Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session M12: Topological Insulators: Topological States in Superconductors |
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Sponsoring Units: DCMP Chair: Andrew Wray, Lawrence Berkeley National Laboratory Room: 314 |
Wednesday, March 20, 2013 8:00AM - 8:12AM |
M12.00001: Engineering Majorana modes in MBE grown III-V semiconductor heterostructures Pedram Roushan, Peter O'Malley, Yu Chen, Brooks Campbell, Borzoyeh Shojaei, Javad Shabani, Brian Schultz, Chris Palmstrom, John Martinis Several theoretical proposals for realizing Majorana fermions in condensed matter systems have created much excitement and are being intensely followed by experimental groups. A common feature of all these proposals is the large size of the parameter space. We are pursuing a proposal based on coupling a semiconductor nanowire with strong spin-orbit coupling to an s-wave superconductor. Considering only the energy landscape, the size of the induced quasiparticle gap depends on the spin-orbit coupling, Zeeman energy, mobility, coupling between the two materials, and the s-wave superconducting gap. We find that Majorana modes can only be realized through carefully engineered materials. We explore this parameter space and discuss the feasibility of realizing Majorana modes based on measured parameters in our MBE grown semiconductor heterostructures. [Preview Abstract] |
Wednesday, March 20, 2013 8:12AM - 8:24AM |
M12.00002: Tunneling spectroscopy of topological superconducting states -- toward detection of Majorana fermions Wan Kyu Park, K. Coughlin, C. Wan, M. Liu, L.H. Greene, J. Schneeloch, R.D. Zhong, Z.J. Xu, G. Gu Topological insulators and superconductors have attracted much research interest recently. These materials are known to possess exotic electronic structures that cannot be adiabatically transformed to topologically trivial ones. The spin-momentum locked (helical) Dirac fermions form surface conduction bands while the bulk is insulating. When they become superconducting, charge-neutral zero-energy modes, the so-called Majorana fermion modes, are predicted to emerge due to the unique quasiparticle properties in such a superconducting state. Aiming at detect them, we investigate two novel superconducting systems using tunneling spectroscopy: i) thin film Nb which is proximity-coupled to the helical Dirac fermions in (Bi,Sb)$_{2}$Se$_{3}$; ii) (Sn,In)Te, a potential topological superconductor. Our measurements reveal unusual conductance features in the background and near zero bias. We will report results on their temperature and magnetic field dependences and discuss their implications. [Preview Abstract] |
Wednesday, March 20, 2013 8:24AM - 8:36AM |
M12.00003: Transport properties of topological superconductor-Luttinger liquid junctions Roman Lutchyn, Jacob Skrabacz Devices involving topological superconductor-Luttinger liquid junctions have been fabricated recently [1,2] to detect Majorana zero-energy modes. One of the signatures of Majoranas in such systems is the so-called ``zero-bias anomaly'' - a quantization of the tunneling conductance at zero temperature. We have developed a framework based on Keldysh formalism to study the corrections to the tunneling conductance due to finite temperature and voltage. Our results are important for understanding the experimental data. \\[4pt] [1] V. Mourik et al., Science 25 May 2012: 336 (6084);\\[0pt] [2] Das et al., arXiv:1205.7073 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 8:36AM - 8:48AM |
M12.00004: Interface currents in topological superconductor-ferromagnet junctions Philip Brydon, Carsten Timm, Andreas Schnyder Both fully gapped and nodal pairing states of noncentrosymmetric superconductors (NCS) display non-trivial topological properties, manifested by topologically protected dispersing and flat-band surface states [1,2]. Using a 2D model of an NCS, we show that the surface states typically have strong spin-polarization $s_{\mu=x,z}(k_y)$, which is odd in the surface-Brillouin-zone momentum $k_y$. Upon placing the NCS in proximity contact with a ferromagnet, the coupling to the exchange field gives a perturbative correction to the energy of these states $\propto s_\mu(k_y)$, thus generating an interface charge current $\propto \partial_{k_y}s_\mu(k_y)$ in the NCS. This is most clearly realized in a nodal NCS, where the weak dispersion acquired by the singly degenerate zero-energy flat bands leads to a strong enhancement of the interface current at low temperatures. We argue that this effect is a ``smoking-gun'' signature of the singly degenerate flat bands.\\[4pt] [1] A. P. Schnyder and S. Ryu, Phys. Rev. B {\bf 84}, 060504(R) (2011).\\[0pt] [2] P. M. R. Brydon, A. P. Schnyder, and C. Timm, Phys. Rev. B {\bf 84}, 020501(R) (2011); A. P. Schnyder, P. M. R. Brydon, and C. Timm, Phys. Rev. B {\bf 85}, 024522 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 8:48AM - 9:00AM |
M12.00005: Majorana fermions in spin-singlet nodal superconductors with coexisting non-collinear magnetic order Ziqiang Wang, Yuan-Ming Lu Realizations of Majorana fermions in solid state materials have attracted great interests recently in connection to topological order and quantum information processing. We propose a novel way to create Majorana fermions in superconductors. We show that an incipient non-collinear magnetic order turns a spin-singlet superconductor with nodes into a topological superconductor with a stable Majorana bound state (MBS) in the vortex core or on the edge. Moreover the topologically-stable point defect of non-collinear magnetic order also hosts a zero-energy MBS. We argue that such an exotic non-Abelian phase can be realized in extended $t$-$J$ models on the triangular and square lattices. Our proposal suggests a new avenue for the search of Majorana fermions in correlated electron materials where nodal superconductivity and magnetism are two common caricatures. [Preview Abstract] |
