Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D26: Focus Session: Iron Based Superconductors -- Electronic Structure |
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Sponsoring Units: DCOMP DMP Chair: Adriana Moreo, University of Tennessee and Oak Ridge National Laboratory Room: D162/164 |
Monday, March 21, 2011 2:30PM - 3:06PM |
D26.00001: Confronting LDA+DMFT results with experiments in the iron pnictide families Invited Speaker: The normal state electronic structure of the pnictides is an important challenge to electronic structure theory . Optical conductivity experiments are indicative of electronic electron correlations (with mass renormalizations of the order of three). Neutron scattering experiments have features characteristic of both itinerant and localized electrons. High energy spectroscopies indicate the absence of satellite peaks. In this talk, we will show how LDA+DMFT allows us to reconcile these apparently inconsistent facts, and trace them to the unique chemical ingredients of these compounds: the iron Hunds rule coupling and the hybridization with very broad arsenide bands. Quantitative results for the different iron pnictide families will be presented and the factors that govern the strength of the correlations in this family of compounds will be discussed. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D26.00002: Electronic structure of iron-pnictide superconductors: just scratching the surface Erik van Heumen, J. Vuorinen, K. Koepernik, F. Massee, Y. Huang, J.B. Goedkoop, M. Lindroos, M. Shi, K. Haule, J. van den Brink, M.S. Golden Angle resolved photoemission (ARPES) and scanning tunneling microscopy (STM) are important tools in the study of iron-pnictide high T$_{c}$ superconductors. These techniques are surface sensitive and one has to ensure that the electronic structure probed in an experiment corresponds to the bulk electronic structure. Using a combination of experimental techniques (STM, ARPES, LEED) and theoretical calculations (LEED simulations, DFT), I will show that the surface structure of BaFe$_{2-x}$Co$_{x}$As$_{2}$ is both reconstructed and distorted. LEED data combined with simulations is used to solve the real surface and sub-surface structure. The impact of the surface on the electronic structure is then determined by comparing DFT slab calculations, based on the real surface structure, with ARPES experiments. The presence of surface states gives a natural explanation for the large k-space broadening observed in these materials. Having identified the surface states and bulk bands, I will address the more fundamental questions with regard to the electronic structure and its role in the mechanism of high temperature superconductivity. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D26.00003: The superconducting phase and electronic excitations of (Rb,Cs)Fe$_{2}$As$_{2}$ J. Kanter, Z. Shermadini, R. Khasanov, A. Amato, Z. Bukowski, B. Batlogg We present specific heat, transport and Muon-Spin Rotation ($\mu $SR) results on (Rb,Cs)Fe$_{2}$As$_{2}$. RbFe$_{2}$As$_{2}$ was only recently found to be superconducting below 2.6 K by Bukowski et al. Compared to the related BaFe$_{2}$As$_{2}$ the electron density is lower and no magnetic order is observed. For the superconducting phase the superfluid density was calculated from $\mu $SR data. The temperature dependence of the superfluid density and the magnetic penetration depth is well described by a multi-gap scenario. In addition the electronic contribution the specific heat was studied for different compositions and magnetic fields and reveals a high value for the Sommerfeld coefficient $\gamma $. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D26.00004: Revealing the degree of magnetic frustration in iron pnictides Cheng-Chien Chen, Ryan Applegate, Brian Moritz, Thomas Devereaux, Rajiv Singh Proposed theories for collinear antiferromagnetism in iron pnictides vary greatly in the amount of magnetic frustration and proximity to a quantum critical point. We discuss how imaging the magnetic fields around a non-magnetic impurity can quantify the degree of magnetic frustration and thereby distinguish various theoretical proposals. It is found that in a frustrated J1-J2 model a non-magnetic impurity strongly reduces its neighboring local moments, and overturned dynamical spins appear close to zero energy. In contrast, the spatially anisotropic J1a-J1b-J2 model produces enhanced local moments on sites neighboring the impurity. In both cases, the disturbance in the magnon local density of states exhibits an anisotropic stripe pattern. These predictions can be tested by experiments such NMR or spin-resolved STM measurements. The results can elucidate the role of frustration in antiferromagnets and help narrow the possible models for the iron-pnictide superconductors. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D26.00005: Theory for Magnetism and Triplet Superconductivity in LiFeAs Maria Daghofer, Philip Brydon, Carsten Timm, Jeroen van den Brink Superconducting pnictides are widely found to feature spin-singlet pairing in the vicinity of an antiferromagnetic phase, for which nesting between electron and hole Fermi surfaces is crucial. LiFeAs differs from the other pnictides by (i) poor nesting properties and (ii) unusually shallow hole pockets. Investigating magnetic and pairing instabilities in an electronic model that incorporates these differences, we find antiferromagnetic order to be absent. Instead we observe almost ferromagnetic fluctuations which drive an instability toward spin-triplet $p$-wave superconductivity.\\ P.M.R. Brydon, M. Daghofer, C. Timm, and J. van den Brink, arXiv:1009.3104 [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D26.00006: Unified Picture for Magnetic Correlations in Iron-Based Superconductors Wei-Guo Yin, Chi-Cheng Lee, Wei Ku The spin-density-wave mechanism due to Fermi surface nesting is widely used to describe magnetism in the parent compounds of iron-based superconductors; however, the field is puzzled by the different magnetic structure of FeTe, which apparently has similar Fermi-surface topology. Here we propose [1] an orbital-degenerate double-exchange model which includes both itinerant electrons and localized spins. The physical properties of the model are governed by the competition between the antiferromagnetic superexchange and Hund's rule coupling. We show that the strength of the effective Hund's rule coupling term strongly depends on the anion height from the iron plane, leading to the collinear (C-type) magnetic order with the ferro-orbital order in iron pnictides and the bicollinear (E-type) magnetic order without orbital ordering in FeTe. This shows that the magnetism in iron-based superconductors can be described in a unified picture and may have a universal origin for all of the materials. Our results reveal the crucial role of Hund's rule coupling for the strongly correlated nature of the system and suggest exploring the interplay of the spin, orbital, charge, and lattice degrees of freedom in promoting high-temperature superconductivity. Work supported by US DOE.\\[0pt] [1] W.-G. Yin, C.-C. Lee, and W. Ku, PRL 105, 107004 (2010). [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D26.00007: Electronic specific heat of the iron chalcogenide superconductor Fe(Te$_{0.55}$Se$_{0.45})$ Jin Hu, Tijiang Liu, Bin Qian, Zhiqiang Mao We report specific heat studies of superconducting Fe(Te$_{0.55}$Se$_{0.45})$[1]. We have obtained the electronic specific heat by subtracting the phonon contribution evaluated from the normalization of the phonon specific heat of a non-superconducting reference sample (Fe$_{0.9}$Cu$_{0.1})$(Te$_{0.55}$Se$_{0.45})$. Our results show that the superconducting ground state is accompanied by unpaired quasiparticles, as in FeAs superconductors, with \textit{$\gamma $}$_{0 }$\textit{$\sim $}2.3 mJ/mol K$^{2}$. The temperature dependence of the electronic specific heat $C_{es}(T)$/$T$ can be well fitted using either a single s-wave gap model with 2\textit{$\Delta $}= 5.2 k$_{B}T_{c}$ or a two-gap model with 2\textit{$\Delta $}$_{1}$/k$_{B}T_{c }$= 5.8 and 2\textit{$\Delta $}$_{2}$/k$_{B}T_{c}$ = 4.0; the two-gap model fitting is slightly better than the single gap fitting. Such large gaps, together with a large specific heat jump $\Delta C(T_{c})$/$T_{c}\sim $57.3 mJ/mol K$^{2}$, suggest a strong-coupling superconducting state. While $C_{es}(T)$/$T$ exhibits isotropic s-wave gap behavior, the magnetic field-induced change in the electronic specific heat $\Delta $\textit{$\gamma $}($H)$ exhibits sublinear field dependence, implying the superconducting pairing in iron chalcogenide superconductors also involves a multiple