Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X35: Focus Session: Iron Pnictides and Other Novel Superconductors XV: Electronic Structure and Magnetism |
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Sponsoring Units: DMP Chair: David Parker, Naval Research Laboratory Room: 405 |
Thursday, March 19, 2009 2:30PM - 3:06PM |
X35.00001: Electronic Structure of Fe-based Superconductors Invited Speaker: Understanding the electronic structure and electronic interactions in the layered Fe superconductors is prerequisite to understanding their superconductivity and other properties. The purpose of this talk is to overview results obtained within band structure approaches in relation to experiment. So far, many puzzles remain. The materials appear to be much more band-like and show much stronger signatures of metallic (Fermi surface related) physics than cuprates, with correspondinly weaker signatures of on-site Hubbard correlations. However, there remain substantial discrepancies between bare band structure calculations and experiment, and interestingly these discrepancies are in the opposite direction from those found in cuprates. These are discussed in the context of spin-fluctuations. This work was done in collaboration with I.I. Mazin, M.H. Du, Lijun Zhang, Alaska Subedi and Michelle Johannes. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X35.00002: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X35.00003: Theory for lattice collapse and frustrated magnetism in FeAs superconductors Rafael Fernandes, Joerg Schmalian We present a theory for the pressure and temperature dependence of the magnetic and structural phase transitions in FeAs superconductors. Magnetic frustration in the FeAs planes leads to an enhanced coupling between lattice and magnetic degrees of freedom and is responsible for the strength of the first order transition from a paramagnetic tetragonal to an antiferromagnetic, orthorhombic phase. We analyze the phase diagram using a large $N$ expansion for the magnetic degrees of freedom coupled to the lattice. Furthermore, we also address the importance of the lattice collapse in the CaFe$_{2}$As$_{2}$ compound and compare our predictions with experiments. Our results demonstrate that it is crucial to simultaneously include lattice and magnetic degrees of freedom for the FeAs systems. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X35.00004: Formation and suppression of Fe magnetism in ferropnictides Igor Mazin, Michelle Johannes First, I will address the issue of how the Fe magnetic moment in HTC pnictides is formed in the DFT calculation, why is it so large (up to 2 $\mu_B$) and why the moments prefer to order in a stripe-like AFM manner. The role of the onsite Hund rule coupling in forming the momentum and the role of one-electron (band) energy in selecting an AFM pattern will be explained and emphasized. The important distinction between AFM interactions local in real space (superexchange, e.g. in cuprates) and local in momentum space (SDW or AFM in ferropnictides, which is {\it not} an SDW in spin-Peierls sense). This part will be largely based upon our preceeding talk. Next, I will present some speculations about possible solitonic fluctuations (dynamic AFM domain boundaries) in this system, and their relations to experiments. This pictures assumes that in the orthorhombic but nonmagnetic state the system consists of dynamic antiphase AFM domains, in the AFM state the domains are frozen (pinned), and in the nonmagnetic state dynamic twin domains dominate. This picture reconciles experiment, thory and first principle calculations in surprisingly many aspects. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X35.00005: First principles study of magnetic interactions and electronic structure in iron chalcogenide superconductors Myung Joon Han, Sergey Y. Savrasov By using first-principles density functional theory combined with linear response theory we investigate the magnetic interaction of the Fe chalcogenide high T$_C$ superconductors, FeSe$_{1-x}$Te$_x$. The calculated exchange interactions are found to be different from those in pnictides, which suggests possibly different superconducting properties. The nearest neighbor antiferromagnetic coupling ($J_{1a}$) is much stronger than the nearest neighbor ferromagnetic ($J_{1b}$) and the next nearest neighbor coupling ($J_2$). The $J_{1a}$ and $J_{2}$ gradually decreases as $x$ increases while $J_{1b}$ increases and becomes to be stronger than $J_{2}$. Total energy calculation results and the electronic structure will be presented and compared to recent experiments. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X35.00006: Microscopic origin of the structural and magnetic transitions in ferropnictide superconductor parent compounds Michelle Johannes, Igor Mazin, Devina Pillay The parent ferropnictide compounds exhibit two transitions: one is an orthorhombic distortion and the other is a magnetic transition. The transitions are simultaneous in the 122 structural type, but the structural transition precedes the magnetic one in the 1111 type. Although this temperature separation implies that the magnetism depends on the distortion, our computational results show that exactly the opposite is true. The structural distortion is fully dependent on the existence of magnetism and will not occur if a magnetic moment is not present. The particularities of the distortion, namely the expansion along the axis containing aligned spins, occurs as a result of minimizing the one-electron (band) energies. We show that the distortion depends not only on the existence of a magnetic moment, but on the particular ordering pattern chosen by the spins. Imposing a checkerboard ordering results in full x/y symmetry, while a so-called stripe ordering results in near perfect agreement with experimental neutron data below the transition temperature. Our results indicate that, even in the doped (superconducting) compounds, the underlying physics is magnetic. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X35.00007: Magnetic Fluctuation and Anisotropy in High-Tc Iron Pnictides Quan Yin, Myung Joon Han, Warren E. Pickett, Sergey Y. Savrasov Using first-principle density functional theory calculations combined with tight-binding method, dynamical mean field theory, and linear response theory, we extensively investigated the electronic structures and magnetic interactions of nine ferropnictides representing three different structural classes. The calculated magnetic interactions are found to be short-range, and the nearest ($J_{1a}$) and next-nearest ($J_{2}$) exchange constants follow the universal trend of $J_{1}$/$2J_{2}\sim 1$, despite their extreme sensitivity to the z-position of As. This suggests magnetic frustration as the key factor in stabilizing the superconducting ground state. The calculated spin wave dispersions show strong magnetic anisotropy in the Fe plane, in contrast to cuprates. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X35.00008: Role of covalent Fe-As bonding in the magnetic moment formation and exchange mechanisms in iron-pnictide superconductors Kirill Belashchenko, Vladimir Antropov The electronic origin of the huge magnetostructural effect in layered Fe-As compounds is elucidated using LiFeAs as a prototype. The crucial feature of these materials is the strong covalent bonding between Fe and As, which tends to suppress the exchange splitting. The bonding-antibonding splitting is very sensitive to the distance between Fe and As nuclei. We argue that the fragile interplay between bonding and magnetism is universal for this family of compounds. The exchange interaction is analyzed in real space, along with its correlation with covalency and doping. The range of interaction and itinerancy increase as the Fe-As distance is decreased. Superexchange makes a large antiferromagnetic contribution to the nearest-neighbor coupling, which develops large anisotropy when the local moment is not too small. This anisotropy is very sensitive to doping. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X35.00009: The magnetic interactions in iron pnictides Jiji Pulikkotil, Vladimir Antropov, Mark van Schilfgaarde Using static linear response theory we studied the pair wise magnetic interaction parameters in many typical families of iron pnictides. Parameters have been obtained for a wide range of volumes and distance between Fe and As atoms. We demonstrate that two nearest neighbor couplings in plane dominate, with a third and fourth nearest neighbor coupling responsible for the appearance of non-collinear ordering when the magnetic moment is small. We found that the ratio between first and a second neighbor coupling is not universal and greatly varies as a function of pressure or Fe-As distance. A small interplane coupling is found, and it varies by a factor of 10-20 among pnictides. We analyze the Neel temperatures, adiabatic spin wave spectrum and a nature of magneto-structural transitions in different classes of pnictides. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X35.00010: The magnetic phase diagram of iron pnictides German Samolyuk, Jiji Pulikkotil, Vladimir Antropov We study the stability of magnetic structures in iron pnictides as a function of doping, external pressure and the amount of defects. Several collinear and non-collinear magnetic structures are found to be stable in all classes of pnictides. This stability however is a result of a fragile competition between several nearest neighbor exchange couplings and depends greatly on doping. We determined that for a relatively small electron doping the non-magnetic instability is developed, while already for a small hole doping the stripe structure is instable in many pnictides and other magnetic structures are stabilized. For a larger hole doping the local magnetic moment phase with ferromagnetic long range order can be stabilized. A transition to non-collinear state at small moments is explained by a competition between the anisotropy of the nearest neighbors exchange couplings and third or forth neighbor couplings. Using very extensive calculations of magnetic stability parameter we build a generic pressure-concentration phase diagram of iron pnictides. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X35.00011: Magnetic excitations in iron pnictides Vladimir Antropov, Liqin Ke, Takao Kotani, Mark van Schilfgaarde We analyze the dynamical spin susceptibility $\chi({\bf q},\omega)$ in the iron pnictides: FeSe, CaFe2As2 and SrFe2As2 and obtain the spectra of spin excitations. In the longwavelength limit we obtain parameters for the adiabatic Heisenberg model and compare it with parameters generated by a static response method. Antiferromagnons are found for a small q, while for the larger q strong Stoner excitations are developed. These results support the claim that iron pnictides are marginally itinerant systems. We also estimate zero-point fluctuations from $\chi$ and find the following contributing mechanisms: adiabatic spin waves, hole-particle Stoner excitations and longitudinal fluctuations. Taking these effects into account improves the agreement between theory and experiment and indicate the importance of itinerant spin fluctuations. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X35.00012: Collinear (Bi-collinear) antiferromagnetic order in iron-pnictides (chalcogenides) Zhong-Yi Lu, Fengjie Ma, Tao Xiang By the first-principles electronic structure calculations, we find that the ground state of the tetragonal $\alpha$-FeTe is in a bi-collinear antiferromagnetic order, in which the Fe local moments ($\sim2.5\mu_B$) align ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe-Fe square lattice. This novel bi- collinear order results from the interplay among the nearest, the next nearest, and the next next nearest neighbor superexchange interactions, mediated by Te $5p$-band. In contrast, the ground state of the other iron pnictides or chalogenides is in a conventional collinear antiferromagnetic order, like LaFeAsO, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions, bridged by As atoms. This finding sheds new light on the origin of magnetic ordering in Fe-based superconductors. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X35.00013: Effective Hamiltonian for FeAs based superconductors Efstratios Manousakis The Fe-pnictide superconductors exhibit unusual properties attributed to electrons and holes occupying the Fe $d$-orbitals and the outermost occupied $s$ and $p$ pnictide orbitals. Starting from the atomic limit, we carry out a strong coupling expansion for the FeAs layer, where the on-site Coulomb repulsion parameters are assumed to be significantly larger than the hopping between Fe $d$ orbitals and the hybridization parameters between the Fe $d$ and As $4s$ or $4p$ orbitals; we derive an effective Hamiltonian that describes the low energy electron/hole behavior. If this condition for strong coupling expansion is not satisfied, still, we believe that our qualitative results capture important aspects of the physics in these materials. The hopping and the hybridization parameters are obtained by fitting the results of our calculations based on the local density approximation to a tight-binding model. The effective Hamiltonian, in the strong coupling limit, consists of three parts which operate on three sub-spaces coupled through Hund's rule and spanned by the following Fe orbitals: (a) the $d_{x^{2}-y^{2}}$; (b) the degenerate orbitals $d_{xz}$ and $d_{yz}$; and (c) the $d_{xy}$ and $d_{z^{2}}$. Each of these parts is an extended $t-t^{\prime}-J-J^{\prime}$ model and is characterized by different coupling constants and filling factors. For the undoped material the second subspace alone prefers a ground state characterized by a spin-density-wave order similar to that observed in recent experimental studies, while the other two subspaces prefer $(\pi,\pi)$ antiferromagnetic order. The observed spin-density-wave order is imposed by the $d_{xz}/d_{yz}$ subspace as the ground state of the total Hamiltonian of the undoped parent compounds. However, due to the above mentioned frustration the magnetic moment is small in agreement with observation. Our calculation illustrates in a simple manner the reason for the difference in the magnetic ordering between the Fe-pnictides and the cuprates. It also suggests a different evolution of the magnetic order upon electron versus hole doping. [Preview Abstract] |
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