Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session P39: Focus Session: Iron Based Superconductors: Theory II |
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Sponsoring Units: DMP DCMP Chair: Michelle Johannes, Navel Research Laboratory Room: F150 |
Wednesday, March 17, 2010 8:00AM - 8:12AM |
P39.00001: Ginzburg-Landau theory of layered superconductors: application to iron pnictides James Murray, Zlatko Tesanovic We use the Ginzburg-Landau approach for a layered superconductor with a Josephson coupling between the layers to study the effects of fluctuations in recently discovered iron-based high temperature superconductors. While such effects are theoretically well understood in three- and particularly in two-dimensional systems, the special challenge posed by iron pnictides is their layered but still effectively three dimensional character. Using an approximation devised for this particular situation, we derive theoretical expressions for the fluctuation contributions to the free energy, magnetization, specific heat, and electrical conductivity. We compare our results with recent experimental data on the iron pnictide superconductors. [Preview Abstract] |
Wednesday, March 17, 2010 8:12AM - 8:24AM |
P39.00002: Itinerant Antiferromagnetism in the Fe-Pnictides Vladimir Cvetkovic, Yan He, Chandra Varma We propose a model for the itinerant antiferromagnetism in the Fe-pnictides and, as a test for it, calculate the magnetic phase diagram. The model includes kinetic energy for the five orbitals of Fe and local Hund's rule and repulsion. We also hope to derive a free energy for the model so as to calculate the fluctuation spectra and explain the remarkable universal temperature dependence of the susceptibility above T$_N$. [Preview Abstract] |
Wednesday, March 17, 2010 8:24AM - 8:36AM |
P39.00003: Coexistence of superconductivity and spin-density wave in pnictide superconductors: Effect of a transverse Zeeman field Pouyan Ghaemi, Ashvin Vishwanath In several members of the ferro-pnictides, spin density wave (SDW) order coexists with superconductivity over a range of dopings. Contrary to naive expectations, it has been shown that extended s-wave superconductivity can coexist with a SDW. Here we study the unusual magnetic field response of this state. We focus on the effect of the Zeeman splitting, which is expected to be enhanced due to the transverse susceptibility of the SDW. On increasing the field, a strongly anisotropic reduction of superconducting gap is found. The signature of this effect in quasiparticle interference measured by STM, as well as heat transport in magnetic field is discussed. We show that it can be used as an evidence of extended s-wave gap structure in pnictide superconductors. [Preview Abstract] |
Wednesday, March 17, 2010 8:36AM - 8:48AM |
P39.00004: Magnetic fluctuation and Gap symmetry in Iron-Pnictide Superconductors Hiroaki Ikeda, Ryotaro Arita Since the recent discovery of high-$T_c$ superconductivity in LaFeAsO$_{1-x}$F$_x$, the Fe-based superconductors have been intensively investigated. In order to understand the overall feature of the phase diagram, we investigate the unfolded 5-band Hubbard model, which can describe the band structure near the Fermi level in LaFeAsO, within the fluctuation-exchange approximation. We find the enhancement of the antiferromagnetic (AF) spin fluctuation in the hole-doped region and the pseudogap behavior of NMR-$1/T_1$ in the electron-doped region. Evaluating the Eliashberg equation, we find that the sign-reversing $s_\pm$-wave pairing state appears in close proximity to the AF phase. Roughly speaking, the gap function is fully-gapped in the hole-doped region, and remarkably anisotropic in the electron-doped region. The eigenvalue $\lambda$ is relatively small, and insensitive to carrier doping for small Hund coupling $J$. On the contrary, for large $J$, it is relatively large, and sensitive to carrier doping and the presence of the AF phase. [Preview Abstract] |
Wednesday, March 17, 2010 8:48AM - 9:00AM |
P39.00005: Comparison of \textit{Ab initio} Low-Energy Models for LaFePO, LaFeAsO, BaFe$_{2}$As$_{2}$, LiFeAs, FeSe, and FeTe Kazuma Nakamura, Takashi Miyake, Ryotaro Arita, Masatoshi Imada We present effective low-energy models for LaFePO and LaFeAsO (1111 family), BaFe$_{2}$As$_{2}$ (122), LiFeAs (111), and FeSe and FeTe (11) [1], based on \textit{ab initio} downfolding scheme, a constrained random-phase-approximation method combined with maximally localized Wannier functions. Comparison among the effective models, derived for 5 Fe-3$d$ bands, provides a basis for interpreting physics/chemistry; material dependences of electron correlations, a multiband character entangled by the 3$d$ orbitals, and the geometrical frustration depending on hybridizations between iron and pnictogen/chalcogen orbitals. We found that LaFePO in the 1111 family resides in the weak correlation regime, while LaFeAsO and 111/122 compounds are the intermediate region and FeSe and FeTe in the 11 family are located in the strong correlation regime. A principal parameter relevant to the physics is clarified to be the pnictogen/chalcogen height from the iron layer. Implications in low-energy properties including magnetism and superconductivity are discussed. [1] T. Miyake, K. Nakamura, R. Arita, and M. Imada, arXiv:0911.3705. [Preview Abstract] |
