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: SpinOrbit 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 spinorbit 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 2dimensional 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 nondegenerate quasiparticle energy spectrum is larger than the energy scale of the superconducting gap, i.e., $\alpha > k_{B}T_{c}$, only one spintriplet 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 spinsinglet and protected spintriplet paring are similar, making it difficult to determine the pairing type through Knight shift measurements. However, Anderson's theorem concerning nonmagnetic 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 nondegenerate bands, the spintriplet and the spinsinglet 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 nondegenerate 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 $^3$HeA 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 spintriplet pairing symmetry in Sr$_2$RuO$_4$ are characterized by a ${\bf d}({\bf k})$ vector aligned along the fourfold symmetry axis. Two well known consequences of all such theories are that the inplane 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: TripletSinglet Mixed Phases in HighTc Superconductors Andrei Lebed We reveal experimental conditions, where a triplet component of a superconducting order parameter appears in layered singlet dwave superconductors. We describe exotic properties of such singlettriplet mixed phases and discuss experimental methods to detect the triplet component in dwave highTc compounds. [Preview Abstract] 
Tuesday, March 22, 2005 12:27PM  12:39PM 
J32.00007: Symmetry of the order parameter in noncentrosymmetric 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 spinorbit coupling. We consider general symmetry properties of the pairing function $\Delta(k)$ in noncentrosymmetric superconductors with strong spinorbit 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 twodimensional Dwave superconductors Anton Vorontsov, Matthias Graf, James Sauls We studied the FuldeFerrellLarkinOvchinnikov (FFLO) state for twodimensional Dwave superconductors with magnetic field parallel to the superconducting planes. This state occurs at high fields near the PauliClogston limit and is a consequence of the competition between the pair condensation and Zeeman energy. We used the quasiclassical theory to selfconsistently 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 Swave case. Contrary to the Swave case, we find that the FFLO state of a Dwave 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 FuldeFerrellLarkinOvchinnikov superconducting phase with nonzero Q pairs: $\kappa$(BEDTTTF)$_{2}$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: Firstprinciples study of the pressure effects on (BEDTTTF)2XCl2 (X= I or Au) Tsuyoshi Miyazaki, Hiori Kino We have calculated the lattice parameters, internal coordinates, and electronic structures of (BEDTTTF)$_{2}X$Cl$_{2}$ ($X$ = I or Au) using the density functional theory within the generalized gradient approximation (GGA). Although these two salts both show nonmetallic behaviors at ambient pressure, their transport properties at high pressures are completely different. ICl$_{2}$ salt shows nonmetalmetal and superconducting transitions under the pressure around 8 GPa, while the AuCl$_{2}$ salt keeps its nonmetallic 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 intermolecule 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 $\beta'$BEDTTTF salts Hiori Kino, Hiroshi Kontani, Tsuyoshi Miyazaki We present theoretical studies on the phase diagram of layered organic charge transfer salts, $\beta'$(BEDTTTF)$_2$ICl$_2$ and $\beta'$(BEDTTTF)$_2$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 fluctuationexchange (FLEX) approximation based on the results of the firstprinciples calculations in DFT/GGA under applied pressures. In the obtained phase diagram of $\beta'$(BEDTTTF) $_2$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 twodimensional organic charge transfer salts Jun Liu, Joerg Schmalian, Nandini Trivedi We introduce and analyze a variational wave function for quasi twodimensional 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 2leg tJ Ladder Douglas Scalapino, Didier Poilblanc The wave vector and frequency dependence of the gap function $\phi(k,\omega)$ for a doped 2leg tJ 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: WeakCoupling BCS Theory of Skutterudite PrOs$_4$Sb$_{12}$ David Parker, Kazumi Maki, Stephan Haas A weakcoupling theory of the A and Bphases of the recently discovered heavyfermion superconductor PrOs$_4$Sb$_{12}$ is developed. The BCS gap equation is solved assuming a p+hwave pairing symmetry with an Aphase 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 Bphase 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 Bphase order parameter has similar thermodynamic properties as the recently proposed spinsinglet s+gwave 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 2dimensional or a 3dimensional representation of the crystal point group. We show that the superconducting phase always breaks the timereversal symmetry for singlet multicomponent superconductors. For triplet superconductors, the superconducting order parameter turns out to be always nonmagnetic in the cubic group, ferromagnetic in tetragonal, and either ferromagnetic ($E_{1u}$ representation) or nonmagnetic ($E_{2u}$ 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] 

J32.00016: SpinWave Excitations and Superconductivity Yuriy Malozovsky, J.D. Fan We study in terms of the effective twoparticles $T$matrix pairing by exchanging spinwave 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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