Wednesday, March 20, 2013 9:00AM - 9:12AM |
M12.00006: Tuning between s-wave and p-wave superconductors as well as emerging Majorana fermions in extended Hubbard lattices Kuei Sun, Ching-Kai Chiu, Jiansheng Wu We study spin-half fermions in one dimensional extended Hubbard lattices in which the superconducting pairing orders are induced by the tuning of nearest-neighbor charge and spin interactions. We derive gap equations for three p-wave (triplet) as well as one s-wave (singlet) pairing orders and obtain a phase diagram characterizing these orders as a function of interaction couplings. We find that the system can evolve between s-wave and p-wave pairing states, accompanied with the emergence of Majorana fermions in the p-wave regime, identified as a time-reversal invariant Kitaev Majorana chain. Finally we discuss the effects on the topological non-trivial states when time-reversal or SU(2) symmetry breaks. [Preview Abstract] |
Wednesday, March 20, 2013 9:12AM - 9:24AM |
M12.00007: Robustness of Majorana modes in multiband topological superconductors Shusa Deng, Gerardo Ortiz, Lorenza Viola We investigate the robustness of Majorana modes in a multiband topological superconductor model belonging to symmetry class DIII, against various perturbations. In the three dimensional case, we find that in topological phases where an even number of Kramer pairs of Majorana modes exist on each boundary, these modes may become gapped under a boundary perturbation, despite time-reversal invariance being preserved. Conversely, in two dimensions, the gapless Majorana modes may remain gapless in the presence of certain time-reversal breaking fields or impurities. However, upon changing the strength of an applied longitudinal Zeeman field, a transformation from helical Majorana modes to chiral Majorana modes may be induced, accompanied by a quantum phase transition in the bulk. [Preview Abstract] |
Wednesday, March 20, 2013 9:24AM - 9:36AM |
M12.00008: Majorana end modes in STM Fabricated Atomic Chains on the Surface of a Superconductor: Theory \& Experiment Stevan Nadj-Perge, Ilya Drozdov, Jungpil Seo, Andrei Bernevig, Ali Yazdani The search for Majorana fermions (MF) in solid state devices has been hampered by the possible affects of disorder which may induce signatures similar to those expected by novel MF boundary states. Therefore it is important to identify clean solid state systems in which MF modes can be easily distinguished from disorder related effects. In this talk, we will present theoretical calculations and preliminary experimental results on chains of magnetic atoms on the surface of an s-wave superconductors. The theoretical efforts show that surprisingly short magnetic chains (20 atoms long or more) support MF under specific conditions depending on spins of the magnetic atoms and their coupling. We will describe these theoretical results along with experiments in which a scanning tunneling microscopy (STM) has been used to assemble chains of magnetic atoms (3d transition metals) on Nb and Pb single crystals. Presence of Majorana boundary modes in these structures can be probed using spatially-resolved STM spectroscopy. [Preview Abstract] |
Wednesday, March 20, 2013 9:36AM - 9:48AM |
M12.00009: Topological defects and subgap excitations in two-band superconductors Kirill Samokhin, Michelle Przedborski Phase solitons are topological defects peculiar to two-band superconductors, which are associated with a $2\pi$ winding of the relative phase of the two superconducting condensates. The order parameter phase variation in each of the bands leads to the quasiparticle bound states whose energies are below the bulk gap. We calculate the single soliton energy as well as the interaction energy of two solitons, at arbitrary temperature. Applications to a similar system -- one or more domain walls in a chiral $p$-wave superconductor -- are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 9:48AM - 10:00AM |
M12.00010: Symmetry Protected Majorana fermions in topological superconductors Masatoshi Sato Recently, there are considerable interests in Majorana fermions in topological superconductors. It has been found that promising schemes to realize Majorana fermions is to break some of symmetries of the system. Indeed, by inducing the spin-orbit interaction and the Zeeman coupling which break inversion and time-reversal symmetries, conventional s-wave superconductors may support Majorana fermions on the boundaries. Moreover, by breaking the spin-rotation symmetry, spin-triplet superconductors may support Majorana fermions. Therefore, one might expect that symmetry is an obstruction to detect Majroana fermions. In this talk, however, we will show that this is not always the case. We show that symmetry may protect Majorana fermions in topological superconductors. As an example, we will show that Majorana Ising charater , which gives a detectable signal of Majorana fermion , is stabilized by symmetry of the system. We will also discuss some other roles of symmetry for Majorana fermions in topological crystalline superconductors. [Preview Abstract] |
Wednesday, March 20, 2013 10:00AM - 10:12AM |
