Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J32: Focus Session: Superconductivity: Theory and Computation (Mostly Triplet) |
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Sponsoring Units: DCOMP DCMP Chair: Igor Mazin, Naval Research Lab Room: LACC 507 |
Tuesday, March 22, 2005 11:15AM - 11:27AM |
J32.00001: Spin-Orbit Coupling and Symmetry of the Order Parameter in Strontium Ruthenate James Annett, Grzegorz Litak, Balazs Gyorffy, Karol Wysokinski Determination of the orbital symmetry of the pairing state in the spin triplet superconductor Sr$_2$RuO$_4$ is a challenge of considerable importance. Most of the experiments show that a chiral state of the $\hat{z} (k_x \pm ik_y)$ type is realized and remains stable on lowering the temperature. Here we have studied the stability of various superconducting states of Sr$_2$RuO$_4$ in the presence of spin-orbit coupling. Numerically we found that the chiral state is never the minimum energy. Alone among the five states studied it has $<{\bf \hat L}.{\bf \hat S} >=0$ and is therefore not affected to linear order in the coupling parameter $\lambda$. We found that stability of the chiral state requires spin dependent pairing interactions. This imposes a strong constraint on the pairing mechanism. [Preview Abstract] |
Tuesday, March 22, 2005 11:27AM - 11:39AM |
J32.00002: APRES lineshape analysis of the Fermi liquid Sr$_2$RuO$_4$ Nicholas Ingle, Kyle Shen, Felix Baumberger, Worowat Meevasana, Donghui Lu, Z.-X. Shen ARPES spectra of correlated materials, shown by other measurements to be Fermi liquids, have yet to show a linewidth that is narrower than the binding energy of the quasiparticle -- a key prediction of Fermi liquid theory. We show that although the effects of energy and momentum resolution can be a significant factor in the quantitative description of the scattering rates from APRES, it is possible to effectively account for them. We find that the ARPES linewidth in Sr$_{2}$RuO$_{4}$, a correlated 2-dimensional Fermi liquid, is narrower than its binding energy, and decreases with the expected functional form as the Fermi energy is approached. In combination with the previously determined Fermi surface, these results give the first complete picture of a Fermi liquid via ARPES. [Preview Abstract] |
Tuesday, March 22, 2005 11:39AM - 11:51AM |
J32.00003: Effects of disorder in parity violating superconductors Paolo Frigeri, Manfred Sigrist, Daniel Agterberg, Akihisa Koga In materials without an inversion center of symmetry the spin degeneracy of the conducting band is lifted by spin orbit coupling (SOC). If the splitting of the non-degenerate quasi-particle energy spectrum is larger than the energy scale of the superconducting gap, i.e., $\alpha > k_{B}T_{c}$, only one spin-triplet pairing channel survives to the lack of the inversion symmetry. In the limit of larger $\alpha$, i.e, $\alpha >> k_{B}T_{c}$, the spin susceptibility in case of spin-singlet and protected spin-triplet paring are similar, making it difficult to determine the pairing type through Knight shift measurements. However, Anderson's theorem concerning non-magnetic impurities still holds. This should make possible to discriminate experimentally at least between conventional and unconventional pairing. If the density of states at the Fermi level is different for the two non-degenerate bands, the spin-triplet and the spin-singlet pairing channels mix, modifying further the response of the system on disorder. The impurity dependence of the critical temperature, the spin susceptibility, and the paramagnetic limiting field have been studied using the Born approximation for various combinations of the pairing channels, and different distributions of the conduction electrons on the two non-degenerate bands. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J32.00004: Topological Terms in the effective action of the two dimensional $p_x + i p_y $ superfluid/superconductor Rahul Roy, Michael Stone The effective action of a two dimensional $p_x + i p_y $ superfluid/superconductor, such as the layered Sr$_2$RuO$_4$ superconductor or a thin film of $^3$He-A is known to contain a Hopf term, which is responsible for non abelian statistics. We study the applicability of the Eilenberger formalism which is used to reduce higher dimensional problems to effective one dimensional ones, in computing the effective action. We also investigate the Wess Zumino Witten model in this connection. [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J32.00005: Role of Magnetic Fields on Superconducting Sr$_2$RuO$_4$ Daniel Agterberg, Raminder Kaur, Hiroaki Kusunose, Manfred Sigrist Nearly all theoretical descriptions of the spin-triplet pairing symmetry in Sr$_2$RuO$_4$ are characterized by a ${\bf d}({\bf k})$ vector aligned along the four-fold symmetry axis. Two well known consequences of all such theories are that the in-plane anisotropy of the upper critical field should exist up to $T_c$ and that there must be multiple vortex phases corresponding to changes in the order parameter structure. Neither of these two predictions appear to have strong experimental support. We examine these predictions within an approximate analytic solution of the quasiclassical equations for the vortex phase to provide tenative explanations for this inconsistency. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J32.00006: Triplet-Singlet Mixed Phases in High-Tc Superconductors Andrei Lebed We reveal experimental conditions, where a triplet component of a superconducting order parameter appears in layered singlet d-wave superconductors. We describe exotic properties of such singlet-triplet mixed phases and discuss experimental methods to detect the triplet component in d-wave high-Tc compounds. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J32.00007: Symmetry of the order parameter in non-centrosymmetric superconductors: Implementation for CePt$_3$Si and Cd$_2$Re$_2$O$_7$ Ivan A. Sergienko, Stephanie H. Curnoe In noncentrosymmetric metals, the spin degeneracy of the electronic bands is lifted by spin-orbit coupling. We consider general symmetry properties of the pairing function $\Delta(k)$ in noncentrosymmetric superconductors with strong spin-orbit coupling (NSC). We find that $\Delta(k) = \chi(k) t(k)$, where $\chi(k)$ is an even function which transforms according to the irreducible representations of the crystallographic point group and t(k) is a model dependent phase factor. We consider tunnelling between a NSC and a conventional superconductor. It is found that, in terms of thermodynamical properties as well as the Josephson effect, the state of NSC resembles a singlet superconducting state with gap function $\chi(k)$. We propose the gap functions which may account for the experimental properties of the heavy fermion compound CePt$_3$Si and the distorted pyrochlore Cd$_2$Re$_2$O$_7$. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J32.00008: The FFLO state in two-dimensional D-wave superconductors Anton Vorontsov, Matthias Graf, James Sauls We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state for two-dimensional D-wave superconductors with magnetic field parallel to the superconducting planes. This state occurs at high fields near the Pauli-Clogston limit and is a consequence of the competition between the pair condensation and Zeeman energy. We used the quasiclassical theory to self-consistently compute the inhomogeneous order parameter and the free energy. We mapped out the phase diagram and found that the transitions from the FFLO phase to either the normal state or uniform superconducting state are of second order, analogous to the S-wave case. Contrary to the S-wave case, we find that the FFLO state of a D-wave order parameter breaks translational symmetry along prefered directions. The orientation of the nodes in real space is pinned by the nodes of the basis function in momentum space. We will discuss the experimental consequences of this effect. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J32.00009: Assessment of Possible FFLO States in Real Materials. Alan Kyker, Warren Pickett, Francois Gygi Very recently a few mateirals have been suggested to show realizations of the Fulde-Ferrell-Larkin-Ovchinnikov superconducting phase with nonzero Q pairs: $\kappa$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$, CeCoIn$_5$, UBe$_{13}$, and conceivably ZrZn$_2$. Occurrence of this phase is highly dependent on the degree to which finite pair momentum can nest spin split Fermi surfaces. Histograms of the "projected Fermi velocity density of states" provides a useful method of measuring the degree of nesting possible for a given material, and hence its suitability for supporting an FFLO phase. We present and discuss the nesting properties for a number of materials using projected velocity methods, and critically evaluate the suggestions for FFLO phases. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J32.00010: First-principles study of the pressure effects on (BEDT-TTF)2XCl2 (X= I or Au) Tsuyoshi Miyazaki, Hiori Kino We have calculated the lattice parameters, internal coordinates, and electronic structures of (BEDT-TTF)$_{2}X$Cl$_{2}$ ($X$ = I or Au) using the density functional theory within the generalized gradient approximation (GGA). Although these two salts both show non-metallic behaviors at ambient pressure, their transport properties at high pressures are completely different. ICl$_{2}$ salt shows nonmetal-metal and superconducting transitions under the pressure around 8 GPa, while the AuCl$_{2}$ salt keeps its non-metallic behavior even when the pressure of 10 GPa is applied. We first show that the calculated structures at ambient pressure agree well with the experimental ones for both materials, implying that GGA is reliable for the theoretical determinations of the crystal structure. Then we report that the inter-molecule interactions exhibit various and complex pressure dependence, contrary to the simple suggestions made so far. Although we cannot find sensible differences in the pressure effects on the crystal structures, we have found that the pressure dependence of their band structures is qualitatively different between these two salts. We will show the dimensional change of Fermi surfaces, increase of the band width, and the pressure dependence of TB parameters which reproduces our present DFT results. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J32.00011: Phase diagram of $\beta'$-BEDT-TTF salts Hiori Kino, Hiroshi Kontani, Tsuyoshi Miyazaki We present theoretical studies on the phase diagram of layered organic charge transfer salts, $\beta'$-(BEDT-TTF)$_2$ICl$_2$ and $\beta'$-(BEDT-TTF)$_2$AuCl$_2$. The former shows the highest superconducting transition temperature ($T_{\rm c} $14.2~K under a high hydrostatic pressure) among the organic superconductors. We study an effective model using the fluctuation-exchange (FLEX) approximation based on the results of the first-principles calculations in DFT/GGA under applied pressures. In the obtained phase diagram of $\beta'$-(BEDT-TTF) $_2$ICl$_2$, the superconductivity with $d_{xy}$-like symmetry is realized next to the antiferromagnetic phase. The calculated $T_{\rm c}$ quantitatively coincides well with the experimental one. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J32.00012: Pairing and superconductivity driven by strong quasiparticle renormalization in two-dimensional organic charge transfer salts Jun Liu, Joerg Schmalian, Nandini Trivedi We introduce and analyze a variational wave function for quasi two-dimensional kappa-(ET)2 organic salts containing strong local and nonlocal correlation effects. We find an unconventional superconducting ground state for intermediate charge carrier interaction, sandwiched between a conventional metal at weak coupling and a spin liquid at larger coupling. Most remarkably, the excitation spectrum is dramatically renormalized and is found to be the driving force for the formation of the unusual superconducting state. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J32.00013: The Gap Function for a 2-leg t-J Ladder Douglas Scalapino, Didier Poilblanc The wave vector and frequency dependence of the gap function $\phi(k,\omega)$ for a doped 2-leg t-J ladder is obtained from a Lanczos calculation. Based upon this information, we argue that the pairing interaction is (1) short range in space, (2) retarded, and (3) dominated by the S=1 channel near its onset. [Preview Abstract] |
Tuesday, March 22, 2005 1:51PM - 2:03PM |
J32.00014: Weak-Coupling BCS Theory of Skutterudite PrOs$_4$Sb$_{12}$ David Parker, Kazumi Maki, Stephan Haas A weak-coupling theory of the A- and B-phases of the recently discovered heavy-fermion superconductor PrOs$_4$Sb$_{12}$ is developed. The BCS gap equation is solved assuming a p+h-wave pairing symmetry with an A-phase nodal gap function $\Delta ({\bf k}) \sim e^{\pm i\phi_{i}} (1-\hat{k}_{x}^4 - \hat{k}_{y}^4 - \hat{k}_{z}^4)$ and with a B-phase nodal gap function $\Delta({\bf k})\sim e^{\pm i\phi}(1-\hat{k}_{y}^4)$, where $e^{\pm i\phi_{i}} \sim \hat{k}_ {z} \pm i\hat{k}_{x},\,\, \hat{k}_{x} \pm i\hat{k}_{y}$ or $\hat {k}_{y} \pm i\hat{k}_{z}$ and $e^{\pm i\phi} \sim \hat{k}_{z} \pm i\hat{k}_{x}$. The B-phase order parameter has similar thermodynamic properties as the recently proposed spin-singlet s+g-wave superconductor [Maki et al, Europhys. Lett. {\bf 64} 496 (2003)]. However, there is accumulating evidence for spin- triplet pairing for superconductivity in PrOs$_4$Sb$_{12}$. Analytic limiting case results as well as numerical solutions for the whole temperature range $T \leq T_{c}$ are presented for the order parameter, the specific heat, the thermodynamic critical field, and the superfluid density. We compare these results with data from a few recent experiments. [Preview Abstract] |
Tuesday, March 22, 2005 2:03PM - 2:15PM |
J32.00015: Weak coupling theory of multicomponent superconductors. Valentina Kuznetsova, Victor Barzykin We consider the pairing state due to the BCS mechanism in substances of cubic, hexagonal, or tetragonal symmetry, where the superconducting parameter is multicomponent, i.e., transforms according to either a 2-dimensional or a 3-dimensional representation of the crystal point group. We show that the superconducting phase always breaks the time-reversal symmetry for singlet multicomponent superconductors. For triplet superconductors, the superconducting order parameter turns out to be always non-magnetic in the cubic group, ferromagnetic in tetragonal, and either ferromagnetic ($E_{1u}$ representation) or non-magnetic ($E_{2u}$ representation) in hexagonal group. According to these results, for example, the superconducting phases in both Sr$_2$RuO$_4$ and Na$_x$CoO$_2$-yH$_2$O should be ferromagnetic. [Preview Abstract] |
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J32.00016: Spin-Wave Excitations and Superconductivity Yuriy Malozovsky, J.D. Fan We study in terms of the effective two-particles $T$-matrix pairing by exchanging spin-wave excitations near the ferromagnetic or antiferromagnetic instability. We show that near the ferromagnetic instability the paring due to the exchange the paramagnetic excitations can occur in the $p$-wave state (\textit{$\ell $} = 1) and $m=\pm $1. We show that near the antiferromagnetic instability the pairing due the exchange the paramagnetic excitations occurs also in the $p$-wave state (\textit{$\ell $} = 1) but with $m$ = 0 and that there is no pairing in $d$-wave state. [Preview Abstract] |
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