Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T36: Ruthenates, Iridates, and p-wave Superconductivity |
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Sponsoring Units: DCMP Chair: Harald Jeschke, Universitat Frankfurt Room: 344 |
Thursday, March 21, 2013 8:00AM - 8:12AM |
T36.00001: Theoretical study of novel superconductivity in Ir oxides with large spin-orbit coupling Hiroshi Watanabe, Tomonori Shirakawa, Seiji Yunoki Recently, the 5$d$ transition metal oxide Sr$_2$IrO$_4$ has attracted much attention. In this material, three $t_{2g}$ orbitals of Ir atoms are hybridized with each other by the spin-orbit coupling of 5$d$ electrons. As a result of the quantum entanglement of spin and orbital degrees of freedom, an anomalous $J_{\mathrm{eff}}$=$|L-S|=1/2$ state is realized, which causes interesting properties. To clarify the properties of this system, we have studied the ground state of the three-orbital Hubbard model with a spin-orbit coupling term using variational Monte Carlo method. Here, we study the electronic states when carriers are doped in this three-orbital system and discuss the possibility of superconductivity. The obtained ground state phase diagram reveals the antiferromagnetic state, stable around the electron density $n=5$, is destabilized by carrier doping and the ground state turns to be superconducting under a certain condition. Similar to the high-$T_{\mathrm{c}}$ cuprates, a large asymmetry between electron doping ($n>5$) and hole doping ($n<5$) is also observed. Due to the large spin-orbit coupling, the spin is no longer a good quantum number. Instead, the pseudospins form a Cooper pair and a $d_{x^2-y^2}$-wave ``pseudospin-singlet'' superconductivity is realized. [Preview Abstract] |
Thursday, March 21, 2013 8:12AM - 8:24AM |
T36.00002: Observation of strong spin-orbital entanglement in Sr$_2$RuO$_4$ Andrea Damascelli, C.N. Veenstra, Z.-H. Zhu, B. Ludbrook, A. Nicolaou, M. Raichle, I.S. Elfimov, M.W. Haverkort, B. Slomski, G. Landolt, J.H. Dil, S. Kittaka, Y. Maeno Sr$_2$RuO$_4$ stands out even amongst the unconventional superconductors. The relativistic spin orbit interaction causes a momentum dependent entanglement of orbital and spin quantum numbers. Using circularly polarized light combined with spin and angle resolved photoemission spectroscopy, we directly observe this entanglement in good agreement with relativistic band-structure calculations. The presence of spin-charge entangled states inherently has a profound influence on the description of the superconducting state. These entangled states are not well described by a product of an orbital and spin wave-function, thereby blurring the distinction between triplet and singlet states. [Preview Abstract] |
Thursday, March 21, 2013 8:24AM - 8:36AM |
T36.00003: Dislocations and the enhancement of superconductivity in odd-parity superconductor Sr$_2$RuO$_4$ Yiqun Ying, Neal Staley, Xinxin Cai, Ying Liu, Yan Xin, Kai Sun, David Fobes, Tijiang Liu, Zhiqiang Mao We investigated the 3-K phase of spin-triplet, odd-parity superconductor Sr$_2$RuO$_4$, which was usually referred to the eutectic phase of Ru and Sr$_2$RuO$_4$ featuring Ru islands embedded in single crystalline Sr$_2$RuO$_4$. Using single-crystal flakes of Sr$_2$RuO$_4$ of mesoscopic size free of Ru, we observed an enhancement of superconducting transition temperature ($T_c$) up to about twice of that of the bulk when lattice dislocations were found in the samples, a surprising result given the well known sensitivity of superconductivity in Sr$_2$RuO$_4$ to disorder. We formulated a phenomenological theory taking into account the crystalline as well as the pairing symmetry of Sr$_2$RuO$_4$ and showed that the enhanced $T_c$ can be attributed to symmetry reduction in superconductors with a two-component order parameter. We found that our experimental results are consistent with the theoretical predictions. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 8:48AM |
T36.00004: Numerical study of the stability of half-quantum vortices in superconducting Sr$_2$RuO$_4$ Kevin Roberts, Raffi Budakian, Michael Stone We numerically solve the coupled Landau-Ginzburg-Maxwell equations for a model of a $p_x+ip_y$ superconductor in which whole or half-quanta of flux threads through a hole. We recover the pattern of stable and unstable regions for the half-flux observed in the experiments of Jang et al [1].\\[4pt] [1] J. Jang, et al, Observation of half-height magnetization steps in Sr$_2$RuO$_4$, \textit{Science}, \textbf{331}, 186-188(2011) [Preview Abstract] |
