Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session E28: Topological SuperconductivityFocus
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Sponsoring Units: DMP Chair: Abhay Pasupathy, Columbia University Room: 327 |
Tuesday, March 15, 2016 8:00AM - 8:36AM |
E28.00001: Emergent surface superconductivity in a 3D topological insulator. Invited Speaker: Lia Krusin-Elbaum Surfaces of three-dimensional topological insulators have emerged as one of the most remarkable states of condensed quantum matter where exotic charge and spin phases of Dirac particles could form. This work reports on novel mesoscopic superconductivity in the topological insulator Sb$_{2}$Te$_{3}$ with transition to zero resistance induced through a minor tuning of growth chemistry that depletes bulk conduction channels [1]. The depletion shifts Fermi energy towards the Dirac point as witnessed by a factor of 300 reduction of bulk carrier density and by the largest carrier mobility (\textgreater 25, 000 cm$^{2}$V$^{-1}$s$^{-1})$ found in any topological material of this class. Direct evidence from transport, the unprecedentedly large diamagnetic screening, and the presence of $\sim $ 25 meV gaps detected by scanning tunneling spectroscopy reveal the superconducting condensate to emerge first in surface puddles at unexpectedly high temperature of $\sim $ 50 K, with the onset of global phase coherence at $\sim $ 9 K. The unconventional spin response of Sb$_{2}$Te$_{3\, }$[2] and the presence of subsurface 2DEG quantum well states arising from charge transfer to the surface [3] are likely to play a role in the emergent superconducting state. The rich structure of this state lends itself to manipulation via growth conditions and the material parameters such as Fermi velocity and mean free path. [1] Emergent surface superconductivity in the topological insulator Sb$_{2}$Te$_{3}$, L. Zhao, H. Deng, I. Korzhovska, J. Secor, M. Begliarbekov, Z. Chen, E. Andrade, E. Rosenthal, A. Pasupathy, V. Oganesyan, and Lia Krusin-Elbaum, \textit{Nature Comm.} \textbf{6}, 8279 (2015); DOI:10.1038/ncomms9279. [2] Singular robust room-temperature spin response from topological Dirac fermions, L. Zhao, H. Deng, I. Korzhovska, Z. Chen, M. Konczykowski, A. Hruban, V. Oganesyan and Lia Krusin-Elbaum, \textit{Nature Mat.} \textbf{13}, 580 (2014); DOI: 10.1038/nmat3962. [3] Robust topological interfaces and charge transfer in epitaxial Bi$_{2}$Se$_{3}$/II-VI semiconductor superlattices, Z. Chen, L. Zhao, K. Park, T. A. Garcia, M. C. Tamargo, and Lia Krusin-Elbaum, \textit{Nano Lett}. \textbf{15} (10), 6289 (2015); DOI:10.1021/acs.nanolett.5b01358. [Preview Abstract] |
Tuesday, March 15, 2016 8:36AM - 8:48AM |
E28.00002: Superconducting Proximity Effect in the Weyl Semimetal WTe2 and MoTe2 Wudi Wang, Minhao Liu, Quinn Gibson, R. J. Cava, N. P. Ong WTe2 and MoTe2 are predicted to have type-II Weyl nodes and many novel transport properties have been studied. We investigated the transport of cooper pairs and Andreev reflection in Weyl semimetals by proximitizing WTe2 and MoTe2 nanoflakes with superconducting pads (Nb and Al). We have fabricated superconductor-nanoflakes-superconductor structure with different length. Supercurrent were observed in both materials with junction length up to 700nm. We conducted dc IV curve measurements and got exotic Fraunhofer patterns. We also measured the current-phase relation with a radio frequency-based CPR measurement technique. [Preview Abstract] |
Tuesday, March 15, 2016 8:48AM - 9:00AM |