M12.00011: Majorana fermions in 3DTI with superconductivity Pedro Lopes, Pouyan Ghaemi, Shinsei Ryu We study the problem of a strong 3D topological insulator (TI) with intrinsic superconductivity (SC). Particularly we present microscopic calculations using a low energy model of bulk massive Dirac fermions with mean field s-wave SC pairing. Introducing a kink in the mass in one spatial direction we can verify the appearance of localized (around the kink) states which correspond to the TI surface states and, with the further introduction of a vortex in the SC pairing, we are able to bind Majorana zero-modes (MZM's). The MZM's are known to be elusive particles in the sense that they are hard to detect. We then introduce a Majorana representation to the system Hamiltonian described above and propose an artificial doubling of this system which gives rise to a O(2) symmetry and allows us to define a conserved charge that can be used to probe for the presence of the MZM's. This doubled Majorana system then becomes an interesting playground, allowing us to search for masses which mix the different Hilbert spaces and study the behavior of this charge. We finish with a path-integral formulation of the problem through which we can integrate out the fermions and find an effective action for both, the electromagnetic as well as the corresponding to the O(2) conserved charge, gauge fields. [Preview Abstract] |
Wednesday, March 20, 2013 10:12AM - 10:24AM |
M12.00012: Josephson-Majorana cycle in topological single-electron hybrid transistors Nicolas Didier, Marco Gibertini, Ali G. Moghaddam, Juergen Koenig, Rosario Fazio Charge transport through a small topological superconducting island in contact with a normal and a superconducting electrode occurs through a cycle which involves coherent oscillations of Cooper pairs and tunneling in/out the normal electrode through a Majorana bound state, the Josephson-Majorana cycle. We illustrate this mechanism by studying the current-voltage characteristics of a superconductor - topological superconductor - normal metal single-electron transistor. At low bias and temperature the Josephson-Majorana cycle is the dominant mechanism for transport. We discuss a three-terminal configuration that constitutes a direct probe of the non-local character of the Majorana bound states. Non-local cotunneling dominates over the local contributions and the current noise is maximally correlated independently of the length of the wire. Preprint: arXiv:1202.6357 [Preview Abstract] |
Wednesday, March 20, 2013 10:24AM - 10:36AM |
M12.00013: Superconducting Klein tunneling and AC Josephson effect in superconductor/topological insulator/superconductor junctions Ewelina Hankiewicz, Grigory Tkachov We consider superconductor(S)/surface state of topological insulator(TI)/superconductor junctions (S) where the S regime describes the surface state of the TI with the proximity with the s-wave superconductor. The novelty of such S/TI/S junctions originates from the electron spin helicity (locking of the mometum and the spin for a surface of TIs) which leads to both the s-wave singlet and the p-wave triplet pairing on the surface underneath the superconductor. Existence of these two superconducting channels lead to interesting features in transport through these junctions. In particular we show that superconducting Klein tunneling and topological Andreev bound state (ABS) (state of hybridized two Majorana fermions)) occur for the normal incidence where ABS is protected against backscattering. For transport channels different than for the normal incidence, the scattering from the junction barrier generates an energy gap in the spectrum supporting non-topological ABSs. Due to mixed order parameter, the AC Josephson effect is fractional showing higher odd harmonics. We conclude that favorable conditions for the observation of the topological ABS exist in narrow TI links with a small number of open channels close to one. [Preview Abstract] |
Wednesday, March 20, 2013 10:36AM - 10:48AM |
M12.00014: Josephson currents through topological insulator surfaces Jens H. Bardarson, Roni Ilan, Heung-Sun Sim, Joel E. Moore Motivated by recent experiments carried out on superconductor -- 3D topological insulator -- superconductor junctions, we study the transport properties of these junctions. Transport is believed to be dominated by the surface states of the topological insulator, and we discuss the effects of the junctions geometry on the Josephson supercurrent in the presence of a magnetic field. [Preview Abstract] |
Wednesday, March 20, 2013 10:48AM - 11:00AM |
M12.00015: X-ray absorption spectroscopy of doped Bi2Se3 and Bi2Te3 Jessica McChesney, Richard Rosenberg, Duck Young Chung, Mercouri Kanatzidis Topological insulators are a prototypical system to investigate correlated electron physics. Analogous to quantum hall states, these remarkable materials have conducting surface/edge states surrounding an insulating in the bulk state. Unlike quantum hall systems the conducting states of topological insulators do no arise from an applied magnetic field but instead emerge as a result of spin-orbit interactions. Furthermore, doping with different 3d-metals can significantly alter the electronic structure, inducing superconductivity in the case of CuxBi2-xSe3, and ferromagnetism in Bi2-xMnxTe3. In an effort to elucidate the role of the local bonding environment on the electronic structure in the chalchogenide topological insulators, Bi2Te3 and Bi2Se3 with various transition metal as dopants, we have preformed a series of soft x-ray absorption spectroscopy measurements. [Preview Abstract] |
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