Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session W37: Focus Session: Fe-based Superconductors: General Theory |
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Sponsoring Units: DMP DCOMP Chair: Rafael Fernandes, University of Minnesota Room: 345/346 |
Thursday, March 21, 2013 2:30PM - 3:06PM |
W37.00001: Gap symmetry and nodal structure of iron-based superconductors Invited Speaker: Maxim Khodas We first analyze the gap symmetry in iron chalcogenides with only electron pockets. Previously, two competing alternatives for the gap symmetry were considered. In the first scenario the order parameter has opposite sign on two pockets which gives rise to a $d$-wave symmetry. In the second scenario the order parameter has a constant sign, resulting in an $s$-wave symmetric state. Experimentally, the $d$-wave is excluded by ARPES, while $s$-wave scenario is inconsistent with the spin resonance as seen by neutrons. We present the third alternative agreeing with both ARPES and neutron scattering. In contrast to the earlier theories we suggest that the pairing of electrons at different pockets is equally or more important than the usual intra-pocket pairing. The inter-pocket pair momentum $(\pi,\pi)$ is supplied by the lattice via the inter-pocket hybridization processes. When the hybridization amplitude exceeds the threshold set by the pocket ellipticity the system is brought into an $s^{\pm}$ state. In this state both intra- and inter-pockets pair condensates are present. We argue that $s^{\pm}$ state is consistent with experiments. We next argue that the hybridization is crucial for the nodal structure of iron pnictides. In these superconductors with both electron and hole pockets the hybridization causes the nodal lines to form a closed nodal loops. This is consistent with ARPES, penetration depth and specific heat measurements. [Preview Abstract] |
Thursday, March 21, 2013 3:06PM - 3:18PM |
W37.00002: S+iS superconductivity in hole-doped Fe-pnictides Saurabh Maiti, Andrey Chubukov The extended s-wave (s+-) symmetry proposed for Fe-pnictides requires flipping of the phase of the superconducting order parameter (the gap) on at least two pockets. In optimally doped BaK-122, the phase is flipped between the hole and electron gaps-but have the same phase on the hole gaps (e.g., both are +). But in the strongly hole doped sample only hole pockets remain, and ARPES experiments were interpreted as evidence for s+- symmetry. This requires flipping of the phase on a pair of hole pockets (one gap is + and another is -). We address this issue of ++ to +- transition of the hole gaps as doping is changed. We find that such a transition occurs via an intermediate phase of s+is type, in which time reversal symmetry is broken (TRSB state). The ++ and +- states are two end points of the s+is state. We show that TRSB state emerges at a single point at $T_c$, but the parameter range over which it exists widens as we go down in temperature down to $T=0$. We investigate the structure of collective phase and amplitude gap fluctuations in the TRSB state and analyze the sensitivity of this state to the angular anisotropy of the interaction. We find that anisotropy-driven accidental gap nodes can survive in s+is state, unlike in s+id state (proposed for electron doped pnictides). [Preview Abstract] |
Thursday, March 21, 2013 3:18PM - 3:30PM |
W37.00003: Enhancement of the London penetration depth in pnictides at the onset of SDW order under superconducting dome Alex Levchenko, Maxim Vavilov, Dushko Kuzmanovski, Maxim Khodas, Andrey Chubukov Recent measurements of the doping dependence of the London penetration depth $\lambda(x)$ in clean samples of isovalent BaFe$_2$(As$_{1-x}$P$_x$)$_2$ at $T \ll T_c$ [Hashimoto et al., Science 336, 1554 (2012)] revealed a sharp peak in $\lambda(x)$ near optimal doping $x=0.3$. This observation points to the existence of the quantum critical point beneath the superconducting dome. We show that quantum magnetic fluctuations, associated with the emerging spin-density-wave order give rise to the observed feature. The effect comes from the dynamic renormalization of the effective mass $m^*$, which is related to $\lambda$ as $\lambda \propto \sqrt{m^*}$. We show that the effective mass has a maximum at the onset of the spin-density-wave order. We argue that the case of pnictides is conceptually different from a one-component Galilean invariant Fermi liquid, for which correlation effects do not cause the renormalization of the London penetration depth at $T=0$. [Preview Abstract] |
Thursday, March 21, 2013 3:30PM - 3:42PM |