E28.00003: Superconducting proximity effect and the Fermi velocity in the surface-state of SmB$_{\mathrm{6}}$ thin films Seunghun Lee, Xiaohang Zhang, Richard L. Greene, Ichiro Takeuchi SmB$_{\mathrm{6}}$ recently has been predicted to be topological Kondo insulator. Here, we investigate the Fermi velocity ($v_{F})$ of SmB$_{\mathrm{6}}$ using transport measurements and a study on the superconducting proximity effect, independently. In the transport measurement, SmB$_{\mathrm{6}}$ thin films show thickness-independent transport characteristics at low temperatures, which is a strong evidence for the presence of the surface conducting channel as well as the insulting bulk state as the nature of Kondo insulator. We estimate the thickness of the surface-state to be $\approx $ 7 nm and the $v_{F}$ to be \textasciitilde 10$^{\mathrm{5}}$ m/s. In order to carry out the proximity effect investigation, we fabricated superconducting Nb/SmB$_{\mathrm{6}}$ bilayers \textit{in-situ. }We performed Usadel fitting to the variation of critical temperatures of the Nb layers due to the proximity effect. Interestingly, only the fitting regarding a 2D surface model yielded the consistent value of the $v_{F}$ with the value obtained from the transport measurement as well as the reported value from the quantum oscillation measurement. These results indicate that SmB$_{\mathrm{6}}$ has a true 2D surface-channel responsible for the observed proximity effect. [Preview Abstract] |
Tuesday, March 15, 2016 9:00AM - 9:12AM |
E28.00004: Probing the Superconducting Proximity Effect of Topological Insulator $\textrm{Bi}_2\textrm{Se}_3$ Using Scanning Tunneling Microscopy Ian Dayton, Nicholas Sedlmayr, Victor Ramirez, Thomas Chasapis, Reza Loloee, Alex Levchenko, Mercouri Kanatzidis, Stuart Tessmer Placing a 3D topological insulator (TI) in proximity to an s-wave superconductor is theoretically predicted to induce 2D p-wave superconductivity into the topologically protected surface state (TSS). In this talk, we will present cryogenic scanning tunneling microscopy measurements of a large $\textrm{Bi}_2\textrm{Se}_3$ crystal with nanometer scale islands of PbBi deposited on the surface. Local density of states measurements are consistent with p-wave superconductivity in the top $\textrm{Bi}_2\textrm{Se}_3$ quintuple layer, with coherence length of $540 \pm 50$ nm in the direction parallel to the layer. We see indications of a reverse proximity effect as well, where the TSS from the TI leaks back into local density of states measured on the superconducting islands. The density of states curves also exhibit structure which we interpret as McMillan-Rowell oscillations due to Andreev confinement perpendicular to the layer. [Preview Abstract] |
Tuesday, March 15, 2016 9:12AM - 9:24AM |
E28.00005: Experimental consequences of candidate pairings for superconducting topological insulators. Lei Hao, Ting-Kuo Lee, Wei-Feng Tsai, Jun Wang, Chin-Sen Ting Genuine pairing symmetry of the superconducting topological insulators, including Cu$_{\mathrm{x}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, and Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$, are under debate. In this work, we make an extensive comparison on the experimental consequences of several candidate pairings. The physical quantities studied include the surface spectral functions, the surface local density of states, and the electronic thermal conductivities. Apparent differences in the results are found and can be used to identify the actual pairing symmetry. In particular, for model and parameters used in this work which are obtained by fitting first-principles calculations, we get interesting new results such as a segmental flat band of surface Andreev bound states for a spin-singlet pairing. A combination of several experimental techniques should be able to identify the genuine pairing symmetry. [Preview Abstract] |
Tuesday, March 15, 2016 9:24AM - 9:36AM |