Wednesday, March 17, 2010 9:00AM - 9:12AM |
P39.00006: Kinetic Magnetism and Orbital Order in Iron Telluride Ari Turner, Ashvin Vishwanath Iron Telluride (FeTe), related to the new Iron-pnictide high temperature superconductors, has an interesting low-temperature phase. This phase has an unusual magnetic order, as well as a structural distortion, and it is a conductor. This talk presents a model in which these facts are related to one another and to orbital ordering. Evidence for or against this hypothesis can be acquired from measurements of conductivity, ARPES, neutron scattering and evolution of the phase diagram on changing chemical composition. [Preview Abstract] |
Wednesday, March 17, 2010 9:12AM - 9:24AM |
P39.00007: Impact of the tetrahedral distortion in the superconducting and magnetic properties of iron pnictides Belen Valenzuela, Maria Jose Calderon, Elena Bascones The origin of magnetism and superconductivity in iron pnictides is unknown. An added complexity in these materials is the strong impact in the electronic properties brought by small distortions of the As-Fe tetrahedra. We have proposed a five orbital tight binding model using the Slater-Koster framework that with {\it just four} parameters reproduce the bands and Fermi surface found with first principle calculations [1]. The good agreement between our results and DFT predictions extends to the orbital weight of each band. Using this model we study the magnetic and superconducting state and analyze how it depends on the distortion of the tetrahedron.\\[4pt] [1] M.J. Calder\'on, B. Valenzuela and E. Bascones, Phys. Rev. B 80, 94531 (2009) [Preview Abstract] |
Wednesday, March 17, 2010 9:24AM - 9:36AM |
P39.00008: Ferro-Orbital Order and Anisotropic Magnetic Structure in Parent Compounds of Iron Pnictides Chi-Cheng Lee, Wei-Guo Yin, Wei Ku Recently observed strong anisotropy of magnetic excitations [1] imposes a serious challenge to the current understanding of the electronic structure of parent compounds of iron pnictides. Here we examine the electronic structure of the representative LaOFeAs, using a first-principles Wannier-function analysis [2]. Without resorting to the widely employed frustration or nesting picture, a robust ferro-orbital ordering is found to give rise to the observed strongly anisotropic magnetic coupling, and drive both magnetic and lattice phase transitions. The revealed necessity of the additional orbital physics leads to a correlated electronic structure fundamentally distinct from that of the cuprates. In particular, the strong coupling to the magnons advocates the active roles of light orbitons in spin dynamics and electron pairing in iron pnictides. \\[4pt] [1] Jun Zhao et. al., Nature Physics, 5, 555 (2009)\\[0pt] [2] Chi-Cheng Lee et. al., http://arxiv.org/abs/0905.2957 [Preview Abstract] |
Wednesday, March 17, 2010 9:36AM - 9:48AM |
P39.00009: First-principles study of the iron pnictide superconductor BaFe2As2 Ethem Akturk, Salim Ciraci This work presents our study on the atomic, electronic, magnetic structures, and phonon modes of the low-temperature orthorhombic phase of undoped BaFe2As2 crystal. The electronic structure is characterized by a sharp Fe-3d peak close to the Fermi level and is dominated by Fe-3d- and As-4p-hybridized states. Ba contribution occurs only at lower energies. The spin ordering of the magnetic ground state, which is determined by minimizing the total energy of different spin alignments on Fe atoms in the conventional cell, is in agreement with experimental findings but is different from the antiferromagnetic spin ordering obtained by assigning antiparallel spin directions on two Fe atoms in the primitive unit cell. Valuable information about the charge transfer and bonding is revealed through the analysis of the charge density. Electrons are transferred from Ba to Fe-As layers and also from Fe to As atoms. The magnetic phonon calculations of the ground state are carried out to predict Raman and infrared-active modes. Softening of some calculated spin-dependent phonon modes corroborates the contribution of spin-lattice coupling to the structural phase transition from I4/mmm to Fmmm. [Preview Abstract] |
Wednesday, March 17, 2010 9:48AM - 10:00AM |
P39.00010: Electron correlation and spin density wave order in iron pnictides Sen Zhou, Ziqiang Wang We study the correlation effects on the electronic structure and spin density wave order in Fe-pnictides. Using the multiorbital Hubbard model and Gutzwiller projection, we show that nonperturbative correlation effects are essential to stabilize the metallic spin density wave phase for the intermediate correlation strengths appropriate for pnictides. We find that the ordered moments depend sensitively on the Hund's rule coupling $J$ but weakly on the intraorbital Coulomb repulsion $U$, varying from $0.3\mu_B$ to $1.5\mu_B$ in the range $J=0.3\sim0.8$ eV for $U=3\sim4$ eV. We obtain the phase diagram and discuss the effects of orbital order and electron doping, the evolution of the Fermi surface topology with the ordered moment, and compare to recent experiments. [Preview Abstract] |