Thursday, March 21, 2013 8:48AM - 9:00AM |
T36.00005: Unravelling the Surface-to-Bulk Progression of the Electronic Structure in Sr$_2$RuO$_4$ Christian N. Veenstra, Z.-H. Zhu, B. Ludbrook, M. Capsoni, G. Levy, A. Nicolaou, J.A. Rosen, R. Comin, I.S. Elfimov, A. Damascelli, S. Kittaka, Y. Maeno We revisit the normal-state electronic structure of Sr$_2$RuO$_4$ by angle-resolved photoemission spectroscopy (ARPES) with improved data quality, as well as ab-initio band structure calculations in the local-density approximation (LDA) with the inclusion of spin-orbit coupling (SO). We find that the current model of a single surface layer $(\sqrt{2} \times \sqrt{2})$R45$^{\circ}$ reconstruction does not explain all detected features. The observed depth-dependent signal degradation, together with the close quantitative agreement with LDA+SO slab calculations based on the surface crystal structure as determined by low-energy electron diffraction (LEED), reveal that -- at a minimum -- the subsurface layer also undergoes a similar although weaker reconstruction. This model accounts for all features -- a key step in understanding the electronic structure - and indicates a surface-to-bulk progression of the electronic states driven by structural instabilities, with no evidence for other phases stemming from either topological bulk properties or the interplay between SO and the broken symmetry of the surface. [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:12AM |
T36.00006: Quantifying covalency and metallicity in pyrochlore ruthenates undergoing metal-insulator transitions Ashish Chainani, Ayako Yamamoto, Masaharu Matsunami, Ritsuko Eguchi, Munetaka Taguchi, Yasutaka Takata, Hidenori Takagi, Shik Shin, Yoshinori Nishino, Makina Yabashi, Kenji Kenji Tamasaku, Tetsuya Ishikawa We use bulk-sensitive hard x-ray photoelectron spectroscopy to investigate the electronic structure of the cubic pyrochlore ruthenates Tl$_2$Ru$_2$O$_7$ and Hg$_2$Ru$_2$O$_7$, which show first-order temperature(T)-dependent metal-insulator transitions(MITs). Ru 3d core-level spectroscopy shows drastic changes as a function of T. The metallic-origin features in core-level spectra get quenched upon gap formation in valence band spectra. The results establish temperature-driven Mott-Hubbard MITs in three-dimensional ruthenates and reveals three energy scales : (a) $4d$-electronic changes occur on the largest ($\sim$eV) energy scale, (b) the band gap energies/charge gaps (E$_g$ $\sim$160-200 meV) are intermediate, and (c) the lowest energy scale corresponds to the transition temperature T$_{MIT}$($\sim$10 meV), which is also the spin gap energy of Tl$_2$Ru$_2$O$_7$ and the magnetic-ordering temperature of Hg$_2$Ru$_2$O$_7$. The results identify and quantify the role of covalency and metallicity in the pyrochlore ruthenates undergoing T-dependent metal-insulator transitions. [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:24AM |
T36.00007: Weak-coupling analysis of quasiparticle excitations in strontium ruthenate John Deisz, Tim Kidd We report FLEX calculations for the quasiparticle properties of pure and electron-doped strontium ruthenate. Through self-consistent calculations of energy- and band-dependent linewidths and effective masses, the specific heat coefficient and superconducting $T_c$, we assess the effectiveness of this weak coupling approach for consistently describing the electron-electron correlations in this material. We also analyze the impact of the momentum dependence of the electron self-energy in describing the significant correlation effects observed in strontium ruthenate. [Preview Abstract] |
Thursday, March 21, 2013 9:24AM - 9:36AM |
