Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session L7: Focus Session: Topological Superconductivity and Majorana Fermions |
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Sponsoring Units: DMP DCMP Chair: Taylor Hughes, University of Illinois Room: 006B |
Wednesday, March 4, 2015 8:00AM - 8:12AM |
L7.00001: Multiple signatures of topological phase transitions in a finite superconducting nanowire with intrinsic interactions Y.-H. Chan, Ching-Kai Chiu, Kuei Sun We study a finite chain model with spinless fermions that describes a p-wave superconducting nanowire with proximity-induced pairing gap and intrinsic interactions. By systematically tracking various physical quantities such as ground state energy, compressibility, entanglement spectrum, Cooper pair size, and pair condensate density, we obtain multi-signature of topological phase transitions between strong and weak pairing states. Some of the signatures are stable against finite-size effects. In addition, we explore the possibility of the topological transition at fixed volume, number of particles, and number of condensed pairs. The results would help explore a fundamental question: whether or not must a topological phase transition accompany with the change of extensive thermodynamic quantities? [Preview Abstract] |
Wednesday, March 4, 2015 8:12AM - 8:24AM |
L7.00002: Effect of coupling to Majorana bound states on Kondo physics in a strongly correlated quantum-dot device Tathagata Chowdhury, Kevin Ingersent Majorana bound states are non-Abelian quasiparticle excitations proposed to exist in topological phases. We employ the numerical renormalization group to study an Anderson impurity model of a strongly interacting quantum dot connected to one end of a quasi-one-dimensional topological superconductor, as well as to one or two normal metal leads. We elucidate the changing physics under variation of (1) the tunneling strength between the quantum dot and a Majorana mode localized at one end of the superconductor, and (2) the coupling between that Majorana mode and a second one localized at the opposite end of the superconductor. We characterize the many-body ground state and low-energy excitations of the system, identify the intermediate-temperature regimes and crossover scales of the problem, and determine the temperature dependence of the linear conductance. [Preview Abstract] |
Wednesday, March 4, 2015 8:24AM - 8:36AM |
L7.00003: Classification of reflection symmetry protected topological semimetals and nodal superconductors Ching-Kai Chiu, Andreas Schnyder While the topological classification of insulators, semimetals, and superconductors in terms of nonspatial symmetries is well understood, less is known about topological states protected by crystalline symmetries, such as mirror reflections and rotations. In this work, we systematically classify topological semimetals and nodal superconductors that are protected, not only by nonspatial (i.e., global) symmetries, but also by a crystal reflection symmetry. We find that the classification crucially depends on (i) the codimension of the Fermi surface (nodal line or point) of the semimetal (superconductor), (ii) whether the mirror symmetry commutes or anticommutes with the nonspatial symmetries and (iii) how the Fermi surfaces (nodal lines or points) transform under the mirror reflection and nonspatial symmetries. The classification is derived by examining all possible symmetry-allowed mass terms that can be added to the Bloch or Bogoliubov-de Gennes Hamiltonian in a given symmetry class and by explicitly deriving topological invariants. [Preview Abstract] |
Wednesday, March 4, 2015 8:36AM - 8:48AM |
L7.00004: Signatures of topological phase transition from fluctuating vortices in superconducting doped topological insulators Pedro Lopes, Pouyan Ghaemi We study the interplay between low energy vortex bound modes in superconducting doped topological insulators and dynamical fluctuations of the vortex position. We show how this interaction leads to corrections in the local density of states close to the vortex core signaling a topological vortex phase transition. We also present a detailed analysis of the low energy vortex bound modes, with analytic and numerical approximations, which may be used to access the quantities of physical interest. [Preview Abstract] |
(Author Not Attending)
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L7.00005: Quantum interference of edge supercurrents in a two-dimensional topological insulator Grigory Tkachov, Pablo Burset, Bjoern Trauzettel, Ewelina Hankiewicz Josephson weak links made of two-dimensional topological insulators (TIs) exhibit magnetic oscillations of the supercurrent that are reminiscent of those in superconducting quantum interference devices. We propose a microscopic theory of such a TI SQUID effect [1]. The key ingredient of our model is the exact treatment of the influence of an external magnetic field on the edge supercurrents. We show that this influence has the form of a 1D Doppler effect that describes the flux-controlled interference of the edge currents with superimposed suppression of Andreev reflection. Both long and short junctions are discussed. In particular, for long junctions the theory shows a temperature-driven crossover from the normal $\Phi_0$-periodic SQUID pattern to a 2 $\Phi_0$- quasiperiodic pattern consisting of a series of alternating even and odd peaks (where $\Phi_0$=ch/2e is the magnetic flux quantum). The predicted even-odd effect is the signature of gapless (protected) Andreev bound states with a sawtooth dependence on the magnetic flux. Our findings may shed some light on the recently observed even-odd interference pattern in InAs/GaSb-based TI Josephson junctions, suggesting new operation regimes for nano-SQUIDs.\\[4pt] [1] G. Tkachov, P. Burset, B. Trauzettel, and E.M. Hankiewicz, arXiv:1409.7301. [Preview Abstract] |
