8:12AM 
A10.00001: Composite Dirac liquids: parent states of symmetric surface topological orders Andrew Essin, David Mross, Jason Alicea In the absence of interactions, topological insulators surfaces must be gapless or break symmetry. With the addition of strong interactions at the surface, a third possibility is a gapped, symmetric surface that supports anyons, as has been recognized in a number of recent developments. The composite Dirac liquid (CDL) provides a natural stepping stone to identifying such states. The CDL consists of neutral, fractional Dirac fermions coupled to gapped charges, and the addition of pairing to the neutral sector produces a gap for all excitations without breaking any symmetry. The quasi1d technology we have used in the study of the CDL also allows us to construct and characterize such gapped surface phases, and generalizes naturally to (bosonic) symmetry protected topological phases as well. [Preview Abstract] 
Monday, March 2, 2015 8:12AM  8:24AM 
A10.00002: Composite Dirac liquids: exotic gapless states at an electronic topological insulator surface David Mross, Andrew Essin, Jason Alicea We introduce exotic gapless states `composite Dirac liquids' that can appear at a strongly interacting surface of a threedimensional electronic topological insulator. Composite Dirac liquids exhibit a gap to all charge excitations but nevertheless feature a single massless Dirac cone built from emergent electrically neutral fermions. These states thus comprise electrical insulators that preserve all symmetries and, interestingly, retain thermal properties similar to those of the noninteracting topological insulator surface. To controllably access the composite Dirac liquid we exploit a quasi1D deformation of the original electronic Dirac cone that enables us to analytically address the fate of the strongly interacting surface. [Preview Abstract] 
Monday, March 2, 2015 8:24AM  8:36AM 
A10.00003: Axion field theory and $Z_{16}$ classification of time reversal invariant topological superconductors Yingfei Gu, Xiaoliang Qi Timereversal invariant topological superconductors (TRI TSC) are gapped superconductors with topologically robust gapless modes on the boundary. In the work by X. L. Qi et al, [PRB, 87, 134519(2013)], a topological field theory description was proposed for 3+1dimensional TRI TSC, which contains an axionic coupling between superconducting phase and electromagnetic field. In my talk, I will describe a generalization of this theory to include interaction effects which provides a physical explanation why the integer classification is reduced to $Z_{16}$. I will also attempt to generalize our results to higher dimensions and give constraints on the possible collapsing of topological classification induced by interaction effects. [Preview Abstract] 
Monday, March 2, 2015 8:36AM  8:48AM 
A10.00004: Possible realization of interacting symmetryprotected topological phases in topological crystalline insulators Hiroki Isobe, Liang Fu The effects of electronelectron interaction in edge states of mirrorsymmetry protected topological crystalline insulators (TCI's) are discussed. The analysis is performed by using bosonized Hamiltonian following the TomonagaLuttinger liquid theory. When two pairs of helical edge states exist, electronelectron interaction could gap out one edge mode, which is a possible realization of interacting symmetryprotected topological (SPT) phases. This type of SPT phase is closely related to a LutherEmery liquid in spinful 1D system. We also propose a method of detecting the SPT phases by STM. The other focus of the study is the classification of SPT phases in mirrorsymmetry protected TCI's. By adopting the ChernSimons theory, we find that electronelectron interaction reduces the classification from $Z$ to $Z_4$. It means that the edge states can be gapped out when four pairs of edge states exist. In other cases, the edge modes cannot be fully gapped. Each of these states corresponds to a different SPT phase depending on the relevant interaction process. [Preview Abstract] 
Monday, March 2, 2015 8:48AM  9:00AM 
A10.00005: Topological crystalline insulators from crystal field effect in monolayer IVVI semiconductors Junwei Liu, Xiaofeng Qian, Liang Fu Twodimensional (2D) topological crystalline insulators (TCIs) were recently predicted in thin films of the SnTe class of IVVI semiconductors, which can host metallic edge states protected by mirror symmetry. As thickness decreases, quantum confinement effect will increase and surpass the inverted gap at a critical thickness, turning 2D TCIs into normal insulators. Surprisingly, based on firstprinciples calculations, here we demonstrate that (001) monolayers of rocksalt IVVI semiconductors XY (X=Ge, Sn, Pb and Y= S, Se, Te) are 2D TCIs with the fundamental band gap as large as 260 meV in monolayer PbTe. This unexpected nontrivial topological phase stems from the strong crystal field effect in the monolayer, which lifts the degeneracy between $p_{x,y}$ and $p_z$ orbitals and leads to band inversion between cation and anion $p_z$ orbitals. Our work offers a new strategy to find atomically thin 2D topological materials. [Preview Abstract] 