Wednesday, March 17, 2010 10:00AM - 10:12AM |
P39.00011: Two magnon Raman scattering in a $J_1-J_2$ antiferromagnet and its implications for iron arsenides Elihu Abrahams, Pallab Goswami, Qimiao Si The observation spin-wave excitations at the zone boundary and up to energies $200$ meV in inelastic neutron scattering measurements in iron arsenides have highlighted the relevance of quasi-local moments and $J_1-J_2$ magnetic frustration in these materials. Motivated by these experimental observations, we have studied two magnon Raman scattering intensity of a $J_1- J_2$ Heisenberg antiferromagnet for different experimental geometries. We demonstrate that careful measurements of Raman intensity in different geometry can be useful in understanding the role of local moments, and magnetic frustration in iron arsenides. [Preview Abstract] |
Wednesday, March 17, 2010 10:12AM - 10:24AM |
P39.00012: Exchange interactions in iron pnictides: beyond long wave approximation Vladimir Antropov, Liqin Ke, Mark van Schilfgaarde The character of magnetism in the recently discovered iron pnictides is unusual. In particular, the exchange coupling parameters have attracted a lot of attention. However, calculation of these parameters have been based on methods or approximations suitable only for localized magnetic moment systems. Such approximations greatly underestimate the amount of possible itinerant magnetic excitations and can result not only in large quantitative errors, but they often create a wrong picture of magnetic interactions in systems with itinerant magnetic electrons. We present new calculations of the exchange coupling parameters, going beyond the widely accepted long-wave approximation with application for most popular iron pnictide systems. We compare it with earlier calculations and the experimental neutron analysis. The criteria of applicability of long wave approximations have been estimated for a wide range of magnetic systems. [Preview Abstract] |
Wednesday, March 17, 2010 10:24AM - 10:36AM |
P39.00013: Is Sr2VO3FeAs a New Paradigm for Fe-based Superconductors? Igor Mazin One of the most popular scenarios for the superconductivity in Fe-based superconductors (FeBSC) posits that the bosons responsible for electronic pairing are spin-fluctuations with a wave vector spanning the hole Fermi surfaces (FSs) near $\Gamma$ and the electron FSs near M points. So far all FeBSC for which neutron data are available do demonstrate such excitations, and the band structure calculations so far were finding quasi-nested FSs in all FeBSC, providing for a peak in the spin susceptibility at the desired wave vectors. However, the newest addition to the family, Sr$_{2}$VO$_{3}$FeAs, has been calculated to have a very complex FS with no visible quasi-nesting features. It was argued therefore that this material does not fall under the existing paradigm and calls for revisiting our current ideas about what is the likely cause of superconductivity in FeBSC. I show that the visible complexity of the FS is entirely due to the V-derived electronic states. Assuming that superconductivity in Sr$_{2}$VO$_{3}$FeAs, as in the other FeBSC, originates in the FeAs layers, and the superconducting electrons are sensitive to the susceptibility of the FeAs electronic subsystem, I recalculate the bare susceptibility, weighting the electronic states with their Fe character, and obtain a susceptibility that fully supports the existing quasi-nesting model. I also present results of magnetic calculations with 5 different patterns and discuss possible peculiarities of the spin fluctuation in this system. [Preview Abstract] |
Wednesday, March 17, 2010 10:36AM - 10:48AM |
P39.00014: Tight-binding Hamiltonian for LaFeAsO D. A. Papaconstantopoulos, Michael Mehl, Michelle Johannes, David Parker We construct a realistic tight-binding Hamiltonian (TB) fitted to first-principles band structure calculations of the pnictide parent compound LaFeAsO. We have come to the conclusion that most previously derived (TB) Hamiltonians poorly reproduce the first-principles results. Here we use the NRL-TB method to fit our LAPW results as a function of volume and As displacement. The TB basis includes the Fe d, As p, and O p-orbitals. We omit the La d-orbitals, which only contribute to states well above the Fermi level. We find a TB band structure that fits an LAPW range of energies with a width of 3.5 eV. The bands near the Fermi level fit the LAPW results very well, reproducing the Fermi surface near $\Gamma$ and M. The TB densities of states agree with first-principles results. We also study the variation of the total energy with respect to the position of the As atoms. We find that the TB total energies fit the LAPW values very well even for structures not included in our fit. We use our accurate hopping parameters to uncover the origin of the so far unexplained pseudogap near the Fermi energy and explore the evolution of this gap with magnetism. [Preview Abstract] |
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