Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L13: Focus Session: Fe Based Superconductors-Pairing Symmetry |
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Chair: Peter Hirschfield, Unversity of Florida Room: 207 |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L13.00001: Superconducting gap in LiFeAs from three-dimensional spin-fluctuation pairing calculations Yan Wang, Andreas Kreisel, Peter Hirschfeld, Volodymyr Zabolotnyy, Sergey Borisenko, Bernd B\"uchner, Thomas Maier, Douglas Scalapino The lack of nesting of Fermi-surface sheets in the Fe-based superconductor LiFeAs, with a $T_c$ of 18 K, has led to questions as to whether the origin of superconductivity in this material might be different from other Fe-based superconductors. Here we present calculations of the superconducting gap and pairing in the random-phase approximation using Fermi surfaces derived from ARPES. The gaps obtained are qualitatively different from previous 2D theoretical works and in good agreement with ARPES on the main Fermi-surface pockets. We analyze the contributions to the pairing vertex thus obtained and show that the scattering processes between electron and hole pockets still dominate the pairing as in other Fe-based superconductors despite the lack of nesting, leading to gaps with anisotropic $s_\pm$ structure. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L13.00002: Orbital fluctuation mediated S$_{++}$ wave state based on the SC-VC$_{\Sigma}$ method -- Impact of vertex correction for the gap equation Seiichiro Onari, Hiroshi Kontani We develop the theory of superconductivity by introducing the vertex correction (VC). Although the Coulomb interaction violates Migdal's theorem, the VC for the gap equation have usually been neglected for simplicity. However, the VC is inevitable to satisfy Ward identity. Here, we show that S$_{++}$ wave state mediated by the orbital fluctuation is favored by the VC for the gap equation. Previously, we have shown that the orbital fluctuations are strongly enhanced due to the spin-orbital mode-coupling described by the VC for the irreducible susceptibility. The structural phase transition and softening of shear modulus C$_{66}$ are naturally explained by the orbital fluctuations. In this study, both the VC and the self-energy ($\Sigma$) are obtained self-consistently (SC-VC$_{\Sigma}$ method). By using the SC-VC$_{\Sigma}$ method, we aim to understand whole phase diagram including the superconducting state in H-doped LaFeAsO and LiFeAs. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L13.00003: Effects of magnetic flux in a loop formed by an s-wave superconductor and an $\mathrm{s_{\pm}}$ superconductor Rosa Rodriguez-Mota, Tami Pereg-Barnea Identifying the correct order parameter structure of the iron based superconductors will provide insight into the pairing mechanism in these materials. Due to the multi-orbital band structure of these materials and the proximity of the superconducting phase to an anti-ferromagnetic phase, most theories favor magnetic fluctuations as the pairing mechanism and an order parameter with the so-called $\mathrm{s_{\pm}}$ symmetry. However, it is experimentally challenging to distinguish the $\mathrm{s_{\pm}}$ symmetry from conventional s-wave symmetry; thus, the $\mathrm{s_{\pm}}$ structure remains unconfirmed. In 2010, Chen \textit{et al} showed evidence of integer and half integer flux quantum transitions in an $\mathrm{Nb}$-$\mathrm{NdFeAsO_{0.88}F_{0.12}}$ loop excited by electromagnetic pulses [1]. We present a theoretical study of the effects of magnetic flux in a superconducting s/$\mathrm{s_{\pm}}$ loop inspired by these results. Our findings are in agreement with preliminary results of a phenomenological Ginzburg Landau model [2], and help clarify the relation between the transitions observed in the experiment and the $\mathrm{s_{\pm}}$ symmetry.\\[4pt] [1] C.-T. Chen \textit{et al}, Nature Physics 6, 260 (2010).\\[0pt] [2] Berg, Lindner, Pereg-Barnea, Phys. Rev. Lett. 106, 1470. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L13.00004: Topological defects in s$+$is and d$+$id superconductors Egor Babaev, Julien Garaud Recently arguments were advanced that various compounds can have s$+$is or d$+$id superconductivity. I will discuss topological defects which can arise in such superconducting states, their properties and experimental signatures. The allowed defects: vortices, domain walls and Skyrmions all have distinct magnetic features so their observation can be a confirmation of the s$+$is or d$+$id nature of superconducting states. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L13.00005: Orbital-Parity Selective Superconducting Pairing Structures of Fe-based Superconductors under Glide Symmetry Chiahui Lin, Chung-Pin Chou, Wei-Guo Yin, Wei Ku We show that the superconductivity in Fe-based superconductors consists of zero and finite momentum $(\pi,\pi,0)$ Cooper pairs with the same and different parities of the Fe $3d$ orbitals respectively. The former develops the distinct gap structures for each orbital parity, and the latter is characteristic of spin singlet, spacial oddness and time reversal symmetry breaking. This originates from the unit cell containing two Fe atoms and two anions of staggered positioning with respect to the Fe square lattice. The in-plane translation is turned into glide translation, which dictates orbital-parity selective quasiparticles. Such novel pairing structures explain the unusual gap angular modulation on the hole pockets in recent ARPES and STS experiments. Work supported by DOE DE-AC02-98CH10886 and Chinese Academy of Engineering Physics and Ministry of Science and Technology. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L13.00006: On the internal $d$-wave structure of s$^{\pm}$ pairs in Iron-based Superconductors Tze Tzen Ong, Piers Coleman A key issue in understanding the high temperature iron-based superconductors concerns the mechanism by which the paired electrons minimize their strong mutual Coulomb repulsion. Whereas electronically paired superconductors generally avoid the Coulomb interaction through the formation of higher angular momentum pairs, iron based superconductors involve s-wave (s$^{\pm}$) pairs with zero angular momentum. By taking account of the orbital degrees of freedom of the iron atoms, here we show that the s$^{\pm}$ pairs in these materials possess hidden d-wave symmetry, forming orbital triplets in which the the d-wave angular momentum of the pairs is compensated by the internal angular momentum of the orbitals. The recent observation of a gap with octahedral structure in KFe$_{2}$As$_{2}$ materials can be understood as a transition to a ``high spin'' configuration of the d-wave orbital triplets, through the alignment of the two angular momentum components of the pair. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L13.00007: Similarities, diferences and the pairing interaction of the Fe-based and cuprate superconductors Invited Speaker: Douglas Scalapino The undoped multi-orbital, multi-Fermi surface Fe-based superconductors exhibit metallic antiferromagnetism while the undoped cuprates are insulating Mott antiferromagnets which when optimally doped have one large Fermi surface. Nevertheless, both systems exhibit a neutron spin resonance in the superconducting state providing evidence of an unconventional sign changing superconducting gap which appears in proximity or coexisting with antiferromagentism. Here we will examine what the similarities and differences of these two classes of materials tell us about the pairing mechanism. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L13.00008: Symmetry measurements of the order parameter of BaFe$_{2}$As$_{2}$ superconductors Juan Atkinson, Dale Van Harlingen, Paul Canfield Since the discovery of the Fe-pnictide superconductors, extensive efforts have been directed toward understanding the symmetry and mechanism of the superconducting pairing. Extended s-wave models, predominantly the s$\pm$ model, are predicted by many theories, but a definitive experimental verification has been elusive. In this case, the phase-sensitive Josephson interferometry measurements that have been effective in determining the symmetry of many superconductors do not give a definitive signature. As an alternative approach, we are searching for proximity-induced structure in the density-of-states of an s-wave superconductor proximity-coupled to Co-doped BaFe$_2$As$_2$ superconductive crystals. Unique features are predicted to arise for s$\pm$ pairing that allow this symmetry to be distinguished from other order parameter forms such as s++ or d-wave. Observation of this structure would provide both magnitude and phase information about the multigap structure of the BaFe$_2$As$_2$ crystal. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L13.00009: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L13.00010: Time-reversal symmetry broken metallic states in multiband superconductors Troels Arnfred Bojesen, Egor Babaev, Asle Sudbo The recent discovery of so-called multiband superconductors, like the iron pnictides, has spurred a surge in interest for superconductors with several bands crossing the Fermi level. The reason for this is that frustration in interband couplings may lead to a broken $Z_2$ (``time reversal'') symmetry in addition to the ``ordinary'' breaking of the $\mathrm{U}(1)$ symmetry in single band superconductors, opening up for the possibility of new forms of topological excitations and interesting new physics. We have investigated phase diagrams and phase transitions of $\mathrm{U}(1)\times Z_2$ superconductors in 2D and 3D beyond mean-field approximation, using large-scale Monte Carlo simulations. In addition to the superfluid $\mathrm{U}(1)\times Z_2$ and $\mathrm{U}(1)$ broken states, we find, in a certain parameter regime, a new, non-superfluid (metallic) $Z_2$ broken (but $\mathrm{U}(1)$ symmetric) state where time-reversal symmetry is spontaneously broken. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L13.00011: Improper s-wave symmetry for the electronic pairing in iron-based superconductors by first-principles calculations Michele Casula, Sandro Sorella By means of space-group symmetry arguments, we argue that the electronic pairing in iron-based high temperature superconductors shows a structure which is a linear combination of planar s-wave and d-wave symmetry channels, both preserving the 3-dimensional $A_{1g}$ irreducible representation of the corresponding crystal point-group. We demonstrate that the s- and d-wave channels are determined by the parity under reflection of the electronic orbitals through the iron planes, and by improper rotations around the iron sites. We provide evidence of these general properties by performing accurate quantum Monte Carlo ab-initio calculations of the pairing function for a FeSe lattice. In order to achieve a higher resolution in momentum space we introduce a BCS model that faithfully describes our QMC variational pairing function. This allows us to provide a k-resolved image of the pairing function, and show that non-isotropic contributions in the BCS gap function are related to the improper s-wave symmetry. Our theory can rationalize and explain a series of contradictory experimental findings, such as the observation of twofold symmetry in the FeSe superconducting phase and the $s$-to-$d$-wave gap transition in BaFe$_2$As$_2$ under K doping. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L13.00012: Field Induced Superlattice Modulation in Hole-Doped Iron-Pnictide Superconductors Hong-Yi Chen, C.S. Ting Based upon a phenomenological model with competing spin-density-wave and extended s$\pm$ pairing superconductivity, the vortex states in $Ba_{1-x}K_xFe_2As_2$ are investigated by solving Bogoliubov-de Gennes equations. Our results for the optimally doped compound with slightly induced SDW at the center of the vortex are in agreement with STM experiments. We also propose that in the underdoped compound the field induced superlattice modulation. The emergence of the superlattice modulation with period $12a_0$ results in a band reconstruction around the Fermi energy. We found out that the reconstructed band has particle-hole symmetry. The symmetric particle-hole band causes a zero bias resonance peak. The band reconstruction can be also used to explain the absence of the peak at the vortex center in $Ba(Fe_{1-x}Co_x)_2As_2$ electron doped pnictide. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L13.00013: Zero energy Andreev Bound states in odd parity pairing superconductors Chandan Setty, Jiangping Hu We study the properties of zero energy Andreev bound states in SNS junctions. The superconductors on either sides of the normal metal are assumed to have two sub-lattices with either odd or even parity pairing within each sub-lattice. In addition, we add a uniform even parity pairing between the two sub-lattices and study its effect on the Andreev zero energy bound state. In general, we find that the even parity pairing tends to weaken/destroy the zero bias peak (ZBP). We point out the relevance of our results to a recently proposed superconducting ground state in Iron based superconductors (FeSCs) [1]. \\[4pt] [1] Jiangping Hu PRX, 3, 031004 (2013) [Preview Abstract] |
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