A10.00006: ABSTRACT WITHDRAWN 
Monday, March 2, 2015 9:12AM  9:24AM 
A10.00007: Probing Topological Superconductors David Schmeltzer The presence of attractive interaction on the surface of a 3D topological insulator which is characterized by spinors carrying a Berry phase of $\pi$ gives rise to superconductivity that support space time half vortices ( Majorana zero modes). We construct the effective dual action for the superconductor with the vortices, and show that the $2n$ Majorana fermions are localized and can be replaced with $n$ spinless fermions. The effect of the Majorana zero modes can be observed trough the the Andreev cross reflection when metallic leads are attached to the superconductor. The presence of the Majorana fermions can be detected with transverse sound waves. We have computed the effect of elastic strain fields and obtain an anomalous response indicating the presence of the Majorana fermions. [Preview Abstract] 
Monday, March 2, 2015 9:24AM  9:36AM 
A10.00008: Timereversal invariant topological superconductivity in doped Weyl semimetals Pavan Hosur, Xi Dai, Zhong Fang, XiaoLiang Qi Timereversal invariant topological superconductors are a new state of matter which have a bulk superconducting gap and robust Majorana fermion surface states. These have not yet been realized in solid state systems. In this paper, we propose that this state can be realized in doped Weyl semimetals or Weyl metals. The Fermi surfaces of a Weyl metal carry Chern numbers, which is a required ingredient for such a topological superconductor. By applying the fluctuationexchange approach to a generic model of timereversal invariant Dirac and Weyl semimetals, we investigate what microscopic interactions can supply the other ingredient, viz., sign changing of the superconducting gap function between Fermi surfaces with opposite Chern numbers. We find that if the normal state is inversion symmetric, onsite repulsive and exchange interactions induce various nodal phases as well as a small region of topological superconductivity on the phase diagram. Unlike the He3B topological superconductor, the phase here does not rely on any special momentum dependence of the pairing amplitude. Breaking inversion symmetry precludes some of the nodal phases and the topological superconductor becomes much more prominent, especially at large ferromagnetic interaction. [Preview Abstract] 
Monday, March 2, 2015 9:36AM  9:48AM 
A10.00009: Topological Crystalline Metal in Orthorhombic Perovskite Iridates Yige Chen, YuanMing Lu, HaeYoung Kee Since topological insulators were theoretically predicted and experimentally observed in semiconductors with strong spinorbit coupling, more and more attention has been drawn to topological materials which host exotic surface states. These surface excitations are stable against perturbations since they are protected by global or spatial/lattice symmetries. Succeeded in achieving various topological insulators, a tempting challenge now is to search for metallic materials with novel topological properties. Here we predict that orthorhombic perovskite iridates realize a new class of metals dubbed topological crystalline metals, which support zeroenergy surface states protected by certain lattice symmetry. These surface states can be probed by photoemission and tunnelling experiments. Furthermore, we show that by applying magnetic fields, the topological crystalline metal can be driven into other topological metallic phases, with different topological properties and surface states. [Preview Abstract] 
Monday, March 2, 2015 9:48AM  10:00AM 
A10.00010: Crystalline Topological Insulators and Superconductors  The Role of Nonsymmorphic Symmetries Daniel Varjas, Fernando de Juan, YuanMing Lu We investigate how the presence of nonsymmorphic lattice symmetries affects the classification of topological insulators and superconductors in 2 and 3 dimensions. We use the representation theory of space groups, the classification of 1D fermionic symmetry protected topological phases and analyze tight binding models that exhibit gapless surface modes protected by unitary or antiunitary space group symmetries. [Preview Abstract] 
Monday, March 2, 2015 10:00AM  10:12AM 
A10.00011: New classes of threedimensional topological crystalline insulators with unpinned surface Dirac cones Chen Fang, Timothy Hsieh, Liang Fu We theoretically predict two new classes of 3D topological crystalline insulators (TCI) that have protected, robust surface states. In first class, the surface states are protected by a \emph{single} glide mirror symmetry. On a symmetrypreserving surface, a single Dirac point can appear at any position along either one of the two mirror symmetric lines inside the surface Brillouin zone (SBZ). In the second class, the surface Dirac point is protected by a combination of twofold rotation and timereversal symmetry, and appears on the crystal surface perpendicular to the rotation axis. Its position in the SBZ is completely free to move by symmetrypreserving perturbations. In each class, we prove the existence of a $Z_2$ bulk invariant and find its explicit analytic expression. These new classes of TCI do not presume the presence or the absence of spinorbital coupling. [Preview Abstract] 