E28.00006: Stability of Majorana vortex bound states on the surface of superconducting topological insulators Junyi ZHANG, Jennifer Cano, Titus Neupert Fu and Kane showed that superconductivity induced on the surface of a 3D topological insulator results in isolated Majorana bound states that appear in the cores of vortices. Many efforts to realize this idea are based on proximity-induced superconducting order in a heterostructure. Recently, superconductivity has been observed in $\text{PbTaSe}_2$, which has the band topology of a topological insulator with Dirac cone surface states. Hence, it nourishes the vision of realizing the Fu and Kane proposal in a stoichiometric material without the need for doping or fabricating heterostructures. Motivated by this possibility, we give a comprehensive analysis of stability and localization properties of the vortex Majorana modes in such a topological superconducting material. In particular, we address the experimentally relevant questions regarding (i) the energy separation between the vortex bound and excited states, (ii) the dependence of the hybridization between Majorana modes from opposite surfaces on the thickness of a thin-film sample, (iii) the influence of the bulk superconducting pockets on the Majorana states. [Preview Abstract] |
Tuesday, March 15, 2016 9:36AM - 9:48AM |
E28.00007: Magnetic Ordering In Superconducting Nb-doped Bi$_{2}$Se$_{3}$ Paul Corbae, Benjamin Lawson, Gang Li, Fan Yu, Tomoya Asaba, Colin Tinsman, Yusheng Qui, yew San Hor, Lu Li Coexistence of superconductivity and magnetic order has been suggested by early studies of topological superconductor candidate, niobium doped Bi$_{2}$Se$_{3}$. In order to elucidate the interesting physics of this coexistence, we performed highly sensitive torque magnetometry to study the material’s magnetization. We observed a bump feature in the magnetization around 8 Tesla in both the superconducting and non-superconducting samples. This is distinct from the paramagnetic torque response of the parent compound, Bi$_{2}$Se$_{3}$, suggesting some interesting magnetic order in Nb-doped Bi$_{2}$Se$_{3}$. [Preview Abstract] |
Tuesday, March 15, 2016 9:48AM - 10:00AM |
E28.00008: Multi-orbits observed in superconducting Nb-doped Bi$_{2}$Se$_{3}$ Benjamin Lawson, Paul Corbae, Gang Li, Fan Yu, Tomoya Asaba, Colin Tinsman, Yunsheng Qiu, Yew San Hor, Lu Li Recently discovered superconducting niobium doped Bi$_{2}$Se$_{3}$ shows promise to realize new physical phenomenon including the coexistence of superconductivity and magnetic ordering and possibly topological superconductivity. To understand the new physics showcased in this system, a detailed knowledge of the electronic structure is needed. We present the first observation of quantum oscillations in the magnetization (the de Haas-van Alphen effect) of Nb-doped Bi$_{2}$Se$_{3}$. In the fully superconducting crystal, two distinct orbits are observed, in sharp contrast to Bi$_{2}$Se$_{3}$, Cu-doped Bi$_{2}$Se$_{3}$, and Sr-doped Bi$_{2}$Se$_{3}$. The multiple frequencies observed in our quantum oscillations, combined with our electrical transport studies, indicate the multi-orbit nature of the electronic state of Nb-doped Bi$_{2}$Se$_{3}$. [Preview Abstract] |
Tuesday, March 15, 2016 10:00AM - 10:12AM |
E28.00009: Lattice melting and surface Topological order in 3D Time reversal invariant topological superconductor YIZHI You In this talk, we start from the eight copies of 3D Time reversal invariant topological superconductor on a crystal. By proliferating and condensing the disclinations, we can therefore restore the spatial rotation symmetry of the lattice crystal. During the procedure of lattice melting, the fermion acquires a $\pi$ Berry Phase when winding around the $2\pi$ disinclination and therefore the fermions get confined when disclination loop proliferates. After the disclination condensed, we obtained a 3D bosonic SPT phase with by $T$ and spatial rotation symmetry.In addition, we investigate the surface state of this Bosonic SPT. If we break the $T$ symmetry on the surface, the 8 majorana cones are gapped and the $2\pi$ disclination has semion statistics. To obtain a $T$ and rotation invariant gapped surface state, we first turn on Fulde-Ferrell superfluid order of the surface to gap the surface fermion and then condensed the vortex/disclination of the superfluid to restore the $T$ and rotation symmetry. The disclination dipole and vortex exhibit fractional statistics and therefore vortex condensation and lattice melting give rise to new surface topological order. [Preview Abstract] |
