Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B35: Focus Session: Iron Pnictides and Other Novel Superconductors III: General Theory |
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Sponsoring Units: DMP Chair: Alex Koshelev, Argonne National Laboratory Room: 405 |
Monday, March 16, 2009 11:15AM - 11:27AM |
B35.00001: Valley density-wave (VDW) and Superconductivity in Iron-Pnictides Vladimir Cvetkovic, Zlatko Tesanovic One of the experimentally observed features of iron-pnictide superconductors is the structural transition and SDW ordering occurring at almost the same temperature. Starting from a tight-binding model [1], we construct an effective theory for iron-pnictides with the distinctive two hole and two electron Fermi surfaces. This theory is then mapped onto a negative-U Hubbard model with additional orbital and spin flavors [2]. We demonstrate that the superconducting instability of the attractive Hubbard model --- valley density-wave (VDW) --- corresponds to the observed structural and SDW orders. The deviations from perfect nesting between the hole and electron Fermi surfaces are mapped onto the Zeeman field which causes portions of Fermi surface to remain ungapped. The origin of pnictide superconductivity in this model, and its ties to the VDW are discussed. [1] V. Cvetkovic and Z. Tesanovic, http://arxiv.org/abs/0804.4678. [2] V. Cvetkovic and Z. Tesanovic, http://arxiv.org/abs/0808.3742. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B35.00002: Spin-Density Wave in Iron Pnictides Jian Kang, Valentin Stanev, Zlatko Tesanovic Multi-band Hubbard-like model with appreciable nesting is applied to the study of spin-density wave (SDW) in iron pnictides\footnote{ V. Stanev, J. Kang, and Z. Tesanovic, Phys. Rev. B \textbf{78}, 184509 (2008).}. It is assumed that the SDW particle-hole pairing mechanism arises from the short range interaction between hole bands near $\Gamma $ point and electron bands near M. Within the Hubbard-Stratonovich transformation, an auxiliary field is introduced to obtain the effective action. The mean-field solution is obtained by the stationary phase analysis of this action, and results in an itinerant, antiferromagnetically ordered ground state, with the staggered magnetic moment modulation at wavevector M. We study fluctuations of the spin order around M, both in its direction and amplitude. We present detailed results for the propagation velocity of this mode (spin-wave velocity) as a function of the various parameters of our model and compare them to the available experimental observations of the spin-wave spectrum. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B35.00003: Iron-based superconductors: What can we learn from DFT? Lilia Boeri, Oleg Dolgov, Alexander Golubov, Ole Krogh Andersen The discovery of superconductivity in iron pnicticides has initiated an intense theoretical activity. So far, however, not only the pairing mechanism, but even the basic electronic structure of these materials is not well understood. We use Density Functional Theory to understand the electronic and vibrational properties of LaOFeAs, which can be considered a prototype for iron pnictides. First, we calculate the phonon dispersions and electron-phonon coupling using linear response and show that standard Migdal-Eliashberg theory cannot explain the experimental Tc. Then we derive ab-initio an accurate tight-binding Hamiltonian, using downfolding + N-ization (NMTO), which allows us to elucidate the origin of the complicated band structure of iron pnicticides. As a first application of our model, we study magnetism. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B35.00004: Correlations in Ferropnictides Klaus Koepernik, Helmut Eschrig The strength of correlations in the ferropnictide superconductors is still under debate. While arguments for an electron-electron interaction $U$ of $5$eV have been made, some experimental results support a $U$ of merely $1$eV. Density functional theory in the local spin density approximation (LSDA) seems to describe several aspects of the electronic structure quite reasonably, which would also support a smaller $U$. However, the unusually large error of the calculated lattice structure remains a puzzle. We discuss the influence of correlations on the electronic structure and the properties of the ferropnictides in the framework of LSDA+U calculations. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B35.00005: Superconductivity in SrFe$_{2-x}$Co$_x$As$_2$: Internal Doping of the Iron Arsenide Layers Helge Rosner, Andreas Leithe-Jasper, Walter Schnelle, Christoph Geibel In the strontium iron-cobalt arsenides SrFe$_{2-x}$Co$_x$As$_2$ ($0.2\leq x \leq 0.4$) superconductivity with $T_c$ up to 20\,K is observed in magnetic susceptibility, electrical resistivity, and specific heat data. This first observation of bulk superconductivity induced by electron doping in this family of compounds -- despite strong disorder in the Fe-As layer -- favors an itinerant electronic theory in contrast to the strongly correlated cuprates and renders a $p$- or $d$-wave pairing unlikely. The magnetic ordering present in SrFe$_2$As$_2$ is rapidly suppressed by substitution of Fe by Co. DFT calculations show that this is due to a rigid down-shift of the Fe-3$d_{x^2-y^2}$-related band edge in the density of states. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B35.00006: Linear temperature dependence of the spin susceptibility in Fe-pnictides. Dmitri V. Efremov, Andrey V. Chubukov, Ilya M. Eremin, Maxim M. Korshunov, Dmitri L. Maslov We argue that linear $T$ dependence of the spin susceptibility $\chi >T$ observed in Fe pnictides can be explained within the itinerant Fermi liquid model of hole and electron bands. The spin susceptibility is linear in $T$ in a generic Fermi liquid in 2D. We show that for pnictides, the prefactor for the $T$ term comes chiefly from intra-band scattering and is strongly enhanced compared to an ordinary Fermi liquid as it contains precisely the same interaction that gives rise to spin-density-wave ordering. We compare theoretical slope with the data. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B35.00007: Theory of novel and superconducting properties of Fe-based superconductors Invited Speaker: I will discuss antiferromagnetism and superconductivity in novel $Fe-$based superconductors within the itinerant model of small electron and hole pockets near $(0,0)$ and $(\pi,\pi)$. I will argue that the effective interactions in both channels logarithmically flow towards the same values at low energies, {\it i.e.}, antiferromagnetism and superconductivity must be treated on equal footings. The magnetic instability comes first for equal sizes of the two pockets, but looses to superconductivity upon doping. The superconducting gap has no nodes, but changes sign between the two Fermi surfaces (extended $s$-wave symmetry). I will argue that the $T$ dependencies of the spin susceptibility, NMR relaxation rate, and the penetration depth for such state are exponential only at very low $T$, and can be well fitted by power-laws over a wide $T$ range below $T_c$. I will also discuss the type of a transition between spin-density-wave and superconducting states at $T=0$ and at finite $T$, and the linear $T$ dependence of the spin susceptibility in the normal state. \\ Based on the works done with I. Eremin, D. Efremov, M. Korshunov, D. Maslov, M. Vavilov, and A. Vorontsov. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B35.00008: Nodal Spin Density Wave and band topology of the FeAs based materials Hui Zhai, Ying Ran, Fa Wang, Ashvin Vishwanath, Dung-Hai Lee The recently discovered FeAs-based materials exhibit a $(\pi,0)$ Spin Density Wave (SDW) in the undoped state, which gives way to superconductivity upon doping. Here we show that due to an interesting topological feature of the band structure, the SDW state cannot acquire a full gap. This is demonstrated within the SDW mean-field theory of both a simplified two band model and a more realistic 5-band model. The positions of the nodes are different in the two models and can be used to detected the validity of each model. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B35.00009: Normal State Spin Dynamics of Five-band Model for Iron-pnictides Toshikaze Kariyado, Masao Ogata Normal state (assuming absence of SC or AF order) spin dynamics of iron-pnictide superconductors is discussed by calculating spin structure factor $S(q,\omega)$ in an itinerant five-band model within RPA approximation. Due to the characteristic Fermi surface structure of iron-pnictide, column like response is found at $(\pi,0)$ in extended Brillouin zone. This is consistent with recent neutron experiments. Furthermore, we show that the temperature dependence of inelastic neutron scattering intensity is reproduced if we set interaction parameters appropriately. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B35.00010: Symplectic fermion approach to the striped magnetism in the iron arsenides Xun Xue, Jianhui Dai Based on the fact that the near transition temperature of striped SDW and structure distortion in iron pnictides, we propose a symplectic fermion approach to account for this kind of antiferromagnetic properties. The model is expecting for better understanding of the experimental results. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B35.00011: Parquet formalism applied to pnictide superconductors Jun Liu, Karlis Mikelsons, Shuxiang Yang, Herbert Fotso , Mark Jarrell DMFT combined with Parquet approximation is used to study the single particle property of pnictide superconductors (such as FeSe, SrFe2As2,...) in an attemp to understand the enhancement of superconducitivity under pressure. By tracking the evolution of one-particle spectral function, pressure dependence of this type of compound is studied in depth. In the study, inhomogeneous frequency grid is used to high frequency summation. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B35.00012: Jahn-Teller Effect, Structural Phase Transition and Resistivity Anomaly in Iron Pnictides Weicheng Lv, Jiansheng Wu, Philip Phillips We attribute the structural phase transition (SPT) in the parent compounds of iron pnictides to a Jahn-Teller distortion. Due to the anisotropy of the $d_{xz}$ and $d_{yz}$ orbitals in the $xy$ plane, some orbital ordering will make the orthorhombic structure more energetically favorable, thus inducing the SPT. In an orbital ordered system, the sites with orbitals that do not order act as scattering impurities, causing a resistivity anomaly upon the onset of the SPT. Below the SPT, we find that the resistivity displays a $\ln{T}$ divergence. All of these are in agreement with the experiments. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B35.00013: Theory of the Magnetic Moment in Iron Pnictides Jiansheng Wu, Philip Phillips, Antonio Castro-Neto We show that the combined effects of spin-orbit, monoclinic distortion, and p-d hybridization in tetrahedrally coordinated Fe in LaFeAsO invalidates the naive Hund's rule filling of the Fe d-levels. The two highest occupied levels have one electron each but as a result of differing p-d hybridizations, the upper level is more itinerant while electrons in the lower level are more localized. The resulting magnetic moment is highly anisotropic with an in-plane value of $0.25-0.35\mu_B$ per Fe and a z-projection of $0.06\mu_B$, both of which are in agreement with experiment. [Preview Abstract] |
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