band effect, as seen in pnictide superconductors. [1] M. H. Fang, H. M. Pham, B. Qian, T. J. Liu, E. K. Vehstedt, Y. Liu, L. Spinu, and Z. Q. Mao, Superconductivity close to magnetic instability in Fe(Se$_{1-x}$Te$_{x})_{0.82}$, Phys. Rev. B 78, 224503 (2008). [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D26.00008: Electronic correlations and superconducting response in the optical properties of FeTe$_{\mathbf{0.55}}$Se$_{\mathbf{0.45}}$ C.C. Homes, A. Akrap, J.S. Wen, Z.J. Xu, Z.W. Lin, Q. Li, G.D. Gu The in-plane complex optical properties of the iron-chalcogenide superconductor FeTe$_{0.55}$Se$_{0.45}$ have been determined above and below $T_c = 14$~K. At room temperature the conductivity is described by a weakly-interacting Fermi liquid with $\omega_{p,D}\simeq 7200$~cm$^{-1}$ and $1/\tau_D\simeq 414$~cm$^{-1}$. Below 100~K the conductivity is no longer described by the Drude model. Adopting the generalized Drude model reveals that $1/\tau(\omega) \propto \omega$ in the terahertz region just above $T_c$, signaling the increasingly correlated nature of this material.\footnote{C. C. Homes {\em et al.}, Phys. Rev. B {\bf 81}, 180508(R) (2010).} For $T \ll T_c$ the superconducting plasma frequency $\omega_{p,S} \simeq 3000$~cm$^{-1}$ ($\lambda_{\it eff} \simeq 5300$~\AA ); $\omega_{p,S}^2/\omega_{p,D}^2 \ll 1$ indicating that this material is not in the clean limit. Allowing $\sigma_{dc}\equiv \sigma_1(\omega \rightarrow 0)$, then $\sigma_{dc}(T\simeq T_c) \simeq 3500\pm 400$ $\Omega^{-1}$cm$^{-1}$ and the superfluid density $\rho_{s0} \equiv \omega_{p,S}^2 \simeq 9\pm 1\times 10^6$~cm$^{-2}$ places material close to the scaling line $\rho_{s0}/8 \simeq 8.1\,\sigma_{dc}T_c$ for a BCS dirty-limit superconductor. Below $T_c$ the optical conductivity reveals two energy scales for the superconductivity at $\Delta_1(0) \simeq 2.5$~meV and $\Delta_2(0) \simeq 5.1$~meV, consistent with the $s^\pm$ model. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D26.00009: Microwave surface impedance measurements of LiFeAs and LiFe(As,P) single crystals Y. Imai, H. Takahashi, T. Okada, A. Maeda, K. Kitagawa, K. Matsubayashi, M. Takigawa, Y. Uwatoko, N. Nakai, Y. Nagai, M. Machida We report results of microwave surface impedance measurements in LiFeAs and LiFe(As,P) single crystals [1]. These crystals were grown by self-flux method. The surface impedances of crystals were measured by a cavity perturbation technique. The in-plane penetration depth calculated from the surface reactance shows an exponential temperature dependence at low temperatures in both of LiFeAs and LiFe(As,P). This indicates that these materials do not have any nodes in the superconducting gap. The temperature dependence of the superfluid density indicates that LiFeAs and LiFe(As,P) are multi-gap superconductors with at least two isotropic gaps. In addition, the real part of complex conductivity exhibits an enhancement below $T_{\rm {c}}$, which is different from the so-called coherence peak. This is due to the rapid increase of the relaxation time of the quasiparticle below $T_{\rm {c}}$. We believe that this enhancement is rather common to all superconductors where an inelastic scattering is dominant above $T_{\rm {c}}$, irrespective of the strength of the electron correlation.\\[4pt] $\left[ 1 \right]$ Y. Imai $et$ $al.$, J. Phys. Soc. Jpn, $in$-$press$.(arXiv: 1009.4628.) [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D26.00010: Electromagnetic response of Fe(Se,Te) in the superconducting sate A. Maeda, H. Takahashi, D. Nakamura, T. Akiike, F. Nabeshima, Y. Imai, S. Komiya, I. Tsukada We investigate the electromagnetic response of Fe(Se,Te) from microwave to THz region, with special interests in the superconducting state. Samples were epitaxial thin films or bulk single crystals, depending on the technique of measurements. The results obtained are the followings. (1) Temperature dependence of the penetration depth was almost flat, with a very small contribution of $T^2$, suggesting nodeless gap plus some disorder induced pair breaking. (2) Temperature dependence of superfluid density is rather different from that of, eg, LiFeAs, suggesting weak interband scattering. (3) Quasiparticle (QP) conductivity, $\sigma (\omega,T)$ shows a broad peak below $T_c$. Both of temperature dependence and frequency dependence show that this peak is not the coherence peak in conventional $s$-wave superconductors, but is due to the enhancement of the QP scattering time. These strongly suggest that the symmetry of superconducting wave function is sign-changed s-wave. (4) Sharp peak around $T_c$ was invisible in $\sigma(T)$, suggesting very small superconductivity fluctuation. (5) Conductivity spectra ($\sigma (\omega)$) suggest that the superconducting gap, 2$\Delta$ is 1.2 meV, leading to 2$\Delta/k_BT_c$ is 1.37. We will discuss possible reasons for the small gap value. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D26.00011: Iron isotope effect on the superconducting transition temperature and the crystal structure of FeSe$_{1-x}$ Markus Bendele, Rustem Khasanov, Kazimierz Conder, Ekaterina Pomjakushina, Vladimir Pomjakushin, Annette Bussmann-Holder, Hugo Keller The Fe isotope effect (Fe-IE) on the transition temperature Tc and the crystal structure was studied in the Fe chalcogenide superconductor FeSe$_{1-x}$ by means of magnetization and neutron powder diffraction (NPD). The substitution of natural Fe (containing $\simeq92$\% of $^{56}$Fe) by its lighter $^{54}$Fe isotope leads to a shift in T$_{\rm c}$ of 0.22(5) K corresponding to an Fe-IE exponent of $\alpha_{\rm Fe}=0.81(15)$. Simultaneously, a small structural change with isotope substitution is observed by NPD. This may help to clarify the currently controversal results of the Fe-IE. Upon correcting the isotope effect exponent for these structural effects, an almost unique value of $\alpha \sim0.35-0.4$ is observed for at least three different families of Fe-based HTS. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D26.00012: Magnetism of SmFeAsO1-xFx Yoichi Kamihara, Yasuhiro Kobayashi, Shinji Kitao, Yoshitaka Yoda, Makoto Seto, Hideo Hosono Magnetic properties of superconducting SmFeAsO$_{1-x}$F$_{x}$ are demonstrated by $^{57}$Fe Mossbauer spectroscopy (MS) and $^ {149}$Sm Nuclear resonant forward scattering (NRFS). Polycrystalline SmFeAsO$_{1- x}$F$_{x}$ samples were synthesized using two-step solid state reaction described elsewhere. [New J. Phys. 12, 033005 (2010).] Purity of samples was checked by X-ray diffraction patterns using Cu K-alpha radiation. Resistivity and magnetization measurements, as well as by $^{57}$Fe MS and $^ {149}$Sm NRFS spectroscopy, at various temperatures were performed to define superconducting, magnetic ordering temperatures. [Phys. Rev. B 78, 184512 (2008) {\&} J. Phys. Soc. Japan 77, 103706 (2008).] A magnetic phase diagram we have proposed is closer to that by Hess et al [Europhys. Lett. 87, 17005 (2009).]; that is long-range AF ordering of Fe (a static magnetism) does not persist in the superconducting regime. Such a relation between spin dynamics and SC is a common feature among LnFeAsO$_{1-x}$F$_{x}$ (Ln = La, Ce, Pr, Nd, and Sm). Our results indicate that the relation between the static magnetism and $T_{c}$ of LnFeAsO$_{1-x}$F$_{x}$ shows similar topology to that of copper-based high-Tc superconductors. [Phys. Rev. B 42, 7981 (1990).] [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D26.00013: Surface geometric and electronic structures of A(Fe, Co)$_{2}$As$_{2}$ (A=Ba,Ca) Guorong Li, V.B. Nascimento, Xiaobo He, Amar B. Karki, Jiandi Zhang, Rongying Jin, A.S. Sefat, M.A. McGuire, B.C. Sales, D. Mandrus, Ward Plummer We utilize Low Energy Electron Diffraction (LEED) to determine the surface structure combined with real-space scanning tunneling microcopy/spectroscopy (STM/STS), to investigate the local geometric and electronic structures at the (001) surface of the compounds of AFe$_{2}$As$_{2}$ (A= Ba, Ca). In general two competing surface reconstructions are observed with either a 1x2 or a ($\surd $2x$\surd $2)R45$^{\circ}$ (tetragonal notation) structure. The ($\surd $2x$\surd $2)R45$^{\circ}$ structure corresponds to the 1x1 orthorhombic phase. While the ($\surd $2x$\surd $2)R45$^{\circ}$ phase always present for A=Ba, the 1x2 structure dominates for A=Ca. We will discuss the detailed structural change with Co doping, thermal cycling, contamination, electron beam induced damage, and cleaving temperature. Specifically, 1x2 phase is sensitive to the thermal processing, with indications of a temperature dependence phase transition. *Supported by NSF DMR-1002622 [Preview Abstract] |
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