W37.00004: Prediction for fingerprints of bosonic modes through self-energy effects in LiFeAs Kyungmin Lee, Mark Fischer, Eun-Ah Kim The role of bosonic modes has been of great interest in the research of Fe-pnictides. We aim at identifying fingerprints of specific bosonic modes in the spectral properties of the multi-orbital superconductor LiFeAs. For this, we contrast the lowest order contributions to the self energy of Bogoliubov quasiparticles from two bosonic modes: antiferromagnetic(AF) fluctuation and $E_g$ phonon. Focusing on the largest hole pocket in LiFeAs, we find that $E_g$ phonon leads to an almost completely isotropic self energy. In contrast, AF mode leads to a pronounced angle dependent self energy. We predict signatures of such self-energy in ARPES and quasiparticle interference measured by spectroscopic imaging STM. [Preview Abstract] |
Thursday, March 21, 2013 3:42PM - 3:54PM |
W37.00005: Quantum Monte Carlo study of a dominant $s$-wave pairing symmetry in iron-based superconductors Tianxing Ma, Hai-Qing Lin, Jianping Hu We perform a systematic quantum Monte Carlo study of the pairing correlation in the $S_4$ symmetric microscopic model for iron-based superconductors. It is found that the pairing with an extensive $s$-wave symmetry robustly dominates over other pairings at low temperature in reasonable parameter region regardless of the change of Fermi surface topologies. The pairing susceptibility, the effective pairing interaction and the $(\pi,0)$ antiferromagnetic correlation strongly increase as the on-site Coulomb interaction increases, indicating the importance of the effect of electron-electron correlation. Our non-biased numerical results provide a unified understanding of superconducting mechanism in iron-pnictides and iron-chalcogenides and demonstrate that the superconductivity is driven by strong electron-electron correlation effects. [Preview Abstract] |
Thursday, March 21, 2013 3:54PM - 4:06PM |
W37.00006: Magnetic and superconductivity structures near the twin boundaries in low doped Fe-pnictides Bo Li, Jian Li, Kevin Bassler, Chin-Sen Ting The spatial distributions of the magnetic, superconducting (SC) and charge orders near twin boundaries (TBs) in slightly electron-doped Ba(Ca)(FeAs)$_2$ superconductors are investigated. Two different types of TBs, which respectively correspond to the 90-degree lattice rotation and asymmetrically placement of As atoms, are considered. We find that the domain walls, which spatially separate different magnetic regions, can be formed under a relatively small Coulomb interaction due to the existence of TBs. We show that the SC is enhanced on the TBs of the first type, while on the TBs of the second type, the SC is always suppressed. [Preview Abstract] |
Thursday, March 21, 2013 4:06PM - 4:18PM |
W37.00007: Oriented gap opening in the magnetically ordered state of Iron-pnicitides: an impact of intrinsic unit cell doubling on the Fe square lattice by As atoms Ningning Hao, Yupeng Wang, Jiangping Hu We show that the complicated band reconstruction near Fermi surfaces in the magnetically ordered state of iron-pnictides observed by angle-resolved photoemission spectroscopies (ARPES) can be understood in a meanfield level if the intrinsic unit cell doubling due to As atoms is properly considered as shown in the recently constructed S4 microscopic effective model. The (0,pi) or (pi,0) col-linear antiferromagnetic (C-AFM) order does not open gaps between two points at Fermi surfaces linked by the ordered wave vector but forces a band reconstruction involving four points in unfolded Brillouin zone (BZ) and gives rise to small pockets or hot spots. The S4 symmetry naturally chooses a staggered orbital order over a ferro-orbital order to coexist with the C-AFM order. These results strongly suggest that the kinematics based on the S4 symmetry captures the essential low energy physics of iron-based superconductors. [Preview Abstract] |
Thursday, March 21, 2013 4:18PM - 4:30PM |
W37.00008: Local Quantum Criticality in an Iron-Pnictide Tetrahedron Tzen Ong, Patrick Semon, Andr\'e-Marie Tremblay, Piers Coleman The iron-based superconductors display a close experimental relationship between the Tc values and the tetrahedral bond angle of the As-Fe-As layer, with optimal Tc clustering close to the ideal tetrahedron geometry. This motivates a study of the local physics of an Fe atom within an As tetrahedron, and we find a strong interplay between spin and orbital degrees of freedom. The d-orbitals are crystal field split, and the lower eg orbitals have an SU(2) $\times$ SU(2) symmetry with both a spin and orbital Kondo interaction. The spin Kondo coupling is strongly reduced by the Hund's coupling; hence the system flows to an over-screened orbital Kondo state. A perturbative RG analysis of the strong-coupling fixed point is done using a Majorana fermion representation of the SU(2) $\times$ SU(2) symmetry. The low-temperature physics, and the possibility of a Marginal Fermi Liquid ground state, is carefully studied using the CTQMC method, taking into account the effect of Hund's coupling on the Kondo physics. [Preview Abstract] |
Thursday, March 21, 2013 4:30PM - 4:42PM |
