Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F43: Topological Superconductivity: Theory |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Sasha Balatsky, Los Alamos National Laboratory Room: Mile High Ballroom 4B |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F43.00001: Detecting Perfect Transmission in Josephson Junctions on the Surface of Three Dimensional Topological Insulators Jens H. Bardarson, Roni Ilan, H.-S. Sim, Joel E. Moore We consider Josephson junctions on surfaces of three dimensional topological insulator nanowires. We find that in the presence of a parallel magnetic field, short junctions on nanowires show signatures of a perfectly transmitted mode capable of supporting Majorana fermions. Such signatures appear in the current-phase relation in the presence or absence of the fermion parity anomaly, and are most striking when considering the critical current as a function of flux $\Phi$, which exhibits a peak around $\Phi = h/2e$. The peak sharpens in the presence of disorder at low but finite chemical potentials, and can be easily disentangled from weak-antilocalization effects. The peak also survives at small but finite temperatures, and represents a realistic and robust hallmark for perfect transmission and the emergence of Majorana physics inside the wire. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F43.00002: Proximity-induced unconventional superconductivity in topological insulators Annica Black-Schaffer, Alexander Balatsky We study proximity-induced superconducting pairing in a three-dimensional topological insulator - superconductor hybrid structure for superconductors with different pairing symmetries. The Dirac surface state in the topological insulator gives rise to a coupling between spin-singlet and spin-triplet pairing amplitudes as well as pairing that is odd in frequency for $p$-wave superconductors. We also find that all superconductors induce pairing that is odd in both frequency and orbital (band) index, with a complete reciprocity between pairing in orbital index and frequency. We show that the different induced pairing amplitudes significantly modify the density of states in the TI surface layer. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F43.00003: Dirac Fermion induced Parity Mixing in Superconducting Topological Insulators Takeshi Mizushima, Ai Yamakage, Masatoshi Sato, Yukio Tanaka We self-consistently study surface states of the recently discovered superconducting topological insulator Cu$_{\mathrm{x}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$. We demonstrate that if a topologically trivial bulk s-wave paring symmetry is realized, parity mixing of pair potential near the surface is anomalously enhanced by surface Dirac fermions, opening an additional surface gap larger than the bulk one. Contrary to classical s-wave superconductors, the resulting surface density of state hosts an extra coherent peak at the induced gap besides a conventional peak at the bulk gap. In contrast, no such a surface parity mixing is induced by Dirac fermions for topological odd-parity superconductors. Our calculation suggests that a simple U-shaped spectrum of scanning tunneling microscope is not originated from s-wave superconductivity of Cu$_{\mathrm{x}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F43.00004: Anomalous Topological Pumps and Fractional Josephson Effects Fan Zhang, C.L. Kane We discover novel topological pumps in the Josephson effects for superconductors. The phase difference, which is odd under the chiral symmetry defined by the product of time-reversal and particle-hole symmetries, acts as an anomalous adiabatic parameter. These pumping cycles are different from those in the ``periodic table,'' and are characterized by $Z \times Z$ or $Z_2 \times Z_2$ strong invariants. We determine the general classifications in class AIII, and those in class DIII with a single anomalous parameter. For the $Z_2 \times Z_2$ topological pump in class DIII, one $Z_2$ invariant describes the coincidence of fermion parity and spin pumps whereas the other one reflects the non-Abelian statistics of Majorana Kramers pairs, leading to three distinct fractional Josephson effects. For the $Z \times Z$ topological pump in class DIII, Weyl or/and Dirac fermions appear in the Andreev spectrum. [arXiv:1310.5281] [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F43.00005: 4pi periodic Josephson current through a Quantum Spin-Hall edge Jan Dahlhaus, Carlo Beenakker, Dmitry Pikulin, Timo Hyart, Henning Schomerus The helical edge state of a quantum spin-Hall insulator can carry a supercurrent in equilibrium between two superconducting electrodes (separation L, coherence length ?). We calculate the maximum (critical) current Ic that can flow without dissipation along a single edge, going beyond the short-junction restriction L?? of earlier work, and find a dependence on the fermion parity of the ground state when L becomes larger than ?. Fermion-parity conservation doubles the critical current in the low-temperature, long-junction limit, while for a short junction Ic is the same with or without parity constraints. This provides a phase-insensitive, dc signature of the 4?