Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session R13: New Materials for Topological Superconductivity |
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Sponsoring Units: DMP Chair: Gleb Finkelstein, Duke University Room: LACC 304A |
Thursday, March 8, 2018 8:00AM - 8:12AM |
R13.00001: Superconductivity in the t-J model on the square lattice Hong-Chen Jiang , Zheng-Yu Weng , Steven Kivelson We report a density-matrix renormalization group study of lightly hole-doped t-J model on the square lattice. We observe an interesting interplay between d-wave superconductivity and spin and charge density wave (CDW) order. Despite the tendency of CDW order is strong, our results on finite-width cylinders strongly suggest that the ground state of the system is a Luther-Emery liquid with gapped spin excitation. There is no long-range CDW order no matter its pattern is commensurate or incommensurate. Moreover, we find that the superconducting pair-pair correlation decays power-law with an exponent close to one. The connection of our results to two dimensions is also discussed. |
Thursday, March 8, 2018 8:12AM - 8:24AM |
R13.00002: Critical current and vortex properties in FeS_{1-x}Se_{x} (x = 0, 0.06) Aifeng Wang , Lijun Wu , V. N. Ivanovski , J. B. Warren , Jianjun Tian , Yimei Zhu , Cedomir Petrovic We report the critical current, vortex pinning mechanism, and irreversibility field in FeS_{1-x}Se_{x} (x = 0, 0.06) single crystals. Critical current in FeS is comparable to that in K_{x}Fe_{2-y}Se_{2} with much higher T_{c}. Fitting reduced field dependence of pinning force with Dew-Hughes model indicates that pinning center is core-normal surfacelike. Reduced temperature dependence of critical current indicates that dominant interaction of vortex cores and pinning centers is via scattering of charge carriers with reduced mean free path (δl). Crossover behavior in field dependence of depinning energy U_{0} was observed. The proximity of the irreversibility lines to the upper critical field (H_{c2}) is consistent with strong pinning in FeS and FeS_{0.94}Se_{0.06}. Large critical current and strong pinning suggest that FeS can serve as building blocks for new superconductors with high critical current. |
Thursday, March 8, 2018 8:24AM - 8:36AM |
R13.00003: Thermally activated flux flow in pristine and proton-irradiated Ca_{8.5}La_{1.5}(Pt_{3}As_{8})(Fe_{2}As_{2})_{5} single crystals Woo-Jae Choi , Yuil Seo , Dawood Ahmad , YongSeung Kwon We investigate the influence of the proton irradiation on the thermally activation flux flow (TAFF) on the mixed state in Ca_{8.5}La_{1.5}(Pt_{3}As_{8})(Fe_{2}As_{2}) single crystals using the magnetoresistivity in magnetic fields up to 13T for B//c-axis and B//ab-plane. We found that the effect of thermally activated flux flow is well described by the nonlinear temperature-dependent activation energy in both pristine and proton irradiated sample. The magnetic field dependence of the activation energy follows a power law of U_{0}(B)∼B^{−}^{α} where the exponent α is changed from crossover field of around 3T, indicating that the strength of vortex pinning is weaker as a result of proton irradiation. We suggest that the pristine and proton irradiated sample dominate 2D vortex phase in the TAFF region. |
Thursday, March 8, 2018 8:36AM - 8:48AM |
R13.00004: Vacancy Disorder in K_{x}Fe_{2-y}Se_{2} Iron Based Superconductor Chunruo Duan , Junjie Yang , Yang Ren , Despina Louca The potassium doped iron selenide K_{x}Fe_{2-y}Se_{2} superconductor has been intensively studied since its discovery because of phase separation driven by the Fe vacancy order. Above the structural phase transition the system has randomly distributed Fe vacancies. Below, the system phase separates into the √5×√5×1 Fe vacancy order phase with space group I4/m, and the Fe vacancy free phase with space group I4/mmm. Post-annealing and quenching can increase the sample’s Meissner shielding fraction by orders of magnitude. We studied the structures of two K_{x}Fe_{2-y}Se_{2} single crystals, one quenched and superconducting and the other as-grown and non-superconducting, using X-ray diffraction carried out at 11-ID-C at the Advanced Photon Source. The structure of the quenched crystal shows a partially ordered I4/m phase while the as-grown sample has fully ordered I4/m phase. A Monte-Carlo simulation was performed to simulate the effects of thermal treatment on Fe vacancy distribution. The simulation revealed that the high temperature I4/mmm phase can be frozen during quenching, leaving the I4/m phase mixed with vacancy disorder I4/mmm phase. We propose that this vacancy disorder phase is the superconducting phase of K_{x}Fe_{2-y}Se_{2}. |
Thursday, March 8, 2018 8:48AM - 9:00AM |
