Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A26: Focus Session: Iron Based Superconductors -- Theory |
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Sponsoring Units: DMP DCOMP Chair: Qimiao Si, Rice University Room: D162/164 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A26.00001: Domain Walls in Normal and Superconducting States of Iron Pnictides Huaixiang Huang, Degang Zhang, Tao Zhou, C.S. Ting The electronic and magnetic structures in the normal and superconducting states of iron pnictides are investigated by solving self-consistently the Bogoliubov-de Gennes equation. It is shown that strong electron correlations can induce domain walls, which separate regions with different spin density wave orders. At zero or low electron doping, $90^{\circ}$ domain walls are formed while anti-phase domain walls are produced at higher electron doping. On the domain walls, there always exist larger electron densities. The results agree qualitatively with recent observations of scanning tunneling microscopy and superconducting quantum interference device microscopy. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A26.00002: Correlation, magnetization and conduction in iron pnictides and iron chalcogenides Zhiping Yin, Kristjan Haule, Gabriel Kotliar By combining density functional theory (DFT) and dynamical mean field theory (DMFT), we study the electronic properties of iron pnictides and iron chalcogenides in both the paramagnetic and magnetic states. With ab initio derived realistic Coulomb interaction U and Hund's exchange coupling J, we find detailed agreements bewtween our calculations and many experimental observations in these compounds, including ARPES, magnetic properties, optical conductivity and anisotropy, and so on, WITHOUT any adjustment such as shifting of atomic positions, Fermi level and bands and renormalizations of bands which are commonly needed in DFT calculations in order to compare with experiments. Our theory explains the origin of the different magnetizations in FeTe and other iron pnictides and provides a unique physical picture. We find that in the magnetic phase of the iron pnictides, both the spin and the orbital polarization are strongly energy dependent. The spin polarization becomes weaker around Fermi level when the orbital polarization is stronger and vice verse at high energies. We stress on the role of the Hund's J rather than the Coulomb U and show how the iron pnictides and iron chalcogenides differ from other compounds. [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A26.00003: Charged Stripes in the Two-Orbital Hubbard Model for Pnictides Dao-Xin Yao, Qinlong Luo, Thomas Prestel, Maria Daghofer, Adriana Moreo, Elbio Dagotto The two-orbital Hubbard model for the pnictides is studied numerically in the real-space Hartree-Fock approximation. Upon electron doping, states with a nonuniform ditribution of charge are stabilized. The patterns observed correspond to charge stripes that run perpendicular to the direction of the spin stripes of the undoped magnetic ground state. These striped states are robust when the undoped state has a gap, although with a decreasing amplitude as the gap decreases. Results for hole doping and implications for recent experiments that reported electronic nematic states and spin incommensurability in the pnictides are also discussed. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 9:12AM |
A26.00004: Computational studies of models for the magnetism and superconductivity in iron pnictides Invited Speaker: The properties of multiorbital electronic model Hamiltonians for the pnictides are explored using a variety of many-body techniques. Via mean-field approximations, a regime where the undoped system develops $(\pi,0)$ magnetic order while remaining metallic is found at intermediate values of the Hubbard repulsion $U$. Comparison of our results against ARPES and neutron scattering data allows us to determine a range of realistic values for the parameters in the models [1]. The orbital spectral weight redistribution that occurs near the Fermi surface in the $(\pi,0)$ magnetic state without long-range orbital order is also discussed [2]. The two-orbital ``$t$-$U$-$J$'' Hubbard model at intermediate $U$, with magnetic order and pairing tendencies enhanced by the addition of Heisenberg terms that arise from the strong coupling expansion, is studied via exact diagonalization. At intermediate couplings and considering two extra electrons added to the undoped system, an $A_{1g}$ bound state is found compatible with the ``extended s$\pm$'' pairing discussed in the RPA approximation. Bound states with $B_{2g}$ symmetry, involving intra- and inter-band components, are also stable in portions of the phase diagram, while states with $B_{1g}$ symmetry are close in energy, suggesting that small changes in parameters may render any of the three channels stable [3]. Finally, using the real-space Hartree-Fock approximation on finite clusters the presence of charge stripes at intermediate $U$ is also observed for electron-doped systems. The patterns of charge, spin, and orbital order, as well as the influence of quenched disorder will be discussed [4].\\[4pt] [1] Q. Luo {\it et al.}, Phys. Rev. B {\bf 82}, 104508(2010). See also R. Yu {\it et al.}, Phys. Rev. B {\bf 79}, 104510 (2009).\\[0pt] [2] M. Daghofer {\it et al.}, Phys. Rev. B {\bf 81}, 180514(R) (2010).\\[0pt] [3] A. Nicholson {\it et al.}, preprint. See also M. Daghofer {\it et al.}, Phys. Rev. Lett. {\bf 101}, 237004 (2008), and A. Moreo {\it et al.