Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M12: Exotic Topological Superconductors II |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Dale Van Harlingen, University of Illinois Room: 007C |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M12.00001: Symbiosis of ferromagnetism and superconductivity in topological insulators Y. S. Hor, Y. Qiu, K. N. Sanders, J. E. Medvedeva, T. Vojta, J. Dai, W. Wu, P. Ghaemi Three-dimensional topological insulators (TIs) have been found to depict topological distinct phases of matter and have attracted great interest due to helical spin texture on the surfaces. By Nb-doping in bismuth selenide, the TI turns into a type-II bulk superconductor while maintaining its helical metallic surface state in its normal state. However, at high magnetic field this doped TI behaves superparamagnetically. Niobium is usually considered a non-magnetic cation but it exhibits a unique magnetic behavior in this doped TI. Moreover magnetic correlations appear in the system when the superconductivity emerges at below 3.2 K, presenting novel magnetic coupling of the Nb cations through modification of supercurrents mediated by Nb magnetic moments. As a consequence, ferromagnetism is induced in the superconducting regime below the upper critical field. Magnetic susceptibility of the TI shows paramagnetic behavior at high field due to destruction of the superconductivity which is the underlying bases for ferromagnetic coupling of Nb moments. The superconductivity and ferromagnetism, which are usually mutually destructive, can mutually benefit each other in the system giving rise to a zero-field magnetization which results in a symbiotic relationship between ferromagnetism and superconductivity. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M12.00002: High field magnetization of Half-Heusler compound LuPdBi Benjamin Lawson, Gang Li, Colin Tinsman, Fan Yu, Tomoya Asaba, Xiangfeng Wang, Johnpierre Paglione, Lu Li Topological insulators and topological superconductors are a novel phases of matter that have been an area of rich new physics in recent years. Half-Heusler compound LuPdBi has been suggested to have a topologically nontrivial phase by theoretical calculations and has been shown to have superconductivity and weak antilocalization in magneto-transport measurements. Given the promise of this compound, we preformed and report here preliminary results of magnetization measurements in LuPdBi at high magnetic fields up to 45T. [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M12.00003: Superconductivity induced by In substitution into the topological crystalline insulator Pb$_{0.5}$Sn$_{0.5}$Te Ruidan Zhong, John Schneeloch, Tiansheng Liu, Fernando Camino, John Tranquada, Genda Gu Indium substitution turns the topological crystalline insulator (TCI) Pb$_{0.5}$Sn$_{0.5}$Te into a possible topological superconductor. To investigate the effect of the indium concentration on the crystal structure and superconducting properties of (Pb$_{0.5}$Sn$_{0.5}$)$_{\mathrm{1-x}}$In$_{\mathrm{x}}$Te, we have grown high-quality single crystals using a modified floating-zone method and have performed systematic studies for indium content in the range 0 $\le $ x $\le $ 0.35. We find that the single crystals retain the rocksalt structure up to the solubility limit of indium (x $\sim $ 0.30). Experimental dependencies of the superconducting transition temperature (T$_{\mathrm{c}})$ and the upper critical magnetic field (H$_{\mathrm{c2}})$ on the indium content x have been measured. The maximum T$_{\mathrm{c}}$ is determined to be 4.7 K at x $=$ 0.30, with $\mu_{\mathrm{0}}$H$_{\mathrm{c2}}$(T $=$0) $\approx $ 5T. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M12.00004: mk-STM study of Cu$_{0.2}$Bi$_{2}$Se$_{3}$ with W and Nb Tips Rami Dana, Wan-Ting Liao, Ireneusz Miotkowski, Yong P. Chen, Michael Dreyer The Cu intercalated Bi$_{2}$Se$_{3}$ is predicted to be a time-reversal invariant topological superconductor with Majorana bound state in the vortex core. The samples are characterized by intrinsic inhomogeneity and disorder. Using mk-STM, a variety of high resolution superconducting gaps and sub-gap structures were observed. Our data from SIN and SIS junctions, using W and Nb tips on Cu$_{0.2}$Bi$_{2}$Se$_{3}$ and while applying of a magnetic field will be discussed in details. [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M12.00005: Quasiparticle Interference Patterns on the Surface of a Topological Insulator with Superconductivity Aaron Farrell, Maxime Beaudry, Tami Pereg-Barnea, Marcel Franz When the electrons on the surface of a strong topological insulator are forced to undergo superconductive pairing (perhaps via proximity effect) a topological superconducting state is formed. Such a state is of interest as it plays host to Majorana modes on its boundaries and in vortex cores. For this reason it is currently of great interest to develop a theoretical understanding of any experimental probes of such systems. In this talk we will discuss the theory of one such probe. Namely, we present results for local density of states modulations in a Dirac system with superconductivity added. These results should be relevant to measurements in Scanning Tunnelling Spectroscopy (STS) on the surface of strong topological insulators with pairing either naturally arising or driven by proximity effect. By considering a variety of different impurity scatterers and tunings of the chemical potential in the system, we discuss different probes of the underlying Dirac physics present in these patterns. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M12.00006: Topological superconductivity with unconventional pairing symmetry characterized by monopole harmonics Yi Li We study the topological structure of Cooper pairing whose symmetry is characterized by the monopole harmonic functions instead of the usual spherical harmonic functions. This pairing symmetry can be either driven by the non-trivial topology of the Fermi surface or by interaction effects. The Bogoliubov quasi-particles are nodal which can either exhibit time-reversal invariant Dirac type spectrum or the time-reversal symmetry breaking Weyl spectra with the winding numbers determined the monopole harmonic functions. The non-trivial electromagnetic responses are also discussed. These exotic pairing symmetry can be realized in 3D Weyl metal and fermion systems with magnetic dipolar interactions. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M12.00007: Low-energy phonons and superconductivity in Sn$_{.8}$In$_{.2}$Te Guangyong Xu, Zhijun Xu, John Schneeloch, Ruidan Zhong, J.A. Rodriguez-Rivera, L. Harriger, Robert Birgeneau, Genda Gu, John Tranquada We present neutron scattering measurements on low-energy phonons from a superconducting ($Tc =$ 2.7 K) Sn$_{.8}$In$_{.2}$Te single crystal sample. The longitudinal acoustic phonon mode and one transverse acoustic branch have been mapped out around the (002) Bragg peak for temperatures of 1.7 K and 4.2 K. We observe a substantial energy width of the transverse phonons at energies comparable to twice the superconducting gap; however, there is no change in this width between the superconducting and normal states. We also confirm that the compound is well ordered, with no indications of structural instability. [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M12.00008: Discovery of superconductivity in Bi$_{2}$Te: evidence of universal behavior in an infinitely adaptive series under compression Ryan Stillwell, Jason Jeffries, Scott McCall, Zsolt Jenei, Sam Weir, Yogesh Vohra The end members of the infinitely adaptive (Bi$_{2})_{\mathrm{n}}$(Bi$_{2}$Te$_{3})_{\mathrm{m}}$ series, Bi and Bi$_{2}$Te$_{3}$, can be experimentally tuned to display topological surface states or superconductivity under appropriate conditions. An intermediate member of the series, Bi$_{2}$Te has been found to superconduct under similar conditions, transitioning from a semiconductor at ambient conditions to a metallic superconductor with a maximum $T_{c} =$ 9.1 K at 14.5 GPa. This represents a move towards universal behavior in the infinitely adaptive (Bi$_{2})_{\mathrm{n}}$(Bi$_{2}$Te$_{3})_{\mathrm{m}}$ series in which all of the superstructures converge into a metallic, superconducting state at high pressure. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under contract DE-AC52-07NA27344. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M12.00009: Superconductivity near a 3-Dimensional Dirac Semimetal: Topological Crystalline Insulator (Pb$_{1-x}$Sn$_x$)$_{1-y}$In$_y$Te near the Inversion Transition Tonica Valla, I. Pletikosic, T. Yilmaz, A.P. Weber, R.D. Zhong, G.D. Gu, B. Sinkovic Superconductivity in topological insulators is expected to show very unconventional features such as $p+ip$ order parameter, Majorana fermions etc... However, so far, the intrinsic superconductivity has been observed only in Cu-intercalated Bi$_2$Se$_3$, where due to strong inhomogeneities, the pairing symmetry is still a matter of debate. Here, we show that in the topological crystalline insulator (TCI) (Pb$_{1-x}$Sn$_x$)$_{1-y}$In$_y$Te, superconductivity occurs near the gap inversion transition, when the system is nearly a 3D Dirac semimetal (DSM). The existence of superconductivity near the 3D DSM is highly unusual. We suggest that it is related to an intrinsic instability of a 3D DSM in (Pb$_{1-x}$Sn$_x$)$_{1-y}$In$_y$Te and ``flattening'' of the bulk valence (conduction) band on the inverted, TCI side of the phase diagram that favors the pairing instability if the chemical potential is pinned to these flat regions. [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M12.00010: Entanglement Entropy and Entanglement Spectrum of Disordered Topological Superconductors Jan Borchmann, Aaron Farrell, Shunji Matsuura, Tami Pereg-Barnea In a spin orbit coupled superconductor model the system may change its topology as a function of model parameters. In a clean system, this has been shown to leave a distinct signature in the entanglement entropy [1]. In the current work, we analyze the entanglement spectrum and entropy of a disordered spin-orbit coupled superconductor with either s-wave or d-wave coupling. We study the effect of disorder on the entanglement entropy and spectrum through disorder averaging. By analyzing these quantities across the phase boundary we show that even in the presence of disorder, the entanglement properties exhibit signatures of the topology. We validate our findings by directly calculating the disorder averaged topological invariant in real space. [1] J. Borchmann, A. Farrell, Shunji Matsuura and T. Pereg-Barnea, arXiv:1407.5980 [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M12.00011: One dimensional parafermionic phases and topological order Nicolas Regnault, Aris Alexandradinata, Chen Fang, Matthew Gilbert, Andrei Bernevig Parafermionic chains are the simplest