Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M41: Topological Superconductors: Bulk and Interfaces |
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Sponsoring Units: DCMP Chair: Wei-Feng Tsai, University of Taiwan Room: Mile High Ballroom 3C |
Wednesday, March 5, 2014 11:15AM - 11:27AM |
M41.00001: Topological Superconductivity and the Strong Coupling Expansion Aaron Farrell, Tami Pereg-Barnea Due to the nontrivial winding of their order parameter phase, topological superconductors are expected to support Majorana Fermions in their vortex cores and for this reason have been an area of intense interest over the past couple decades. Current proposals for a device that may support Majorana Fermions are based on semiconductor heterostructures, where pairing is driven by proximity to a normal superconductor. We have recently found mean field results supporting the existence of topological superconductivity in a model with spin-orbit coupled electrons and pairing induced by interactions rather than proximity effect. This talk will look at developing methods to treat the opposite limit of this model, that of strongly coupled electrons. Results of a strong coupling expansion and the development of an analogy to the Gutzwiller approximation will be presented as well as data from renormalized mean field theory. [Preview Abstract] |
Wednesday, March 5, 2014 11:27AM - 11:39AM |
M41.00002: Topological superconductivity at the edge of transition metal dichalcogenides Gang Xu, Jing Wang, Binghai Yan, Xiao-Liang Qi Time-reversal breaking topological superconductors are new states of matter which can support Majorana zero modes at the edge. In this paper, we propose a new realization of one-dimensional topological superconductivity and Majorana zero modes. The proposed system consists of a monolayer of transition metal dichalcogenides MX$_2$ (M=Mo, W; X=S, Se) on top of a superconducting substrate. Based on first-principles calculations, we show that a zigzag edge of the monolayer MX$_2$ terminated by metal atom M has edge states with strong spin-orbit coupling and spontaneous magnetization. By proximity coupling with a superconducting substrate, topological superconductivity can be induced at such an edge. We propose NbS$_2$ as a natural choice of substrate, and estimate the proximity induced superconducting gap based on first-principles calculation and low energy effective model. As an experimental consequence of our theory, we predict that Majorana zero modes can be detected at the 120$^\circ$ corner of a MX$_2$ flake in proximity with a superconducting substrate. [Preview Abstract] |
Wednesday, March 5, 2014 11:39AM - 11:51AM |
M41.00003: Fully gapped topological surface states in Bi$_2$Se$_3$ films induced by a $\textit{d}$-wave high-temperature superconductor Hao Ding, Eryin Wang, Alexei V. Fedorov, Wei Yao, Zhi Li, Yan-Feng Lv, Kun Zhao, Li-Guo Zhang, Zhijun Xu, John Schneeloch, Ruidan Zhong, Shuai-Hua Ji, Lili Wang, Ke He, Xucun Ma, Genda Gu, Hong Yao, Qi-Kun Xue, Xi Chen, Shuyun Zhou The interplay of superconductivity and topological surface states which are protected by time-reversal symmetry provides a platform for exploring new quantum phenomena, such as Majorana zero modes that may find application in fault-tolerant quantum computation. Here, by growing high-quality topological insulator Bi$_2$Se$_3$ films on a $\textit{d}$-wave superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) using molecular beam epitaxy, we are able to induce high-temperature superconductivity on the surface states of Bi$_2$Se$_3$ films with a large pairing gap up to 15 meV. Interestingly, distinct from the $\textit{d}$-wave pairing of Bi2212, the proximity-induced gap on the surface states is nearly isotropic and consistent with predominant $\textit{s}$-wave pairing as revealed by angle-resolved photoemission spectroscopy. Our work could provide a critical step towards the realization of the long sought Majorana zero modes. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M41.00004: Symmetry-protected topological invariant and Majorana impurity states in time-reversal invariant superconductors Bernd Rosenow, Lukas Kimme, Timo Hyart We address the question of whether individual nonmagnetic impurities can induce zero-energy states in time reversal invariant topological superconductors, and define a class of symmetries which guarantee the existence of such states for a specific value of the impurity strength [1]. These symmetries allow the definition of a position space topological $Z_2$ invariant, which is related to the standard bulk topological $Z_2$ invariant. Our general results are applied to the time reversal invariant $p$-wave phase of the doped Kitaev-Heisenberg model [2], where we demonstrate how a lattice of impurities can drive a topologically trivial system into the non-trivial phase. Finally, signatures of impurity states in the spin-susceptibility are described.