-periodic Josephson effect. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F43.00006: Ac Josephson Effect in Topological Josephson Junctions Julia Meyer, Driss Badiane, Manuel Houzet, Leonid Glazman Topological superconductors admit zero-energy Majorana bound states at their boundaries. In Josephson junctions between two topological superconductors, the presence of these states gives rise to an Andreev bound state whose energy varies $4\pi$-periodically in the superconducting phase difference. An applied voltage bias leads to a dynamically varying phase according to the Josephson relation. Furthermore, it leads to dynamics of the occupation of the bound state via its non-adiabatic coupling to the continuum. While the Josephson relation suggests a fractional Josephson effect due to the $4\pi$-periodicity of the bound state, its observability relies on the conservation of the occupation of the bound state on the experimentally probed time scale. We study the lifetime of the bound state and identify the time scales it has to be compared to. In particular, we are interested in signatures of the fractional Josephson effect in the Shapiro steps and in current noise measurements. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F43.00007: Dynamics of Axion Vortices Akihiro Tanaka, Toru Kikuchi, Muneto Nitta Vortes lines of axion fields - axion strings - can arise in a variety of topological states of matter, e.g. in Weyl semimetals, topological superconductors, and dislocations in topological insulators. We derive the forces exterted on such axion vortices in analogy to the forces which a vortex in motion within a superfluid experiences. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F43.00008: Topological properties of possible singlet chiral superconducting states for URu$_2$Si$_2$ Pallab Goswami, Luis Balicas We show that the current thermodynamic measurements in the superconducting phase of URu$_2$Si$_2$ are compatible with two distinct singlet chiral paired states $k_z(k_x \pm i k_y)$ and $(k_x \pm i k_y)^2$. Despite possessing similar low temperature thermodynamic properties, these two pairings are topologically distinguished by their respective orbital angular momentum projections along the c-axis, $m= \pm 1$ and $m= \pm 2$. The point nodes of these states act as the charge-m monopoles and anti-monopoles of the Berry's gauge flux, which are separated in the momentum space along the c-axis, and the Berry's flux through the ab plane equals m. These topologically nontrivial point nodes, give rise to m copies of protected spin degenerate, chirally dispersing surface states on the ca and the cb planes, which carry surface current, and their energies vanish at the Fermi arcs. The Berry's flux through the ab plane gives rise to anomalous spin and thermal Hall conductivities, and various magnetoelectric effects. However, the clear determination of the bulk invariant can only be achieved by probing the pairing symmetry via a corner Josephson junction measurement, and Fourier resolved surface sensitive measurements of the Fermi arcs. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F43.00009: Robust Transport Signatures of Topological Superconductivity in Topological Insulator Nanowires Fernando de Juan, Roni Ilan, Jens H. Bardarson Finding a clear signature of topological superconductivity in transport experiments remains, to this date, an outstanding challenge. In this work, we propose to exploit the unique properties of nanowires made from three-dimensional topological insulators to generate a normal-superconductor junction in the single-mode regime, where an exactly quantized $2e^2/h$ zero-bias conductance can be observed over a wide range of realistic system parameters. Magnetic fields allow to reach the single mode regime in the normal part and to tune into and out of the topological regime in the superconducting region independently. The measurement proposed is insensitive to disorder, and the quantization of conductance survives at finite temperatures. Our proposal may be understood as an experimentally feasible variant of a Majorana interferometer. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F43.00010: Quantum anomalous Hall effect and novel topological superconductivity realized in thin-film SnTe Chen Fang, Matthew Gilbert, B. Andrei Bernevig The newly discovered topological crystalline insulator SnTe has surface states described by four Dirac cones protected by mirror symmetries. The properties of these novel topological surface states make realizing new topological phases possible. Here we propose that by using magnetic dopants and utilizing piezoelectric deposition, an anomalous Hall state can be obtained with Chern number tunable between $\pm4$ in thin-film SnTe. In another proposal, we propose that by proximity induced superconductivity in the thin film, a new kind of topological superconductivity can be obtained which hosts two Majorana states at a single superconducting vortex of unit flux, protected from hybridization (gapping) by magnetic group symmetries. The results are extended to other topological crystalline insulators. