Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y13: Focus Session: Fe Based Superconductors-Correlation Effects |
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Sponsoring Units: DMP Chair: Rafael Fernandes, University of Minnesota Room: 207 |
Friday, March 7, 2014 8:00AM - 8:12AM |
Y13.00001: A DFT+DMFT Investigation on Electron-Phonon Coupling in FeSe Subhasish Mandal, R.E. Cohen, K. Haule The dramatic increase of superconducting temperature with external pressure in undoped FeSe has opened up a new route to investigate the mechanism of superconductivity. Using the self-consistent density functional theory-dynamical mean field theory (DFT-DMFT) method for paramagnetic FeSe as a function of compression, we find that there is greatly enhanced coupling between some correlated electron states between $\Gamma$ and Z points and the $A_{1g}$ lattice distortion. Except at high pressure, the maximum deformation potential in DFT for this mode is insensitive to increasing pressure, whereas the corresponding DFT-DMFT maximum deformation potential shows the same behavior with pressure as the experimental $T_c$, which first increases and then decreases with pressure. The Fermi surface average of the deformation potential in DFT-DMFT method can increase up to 50\% when compared to standard DFT [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y13.00002: Spin dynamics and a novel orbital-antiphase pairing symmetry in iron-based superconductors Zhiping Yin, Kristjan Haule, Gabriel Kotliar We use first-principles many-body method, including ab initio determined two-particle vertex function, to study the spin dynamics and superconducting pairing symmetry in a large number of Fe-based superconductors. In Fe compounds with high transition temperature, we find both the dispersive high-energy spin excitations, and very strong low energy commensurate or nearly commensurate spin response, suggesting that these low energy spin excitations play the dominate role in cooper pairing. We find three closely competing types of pairing symmetries, which take a very simple form in the space of active Fe $3d$ orbitals, and differ only in the relative quantum mechanical phase of the $xz$, $yz$ and $xy$ orbital contributions. The extensively discussed s$^{+-}$ symmetry appears when contributions from all orbitals have equal sign, while the opposite sign in $xz$ and $yz$ orbitals leads to the $d$ wave symmetry. A novel orbital antiphase $s^{+-}$ symmetry emerges when $xy$ orbital has opposite sign to $xz$ and $yz$ orbitals. We propose that this orbital-antiphase pairing symmetry explains the puzzling variation of the experimentally observed superconducting gaps on all the Fermi surfaces of LiFeAs. This novel symmetry of the order parameter may be realized in other Fe superconductors. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y13.00003: In-plane anisotropic resistivity in the antiferromagnetic phase of Fe-based superconductors Koudai Sugimoto, Peter Prelov\v{s}ek, Eiji Kaneshita, Takami Tohyama The parent compound of Fe-based superconductors has a stripy-ordered antiferromagnetic phase showing in-plane anisotropy of resistivity. Recent experiments have suggested that impurities such as Co substituted for Fe play a crucial role in the anisotropy. We start with a five-orbital Hubbard model with mean-field approximation. We examine the anisotropy of resistivity in the antiferromagnetic phase by applying the memory-function approach treating the isotropic nonmagnetic impurities. Near undoped region, where the Drude weight gives anisotropy opposite to experimental observation, the memory-function approach yields a proper anisotropic behavior: The resistivity in the antiferromagnetically ordered direction is smaller than that in the ferromagnetic direction and the anisotropy reverses when holes are introduced. The origin of the anisotropy can be understood from the interplay of impurity scattering and the character of Fermi surface. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y13.00004: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y13.00005: Theory of disorder-limited transport anisotropy in Fe-based superconductors Peter Hirschfeld, Yan Wang, Indranil Paul The unusual temperature dependence of the resistivity and its in-plane anisotropy observed in the Fe-based superconducting materials, particularly BaFe$_2$As$_2$, has been a longstanding puzzle. Here we construct a crude phenomenological model of the scattering of electrons from dopant induced nematic impurity states which explains many qualitative features of these experiments. Within this model, the high-temperature transport anisotropy near the magnetic and structural transitions is due almost entirely to the disordered nematogens, while below $T_N$. Fermi surface reconstruction competes with this effect. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y13.00006: Collective excitations in AFe2Se2 superconductors: application to Raman Maxim Khodas, Andrey Chubukov We present studies of Raman scattering in A$_x$Fe$_{2-y}$Se$_2$ (A=K,Rb,Cs) superconductors with only electron pockets. The pairing symmetry consistent with both ARPES and neutron scattering is hybridization induced $s^{+-}$ state. Such order parameter changes sign between the hybridized pockets. The peak in calculated $B_{2g}$ Raman intensity signifies the in-gap charge excitation of a $d$-wave symmetry. A single $B_{2g}$ mode emerges as a result of strong mixing between the $d$-wave superconducting fluctuations (Bardasis-Schrieffer modes) and the charge nematic fluctuations. Increase in pocket ellipticity promotes $d$-wave ordering. As a result the $B_{2g}$ Raman mode softens, and becomes critical at the transition to an $s+id$ state with broken time reversal symmetry. The reported $B_{2g}$ excitation corresponds to the in-gap feature in experimentally observed Raman spectra. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y13.00007: The iron pnictides and chalcogenides, a DMFT perspective Invited Speaker: Kristjan Haule The complex multi-band nature of iron pnictides and chalchogenides makes the interplay of superconductivity with spin and orbital dynamics very intriguing, leading to very material dependent magnetic excitations, and pairing symmetries. We use the first-principles Dynamical Mean Field method, including ab-initio determined two-particle vertex function, to study the spin dynamics and superconducting pairing symmetry in a large number of iron-based superconductors. In iron compounds with high transition temperature, we find both the dispersive high-energy spin excitations, and very strong low energy commensurate or nearly commensurate spin response, suggesting that these low energy spin excitations play the dominat role in cooper pairing. We find three closely competing types of pairing symmetries, which take a very simple form in the space of active iron $3d$ orbitals, and differ only in the relative quantum mechanical phase of the $xz$, $yz$ and $xy$ orbital contributions. The extensively discussed s$^{+-}$ symmetry appears when contributions from all orbitals have equal sign, while the opposite sign in $xz$ and $yz$ orbitals leads to the $d$ wave symmetry. A novel orbital antiphase $s^{+-}$ symmetry emerges when $xy$ orbital has opposite sign to $xz$ and $yz$ orbitals. We propose that this orbital-antiphase pairing symmetry explains the puzzling variation of the experimentally observed superconducting gaps on all the Fermi surfaces of LiFeAs. This novel symmetry of the order parameter may be realized in other iron superconductors. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y13.00008: Magnetic impurities in the two-band model for Fe-based superconductors M.M. Korshunov, O.V. Dolgov, A.A. Golubov, D.V. Efremov Superconductors with different gap symmetries behave differently being subject to the disorder. It is especially important to determining this exact behavior in the Fe-based materials where both the order parameter symmetry and the mechanism of superconductivity are unknown. Here we analyze how the magnetic disorder affects the low-energy properties of the two-band $s_\pm$ and $s_{++}$ models. In a general case, $T_c$ is suppressed approximately following the Abrikosov-Gor'kov trend. There are, however, few exceptional cases with the saturation of $T_c$ for the finite amount of impurities: 1) $s_\pm$ superconductor with the purely interband impurity scattering potential or with the unitary impurities, 2) $s_{++}$ state with the purely interband scattering. We show that the latter unusual behavior is due to the $s_{++} \to s_\pm$ transition. Similar to the case of non-magnetic impurities in a two-band superconductor, the transition occurs depending on the sign of the average coupling constant $\langle \lambda \rangle$. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y13.00009: Magnetic Domain Walls Induced by Twin Boundaries in Low Doped Fe-pnictides Bo Li, Jian Li, Kevin Bassler, C.S. Ting Inspired by experimental observations of the enhancement of superconductivity at the twin-boundary (TB) in slightly electron doped Ba(Ca)(FeAs)$_2$ where a strong $2\times1$ antiferromagnetic (AF) collinear order is in presence, we investigate theoretically the effects of TBs on the complex interplay between magnetism and superconductivity using a minimum phenomenological two-orbital model. The spatial distributions of the magnetic, superconducting and charge density orders near two different types of TBs are calculated. Each of the TBs has two different orientations. We find that the first type TBs, which corresponds to a $90^\circ$ lattice rotation in the $a-b$ plane, enable magnetic domain walls (DWs) to be pinned at them, and that superconductivity is enhanced at such TBs or DWs. This result is consistent with experiments for a TB with an orientation of $45^\circ$ from the x-axis. Contrastingly, we predict that superconductivity is suppressed at the second type of TBs which correspond to an asymmetrical placement of As atoms on the opposite sides of the TB. Furthermore, the lattice-mismatch effect across the TBs is investigated. The comparison of our results with the observations from the nuclear-magnetic-resonance (NMR) experiments is also discussed. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y13.00010: Disorder effects in a multiband superconductor in the presence of competing order : Implications for underdoped pnictides Vivek Mishra In unconventional superconductors superconductivity emerges at the onset of a magnetic order, and in many cases with a co-existing region of superconductivity and magnetism in the phase diagram. Here I consider the effect of disorder in a multiband superconductor appropriate for ferro-pnictide superconductors. I consider both interband and intraband scattering for a two band model within a self consistent T-matrix approximation. I calculate the effect of disorder on the critical temperature and on the low energy excitation spectrum in the superconducting state with different possible order parameters. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y13.00011: Localization Effects of Cu Substitutions in FeSe: an Improved Green's Function Decoupling Approach Yang Liu, Yun Song We study the substitution effects of Cu in FeSe system by using a Green's function decoupling method based on an extended Hubbard-I approximation. A three-band Hubbard Hamiltonian with randomly distributed impurities is employed to describe the iron superconductor Fe$_{1-x}$Cu$_{x}$Se. The parameters of the three-band model are determined by fitting the Fermi structures with the results of experiments. With the increasing of Cu substitution, the nesting between hole-like and electron-like Fermi pockets is destroyed completely. We also find that Cu substitution can introduce strong localization effects and lead the system to have a metal- insulator transition. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y13.00012: Energy-dependent modulations in the local density of states in the under-doped and optimally doped NaFe$_{1-x}$Co$_{x}$As Lihua Pan, C.S. Ting Motivated by recent scanning tunneling microscopy experiment investigating the quasiparticle interference (QPI) patterns in NaFe$_{1-x}$Co$_{x}$As, we investigate the energy-dependent modulation of local density of states induced by a weak defect using the first-order T-matrix approximation. In the under-doped sample with spin-density-wave ordering, the electrons disperse along the antiferromagneitc direction but remain static along the ferromagnetic direction. The optimally doped sample exhibits square-like QPI patterns. The corresponding QPI in $\textbf{k}$ space is also presented. These numerical results exhibit essential features as those measured by the experiment. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y13.00013: Phase diagram with doping dependence in phosphorous-doped iron-based superconductor and magnetic order study Yuanyuan Zhao, Yuan-Yen Tai, C.S. Ting Recent experiments discover the isovalent doping can induce superconductivity in a unique iron-based superconductor, BaFe2(As1-xPx)2, through substitution of phosphorous (P) for arsenic (As). While the phosphorous (P-) doping is often considered not to alter the occupation of Fe-3d bands, surprisingly, it has the similar phase diagram just like the heterovalent doped cases: with the P-doping, the magnetic order is suppressed and the superconductivity emerges. Here we theoretically give a possible explanation of the phase diagram based on a recent minimal two-orbital tight-binding model. Moreover, through the spin susceptibility calculation, we try to study the effect of the hopping parameters on the magnetic order. [Preview Abstract] |
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