\\[4pt] [1] L.~Kimme, T.~Hyart, and B.~Rosenow, arXiv:1308.2496, (2013).\\[0pt] [2] T.~Hyart, A.R.~Wright, G.~Khaliullin, and B.~Rosenow, Phys. Rev. B 85, 140510(R) (2012). [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M41.00005: Topological Blount's theorem of odd-parity superconductors Shingo Kobayshi, Ken Shiozaki, Masatoshi Sato, Yukio Tanaka Nontrivial nodal structures are one of the most salient features of gap functions of the unconventional superconductors. In a system with spin-orbit coupling and crystal field, the group theory plays a key role to determine the node of the gap function [1]. From the group theoretical ground, Blount proved that the line node is ``vanishingly improbable'' in spin-triplet superconductors [2]. Namely, it is impossible to create a stable line node in odd-parity superconductors. Our motivation is to compare the group theoretical result with topological stability of nodes by K-theory [3] As a result, we found that K-theory not only rebuilds the original Blount's argument but also exhibits counterexamples with the stable line node. In this talk, we will show the physical interpretation of them. [1] M. Sigrist and K. Ueda, Rev. Mod. Phys. \textbf{63}, 239 (1991). [2] E. I. Blount, Phys. Rev. B, \textbf{32}, 2935 (1985). [3] P. Horava, Phys. Rev. Lett. \textbf{95}, 016405 (2005); Y. X. Zhao and Z. D. Wang, Phys. Rev. Lett. \textbf{110}, 240404 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M41.00006: Quantum oscillations of $Cu_{x}Bi_{2}Se_{3}$ in intense magnetic field Benjamin Lawson, Gang Li, Tomoya Asaba, Fan Yu, Ziji Xiang, Colin Tinsman, Y.S. Hor, Lu Li Quantum oscillations are generally studied to resolve the electronic structure of topological insulators. Recently there has been much interest in resolving the Fermi Surface of $Cu_{x}Bi_{2}Se_{3}$ to shed light on the nature of its superconducting state - in particular to determine if it is a topological superconductor, an exotic class of material. Using torque magnetometry, quantum oscillations in magnetization (the de Haas--van Alphen effect) were observed [1] in $Cu_{x}Bi_{2}Se_{3}$ up to 90 degrees in polar angle with respect to the sample surface. The doping of Cu in $Bi_{2}Se_{3}$ increases the carrier density and its ellipsoidal Fermi Surface becomes increasingly elongated. The detailed study of the temperature dependence at different tilt angles reveals strong effective mass anisotropy. The comparison of oscillation data in magnetization with that in magnetoresistance [2] helps elucidate the electronic structure of this interesting material. \\[4pt] [1] B.J. Lawson, Y.S. Hor, Lu Li, Phys. Rev. Lett. {\bf 109}, 226406 (2012).\\[0pt] [2] E. Lahoud, {\it et al.}, Phys. Rev. B {\bf 88}, 195107 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M41.00007: Double Berry monopoles and topological surface states in the superconducting B-phase of UPt$_3$ Andriy Nevidomskyy, Pallab Goswami The recent phase sensitive measurements in the superconducting B-phase of UPt$_3$ provide strong evidence for the triplet, chiral $k_z(k_x \pm ik_y)^2$ pairing symmetries, which endow the Cooper pairs with orbital angular momentum projections $L _z= \pm 2$ along the c-axis. Such pairing can support both line and point nodes, and both types of nodal quasiparticles possess nontrivial topology in the momentum space. We show that the point nodes located at the intersections of the closed Fermi surfaces with the c-axis, act as the double monopoles and the antimonopoles of the Berry curvature, and generalize the notion of Weyl quasiparticles. Consequently, the B phase should support an anomalous thermal Hall effect, various magnetoelectric effects such as the polar Kerr effect, in addition to the protected Fermi arcs on the (1,0,0) and the (0,1,0) surfaces. The line node at the Fermi surface equator acts as a vortex loop in the momentum space and gives rise to the zero energy, dispersionless Andreev bound states on the (0,0,1) surface. At the transition from the B-phase to the A-phase, the time reversal symmetry is restored, and only the nodal ring survives inside the A-phase. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M41.00008: Absence of zero-energy surface bound states in Cu$_{\mathrm{x}}$Bi$_{2}$Se$_{3}$ via a study of Andreev reflection spectroscopy Haibing Peng, Debtanu De, Bing Lv, Fengyan Wei, Ching-Wu Chu Cu$_{x}$Bi$_{2}$Se$_{3}$ has been proposed as a potential topological superconductor characterized by an odd-parity full bulk superconducting gap and zero-energy surface Andreev bound states (Majorana fermions). A predicted consequence of such Majorana fermions is a peak in the zero-energy density of states which should lead to a persistent zero-bias-conductance-peak (ZBCP) in Andreev reflection (AR) or tunneling experiments. Here we employ a newly developed nanoscale AR spectroscopy method to study normal metal/superconductor (N-S) devices featuring Cu$_{x}$Bi$_{2}$Se$_{3}$. The results show that a ZBCP can be tuned in or out from Cu$_{x}$Bi$_{2}$Se$_{3}$ samples depending on the N-S barrier strength. While the appearance of ZBCP may be traced to different origins, its absence under finite barrier strength represents the absence of zero-energy Majorana fermions. The present observations thus call for a reexamination of the intriguing superconductivity in Cu$_{x}$Bi$_{2}$Se$_{3}$. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M41.00009: Point Contact Spectroscopy in Half-Heusler Compounds Steven Ziemak, Rongwei Hu, Yasuyuki Nakajima, Paul Syers, Johnpierre Paglione The half-Heusler family of compounds have been predicted to exhibit topologically non-trivial behavior. Some members, including YPtBi and LuPtBi, exhibit superconductivity, suggesting the possibility of a topological superconductor. We have performed soft point contact spectroscopy measurements on the superconducting half-Heusler compound YPtBi to investigate gap structure and density of states as a function of temperature and magnetic field. We report properties of the conductivity spectra and discuss implications of the superconducting gap features for interpretation of the nature of superconductivity in these compounds. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M41.00010: Fully gapped topological surface states in Bi$_2$Se$_3$ films induced by a $d$-wave high-temperature superconductor Eryin Wang, Hao Ding, Wei Yao, Xi Chen, Qi-Kun Xue, Shuyun Zhou, Hong Yao, Alexei V. Fedorov Topological insulators are a new class of materials which are insulating in bulk but exhibit robust conducting surface states protected by time-reversal symmetry. The coupling between such symmetry-protected surface states and symmetry-broken states (for example, superconductivity) may lead to novel quantum phenomena, such as Majorana zero modes which are crucial for fault-tolerated quantum computation. Using molecular beam epitaxy, we have successfully grown high quality topological insulator Bi$_2$Se$_3$ films on high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. In this talk, I will present our recent work on superconducting proximity effect in Bi$_2$Se$_3$ films induced by high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. Using angle-resolved photoemission spectroscopy, we observe a proximity-induced gap up to 15 meV on the topological surface states of Bi$_2$Se$_3$ [1]. Bi$_2$Se$_3$/Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ heterostructure not only provides new opportunities for investigating the intriguing coupling between a topological insulator thin film and a d-wave superconductor, but also may be a new system for realizing Majorana zero modes. \\[4pt] [1] Eryin Wang et.al, {\it Nature Physics.}{\bf 9}, 621 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M41.00011: Normal state electrodynamics of superconducting Cu$_x$Bi$_2$Se$_3$ Kenneth Burch, Luke Sandilands, Anjan Reijnders, Markus Kriener, Yoichi Ando Using infrared spectroscopy, we have studied the bulk electronic structure of superconducting Cu$_x$Bi$_2$Se$_3$ (x = 0.15, 0.2, 0.3, 0.4), a candidate topological superconductor. The screened plasma frequency is observed to red-shift monotonically with doping, ranging from 198 meV for x = 0.15 to 156 meV for x = 0.4. We have also investigated the effects of electron-boson coupling in this compound. An extended Drude analysis of the free carrier charge dynamics suggests a mass enhancement $m^*/m_b$ of roughly 1.2 for x = 0.2. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M41.00012: Point-contact spectroscopy study of the pairing symmetry of candidate topological superconductors Xunchi Chen, Chao Huan, Yew San Hor, Satoshi Sasaki, Mario Novak, Kouji Segawa, Yoichi Ando, Carlos Sa de Melo, Zhigang Jiang The recently proposed topological superconducting materials are predicted to have odd parity paring and host Majorana fermions on the surface. Here we investigate the pairing symmetry of candidate topological superconductors, including CuxBi2Se3, Sn1-xInxTe, etc., via point-contact spectroscopy. The measurements are performed using both normal-metal gold tips and s-wave superconducting niobium tips. For samples with s-wave pairing, one would expect standard Andreev reflection in gold tip case and supercurrent-like behavior in niobium tip case. For CuxBi2Se3, however, we observe robust zero-bias conductance peak (ZBCP) in the differential conductance spectra with gold point contact, while with niobium point contact we find the height of the peak exhibiting an unusual non-monotonic temperature dependence. We argue that both observations cannot be explained by Andreev reflection within the standard BTK model, but signifying unconventional superconductivity in this material. For Sn1-xTnxTe samples, we observe ZBCP in the differential conductance spectra with gold point contact, while with niobium point contact, the temperature dependence of ZBCP is monotonic as expected from conventional theory, leaving the nature of the superconductivity of Sn1-xTnxTe still an open question. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M41.00013: Proximity effect in the 3D topological insulator Bi2Te3 Zhuo Wang, Tianyu Ye, Ramesh Mani Topological insulators (TI) are electronic materials with a bulk band gap that is supplemented by protected conducting states on their edges or surfaces in the 2- and 3- dimensional cases, respectively. This study reports the magnetotransport response observed in the 3D topological insulator Bi$_{2}$Te$_{3}$ with indium superconducting electrodes, and demonstrates two critical transitions in the magnetoresistive response with decreasing temperatures below T$=$ 3.4K. Here, the first transition is attributed to superconductivity in the In electrodes, as the second transition is attributed to the proximity effect in this hybrid TI/SC structure. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M41.00014: Classification of Two Dimensional Topological Crystalline Superconductors and Majorana Bound States at Disclinations Wladimir Benalcazar, Jeffrey Teo, Taylor Hughes We classify discrete-rotation symmetric topological crystalline superconductors (TCS) in two dimensions and provide the criteria for a zero energy Majorana bound state (MBS) to be present at composite defects made from magnetic flux, dislocations, and disclinations. In addition to the Chern number that encodes chirality, discrete rotation symmetry further divides TCS into distinct stable topological classes according to the rotation eigenspectrum of Bogoliubov-de Gennes quasi-particles. Conical crystalline defects are shown to be able to accommodate robust MBS when a certain combination of these bulk topological invariants is non-trivial as dictated by the index theorems proved within. The number parity of MBS is counted by a $Z_2$-valued index that solely depends on the disclination and the topological class of the TCS. We also discuss the implications for corner-bound Majorana modes on the boundary of topological crystalline superconductors [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M41.00015: Study of the Topological Crystalline Insulator, SnTe and Sn-In-Te systems in the form of nanomaterials M. Saghir, M.R. Lees, S.J. York, G. Balakrishnan We have studied the Topological Crystalline Insulator SnTe and the superconducting variant which arises due to the partial substitution of Sn atoms with indium, Sn$_{\mathrm{1-x}}$In$_{\mathrm{x}}$Te, in this rock salt structure. The observable topological features are thought to be enhanced by increasing the surface area to volume ratio of the materials and therefore suppressing contributions from the bulk. We report the first evidence for the growth of SnTe and SnInTe nanowires starting from bulk crystals. The nanowires produced are typically 20 $\mu$m in length and 20 nm wide. The stoichiometries of these structures have been determined to compare with that of the source material. Various morphologies of nanomaterials are observed and the optimal conditions and processes involved to obtain these are discussed. [Preview Abstract] |
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