Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G25: Exotic Topological Superconductivity I |
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Sponsoring Units: DCMP Chair: Hongyi Xie, Rice University Room: 203B |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G25.00001: Majorana Gun: An On-Demand Single-Majorana Fermion Source Jimmy Hutasoit, Brian Tarasinski, Denis Chevallier, Shuo Mi, Carlo Beenakker We propose a device for the Majorana fermion analog of the single-photon gun. This on-demand single-Majorana fermion source can be thought of as a generalization of the scheme proposed by Keeling \textit{et. al.} [Phys. Rev. Lett. \textbf{101}, 196404 (2008)] to a particle-hole symmetric system. In particular, for a linear voltage sweep, we show that one can obtain a single Majorana fermion regardless of the sweep velocity. [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G25.00002: Topological Pair-Density-Wave Superconducting States Rodrigo Soto Garrido, Gil Young Cho, Eduardo Fradkin We show that the pair-density-wave (PDW) superconducting state emergent in extended Heisenberg-Hubbard models in two-leg ladders is topological in the presence of an Ising spin symmetry. This topological PDW state supports a Majorana zero mode (MZM) at an open boundary and at a junction with a uniform one-dimensional $d$-wave superconductor. Similarly to a conventional finite-momentum paired state, the order parameter of the topological PDW state is a charge-$2e$ field with finite momentum, and a subleading charge $4e$ uniform SC order. However, the topological PDW order parameter here is a $quartic$ electron operator and conventional mean-field theory cannot be applied to study this state. We use bosonization to show that the 1D PDW state has a MZM at a boundary. [Preview Abstract] |
Tuesday, March 3, 2015 11:39AM - 11:51AM |
G25.00003: Accessing topological superconductivity via a combined STM and renormalization group analysis Werner Hanke, Lars Elster, Christian Platt, Ronny Thomale, Ewelina Hankiewicz The search for topological superconductors (SC) has recently become a key issue in condensed matter physics, because of their possible relevance to Majorana states, non-Abelian statistics and fault-tolerant quantum computing. A new scheme is proposed here, which links as directly as possible the experimental search to a material-based microscopic theory. To this, the scanning tunneling microscopy (STM), which typically uses a phenomenological Ansatz for the SC gap functions is elevated to a theory, where a multi-orbital functional RG (fRG) analysis allows for an unbiased microscopic determination of the material-dependent pairing potentials. The conductance spectra are predicted for a normal metal-insulator-topological superconductor (N-I-S) junction, which imitates the STM setup and, therefore, is directly accessible to spectroscopic experiments. The strength of the combined approach is demonstrated for hexagonal systems, i.e. doped graphene and water-intercalated sodium cobaltates, where the lattice symmetry and eletronic correlations can lead to a time-reversal symmetry breaking (chiral) topological SC state. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:03PM |
G25.00004: Low-Temperature Thermal Transport at the Interface of a Topological Insulator and a d-Wave Superconductor Adam Durst We consider the low-temperature thermal transport properties of the 2D proximity-induced superconducting state formed at the interface between a topological insulator (TI) and a d-wave superconductor (dSC). This system is a playground for studying massless Dirac fermions, as they enter both as quasiparticles of the dSC and as surface states of the TI. For TI surface states with a single Dirac point, the four nodes in the interface-state quasiparticle excitation spectrum coalesce into a single node as the chemical potential, $\mu$, is tuned from above the impurity scattering rate ($|\mu| \gg \Gamma_{0}$) to below ($|\mu| \ll \Gamma_{0}$). We calculate, via Kubo formula, the universal limit ($T \rightarrow 0$) thermal conductivity, $\kappa_{0}$, as a function of $\mu$, as it is tuned through this transition. In the large and small $\mu$ limits, we obtain disorder-independent, closed-form expressions for $\kappa_{0}/T$. The large-$\mu$ expression is exactly half the value expected for a dSC, a demonstration of the sense in which the TI surface topological metal is half of an ordinary 2D electron gas. Our numerical results for intermediate $\mu$ illustrate the nature of the transition between these limits, which is shown to depend on disorder in a well-defined manner. [Preview Abstract] |
Tuesday, March 3, 2015 12:03PM - 12:15PM |
G25.00005: Interface effects and electronic transport in superconducting heterostructures with spin-orbit coupling Nayana Shah, Kuei Sun Study on electronic transport in superconducting heterostructures with spin-orbit coupling (SOC) has become an active field for exploring spintronic properties or topological states of matter. For example, recently observed zero-bias conductance peaks in one-dimensional superconductor-semiconductor heterostructures in the presence of SOC and magnetic field are of interest for realizing Majorana fermions. However, most study considers SOC only in bulk materials, while the interface effects, which can significantly alter the physics in spin-active systems, have not been fully analyzed for systems with SOC. Here we construct a general model to describe interfaces in a superconductor-normal metal junction with bulk SOC. We also systematically investigate the trend of conductance as the system parameters vary. Our results are of interest for understanding a variety of transport behaviors in the presence of SOC and are of relevance also for characterizing interfaces in experimental systems. [Preview Abstract] |