Wednesday, March 4, 2015 9:00AM - 9:12AM |
L7.00006: Topologically stable gapless phases in nonsymmorphic superconductors Shingo Kobayashi, Masatoshi Sato Nontrivial node structures are a salient feature in the unconventional superconductors (SCs), providing valuable clues to an understanding of the symmetry of Cooper pairs. In the presence of spin-orbital coupling, the node structures are determined by the group theory [1] where the symmetry operation in a crystal lattice is followed by spin. Such a node is stabilized by crystal symmetry. Especially, as the counterexample of the Blount's theorem, Micklitz and Norman indicated that there exists a stable line node in nonsymmorphic SCs with odd parity [2] In our previous study [3], we found that the topological classification not only includes the Blount's theorem but also updates the instability of line node via the bulk-boundary correspondence. In this talk, taking into account the nonperiodic boundary condition on a tight-binding Hamiltonian, we extend the topological node stability to nonsymmorphic SCs and show that the stable line node suggested by Micklitz and Norman is also the topological object. [1] M. Sigrist and K. Ueda, Rev. Mod. Phys. \textbf{63}, 239 (1991). [2] M. R. Norman, Phys. Rev. B \textbf{32}, 15093 (1995); T. Micklitz and M. R. Norman, \textit{ibid}. \textbf{80}, 100506(R) (2009). [3] SK, K. Shiozaki, Y. Tanaka, and M. Sato, Phys. Rev. B, \textbf{90}, 024516 (2014). [Preview Abstract] |
(Author Not Attending)
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L7.00007: Dynamical Generation of Floquet Majorana Flat Bands in s-Wave Superconductors Amrit Poudel, Gerardo Ortiz, Lorenza Viola We present techniques to dynamically engineer flat bands of symmetry-protected Majorana edge modes in s-wave superconductors. Specifically, we show how time-dependent periodic control may be employed for designing time-independent effective Hamiltonians, which support Floquet Majorana flat bands, starting from topologically trivial equilibrium conditions. In the first approach, a suitably chosen modulation of the chemical potential simultaneously induces Majorana flat bands and dynamically ``activates'' a pre-existing chiral symmetry which is responsible for their protection. In the second approach, a desired chiral symmetry is dynamically generated by suppressing a chirality-breaking term in the static Hamiltonian. In the process, we also show how a non-equilibrium topological state of matter may be reached, that has no known equilibrium counterpart. [Preview Abstract] |
Wednesday, March 4, 2015 9:24AM - 9:36AM |
L7.00008: Superconducting States in Doped Topological Materials Masatoshi Sato There are considerable interests in topological superconductivity in condensed matter physics. In this talk, I will present our recent works on topological superconductors and the related phenomena. In particular, I will discuss how topological non-trivial structures in normal states may provide non-trivial quantum phenomena in the superconducting states. As examples, I will discuss odd parity superconductors, superconducting states in doped topological insulators and Weyl semi-metals. In the latter case, I will show how synergy effects of symmetry and surface states in the normal states give rise to novel topological quantum phenomena in the superconducting states. [Preview Abstract] |
Wednesday, March 4, 2015 9:36AM - 9:48AM |
L7.00009: SNS junctions in nanowires with spin-orbit coupling: role of confinement and helicity on the sub-gap spectrum Jorge Cayao, Elsa Prada, Pablo San-Jose, Ram\'on Aguado We study normal transport and the sub-gap spectrum of superconductor-normal-superconductor (SNS) junctions made of semiconducting nanowires with strong Rashba spin-orbit coupling. We focus, in particular, on the role of confinement effects in long ballistic junctions. In the normal regime, scattering at the two contacts gives rise to two distinct features in conductance, Fabry-Perot resonances and Fano dips. The latter arise in the presence of a strong Zeeman field $B$ that removes a spin sector in the leads (\emph{helical} leads), but not in the central region. Conversely, a helical central region between non-helical leads exhibits helical gaps of half-quantum conductance, with superimposed helical Fabry-Perot oscillations. These normal features translate into distinct subgap states when the leads become superconducting. In particular, Fabry-Perot resonances within the helical gap become parity-protected zero-energy states (parity crossings, related to Yu-Shiba-Rusinov bound states), well below the critical field $B_c$ at which the superconducting leads become topological. As a function of Zeeman field or Fermi energy, these zero-modes oscillate around zero energy, forming characteristic loops, which evolve continuously into Majorana bound states as $B$ exceeds $B_c$. [Preview Abstract] |
Wednesday, March 4, 2015 9:48AM - 10:00AM |