}, Phys. Rev. B {\bf 79}, 134502 (2009).\\[0pt] [4] Q. Luo {\it et al.}, preprint. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A26.00005: Order-Parameter Anisotropies in the Pnictides - An Optimization Principle for Multi-Band Superconductivity Christian Platt, Ronny Thomale, Werner Hanke Using general arguments of an optimization taking place between the pair wave function and the repulsive part of the electron-electron interaction, we analyze the superconducting gap in materials with multiple Fermi-surface (FS) pockets, with application to two proto-type (P-based and As-based) ferropnictides. The main point of our work is to show that the SC state, its gap and, in particular, its anisotropy in momentum space is determined by an optimization, which balances the interplay between the attractive interaction in the sign-reversing $s_{\pm}$-channel and the Coulomb repulsion. This Coulomb repulsion, as discussed below, is unavoidable in a multi-band SC situation: it appears because of a kind of frustration in the $s_{\pm}$-channel, when more than two FS-pockets are involved in setting up the pairing interaction. On the basis of functional Renormalization Group (fRG) calculations for a wide parameter span of the bare interactions and for the different FS topologies applying to these two characteristic Fe-based superconductors, we show that the symmetry of the gap and the nodal versus nodeless behavior is driven by this optimization requirement. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A26.00006: Electronic Structure of High-$T_c$ Iron-Pnictide Superconductors from the Strong Correlation Limit Jose Rodriguez A two-orbital t-J model for a square lattice of iron (pnictide) atoms that includes magnetic frustration and Hund's rule coupling is studied in the limit where inter-orbital hopping of holes is prohibited. A hidden half-metal phase is predicted at weak enough Hund's rule coupling, where holes move coherently through opposing ferromagnetic spin arrangments that are assigned to each orbital. In particular, two Fermi surface hole pockets centered at zero momentum that have unrenormalized Fermi velocities are predicted. Next, the same model is studied at the quantum critical point that separates the hidden ferromagnet from the commensurate spin-density wave (cSDW), where low-energy spinwaves disperse anisotropically away from cSDW wave numbers. Composite hole-spinwave excitations result in ``shadow'' hole Fermi surfaces that are centered at cSDW wave numbers. We explore the possibility that these ``shadow'' bands are intrinsically diffuse enough that they simulate electron bands. Last, determinations of the low-energy spectrum of one hole by numerical exact diagonalization confirms the existence of degenerate ground states at momenta $(0,0)$ and $(\pi,0)$ at a quantum critical point. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A26.00007: Pair hopping mechanism of enhancement in $T_{c}$ for layered superconductors Koichi Kusakabe Two body effective interactions coming from the quantum charge fluctuation may induce pairs tunneling between adjacent layers in high-Tc materials including cuprates, iron-pnictides, MgB$_{2}$, and MNX. This mechanism [1] is favored when 1) the one-body Hamiltonian shows negligible inter-layer single electron hopping for the 2D liquid around the Fermi level, and 2) unfilled extended orbitals support the pair tunneling via local two-electron scattering. Localized nature of the 2D liquid is essential. The density functional theory (DFT) can prove this picture in two steps. The Kohn-Sham scheme tells that the single-particle effective Hamiltonian possess these aspects most clearly for the highest $T_{c}$ material. The multi-reference generalization of DFT allows us to evaluate existence and relevance of the super pair tunneling. A possible mechanism for layered organic superconductors is also discussed. \\[4pt] [1] K. Kusakabe, J. Phys. Soc. Jpn., \textbf{78} (2009) 114716. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A26.00008: Charge Density Wave Induction by Spin Density Wave in Iron-Based Superconductors Alexander Balatsky, Dmitri Basov, Jian-Xin Zhu We argue that spin density wave (SDW) phase in ferrous superconductors contains charge density wave (CDW) with the modulation momentum that is a double of characteristic momenta of SDW [1]. We discuss symmetry constraints on allowed momenta of CDW generated by coupling to spin modulations. To be specific we considered the CDW that could be realized in Fe-11 (e.g., FeTe) and Fe-122 (e.g., BaFe2As2) compounds. In case of commensurate SDW, the CDW modulation vector is at the Bragg peaks positions and could be revealed by local scanned probes. In case of incommensurate SDW, the CDW is incommensurate and can be seen also by x-ray and elastic neutron scattering. We also discuss observable charge modulation due to CDW formation near defects and twin boundaries.