W37.00009: Cooper Pair Formation from Quantum Magnetism in Iron-Pnictide High-Tc Superconductors Jose Rodriguez We study how spin fluctuations mediate the formation of Cooper pairs in iron-pnictide high-$T_c$ superconductors via a Schwinger-boson-slave-fermion analysis of a two-orbital $t$-$J$ model for a square lattice of iron atoms that includes magnetic frustration and Hund's Rule coupling. The starting point is a hidden half-metal state across the two-orbitals that recovers correct nested Fermi surfaces at a quantum-critical transition with a commensurate spin density wave (cSDW) metal [1]. A mean-field approximation indicates that hidden spinwaves at zero 2D momentum [2] result in an s-wave Cooper-pair instability on the hole Fermi surface pockets centered at 2D momentum $(0,0)$. Proximity to the quantum-critical transition results, additionally, in a simultaneous s-wave Cooper-pair instability on the electron Fermi surface pockets centered at 2D momenta $(\pi,0)$ and $(0,\pi)$, but with a sign change. This mean-field prediction will be checked by extracting the amplitude for such $s_{+-}$ pairing from exact numerical diagonalizations of the two-orbital $t$-$J$ model over the $4\times 4$ lattice with two holes.\\[4pt] [1] J. Rodriguez, M. Araujo \& P. Sacramento, Phys. Rev. B 84, 224504 (2011).\\[0pt] [2] J. Rodriguez, Phys. Rev. B 82, 014505 (2010). [Preview Abstract] |
Thursday, March 21, 2013 4:42PM - 4:54PM |
W37.00010: Anisotropic Superconducting Gap in a Multiorbital t-J$_1$-J$_2$ Model for Iron Pnictides Rong Yu, Qimiao Si We study the anisotropy of the superconducting gaps in the iron pnictides within a five-orbital t-J$_1$-J$_2$ model. We show that the interplay between the multiorbital nature and the magnetic frustration can give rise to an anisotropic superconducting gap with the $A_{1g}$ pairing symmetry. We have also calculated the dynamical spin susceptibility in the superconducting state, and find that the anisotropic gap structure affects the spin dynamics by showing two resonance peaks. We further discuss the connections between our results and recent ARPES and inelastic neutron scattering measurements. [Preview Abstract] |
Thursday, March 21, 2013 4:54PM - 5:06PM |
W37.00011: Detecting pairing symmetry in Fe-based superconductors: Solitons and proximity patch Victor Vakaryuk, Valentin Stanev, Wei-Cheng Lee, Alex Levchenko We suggest a mechanism which promotes the existence of a phase soliton -- topological defect formed in the relative phase of superconducting gaps of a two-band superconductor with $s_{+-}$ type of pairing. This mechanism exploits the proximity effect with a conventional $s$-wave superconductor which favors the alignment of the phases of the two-band superconductor which, in the case of $s_{+-}$ pairing, are $\pi$-shifted in the absence of proximity. In the case of a strong proximity such effect can be used to reduce soliton's energy below the energy of a soliton-free state thus making the soliton thermodynamically stable. Based on this observation we consider an experimental setup, applicable both for stable and metastable solitons, which can be used to distinguish between $s_{+-}$ and $s_{++}$ types of pairing in the iron-based multiband superconductors. [Preview Abstract] |
Thursday, March 21, 2013 5:06PM - 5:18PM |
W37.00012: Intersoliton forces and magnetic response of three band superconductors with broken time reversal symmetry Johan Carlstrom, Julien Garaud, Egor Babaev The recent discovery of iron pnictide superconductors has resulted in a rapidly growing interest in multiband models with more than two bands. The three-band Ginzburg-Landau model does in part of the parameter space exhibit broken time reversal symmetry and degenerate ground states. As was shown in Phys. Rev. Lett. 107, 197001 (2011) these systems possess topological defects in the form of bound states of fractional vortices that are different from ordinary vortices, and lack rotational symmetry. We discuss intersoliton forces, and show that they exhibit a strong orientational dependence and thus can results in nontrivial structures appearing in an applied external field. Such structures can be detected by surface magnetic probes such as scanning SQUID, magnetic force microscopy etc. [Preview Abstract] |
Thursday, March 21, 2013 5:18PM - 5:30PM |
W37.00013: Chiral $CP^2$ skyrmions in three-band superconductors and layered superconducting structures Julien Garaud, Johan Carlstrom, Egor Babaev, Martin Speight Recently discovered iron-based superconductors and well as multilayer structures involving $s_{\pm}$ superconductors can exhibit a spontaneous breaking of the time reversal symmetry. This raises the question of experimental manifestations of this additional broken symmetry. We demonstrate that it can result in formation of experimentally detectable nontrivial flux-carrying excitations which are topologically different conventional vortices. This new kind of solitons can provide an experimental signature of the breaking of time reversal symmetry. [Preview Abstract] |
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