Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V1: Iron Pnictides versus Iron Chalcogenides: Magnetism and Pairing Fluctuation |
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Sponsoring Units: DCMP Chair: Ilya Eremin, Ruhr-University Bochum Room: Ballroom A1 |
Thursday, March 24, 2011 8:00AM - 8:36AM |
V1.00001: Symmetry and structure of the pairing gap in Fe-based superconductors Invited Speaker: I review recent works on the symmetry and structure of superconducting gap in Fe-pnictides and related compounds. I show that the gap very likely has s-wave symmetry, and is either nodal or has nodes along the two electron Fermi surfaces, depending on the parameters. I argue that the nodal gap is most likely outcome in systems with less pronounced tendency towards antiferromagnetism. I compare 4-and 5-pocket models for Fe-pnictides and argue that the parameter range where the gap is nodal is much wider in 4-pocket models. I review recent experiments aimed to understand whether the gap has nodes, e.g., experiments on the variation in the field-induced component of the specific heat C(H) with the direction of the applied field in $FeSe_{0.4}Te_{0.6}$. I show that, for extended s-wave gap, C(H) has $\cos{4 \phi}$ component, where $\phi$ is the angle between H and the direction between hole and electron Fermi surfaces, but only if the gap has no nodes. When the gap has accidental nodes, the $\cos{4\phi}$ variation does not hold. I also plan to discuss the interplay between direct Coulomb interaction at large momentum transfer and spin-fluctuation contribution to the pairing, and the interplay between antiferromagnetism and superconductivity. In particular, I show that in 4-pocket (but not in 5-pocket) model superconductivity becomes the leading instability in some range of parameters even at perfect nesting, i.e, antiferromagnetism is not a pre-condition for superconductivity. This agrees with functional RG studies. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 9:12AM |
V1.00002: Anisotropic Superconducting Gap Revealed by Angle Resolved Specific Heat, Point Contact Tunneling and Scanning Tunneling Microscope in Iron Pnictide Superconductors Invited Speaker: Angle resolved specific heat was measured in FeSe$_{0.55}$Te$_{0.45}$ single crystals. A four-fold oscillation of C/T, with the minimum locating at the Fe-Fe bond direction, was observed when the sample was rotated at 9 T, which can be understood as due to the gap modulation on the electron pocket within the scheme of S$\pm $ pairing. Accordingly, by measuring the point contact Andreev reflection spectrum on the BaFe$_{2-x}$Ni$_{x}$As$_{2}$ single crystals in wide doping regimes, we found a crossover from nodeless to nodal feature of the superconducting gap. In K-doped BaFe$_{2}$As$_{2}$ single crystals, we performed the low temperature STM measurements and observed a well ordered vortex lattice in local region. In addition, the statistics on over 3000 dI/dV spectra illustrate clear evidence of two gaps with magnitude of 7.6 meV and 3.3 meV, respectively. Detailed fitting to the tunneling spectrum shows an isotropic superconducting gap. Work collaborated with B. Zeng, C. Ren, L. Shan, Y. L. Wang, B. Shen, G. Mu, H. Q. Luo, T. Xiang, H. Yang, I. I. Mazin and P. C. Dai. References: \\[4pt] [1] B. Zeng, et al., arXiv:1007.3597, Nature Communications, 2010, in press.\\[0pt] [2] C. Ren, et al., to be published.\\[0pt] [3] L. Shan, et al., arXiv:1005.4038. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:48AM |
V1.00003: From magnetism to superconductivity in FeTe$_{1-x}$Se$_{x}$ Invited Speaker: The iron chalcogenide FeTe$_{1-x}$Se$_{x}$ is structurally the simplest of the Fe-based superconductors and exhibits a Fermi surface similar to iron pnictides. Despite this similarity, the parent compound Fe$_{1+y}$Te orders antiferromagnetically with an in-plane magnetic wave vector ($\pi$,0) with an ordered moment of $\sim$2$\mu_{B}$/Fe, suggestive of a localized rather than itinerant character of the magnetic order. This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave vector ($\pi$,$\pi$) that likely arises from Fermi Surface nesting. Regardless both the pnictide and chalcogenide Fe superconductors exhibit a superconducting spin resonance around ($\pi$,$\pi$) as probed by neutron scattering. A central question in this burgeoning field is therefore how ($\pi$,$\pi$) superconductivity emerges from a ($\pi$,0) magnetic instability ? Using neutron scattering we show that incommensurate magnetic excitations around ($\pi$,$\pi$) are found even in the undoped parent compound Fe$_{1+y}$Te. With increasing $x$, the ($\pi$,0)-type magnetic long-range order becomes unstable and correlates with a weak charge carrier localization, while the mode at ($\pi$,$\pi$) becomes dominant for x$>$0.29. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors. This work was carried out in close collaboration with the groups of W.Bao (Renmin), Arno Hies (ILL), Zhiqiang Mao (Tulane), C. Brohom (John Hopkins) and I. Eremin (MPI-Dresden/Bochum). [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:24AM |
V1.00004: The nature of electronic nematic states in iron-pnictides Invited Speaker: We show that the electronic nematic states in iron-pnictides is driven by frustrated spin fluctuations. A three dimensional effective spin model is constructed to explain the nematicity. This model explains the relation between the structural and magnetic transitions, and the spin excitations measured in recent neutron scattering experiments. Moreover, the model naturally predicts the separation between the two transitions are controlled by the c-axis magnetic exchange coupling , and the existence of an non-collinear magnetic state before spin-glass state upon replacing irons by nonmagnetic impurities. The experimental evidence supporting the predictions and the relation to orbital ordering and superconductivity will also be addressed. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 11:00AM |
V1.00005: Differences in the degree of correlations between Pnictides and Chalcogenides: LaFeAsO vs. FeSe Invited Speaker: The discovery of high-temperature superconductivity in iron-based compounds triggered an enormous amount of research in condensed matter physics. A very intriguing property of these new compounds is the rather high flexibility concerning elemental substitutions, leading to several families of superconductors, termed ``1111,'' ``122,'' ``11,'' and so on, depending on their chemical composition. In this talk we will analyse the single-particle properties of prominent iron-based supercondutors using a combination of density-functional theory with the Dynamical Mean-Field Theory, where also the interaction parameters are calculated ab-initio. This approach enables us to understand also these more complex materials at a first-principle level. We will show that there are significant differences in the electronic properties, when going from more weakly correlated members as LaFeAsO, to more correlated ones like FeSe. For reasonable Coulomb parameters, the properties range from Fermi-liquid like to incoherent bad-metal like. [Preview Abstract] |
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