\\[4pt] [1] A. V. Balatsky, D. N. Basov, and Jian-Xin Zhu, PHYSICAL REVIEW B \textbf{82}, 144522 (2010). [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A26.00009: Theory of Valley-Density Wave and Hidden Order in Iron-Pnictides Jian Kang, Zlatko Tesanovic In the limit of perfect nesting, the physics of iron-pnictides is governed by the density wave formation at the zone-edge vector \textbf{M}. At high energies, various spin- (SDW), charge- (CDW), orbital/pocket- (PDW) density waves, and their mutually orthogonal linear combinations, all appear equally likely, unified within the unitary order parameter of the $U(4)\times U(4)$ symmetry. Nesting imperfections and low-energy interactions reduce this symmetry to that of real materials. Nevertheless, the generic ground state preserves a distinct signature of its highly symmetric origins: an SDW along one axis of the square iron lattice is predicted to \textit{coexist} with a PDW along the perpendicular axis, accompanied by a modulated pattern of weak charge currents on inter-iron bonds. This ``hidden" order induces the tetragonal-orthorhombic structural transition in our theory, naturally insures $T_s \ge T_N $, and leads to other observable consequences. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A26.00010: Topological and Transport Properties of Dirac Fermions in Antiferromagnetic Metallic Phase of Iron-Based Superconductors Takami Tohyama, Takao Morinari, Eiji Kaneshita, Koudai Sugimoto We investigate Dirac fermions in the antifferomagnetic metallic state of iron-based superconductors [1]. Deriving an effective Hamiltonian for Dirac fermions, we reveal that there exist two Dirac cones carrying the same chirality, contrary to graphene, compensated by a Fermi surface with a quadratic energy dispersion as a consequence of a non-trivial topological property inherent in the band structure. We also find that the presence of the Dirac fermions gives the difference of sign-change temperatures between the Hall coefficient and the thermopower. This is consistent with available experimental data. The Dirac ferimons also contribute to in-plane anisotropy of the optical conductivity [2].\\[4pt] [1] T. Morinari, E. Kaneshita, and T. Tohyama, Phys. Rev. Lett. {\bf 105}, 037203 (2010).\\[0pt] [2] K. Sugimoto, E. Kaneshita, and T. Tohyama, submitted to J. Phys. Soc. Jpn. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A26.00011: Quasiparticle states around a nonmagnetic impurity in the spin-density-wave state of iron-pnictide superconductors Tao Zhou, Huaixiang Huang, Yi Gao, Jianxin Zhu, C.S. Ting The quasiparticle states around a non-magnetic impurity in the electron doped iron-based superconductors with the presence of spin-density-wave (SDW) ordering will be investigated as a function of doping and for various impurity strengths. We found that In the undoped sample,two resonance peaks are found to approach the Fermi level on the impurity site with the strength of scattering potential increasing from weak to moderate. For doped samples, where the SDW order and the superconducting order coexist, there are two intra-gap resonance peaks for weak scattering potential. For strong scattering potential, one sharp peak appears near fermi energy in underdoped sample and separates to two peaks for larger dopings. For all the cases, the local density of states exhibits clear $C_2$ symmetry. Our results provide an effective tool to detect the SDW order and probe the coexistence of the SDW and superconducting orders. [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A26.00012: Renormalization group flow, competing phases, and gap structure in multi-band models of Fe-based superconductors Andrey Chubukov, Saurabh Maiti We perform an analytical renormalization group (RG) study to address the role of Coulomb repulsion, the competition between extended s-wave superconducting order ($s\pm$) and the spin density wave (SDW) order, and the angular dependence of the superconducting gap in multi-pocket models of Fe-based superconductors. Previous analytic RG studies considered a toy 2-pockets model (one hole and one electron). We consider more realistic models of 4 and 5 pockets (2 electron and 2 or 3 hole pockets), and also incorporate angular dependences of the interactions caused by the transformation from orbital to band description. In a toy 2-pocket model, SDW order always wins over $s\pm$ order at perfect nesting; $s\pm$ order only appears when doping is finite and RG flow extends long enough to overcome intra-pocket Coulomb repulsion. In multi-pocket models, there are two new effects. First, the pairing interaction projected onto $s\pm$ channel has an attractive component no matter how strong intra-pocket repulsion is. Second, in 4-pocket model (but not in 5-pocket model), $s\pm$ order wins over SDW order even for perfect nesting, if parquet RG flow extends long enough, suggesting that SDW order is not a necessary pre-condition for the $s\pm$ order. Our analytic results are in full agreement with recent numerical functional RG studies by Thomale et. al. [Preview Abstract] |
Monday, March 21, 2011 10:48AM - 11:00AM |
A26.00013: Large D-2 Theory of Superconducting Fluctuations in a Magnetic Field and its Application to Iron Pnictides James Murray, Zlatko Tesanovic A Ginzburg-Landau approach to fluctuations of a layered superconductor in a magnetic field is used to show that the interlayer coupling can be incorporated within an interacting self-consistent theory of a single layer, in the limit of a large number of neighboring layers [1]. The theory exhibits two phase transitions: a vortex liquid-to- solid transition is followed by a Bose-Einstein condensation into the Abrikosov lattice, illustrating the essential role of interlayer coupling. By using this theory, explicit expressions for magnetization, specific heat, and fluctuation conductivity are derived. We compare our results with recent experimental data on the iron-pnictide superconductors.\\[4pt] [1] J. M. Murray and Z. Te\v{s}anovi\'{c}, \textit{Phys. Rev. Lett.} {\bf 105}, 037006 (2010). [Preview Abstract] |
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