R13.00005: The Mechanism of Electrolyte Gating on High-T_{c} Cuprates: the Role of Oxygen Migration and Electrostatics Shengwei Zeng , Ariando Ariando , Thirumalai Venkatesan Electrolyte gating is widely used to induce large carrier density modulation on solid surfaces to explore various properties. However, its mechanism is still being debated, and it is therefore vital to reveal the relationship between the role of electrolyte gating and the intrinsic properties of materials. Here, we report entirely different mechanisms of electrolyte gating on two high-T_{c} cuprates, NdBa_{2}Cu_{3}O_{7-δ} (NBCO) and Pr_{2-x}Ce_{x}CuO_{4} (PCCO), with different crystal structures. We show that field-induced oxygen vacancy formation in CuO chains of NBCO plays the dominant role while it is mainly an electrostatic field effect in the case of PCCO. The possible reason is that NBCO has mobile oxygen in CuO chains while PCCO does not. Our study helps clarify the controversy relating to the mechanism of electrolyte gating, leading to a better understanding of the role of oxygen electro migration which is very material specific. |
Thursday, March 8, 2018 9:00AM - 9:12AM |
R13.00006: Searching for topological superconductors in bismuthides CanLi Song , Yan-Feng Lv , Wen-Lin Wang , Yi-Min Zhang , XUCUN MA , Qikun Xue The search for Majorana fermions in solid state system is one of paramount research targets in physics today. By growing bismuth-containing intermetallic films with state-of-the-art molecular beam epitaxy, we have demonstrated the topological superconducting phase on some of these binaries. Unusual zero bias conductance peaks, possibly from the Majorana zero modes, are also revealed by directly probing quasiparticle density of states within the vortex cores under magnetic field with a cryogenic scanning tunneling microscope. Our findings has pointed to a novel avenue for searching topological pairing states in ordinary superconductors with heavy elements. |
Thursday, March 8, 2018 9:12AM - 9:24AM |
R13.00007: Andreev reflection spectroscopy on Bi_{2}X_{3} (X = Se, Te) topological insulators: Implications for the c-axis superconducting proximity effect Chris Granstrom , Igor Fridman , Hechang Lei , Cedomir Petrovic , John Wei To understand the superconducting proximity effect that occurs across the c-axis interface of |
Thursday, March 8, 2018 9:24AM - 9:36AM |
R13.00008: Magnetization study of superconducting Nb-intercalated topological insulator $Nb_xBi_2Se_3$ Ying Wang , Aleksandr Kazakov , Seng Huat Lee , Neil Dilley , Leonid Rokhinson , Yew San Hor Niobium-intercalated topological insulator $Bi_2Se_3$ is an odd-parity p-wave superconductor.Among different possible pairing mechanisms proposed for this material there is a spin-triplet pairing which would result in topologically non-trivial superconductivity with gapped bulk and gapless Andreev surface states.One of the signatures of this topological state is spontaneous time-reversal symmetry breaking and appearance of residual magnetization in the superconducing state.Here we report magnetization studies of $Nb_xBi_2Se_3$ with Nb atomic ratios x = 0.25 and 0.27.Magnetization hysteresis loops display shift of the central peak in the direction of the field scan,indicating flux trapping.No shift of the central peak in the opposite direction is observed,indicating that magnetization in the system is negligibly small.This observation is consistent with recent reports on the spatial anisotropy of the order parameter,which suggests the presence of nematic order parameter and topologically-trivial superconductivity. |
Thursday, March 8, 2018 9:36AM - 9:48AM |
R13.00009: Non-Helical Spin Textures and Proximity-Induced Superconductivity in Topological Insulators David Alspaugh , Mahmoud Asmar , Daniel Sheehy , Ilya Vekhter We study how interface potentials present at the boundary between a topological insulator (TI) and a superconductor (SC) affect the proximity-induced superconductivity of the TI surface state. We construct an effective Hamiltonian for the TI-SC interface states and analyze how the spin-momentum locking of these states, which reflects the reduced symmetries of the interface, influences the induced topological superconductivity. We consider TIs coupled to both spin-singlet and spin-triplet SC's, and show that the type of superconducting order induced in the TI's surface state sensitively depends on the symmetries which are broken at the TI-SC boundary. We identify the conditions necessary for the proximity coupling to different parent SCs. To consider the experimental consequences of such interface potentials we calculate the dispersion and the density of states for the Andreev edge states, the spin currents carried by these states, and the tunneling conductance spectra for parent SCs with several qualitatively different order parameters. |
Thursday, March 8, 2018 9:48AM - 10:00AM |