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F43.00011: Isotropic 3D f-wave topological Cooper pairing Wang Yang, Yi Li, Congjun Wu We generalize the 3D isotropic p-wave spin triplet Cooper pairing state of the $^3$He-B type into even high orbital partial-wave channels with large-spin fermions. In the spin-3/2 case, the $f$-orbital partial wave channel can support a spin-septet pairing yielding a fully gapped rotationally invariant pairing structure. Its topological properties are analyzed through the calculation of the gapless surface spectra. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F43.00012: Systematic Construction of tight-binding Hamiltonians for Topological Insulators and Superconductors Dong-Ling Deng, Sheng-Tao Wang, Lu-Ming Duan A remarkable discovery in recent years is that there exist various kinds of topological insulators and superconductors characterized by a periodic table according to the system symmetry and dimensionality. To physically realize these peculiar phases and study their properties, a critical step is to construct experimentally relevant Hamiltonians which support these topological phases. We propose a general and systematic method based on the quaternion algebra to construct the tight binding Hamiltonians for all the three-dimensional topological phases in the periodic table characterized by arbitrary integer topological invariants, which include the spin-singlet and the spin-triplet topological superconductors, the Hopf and the chiral topological insulators as particular examples. For each class, we calculate the corresponding topological invariants through both geometric analysis and numerical simulation. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F43.00013: Superconducting Proximity Effect in Topological Metal Kyungmin Lee, Abolhassan Vaezi, Mark H. Fischer, Eun-Ah Kim Much interest in the superconducting proximity effect in 3D topological insulators (TI) has been driven by the potential to induce exotic pairing states at the interface surface. However most candidate materials for 3D TI's are in fact bulk metals, due to the presence of bulk conduction states at the Fermi level. Nevertheless, such systems can have well-defined surface states exhibiting robust spin-momentum locking when the doping level is low enough. For such topological metals (TM), superconducting proximity effect can be qualitatively different from that in TI's. By studying a model topological metal-superconductor (TM-SC) heterostructure within Bogoliubov-de Gennes formalism, we show that the pairing amplitude is not confined to the interface as it is in topological insulator-superconductor (TI-SC) heterostructure and rather it reaches the naked surface. Furthermore, we predict vortex bound state spectra to contain a Majorana zero mode localized at the naked surface, separated from the bulk vortex bound state spectra by a finite gap in such a TM-SC heterostructure. Such naked-surface-bound Majorana modes are amenable to experimental observation and manipulation and hence present experimental advantage of TM-SC structure over TI-SC structure. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F43.00014: Symmetry breaking in topological insulators and high temperature superconductors Gayanath Fernando, Kalum Palandage, Kun Fang, Armen Kocharian We study symmetry breaking in three dimensional topological insulators due to various magnetic and nonmagnetic 3d transition metal dopants such as Cr and V. In addition, variational cluster approximation based Hubbard-Rashba systems are studied in order to identify effects of correlations on various geometries. Spin-Hall effect in small clusters, such as ladders with various boundary conditions, is also addressed. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F43.00015: Signatures of topological superconductivity in quantum spin Hall/superconductor junctions Shu-Ping Lee, Karen Michaeli, Jason Alicea, Amir Yacoby Interfacing s-wave superconductors with quantum spin Hall systems provides a highly favorable route to topological superconductivity and Majorana zero-modes. Indeed, once a proximity effect is successfully induced, topological superconductivity emerges very naturally -- tuning of the chemical potential in the quantum spin Hall system is unnecessary, and moreover disorder effects are greatly suppressed since time-reversal symmetry breaking is not required. The ability to implement such systems raises fundamental questions; for instance, how can one definitively expose the topological superconducting phase experimentally? We provide a possible answer by studying long Josephson junctions in quantum spin Hall systems. In particular, we predict fingerprints of topological superconductivity related to the ``fractional Josephson effect'' that, remarkably, survive even in the presence of parity relaxation processes. [Preview Abstract] |
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