T36.00008: Theory of edge currents in Sr$_2$RuO$_4$: effects of topology and gap anisotropy Samuel Lederer, Srinivas Raghu Substantial experimental evidence suggests that Sr$_2$RuO$_4$ is a chiral p-wave superconductor. Depending on bandstructure, such a system may exhibit topologically protected edge modes, and in general would exhibit intrinsic edge currents. The latter, however, have not been observed in sensitive scanning probe measurements. A possible resolution to this apparent contradiction has been offered by Raghu et al.[1]. They show that, in weak coupling, superconductivity is dominant not on the 2D $\gamma$ band as commonly believed, but on the quasi-1D $\alpha$ and $\beta$ bands, leading to a topologically trivial state, presumably with suppressed currents. They also show that the favored order parameter has sharp gap minima on the Fermi surface. We present calculations of edge currents incorporating these features using two different methods: self-consistent Bogoliubov-de Gennes equations, and Ginsburg-Landau theory. We find that, contrary to expectation, the existence and character of topological edge modes have no effect on edge currents. Multiband effects and gap anistropy yield quantitative reductions, but order 1 edge currents are a generic consequence of chiral p-wave superconductivity at low temperature in Sr$_2$RuO$_4$.\\ $[1]$ S. Raghu, et al., PRL 105, 136401 (2010). [Preview Abstract] |
Thursday, March 21, 2013 9:36AM - 9:48AM |
T36.00009: Superconductivity in Weyl Semimetals Vivek Aji, Huazhou Wei, Sung-Po Chao Weyl fermions are linearly dispersing massless particles in three dimensions. They are chiral in that the projection of their spin along their momenta is a conserved quantum number. Interest in these particles in the condensed matter context was piqued by the possibility of their emergence in the low energy sector of Pyrochlore Iridates. Since then a number of other systems have been suggested that also support such excitations. We discuss the nature of the superconducting phases that arise for chemical potential at the Weyl nodes. Since the density of states vanishes a finite coupling strength is needed to nucleate these phases. Among the possibilities are the finite momentum pairing state (FFLO) and the conventional BCS state. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T36.00010: Quantum quench in a p+ip superfluid: non-equilibrium topological gapless state Matthew Foster, Maxim Dzero, Victor Gurarie, Emil Yuzbashyan Ground state ``topological protection'' has emerged as a main theme in quantum condensed matter physics. A key question is the robustness of physical properties including topological quantum numbers to perturbations, such as disorder or non-equilibrium driving. In this work we investigate the dynamics of a p+ip superfluid following a zero temperature quantum quench. The model describes a 2D topological superconductor with a non-trivial (trivial) BCS (BEC) phase. We work with the full interacting BCS Hamiltonian, which we solve exactly in the thermodynamic limit using classical integrability. The non-equilibrium phase diagram is obtained for generic quenches. A large region of the phase diagram describes strong to weak-pairing quenches wherein the order parameter vanishes in the long-time limit, due to pair fluctuations. Despite this, we find that the topological winding number survives for quenches in this regime, leading to the prediction of a gapless topological state. We speculate on potential realizations, including a proximity effect quench on the surface of 3D topological insulator. [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T36.00011: Phases in two dimensional $p_x+ip_y$ superconducting systems with interactions beyond nearest-neighbor Antonio Russo, Sudip Chakravarty A $p_x+ip_y$ superconducting system with longer range hopping and pairing terms is considered. Chern numbers are calculated numerically, and in a simple, visual way by considering weak superconductor order parameter which is still in the same topological phase. Using nearest, second nearest, and third nearest hoppings and pairings, we find Chern numbers $0$ through $4$, including $3$ which, unlike the other Chern numbers, must be thought of in terms of combinations of different range interactions. These Chern numbers are interpreted as phases, with different properties, in particular, the number of edge states created when a cut is introduces. We also explore the effect of introducing magentic flux (in the extreme type-II limit) through flux tubes (which are vortices in the 2D system). In particular, we look at the effect of varying distances between these vortices on the lowest