R13.00010: STM Probe of the Superconductor/Topological Insulator Interface Eric Goodwin , Michael Gottschalk , Nicholas Sedlmayr , Erik Huemiller , Can Zhang , Dale Van Harlingen , Alex Levchenko , Stuart Tessmer Interfacing a 3D topological insulator with a superconductor has been shown to induce superconductivity into the topological insulator material. In this talk, we will present cryogenic scanning tunneling microscopy measurements done on epitaxially grown Bi2Se3 with various thicknesses of superconducting Nb films deposited on top. In this system, we see indications of the topologically-protected surface states leaking into the superconductor -- evidence for the existence of an inverse topological proximity effect (ITPE). Specifically, both the superconducting energy gap feature and the Dirac cone are superposed in the electronic density of states. Theoretical modeling of the ITPE will be presented. |
Thursday, March 8, 2018 10:00AM - 10:12AM |
R13.00011: Electronic states of possible topological superconductor derived from hole-doped topological insulator Chi Trang , Zhiwei Wang , Daichi Takane , Kosuke Nakayama , Seigo Souma , Takafumi Sato , Takashi Takahashi , Alexey Taskin , Yoichi Ando Topological superconductors (TSCs) have become a research frontier in the study of topologically ordered electronic states of matter. Although there has been a considerable amount of research to realize TSCs, the relationship between the electronic states and the topological superconductivity is under intensive debate. Recently, a new platform of TSCs was discovered in thallium (Tl)-doped Bi_{2}Te_{3}. This material shows the superconductivity of T_{c} = 2.28 K with the SC volume fraction as high as 95% [1]. To elucidate the topological superconductivity from the viewpoint of electronic states, we have performed angle-resolved photoemission spectroscopy (ARPES) of Tl_{x}Bi_{2}Te_{3} (TBT) (x = 0.5), and determined the electronic band structure of bulk and surface states [2]. |
Thursday, March 8, 2018 10:12AM - 10:24AM |
R13.00012: Helical Gap Signatures in Nanowire Devices by Solving the 3D Schroedinger-Poisson Nonlinear Equation Pablo Perez-Piskunow , Adriaan Vuik , Anton Akhmerov , Michael Wimmer We present an efficient framework for obtaining the conductance signatures of the helical gap. By simulating a nanowire quantum point contact (QPC) in an electrostatic environment, we can predict the geometrical and physical parameters that can guide experiments [1]. |
Thursday, March 8, 2018 10:24AM - 10:36AM |
R13.00013: Tip-induced Superconductivity Coexisting with Preserved Topological Properties in Line-nodal Semimetal ZrSiS Leena Aggarwal , Chandan Singh , Mohammad Aslam , Ratnadwip Singha , Arnab Pariari , Sirshendu Gayen , Mukul Kabir , Prabhat Mandal , Goutam Sheet ZrSiS was recently shown to be a new material with topologically non-trivial band structure which exhibits multiple Dirac nodes and a robust linear band dispersion up to an unusually high energy of 2 eV. Such a robust linear dispersion makes the topological properties of ZrSiS insensitive to perturbations like carrier doping or lattice distortion. We have shown that a novel superconducting phase with a remarkably high T_{c} of 7.5K can be induced in single crystals of ZrSiS by a non-superconducting metallic tip of Ag or Pt. From first-principles calculations, we have also shown that the observed superconducting phase might originate from dramatic enhancement of density of states due to the presence of a metallic tip on ZrSiS and that the emerging tip-induced superconducting phase co-exists with the well preserved topological properties of ZrSiS. |
Thursday, March 8, 2018 10:36AM - 10:48AM |
R13.00014: Selective Area Epitaxy of Semiconductor-Superconductor Nanowires Stephen Gill , John Jeffrey Damasco , Malcolm Durkin , Sasa Gazibegovic , Erik Bakkers , Nadya Mason Semiconductor nanowires such as InAs and InSb are promising material for studying Majorana zero-modes and demonstrating non-abelian particle exchange relevant for topological quantum computing. While evidence for Majorana bound states in nanowires has been shown, the majority of these experiments are marked by significant disorder. We take the hybrid InSb nanowire/superconductor platform a step further by demonstrating selective-area epitaxy of Al to InSb nanowires. Epitaxial InSb-Al devices generically demonstrate ballistic 1D superconductivity and a hard superconducting gap, requisites for engineering 1D topological superconductivity. Additionally, we investigate Majorana Islands based on epitaxial InSb-Al nanowires and the dependence of even-odd Coulomb oscillations as a function of island length and magnetic field. |
Thursday, March 8, 2018 10:48AM - 11:00AM |
R13.00015: Reflection symmetric second-order topological insulators and superconductors Josias Langbehn , Yang Peng , Luka Trifunovic , Felix von Oppen , Piet Brouwer Second-order topological insulators are crystalline insulators with a gapped bulk and gapped crystalline boundaries, but topologically protected gapless states at the intersection of two boundaries. Without further spatial symmetries, five of the ten Altland-Zirnbauer symmetry classes allow for the existence of such second-order topological insulators in two |
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