generalizations of the Kitaev chain to a family of $Z_N$ -symmetric Hamiltonians. Parafermions realize topological order and they are natural extensions of Majorana fermions. We propose a less restrictive notion of topological order in 1D open chains, which generalizes the seminal work by Fendley [1]. The first essential property is that the groundstates are mutually indistinguishable by local, symmetric probes, and the second is a generalized notion of zero edge modes which cyclically permute the groundstates. These two properties are shown to be topologically robust, and applicable to a wider family of topologically-ordered Hamiltonians than has been previously considered. An an application of these edge modes, we formulate a new notion of twisted boundary conditions on a closed chain, which guarantees that the closed-chain groundstate is topological, i.e., it originates from the topological manifold of the open chain. \\[4pt] [1] P. Fendley, J. Stat. Mech., P11020 (2012). [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M12.00012: Topological aspects of Josephson pi-junctions in Kitaev wires Christian Sp{\aa}nsl\"att, Eddy Ardonne, Jan Budich, T.H. Hansson We investigate the topological properties of Josephson junctions with a phase shift of $\pi$ in one dimensional topological superconductors. We show that by imposing a reality constraint on the order parameter, the resulting zero energy junction bound state can be mapped onto a soliton solution of a Jackiw-Rebbi type Dirac equation similar to that in polyacetylene. Further, we consider $\pi$-junctions where the order parameter phase winds across the junction so that the aforementioned bound state generically acquires a gap. We relate these observations to the classification of the junctions according to their anti-unitary symmetries. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M12.00013: Topological states in normal and superconducting p-wave chains Mucio Continentino, Heron Caldas, David Nozadze, Nandini Trivedi We study a two-band model of fermions in a 1d chain with an antisymmetric hybridization that breaks inversion symmetry. We find that for certain values of its parameters, the sp-chain maps formally into a p- wave superconducting chain, the archetypical 1d system exhibiting Majorana fermions. The eigenspectra, including the existence of zero energy modes in the topological phase, agree for both models. The end states too share several similarities, such as the behavior of the localization length, the non-trivial topological index and robustness to disorder. However, we show that the excitations in the ends of a finite sp chain are conventional fermions though endowed with protected topological properties. Our results are obtained by a scattering approach in a semi-infinite chain with an edge defect treated within the T-matrix approximation. We present exact numerical diagonalization results that extend our analysis to arbitrary parameters and to disordered systems. Finally, we show that the charge stiffness has a universal value at the topological transition of the sp-chain. [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M12.00014: Surface Majorana fermions and bulk collective modes in superfluid $^3$He-$B$ Joseph Maciejko, YeJe Park, Suk Bum Chung The theoretical study of topological superfluids and superconductors has so far been carried out largely as a translation of the theory of noninteracting topological insulators into the superfluid language, whereby one replaces electrons by Bogoliubov quasiparticles and single-particle band Hamiltonians by Bogoliubov-de Gennes Hamiltonians. Band insulators and superfluids are, however, fundamentally different. In particular, unlike the static energy gap of a band insulator, the gap in a superfluid is due to a dynamical order parameter that is subject to both thermal and quantum fluctuations. We explore the consequences of bulk quantum fluctuations of the order parameter in $^3$He-$B$ on the topologically protected Majorana surface states. We find that one of the three spin-orbit Goldstone modes couples to the surface Majorana fermions, which induces an effective short-range two-body interaction between the Majorana fermions with coupling constant inversely proportional to the strength of the nuclear dipole-dipole interaction. A mean-field theory estimate of the value of this coupling suggests that the surface Majorana fermions in $^3$He-$B$ are in the vicinity of a quantum phase transition to a gapped time-reversal symmetry breaking phase. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M12.00015: Conditions for Non-Abelian Braiding in Time-Reversal Invariant Topological Superconductors Pin Gao, Vic Kam Tuen Law, Xiong-Jun Liu We study the validity of the non-Abelian braiding in 1D time-reversal invariant (DIII class) topological superconductors which host Majorana Kramer's pairs in the ends. It has been shown that braiding two Majorana Kramer's pairs can generically be reduced to two independent braiding processes of two different time-reversed copies, while the local noise or perturbations may lead to decoherence in the braiding operations. Here we examine in detail the braiding of Majorana Kramer's pairs in the presence of disordered local couplings and noises, and show the generic conditions under which the decoherence effects are negligible and the non-Abelian braiding can be validated. [Preview Abstract] |
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