Monday, March 2, 2015 10:12AM  10:24AM 
A10.00012: Topological phase transition in thinfilm topological crystalline insulators Ai Yamakage, Hideyuki Ozawa, Masatoshi Sato, Yukio Tanaka Topological crystalline insulator is one of the recent breakthrough ideas [1], in which Dirac fermions on the surface protected by crystalline symmetry, not by timereversal symmetry. Another direction of the topological expansion is nanofabrication. In this work, we reveal the topological phase diagram of a thinfilm topological crystalline insulator Pb$_{x}$Sn$_{1x}$Te. Odd numbers of layers of Pb$_{x}$Sn$_{1x}$Te exhibit the topological phase transition between twodimensional trivial and topological crystalline insulators protected by the mirrorChern number, which is consistent with the previous work [2]. In addition, we have found a new topological phase in the even numbers of layers, which is protected by the glide symmetry [3]. This glide topological phase can be realized in the thin film not in the bulk system. \\[4pt] [1] L. Fu, Phys. Rev. Lett. 106, 106802 (2011); T. H. Hsieh, et al., Nat. Commun. 3, 982(2012); Y. Tanaka, et al., Nat. Phys. 8, 800 (2012).\\[0pt] [2] J. Liu, et al., Nat. Mater. 13, 178 (2014).\\[0pt] [3] H. Ozawa, A. Yamakage, M. Sato, and Y. Tanaka, Phys. Rev. B 90, 045309 (2014). [Preview Abstract] 
Monday, March 2, 2015 10:24AM  10:36AM 
A10.00013: Interface gapless states from interfacial symmetries Ryuji Takahashi, Shuichi Murakami Previously we have shown that at an interface between two topological insulators with opposite Dirac velocities, gapless interface states protected by mirror symmetry appear [1]. We can calculate the interface dispersion using the FuKaneMele (FKM) tightbinding model, and it typically consists of Dirac cones. In this presentation, we report another kind of interface metallic states; the Fermi surface forms loops (``Fermi loop'') [2], rather than isolated Dirac points, sometimes seen in the interface of the FKM models. Such a degeneracy along a loop is unexpected. This Fermi loop appears when the whole junction system preserves particlehole symmetry, while each system breaks particlehole symmetry. We call this symmetry ``interfacial particlehole symmetry'' (IPHS). We discuss the IPHS in general systems and show that the Fermi loop results from a sign change of a Pfaffian of some matrix, defined only in junctions with IPHS symmetry [2]. \\[4pt] [1] R. Takahashi, S. Murakami, Phys. Rev. Lett. 107,166805 (2011).\\[0pt] [2] R. Takahashi, S. Murakami, to appear in Phys. Rev. Lett.. [Preview Abstract] 
Monday, March 2, 2015 10:36AM  10:48AM 
A10.00014: Quantum anomalous Hall insulators and transitions in spinorbit coupled double perovskites Arun Paramekanti, Ashley Cook, Ciaran Hickey Motivated by the interest with strong spinorbit coupling and ferromagnetism, we study quantum anomalous Hall insulators and their quantum phase transitions in clean systems. We show that certain quantum anomalous Hall critical points support quadratic band touching points, which can lead to interaction induced emergent phases which form a dome around the critical point. This yields the simplest example of the ubiquitously observed formation of novel phases around an underlying metallic quantum critical point. We explore metallic double perovskites with high Tc ferromagnetism and strong spinorbit coupling, including Sr$_2$FeMoO$_6$, as possible candidate materials in which to explore this physics. [Preview Abstract] 
Monday, March 2, 2015 10:48AM  11:00AM 
A10.00015: Transport Signatures of Fermi Surface Topology in BiTeI Linda Ye, Joseph Checkelsky, Fumitaka Kagawa, Yoshinori Tokura The giant bulk Rashba spinorbit coupling in BiTeI makes it not only an interesting spintronics system but also a host of an effect Diraclike electronic structure of fundamental interest.~ As a function of lowering the Fermi level $E_{F} $ across the associated Dirac point ($E_{F} =0)$, the Fermi surface of BiTeI changes its topology: from a spindletorus ($E_{F} >0)$ through a horntorus ($E_{F} =0)$ to a ringtorus ($E_{F} <0)$. Here we report a systematic evolution of the magnetoresistance across these energy boundaries in high quality single crystals exhibiting Shubnikovde Haas oscillations. We further discuss the physical origin of the detailed $E_{F} $/field/temperature dependences and remark on the relevance of this study to band crossings generally occurring in 3D systems. [Preview Abstract] 
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