Tuesday, March 3, 2015 12:15PM - 12:27PM |
G25.00006: Point defects and Majorana doublets in 3D time-reversal invariant topological superconductors and superfluids Mengxing Ye, Zheng-Cheng Gu, Kai Sun In this talk, we study topological point defects in 3D time-reversal invariant topological superconductors and superfluids, focusing on the Majorana zero modes hosted at these defects. We will address the exotic properties of these Majorana modes especially the nontrivial exchange group and its connection to fermion parity. In contrast to 2D chiral topological superconductors, in which the braiding of Majorana zero modes has been studied extensively and well understood, the time-reversal invariant topological superconductors allow richer structures at topological defects and thus result in more sophisticated response as we exchange these defects. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G25.00007: Nonmagnetic impurity effects in a superconducting topological insulator Yuki Nagai, Yukihiro Ota, Masahiko Machida Unconventional features in superconductivity are revealed by responses to impurity scattering. We study nonmagnetic impurity effects in a superconducting topological insulator, focusing on an effective model of Cu-doped topological insulator Bi$_2$Se$_3$. Typically, this superconducting compound is considered to be dirty owing to the copper intercalated process. Using a self-consistent T-matrix approach for impurity scattering, we examine in-gap states in density of states. It is well known that the unconventional superconductors such as p-wave diminish via non-magnetic impurity scattering, different from the robustness of an s-wave state (Anderson's theorem). We show that the impurity effects are well characterized by a simple material variable, which measures relativistic effects in the Dirac Hamiltonian. We find that the topological superconductor has two aspects, p- and s-wave features, depending on the weight of relativistic effects. The topological superconductors can not be simply regarded as one of the {\it conventional} unconventional superconductors. [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G25.00008: Sensitivity of a 3D fully-gapped topological superconductor to non-magnetic impurities Yukihiro Ota, Yuki Nagai, Masahiko Machida Topological superconductors (TSC) are notable materials, owing to the mathematical curiosity and the application potential. The bulk TSC can emerge by copper intercalation into topological insulator Bi$_{2}$Se$_{3}$. In this paper, we theoretically study the non-magnetic impurity effects in the mean-field model of Cu$_{x}$Bi$_{2}$Se$_{3}$, focusing on the odd-parity fully-gapped superconducting state. Calculating the density of states with a self-consistent T-matrix approach, we test the presence of mid-gap states, leading to pair-breaking effects. Remarkably, the sensitivity to non-magnetic impurities strongly depend on a normal-state dispersion character, i.e., either non-relativistic or relativistic dispersion relations. We show unification picture for understanding this intriguing result, deriving a low-energy effective superconducting theory. [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G25.00009: Crossed surface flat bands in superconducting Weyl semimetals Keiji Yada, Bo Lu, Masatoshi Sato, Yukio Tanaka Weyl semimetal is a new phase of matter where topological magnetic monopoles emerge in momentum space at so-called Weyl points.[1] Owing to the presence of the Weyl points, zero energy flat band called Fermi arc appears as a surface bound state between the Weyl points projected to the surface Brillouin Zone.[2] In this talk, we will show that the superconducting state with point nodes in doped Weyl semimetal may have more exotic surface bound states, i.e. ``crossed flat bands''.[3] With the help of the crossed flat bands, the divergent behavior of the normalized conductance at zero bias voltage appears. This divergent behavior has never been seen in other superconducting system with point nodes. We found three conditions for the realization of the crossed flat bands mentioned below. i) a uniform pairing such as BCS $s$-wave pairing, ii) broken time reversal symmetry iii) magnetic mirror reflection symmetry. [1] S. Murakami, New J. Phys. \textbf{9}, 356 (2007) [2] G. Xu et al., Phys. Rev. Lett. \textbf{107}, 186806 (2011) [3] B. Lu, K. Yada, M. Sato and Y. Tanaka, arXiv:1406.3804 [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G25.00010: Generating Giant Spin Currents by Majorana Flat Bands in Nodal Topological Superconductors Noah Fanqi Yuan, Yao Lu, James Jun He, Kam Tuen Law When a normal lead is coupled to a single Majorana fermion, electrons with certain spin polarization can undergo equal spin Andreev reflections, in which the reflected holes have the same spin as the incoming electrons. Moreover, electrons with opposite spin are all normally reflected. This process is called Majorana fermion induced selective equal spin Andreev reflections (SESARs) [1]. As a result of SASARs, spin polarized currents can be generated at the normal lead. Nevertheless, in this case, the maximum conductance of the normal lead/superconductor junction is 2e$^{2}$/h [2]. In this work, we show that Majorana fermions associated with Majorana flat bands in nodal topological superconductors can also induce SESARs. Importantly, due to the large number of Majorana fermions, the conductance at the normal lead/nodal topological superconductor junction is enormous. As a result, giant spin currents can be generated in the normal lead. Particularly, we point out that UPt$_{3}$ can be the candidate to induce giant spin currents.\\[4pt] [1] James J. He, T. K. Ng, Patrick A. Lee, K. T. Law, Phys. Rev. Lett. 112, 037001 (2014).