L7.00010: Observation of topologically protected surface states in a Bi-Pd superconductor Masato Sakano, Kenjiro Okawa, Manabu Kanou, Haruhiko Sanjo, Taichi Okuda, Takao Sasagawa, Kyoko Ishizaka A layered Bi-Pd superconductor is investigated by spin- and angle-resolved photoemission spectroscopy. Beside the spin-degenerate bulk bands, several spin-polarized surface bands, some of which crossing the Fermi level, are clearly observed. These surface states are evaluated to be topologically protected, based on the Z$_{2}$ invariant analysis in analogy to 3-dimensional strong topological insulators. It indicates that this material is likely to be a topological superconductor realized without any carrier doping or applying pressure. [Preview Abstract] |
Wednesday, March 4, 2015 10:00AM - 10:12AM |
L7.00011: Surface transport coefficients for three-dimensional topological superconductors Hong-Yi Xie, Yang-Zhi Chou, Matthew Foster We argue that surface spin and thermal conductivities of three-dimensional topological superconductors are universal and topologically-quantized at low temperature. For a bulk winding number $\nu$, there are $|\nu|$ ``colors'' of surface Majorana fermions. Localization corrections to surface transport coefficients vanish due to time-reversal symmetry (TRS). We argue that Altshuler-Aronov interaction corrections vanish because TRS forbids color or spin Friedel oscillations. We confirm this within a perturbative expansion in the interactions, and to lowest order in a large-$|\nu|$ expansion. We suggest that 3D topological superconductors are a closer analog of the 2D quantum Hall effect than 3D topological insulators. [Preview Abstract] |
Wednesday, March 4, 2015 10:12AM - 10:24AM |
L7.00012: Majorana fermion from weak topological superconductivity: application to SrTiO3 and KTaO3 Suk Bum Chung, Cheung Chan, Hong Yao Much of the current experimental efforts for detecting Majorana zero modes centered on probing the boundary of quantum wires with strong spin-orbit coupling. It is possible to realize the same type of Majorana zero mode at crystalline dislocation in the 2D superconductor, which has non-zero weak topological indices. Unlike at an Abrikosov vortex, at such a dislocation, there will not be midgap states other than the Majorana zero mode that can complicate the experimental detection. We will show that, using the anisotropic dispersion of the Ti / Ta t2g orbitals, such a weak topological superconductivity can be realized when the surface 2DEG of SrTiO3 or KTaO3 becomes superconducting. [Preview Abstract] |
Wednesday, March 4, 2015 10:24AM - 10:36AM |
L7.00013: Probing Majorana-like states in quantum dots and quantum rings Igor Zutic, Benedikt Scharf Engineering topological superconductivity in semiconductor structures offers fascinating ways to obtain and study Majorana modes in a condensed matter context. Here, we theoretically investigate topological superconductivity in quantum dots and quantum rings [1]. Using both analytical as well as numerical methods, we calculate the quasiparticle excitation spectra in these structures and the corresponding excitation amplitudes and charge densities. In the topological regime, we can observe the chiral edge modes localized at the boundaries and possessing finite energy in quantum dots and quantum rings. By applying a magnetic field which is expelled from the quantum ring, but which creates a flux that is an odd integer multiple of $\Phi_0/2=\pi\hbar/e$, Majorana modes, that is, (approximately) degenerate edge modes with zero energy and zero charge density, become possible in the topological regime. Furthermore, we investigate finite-size effects that split these degenerate edge modes as well as the effect of a magnetic field penetrating into the superconducting region that can under certain circumstances still support edge modes with approximately zero energy and charge.\\ \\\quad[1] Benedikt Scharf and Igor Zutic, arXiv:1409.7735 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 10:36AM - 10:48AM |
L7.00014: Superconducting tunneling studies on thin film gold nanowires coupled to a BCS superconductor Peng Wei, Ferhat Katmis, Cui-zu Chang, Patrick Lee, Jagadeesh Moodera The nanowire patterned out of (111)-oriented gold thin film is an excellent candidate for hosting Majorana bound states (MBS) when it is coupled to an s-wave superconductor [1]. The robust MBS is guaranteed by the large Rashba spin-orbit coupling (SOC) of gold surface state, as well as by large spatial separations between the two MBS in fabricated micrometer size long nanowires. In addition, being able to produce complex nanowire circuit, our approach is better streamlined for achieving the braiding circuit of Majorana fermions.[2] We present our experimental approach of growing high quality hetero-layers consisting of epitaxial (111)-oriented gold thin film on vanadium using molecular beam epitaxy (MBE). Unique lithography processes are developed to pattern the top gold thin film into nanowires with a width around 100nm without damaging the hetero-layers such as its topography or superconducting behavior. Superconductive tunneling studies are performed over the gold nanowire using lithographically fabricated planar tunnel junctions. These tunneling characteristics will be discussed.\\[4pt] [1] A. C. Potter {\&} P. A. Lee. Phys Rev B 85, 094516 (2012) [2]T. Hyart et. al., Phys Rev B 88, 035121 (2013) [Preview Abstract] |
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