excitation energies of the system. [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T36.00012: Intra-valley Spin-triplet p+ip Superconducting Pairing in Lightly Doped Graphene Jianhui Zhou, Tao Qin, Junren Shi We analyze various possible superconducting pairing states and their relative stabilities in lightly doped graphene. We show that, when inter-sublattice electron-electron attractive interaction dominates and Fermi level is close to Dirac points, the system will favor intra-valley spin-triplet $p+\mathrm{i}p$ pairing state. Based on the novel pairing state, we further propose a scheme for doing topological quantum computation in graphene by engineering local strain fields and external magnetic fields. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T36.00013: Edge currents in multiband chiral p-wave superconductors Wen Huang, Catherine Kallin, Edward Taylor The superconducting phase of Sr$_2$RuO$_4$ is believed to be a time-reversal symmetry breaking state with spontaneous supercurrents at the edge or domain walls of the sample. Yet Scanning SQUID and related probes have so far failed to detect any signature of such edge currents. Recent theoretical work suggests that the active superconducting bands in Sr$_2$RuO$_4$ are the two quasi-1D bands associated primarily with the d$_{xz}$ and d$_{yz}$ orbitals of Ru$^{4+}$. This contrasts with the more conventional picture in which chiral $p$-wave superconductivity is primarily a single-band effect, with the $\gamma$ band being the active superconducting band. Based on Bogoliubov-de-Gennes calculations for tight-binding models, we study the implications of two-band chiral $p$-wave order on the edge current. The two-band model includes inter-orbital hopping and spin-orbit coupling. In general, the two-band model predicts a net edge current that is at least about an order of magnitude smaller than that from the one-band model. In particular, comparable magnitudes of inter-orbital hopping and spin-orbit coupling lead to substantial reduction of edge current. Also presented are finite temperature calcuations involving all three bands. [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T36.00014: Low-lying electronic structure and possible intrinsic gap control in J $=$ 1/2 Mott insulating perovskite iridate Sr$_3$Ir$_2$O$_7$ Chang Liu, Su-Yang Xu, Nasser Alidoust, Madhab Neupane, M. Zahid Hasan, Tay-Rong Chang, Horng-Tay Jeng, Hsin Lin, Robert Markiewicz, Arun Bansil, Chetan Dhital, Sovit Khadka, Yoshinori Okada, Vidya Madhavan, Stephen Wilson Using angle resolved photoemission spectroscopy, the ground state of perovskite iridate Sr$_{3}$Ir$_{2}$O$_{7}$ is found to be in close vicinity to a metal-to-insulator transition. Photoemission data reveal two bands extending up to surprisingly small binding energies around the Brillouin zone corner X, followed by a van Hove-like flat portion at the top of the valence bands. One of these bands form a saddle point while the other shows apparent spectral weight suppression along the the in-plane antiferromagnetic vector direction ($\Gamma $-$\Sigma )$, signaling a possible electronic response to the additional long range order. The energy scale of the Mott insulating gap shows considerable sample-to-sample variation, which points to possible intrinsic control of low temperature resistivity by apical oxygen deficiency - a process suggested by transport experiments and, importantly, similar to the doping process of the cuprates that gives rise to high temperature superconductivity. [Preview Abstract] |
Thursday, March 21, 2013 10:48AM - 11:00AM |
T36.00015: Orbital angular momentum textures in perovskite oxide materials Wonsig Jung, Wonsig Kyung, Yoonyoung Koh, Yoshiyuki Yoshida, Y.J. choi, Masashi Arita, Kenya Shimada, C. Kim We measured electronic structures of perovskite oxide materials Sr$_{2}$MO$_{4}$ (M$=$Rh, Ru, Ir) with angle-resolved photoemission spectroscopy using circular dichroism (CD) method to investigate orbital characters. We observe large CD which shows complicated orbital structures of Sr$_{2}$MO$_{4}$. CD signal comes from obital angular momentum induced from inversion symmetry breaking at cleaved surfaces. We compare results from various orbitals of 3-, 4- and 5-d. [Preview Abstract] |
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