Tuesday, March 15, 2016 10:12AM - 10:24AM |
E28.00010: Topological Superconductivity in Dirac Semimetals Masatoshi Sato, Shingo Kobayashi Dirac semimetals host bulk band-touching Dirac points and a surface Fermi loop. We develop a theory of superconducting Dirac semimetals. Establishing a relation between the Dirac points and the surface Fermi loop, we clarify how the nontrivial topology of Dirac semimetals affects their superconducting state. We note that the unique orbital texture of Dirac points and a structural phase transition of the crystal favor symmetry-protected topological superconductivity with a quartet of surface Majorana fermions. We suggest the possible application of our theory to recently discovered superconducting states in Cd$_3$As$_2$. [Preview Abstract] |
Tuesday, March 15, 2016 10:24AM - 10:36AM |
E28.00011: Superconducting states of topological surface states in $\beta$-PdBi$_2$ investigated by STM/STS Katsuya Iwaya, Kenjiro Okawa, Tetsuo Hanaguri, Yuhki Kohsaka, Tadashi Machida, Takao Sasagawa We investigate superconducting (SC) states of topological surface states in $\beta$-PdBi$_2$ using very low temperature STM. Characteristic quasiparticle interference patterns strongly support the existence of the spin-polarized surface states at the Fermi level in the normal state. A fully-opened SC gap well described by the conventional BCS model is observed, indicating the SC gap opening at the spin-polarized Fermi surfaces. Considering a possible mixing of odd- and even parity orbital functions in C$_{4v}$ group symmetry lowered from D$_{4h}$ near the surface, we suggest that the SC gap consists of the mixture of $s$- and $p$-wave SC gap functions in the two-dimensional state. [Preview Abstract] |
Tuesday, March 15, 2016 10:36AM - 10:48AM |
E28.00012: Superconductivity in Weyl metals Grigory Bednik, Anton Burkov, Alexander Zyuzin We report on a study of intrinsic superconductivity in a Weyl metal. We show that two distinct superconducting states are possible in this system in principle: a zero-momentum pairing BCS state, with point nodes in the gap function; and a finite-momentum FFLO-like state, with a full nodeless gap. We find that, in an inversion-symmetric Weyl metal the odd-parity BCS state has a lower energy than the FFLO state, despite the nodes in the gap. The FFLO state, on the other hand, may have a lower energy in a noncentrosymmetric Weyl metal, in which Weyl nodes of opposite chirality have different energy. We also discuss the anomalous Hall effect in a superconducting Weyl metal and show that it is not affected by the presence of superconductivity at low enough energies, i.e. when the Fermi surfaces is close enough to the nodes. [Preview Abstract] |
Tuesday, March 15, 2016 10:48AM - 11:00AM |
E28.00013: Topological nodal Cooper pairing in doped Weyl metals Yi Li, F. D. M. Haldane We generalize the concept of Berry connection of the single-electron band structure to the two-particle Cooper pair states between two Fermi surfaces with opposite Chern numbers. Because of underlying Fermi surface topology, the pairing Berry phase acquires non-trivial monopole structure. Consequently, pairing gap functions have the topologically-protected nodal structure as vortices in the momentum space with the total vorticity solely determined by the monopole charge q$_{p}$. The pairing nodes behave as the Weyl-Majorana points of the Bogoliubov-de Gennes pairing Hamiltonian. Their relation with the connection patterns of the surface modes from theWeyl band structure and the Majorana surface modes inside the pairing gap is also discussed. Under the approximation of spherical Fermi surfaces, the pairing symmetry are represented by monopole harmonic functions. The lowest possible pairing channel carries angular momentum number j$=$\textbar q$_{p}$\textbar , and the corresponding gap functions are holomorphic or anti-holomorphic functions on Fermi surfaces. [Preview Abstract] |
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