Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session V35: Focus Session: Iron Pnictides and Other Novel Superconductors XIII: Pairing Symmetry, Theory and Experiment |
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Sponsoring Units: DMP Chair: Igor Mazin, Naval Research Laboratory Room: 405 |
Thursday, March 19, 2009 8:00AM - 8:12AM |
V35.00001: On the magnetic fluctuations and unconventional superconducting pairing in iron pnictides Junhua Zhang, Rastko Sknepnek, Joerg Schmalian We explore the role played by spin and density fluctuations in the FeAs based superconductors using the fluctuation-exchange (FLEX) approach. We calculate the superconducting transition temperature and the fluctuation induced pairing gap. In order to compare with experiments, we evaluate the change of the penetration depth with temperature and the evolution of the order parameter. Because of the multi-band feature in this type of material, the interplay between intra- and inter-band fluctuations gives rise to rich physics. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V35.00002: Signatures of the s+ superconducting gap in electronic Raman Scattering and optical conductivity of Fe-based superconductors Ilya Eremin, Andrey V. Chubukov, Maxim M. Korshunov We analyze the consequences of the extended s-wave symmetry of the superconducting gap, proposed recently in Fe-based superconductors, for the electronic pair-breaking Raman scattering and optical conductivity. We calculate conductivity and Raman intensity for elastic scattering and find that an extended s-wave superconducting gap gives rise to several specific features in optical and Raman response functions. In particular, we find that, for the A1g symmetry of the incoming light, there will be a resonant collective mode in the Raman response function at an energy $\omega <$2$\Delta $. The latter is as a hallmark of the s+ superconductivity. Furthermore, the Cooper-pair weakening due to strong inter-band impurity scattering shifts the 2$\Delta $ features towards higher energies in both Raman scattering and optical conductivity. We argue that these features are present in the experimental data for iron-based superconductors. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V35.00003: Experiments for probing the macroscopic quantum coherence and pairing state in FeAs-based superconductors C.C. Tsuei, Ching-Tzu Chen, M.B. Ketchen, J.R. Rozen, Z.A. Ren, Z.X. Zhao, X.H. Chen The recent discovery of superconductivity in FeAs-based pnictides has added another member to the growing family of high-temperature superconductors. In sharp contrast to cuprates, the new superconductor has a $T_c$ up to $55$ K despite the absence of strong electronic correlation. Furthermore, its physical properties are best described by the multi-orbital electronic band structure. It is therefore of great interest to explore the consequence of these novel characteristics on the nature of Cooper pairing in the new FeAs superconductors. In this talk, we will present the design and implementation of experiments which probe the pairing state and the macroscopic quantum coherence across the interface between a pnictide and a conventional s-wave superconductor. Preliminary results will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V35.00004: Limits on the Superconducting Order Parameter in NdFeAsO$_{1-x}$F$_y$ and SmFeAsO$_{1-x}$ from Scanning SQUID Microscopy Thomas Lippman, Clifford Hicks, Martin Huber, Zhi-An Ren, Zhong-Xian Zhao, Kathryn Moler As a test of the symmetry of the order parameter of the ferric oxyarsenide family of superconductors RFeAsO$_{1-x}$F$_y$, where R is a rare earth, we perform scanning SQUID microscopy on dense polycrystalline samples of NdFeAsO$_{0.94}$F$_{0.06}$ and SmFeAsO$_{0.85}$. Dominant $d$- or $p$-wave orders, for example, would result in direction-dependent phase shifts in tunneling. In well-coupled polycrystalline samples, these phase shifts would result in spontaneous orbital currents and magnetization in the superconducting state. We do not find any spontaneous currents in NdFeAsO$_{0.94}$F$_{0.06}$ or SmFeAsO$_{0.85}$, ruling out order parameters with direction-dependent phase shifts in tunneling. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V35.00005: Effect of disorder on sign reversing s-wave pair state for Fe-pnictides Vivek Mishra, G. Boyd, S. Graser, P. J. Hirschfeld In this work, we consider a phenomenological model for sign changing s-wave order parameters appropriate for the ferropnictide superconductors. We consider both magnetic and non-magnetic impurities within the Born approximation, and study the effects of this disorder on transport properties in superconducting state. We compare our results with experimental data for Fe-pnictides, where such a sign-changing s-wave state may be realized. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V35.00006: Phase fluctuations in a magnetic exchange model of iron-based superconductors F.J. Burnell, B. Andrei Bernevig, Meera M. Parish We analyze the phase fluctuations in a two band model of iron- based superconductors. Multi-band superconductors have a sound- like collective excitation (Leggett mode) due to phase oscillations between different condensates. We calculate the spectrum of this mode in a short-range exchange model of iron pnictide superconductors. Unlike previously studied systems, in this model the superconducting order parameters of the particle and hole Fermi surfaces naturally have opposite sign. We describe the consequences of this for the Leggett mode and outline possible experimental signatures of this sign difference between the two superconducting gaps. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V35.00007: Near degeneracy of several pairing channels in a multi-orbital model for the Fe-pnictides Thomas Maier, Siegfried Graser, Peter Hirschfeld, Douglas Scalapino The experimental evidence regarding the gap structure in different iron pnictide superconductors is currently conflicting. In addition, weak-coupling approaches to the pairing problem in multi-orbital models of the iron pnictides have predicted a wide variety of superconducting ground states. We argue here that this controversy is naturally explained by the near-degeneracy of different pairing channels in superconductors with many distinct Fermi surface sheets. In particular, we will present results for the spin susceptibility and the pairing symmetry within a five-band random phase approximation model. We will discuss the robustness of these results for different dopings, interaction strengths, and variations in the band structure, in the light of recent experiments. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V35.00008: Pairing Symmetry in a Two-Orbital Exchange Coupling Model of Oxypnictides B. Andrei Bernevig, Kangjun Seo, Jiangping Hu We study the pairing symmetry of a two orbital $J_1-J_2$ model for FeAs layers in oxypnictides. We show that the mixture of an intra-orbital unconventional $s_{x^2y^2}\sim \cos(k_x)\cos(k_y)$ pairing symmetry and a small $d_{x^2-y^2}\sim \cos(k_x)-\cos (k_y)$ component is favored in a large part of $J_1-J_2$ phase diagram. A pure $ s_{x^2y^2}$ pairing state is favored for $J_2>>J_1$. The signs of the $d_{x^2-y^2}$ order parameters in the two different orbitals are opposite. While a small $d_{xy} \sim \sin(k_x)\sin(k_y)$ inter-orbital pairing coexists in the above phases, the intra-orbital $d_{xy}$ pairing is not favored even for large $J_2$. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V35.00009: Self-consistent calculations of the local density of states of FeAs superconductor/ferromagnet bilayers Nayoung Lee, Han-Yong Choi We study the local density of states (LDOS) of the superconductor/ferromagnet (S/F) bilayers using the self-consistent Bogoliubov-de Gennes equation, where the S is modeled in terms of the $s\pi $ pairing state. The $s\pi $ pairing is an s-wave pairing state with an internal $\pi $ phase~ between the two condensates of a two band superconductor which seems relevant for the FeAs superconductors. We calculate the pairing and magnetic order parameters self-consistently to obtain LDOS as a function of the energy and position of S/F bilayers. The results will be discussed in terms of the interplay between the internal and external $\pi $ phases. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V35.00010: Possible time-reversal symmetry breaking pairing state in FeAs superconductors Wei-Cheng Lee, Shou-Cheng Zhang, Congjun Wu We investigate the competition between the extended s-wave and d-wave pairing order parameters in the iron-based superconductors. Because of the frustrating pairing interactions among the electron and the hole Fermi pockets, a time reversal symmetry breaking $s+$i$d$ pairing state could be favored. We analyze this pairing state within the Ginzburg-Landau theory, and explore the experimental consequences. In such a state, spatial inhomogeneity induces supercurrent near a non-magnetic impurity and the corners of a square sample. The resonance mode between the s and d-wave order parameters can be detected through the B$_{1g}$-Raman spectroscopy. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V35.00011: Spin fluctuation mediated extended s-wave pairing from multiple Fermi surfaces in iron pnictide superconductors Kazuhiko Kuroki, Seiichiro Onari, Ryotaro Arita, Hidetomo Usui, Yukio Tanaka, Hiroshi Kontani, Hideo Aoki For the superconducting iron pnictides, we have constructed a minimal model, where all the five Fe d bands turn out to be involved[1]. The model is used to investigate the origin of superconductivity with a five-band random-phase approximation for solving the Eliashberg equation. We conclude that the spin fluctuation modes arising from the nesting between the disconnected Fermi pockets realize basically an extended s-wave pairing, but that the gap function is in fact a matrix with significant off-diagonal elements. [1] K. Kuroki, S. Onari, R. Arita, H. Usui, Y. Tanaka, H. Kontani, and H. Aoki, Phys. Rev. Lett. \textbf{101}, 087004 (2008). [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V35.00012: Spin fluctuation dynamics and multiband superconductivity in iron pnictides Valentin Stanev, Jian Kang, Zlatko Tesanovic Multiband superconductivity, involving resonant pair scattering between different bands, has emerged as a likely possibility for the iron pnictides. In this scenario the gap changes sign between the hole and the electron Fermi surfaces (separated by wave-vector M). In the quest to distinguish this extended s- from an ordinary s- wave state, it is essential to use experiments that have momentum space resolution and can probe momenta of order M\footnote{ V. Stanev, J. Kang, and Z. Tesanovic, Phys. Rev. B \textbf{78}, 184509 (2008).}. We study the fluctuation dynamics of these superconducting states, as well as d- and p-wave states. The coupling between spin fluctuations and the quasiparticles of the superconducting state leads to damping of the former. The gap structure leaves a signature in the form of this damping. This can be used to diagnose the order parameter in spin sensitive experiments. We also discuss the case of coexistence of superconductivity and spin-density wave. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V35.00013: Emergent symmetry of superconductivity and antiferromagnetism in the iron pnictides Yong Baek Kim, Daniel Podolsky, Hae-Young Kee We show the presence of an emergent symmetry that unifies superconductivity and magnetism in the iron pnictides. We discuss the expected experimental consequences of this symmetry. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V35.00014: Two-band Model for Unconventional Superconductivity in Iron-Based Superconductors Yan Chen The recent discovery of FeAs superconductors with high Tc has triggered intensive efforts to explore magnetism and superconductivity in this family of materials. We adopt an effective model Hamiltonian including a two-band tight-binding term as well as a spin-spin exchange interactions term. The first term can fit approximately the multi-band dispersions near electron/hole Fermi-pockets while the second term may lead to the appropriate superconducting pairing symmetry as well as the SDW state at low doping. Different pairing symmetry candidates of FeAs materials are evaluated for various model parameters. In particular, we study the local electronic structure at vortex cores in the system subject to an external magnetic field. The relevance to recent STM measurements will be discussed. Moreover, we calculate the tunneling conductance of various superconducting junctions, including the Josephson currents and the Andreev reflection, between the FeAs materials modeled by a two-band superconducting state and various systems include normal metal, a conventional s-wave superconductor, d-wave superconductor, or same FeAs materials. The Josephson current can be decomposed into two parts, an interband and an intraband components. Distinct interference effects can be used to distinguish the pairing symmetry of FeAs system. [Preview Abstract] |
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