\\[0pt] [2] K. T. Law, Patrick A. Lee, T. K. Ng, Phys. Rev. Lett. 103, 237001 (2009). [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G25.00011: Detecting Majorana Fermions in Ferromagnetic Atomic Chains in Proximity to Spin-orbit Coupled $s$-wave Superconductor Tong Zhou, Noah Fanqi Yuan, James Jun He, Yao Lu, Vic Kam Tuen Law, Patrick Lee Recently, it was proposed that Majorana bound states could be observed experimentally in ferromagnetic atomic chains on the surface of two-dimensional superconducting lead (Pb). Using the scanning tunneling microscope (STM), the experimental group obtained the spatially resolved density of states and observed zero bias peaks at the end of the chains. In this work, we study the topological phases of the proposed system. With realistic parameters, we study the transport properties at different positions along the chains. At zero temperature, resonant zero bias peaks emerge at the ends of the chains in the topological regimes, thus confirming the existence of Majorana bound states in the system. However, our transport calculation shows that at finite temperature, conductance peaks at zero bias also arise in the trivial regime due to the presence of low-energy fermionic end states in the system. Therefore, this would suggest that the experimentally observed zero bias peaks can have alternative theoretical explanations. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G25.00012: Tunneling spectroscopy and Majorana modes emergent from topological gapless phases in high-$T_c$ cuprate superconductors Chung-Yu Mou, Jun-Ting Kao, Shin-Ming Huang, Chang C. Tsuei We explore possible signatures for observing Majorana Fermions in the tunneling spectroscopy of high-Tc cuprate superconductors. We find that as long as the Rashba spin orbit interaction is in presence either through the proximity effect due to an electrode made by heavy metal or by the intrinsic nature of cuprates, in addition to the Heisenberg spin exchange interaction, the Dzyaloshinskii-Moriya and spin dipole-dipole interactions are induced. As a result, $p$-wave superconductivity is induced with the gap function $d$-vector being not aligned with the internal magnetic field of the spin-orbit interaction. Most importantly, the ground state goes through transitions into gapless phases with split nodal points. The split nodal structure always results in Majorana modes for any interfaces that are not exactly in (100) or (010) directions. Hence for general interfaces, existence of Majorana bound states is a robust feature. Our results indicate that these Majorana modes would result in a small plateau in tunneling spectrum near zero bias peak and in 4 $\pi$ periodicity in typical SIS' junctions. As a result, it is easy for a $\pi$-ring in tricrystal experiments to hold Majorana Ferimions and exhibit periods of two flux quanta in external magnetic fields. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G25.00013: Time-Reversal-Invariant Topological Superconductivity in n-type Doped BiH Fan Yang, Cheng-Cheng Liu, Yu-Zhong Zhang, Yugui Yao, Dung-Hai Lee Intrinsic and symmetry protected topological states have attracted lots of interest in condensed matter physics recently. In particular, time reversal symmetry protected fermion topological insulators have been theoretically predicted and experimentally verified. However despite considerable experimental and theoretical works, definitive evidence for time reversal invariant topological superconductivity is still lacking. Here we propose that upon electron doping the hydrogenated single bilayer Bi, namely BiH, will exhibit time reversal invariant topological superconductivity. If confirmed experimentally this material will constitute the first example of TRI topological superconductor. [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G25.00014: Vanishing edge currents and orbital angular momentum in non-$p$-wave topological chiral superconductors Wen Huang, Edward Taylor, Catherine Kallin The edge currents of two dimensional topological chiral superconductors with nonzero Cooper pair angular momentum-e.g., chiral $p$-, $d$-, and $f$-wave superconductivity-are studied. Bogoliubov-de Gennes and Ginzburg-Landau calculations are used to show that in the continuum limit, only chiral $p$-wave states have a nonzero edge current and orbital angular momentum. Outside this limit, when lattice effects become important, edge currents in non-$p$-wave superconductors are comparatively smaller, but can be nonzero. Using Ginzburg-Landau theory, a simple criterion is derived for when edge currents vanish for non-$p$-wave chiral superconductivity on a lattice. The implications of our results for putative chiral superconductors such as Sr$_2$RuO$_4$ and UPt$_3$ are discussed. [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G25.00015: Non-topological nature of the edge current in a chiral $p$-wave superconductor Edward Taylor, Wen Huang, Samuel Lederer, Catherine Kallin The edges of time reversal symmetry breaking topological superconductors support chiral Majorana bound states as well as spontaneous charge currents. The Majorana modes are a robust, topological property, but the charge currents are non-topological--and therefore sensitive to microscopic details--even if we neglect Meissner screening. We give insight into the non-topological nature of edge currents in chiral p-wave superconductors using a variety of theoretical techniques, including lattice Bogoliubov-de Gennes equations, the quasiclassical approximation, and the gradient expansion, and describe those special cases where edge currents do have a topological character. While edge currents are not quantized, they are generically large, but can be substantially reduced for a sufficiently anisotropic gap function, a scenario of possible relevance for the putative chiral p-wave superconductor $\mathrm{Sr}_2\mathrm{RuO}_4$. [Preview Abstract] |
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