Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G37: Focus Session: Fe-based Superconductors: RPA and Beyond/Gap Structure |
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Sponsoring Units: DMP DCOMP Chair: Peter Hirchfeld, Univ. Florida Room: 345/346 |
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G37.00001: Development of orbital and spin fluctuations in Fe-based superconductors based on the self-consistent vertex correction (SC-VC) method Invited Speaker: Hiroshi Kontani To achieve unified understanding of the whole phase diagram of Fe-based superconductors, we analyze the multiorbital Hubbard model going beyond the random phase approximation (RPA). The 2nd-order non-magnetic structure transition at $T_{\mbox{S}} (>T_{\mbox{N}} )$, nematic order as well as large softening of shear modulus $C_{66} $ indicate the strong orbital fluctuations in the normal state. However, only the spin fluctuations develop within the RPA. To resolve this discrepancy, we develop the self-consistent vertex correction (SC-VC) method beyond the RPA, and find the mutual development of orbital and spin fluctuations due to the Aslamazov-Larkin VC, which describes the Kugel-Khomskii type spin-orbital coupling [1]. We find that (i) both the antiferro-orbital and ferro-orbital ($=$nematic) fluctuations develop for $J/U > 0.17$ by including the self-energy correction ($=$SC-V$\Sigma $ method): Both fluctuations contribute to the s-wave superconductivity, and the nematic fluctuations are the origin of the structure transition and the softening of $C_{66} $. (ii) The coexistence of orbital and spin fluctuations can induce the loop-shape nodes on the electron-pockets in BaFe$_{2}$(As,P)$_{2}$, as well as (impurity-induced) smooth $s_{\pm } \to s_{++} $crossover with high $T_{\mathrm{c}}$ [2,3]. Also, the horizontal node on the $z^{2}$-orbitlal hole-pocket predicted by RPA is filled by the inter-orbital fluctuations due to the VC, consistently with laser ARPES and other bulk experiments of 122 compounds. (iii) The same orbital nematic fluctuations are obtained in a simple two-orbital model for Sr$_{3}$Ru$_{2}$O$_{7}$, not only by the SC-VC method [4] but also by the two-dimensional RG method [5]. Therefore, the VC is expected to be the origin of novel orbital physics in various multioritital $d$- and $f$-electron systems.\\[4pt] [1] S. Onari and H. Kontani, PRL \textbf{109}, 137001 (2012).\\[0pt] [2] H. Kontani and S. Onari, PRL \textbf{104}, 157001 (2010).\\[0pt] [3] S. Onari and H. Kontani, PRL \textbf{103}, 177001 (2010).\\[0pt] [4] Y. Ohno, M. Tsuchiizu, S. Onari, and H. Kontani, arXiv:1209.3629.\\[0pt] [5] M. Tsuchiizu, S. Onari, and H. Kontani, arXiv:1209.3664. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G37.00002: Theoretical Study of 3D Superconducting Gap Structure in Iron Based Superconductors Tetusro Saito, Seiichiro Onari, Hiroshi Kontani The mechanism and symmetry of the superconducting (SC) gap in Fe-based superconductors have been studied actively, and both the spin fluctuation-mediated $s_{\pm}$-wave SC state and orbital fluctuation-mediated $s_{++}$-wave SC state had been proposed. To obtain important information on the pairing mechanism, we analyze the Eliashberg gap equation using the 3-dimensional 10-orbital model. When we perform the RPA by considering only the Coulomb interaction, only the spin fluctuations develop, and the SC gap of $z^2$-orbital dominant part on the hole pockets is almost zero. The resultant horizontal node is inconsistent with several measurements. However, the orbital fluctuations develop by introducing the quadrupole interaction $g$ (due to the vertex correction) and it is found that (i) the horizontal node disappears and (ii) the crossover from $s_{\pm}$-state to $s_{++}$-state is realized. During the crossover, we obtained the loop-node structures on the electron pockets, which are actually observed by ARPES measurements in BaFe$_2$(As,P)$_2$. We expect that optimally doped BaFe$_2$(As,P)$_2$ is in the crossover regime between s$_{++}$-state and s$_{\pm}$-state. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G37.00003: Pressure dependence of critical temperature of bulk FeSe from spin fluctuation theory Peter Hirschfeld, Andreas Kreisel, Yan Wang, Milan Tomic, Harald Jeschke, Anthony Jacko, Roser Valenti, Thomas Maier, Douglas Scalapino The critical temperature of the 8K superconductor FeSe is extremely sensitive to pressure, rising to a maximum of 40K at about 10GPa [1]. We test the ability of the current generation of fluctuation exchange pairing theories to account for this effect, by downfolding the density functional theory electronic structure for each pressure to a tight binding model. The Fermi surface found in such a procedure is then used with fixed Hubbard parameters to determine the pairing strength using the random phase approximation for the spin singlet pairing vertex. We find that the evolution of the Fermi surface captured by such an approach is alone not sufficient to explain the observed pressure dependence, and discuss alternative approaches.\newline [1] S. Margadonna, \textit{et al.}, Phys. Rev. B \textbf{80}, 064506 (2009); S. Medvedev, \textit{et al.}, Nat. Mater. \textbf{8}, 630 (2009). [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G37.00004: Effect of realistic finite-size impurities on Tc in Fe-based superconductors Youichi Yamakawa, Seiichiro Onari, Hiroshi Kontani Recently, the phase diagram of LaFeAsO$_{1-x}$H$_x$ is reported and two-dome structure of superconducting state, first dome for $x<0.2$ with $T_c^{\rm max}=29$K and second dome for $0.2< x<0.5$ with $T_c^{\rm max}=36$K, has attract great attention[1]. To clarify the origin of the second superconducting dome, we construct tight-binding models for each doping level $x$ and investigate the spin and orbital fluctuations based on the random phase approximation. We fined that the nesting between electron-hole Fermi surfaces is monotonically weakened with $x$ and spin density wave order with momentum ${\bf q}=(\pi,\pi)$ disappears. In the over-doped regime for $x>0.2$, however, the nesting between electron-electron Fermi surfaces increases, and an incommensurate spin density wave order emerges. The orbital order also shows a re-entrant phase diagram. The spin and orbital fluctuations due to the incommensurate nesting would then be the origin of the second superconducting dome reported in the H-over-doped LaFeAsO. The obtained electronic states for $x=0.5$ are very similar to that for KFe$_2$Se$_2$[2], which is a heavily electron doped system(0.5 electron/Fe). [1] S. Iimura, {\it et al.}, Nat. Commumn. {\bf 3}, 943 (2012). [2] T. Saito, {\it et al.}, Phys. Rev. B {\bf 83}, 140512 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G37.00005: Magnetic degeneracy and C$_4$ symmetric magnetic phase in iron-based superconductors Ilya Eremin, Andrey V. Chubukov We analyze the magnetic phase diagram of iron pnictides by going beyond Ginzburg-Landau expansion and solving full non-linear equation for magnetic order parameter within itinerant model with hole pockets centered around (0,0) and electron pockets centered at ($\pi $,0) and (0, $\pi )$ in the unfolded Brillouin zone. We extend our previous analysis of the itinerant model to higher carrier concentrations when Fermi surface nesting is weaker, and find that the phase diagram is richer than previously anticipated. We show that, in addition to stripe SDW order which breaks C$_{4}$ rotational symmetry, there exists, in some range of parameters, a different type of SDW order which preserves C$_{4}$ symmetry. The order parameter in this new phase is equally weighted combination of SDW components with wavevectors Q$_{\mathrm{X}}=(\pi $,0) and Q$_{\mathrm{Y}}=$(0,$\pi )$. The new phase emerges from the stripe phase via a second transition at T \textless\ T$_{\mathrm{N}}$. Such a phase is highly unlikely in the orbital scenario for magnetism in iron-based superconductors, because in this scenario the breaking of the symmetry between Q$_{\mathrm{X}}$ and Q$_{\mathrm{Y}}$ is the pre-condition for the magnetic transition. We discuss recent experiments in which the second low-T magnetic phase which does not break the symmetry between Q$_{\mathrm{X}}$ and Q$_{\mathrm{Y}}$ has been detected and argue that its observation is a strong indication that the magnetic order is of magnetic rather than of orbital origin. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G37.00006: Pairing strength and gap functions in multiband superconductors: 3D effects Andreas Kreisel, Yan Wang, Peter Hirschfeld, Thomas Maier, Douglas Scalapino We examine the superconducting pairing symmetry in Fe-based superconductors using spin-fluctuation pairing theory. It has been shown in multi-orbital models that the different matrix elements of the pairing vertex are essential in determining the symmetry. In our approach we perform a 10-orbital spin-fluctuation calculation to account for the full matrix elements and the 3 dimensional character of the bandstructure which is most important in the systems under consideration (LiFeAs and K$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_2$). Our approach contains both, the deviations from tube-like Fermi surface that also allows different pairing strengths in the z-direction, and the hybridization of the Fermi surface. Starting from the tight-binding Hamiltonian corresponding to the real crystal cell, we find several competing 3D gap structures and compare with ARPES experiments. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G37.00007: Spin fluctuation theory of pairing in AFe$_2$As$_2$ Thomas Maier, Yan Wang, Andreas Kreisel, Peter Hirschfeld, Douglas Scalapino The absence of Fermi surface hole pockets in the alkaline iron selenides has challenged the usual spin fluctuation arguments leading to the most popular s+- superconducting gap structure in the closely related iron pnictide superconductors. Thus they provide a new venue to study the nature of pairing in a system with only electron pockets. Here, we present the results of spin fluctuation calculations of the pairing interaction based on realistic descriptions of the bandstructure of the iron selenides. In particular, we will discuss the predictions of these studies with regard to the gap structure in the absence of hole pockets, its consequences for the magnetic neutron scattering spectrum, and their evolution with doping. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G37.00008: Phase diagram of doped BaFe$_2$As$_2$ superconductor under broken C$_4$ symmetry Yuan-Yen Tai, Jian-Xin Zhu, Matthias J. Graf, C.S. Ting We developed a minimal multi-orbital tight-binding model with realistic hopping parameters that breaks the symmetry of the point group by lowering it from C$_4$ to D$_{2d}$. The model accurately describes the Fermi surface evolution of the electron, BaFe$_{2-x}$Co$_x$As$_2$, and hole, Ba$_{1-y}$K$_y$Fe$_2$As$_2$, doped compounds. Since in this class of materials the competing superconductivity and co-linear antiferromagnetism rely on the evolution of the Fermi surface with doping, we investigated the phase diagram with a mean-field t-U-V Bogoliubov-de Gennes equation. Our results match the experimental electron-doped phase diagram. Furthermore, the model is in reasonable agreement with the experimental hole-doped part with only one set of t, U and V parameters. The self-consistently calculated superconducting order parameter exhibits s+/-d pairing symmetry in the entire doping range. It is the subtle result of competing interactions in the multi-orbital mean-field Hamiltonian based on the broken C$_4$ symmetry and might be observable in STM and ARPES experiments. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G37.00009: Hidden ($\pi$,0) instability as an itinerant origin of bicollinear antiferromagnetism in Fe$_{1+x}$Te Yu-Zhong Zhang, Ming-Cui Ding, Hai-Qing Lin By calculating orbitally resolved Pauli susceptibilities within maximally localized Wannier orbital basis transformed from first principles band structures, we find that magnetism in Fe$_{1+x}$Te still has its itinerant origin even without Fermi surface nesting, provide orbital modulation of particle-hole excitations are considered. This leads to strong magnetic instabilities at wave vector ($\pi$,0)/(0,$\pi$) in d$_{xz}$/d$_{yz}$ orbitals that are responsible for bicollinear antiferromagnetic state as extra electrons donated from excess Fe are considered. Magnetic exchange coupling between excess Fe and in-plane Fe further stabilizes the bicollinear antiferromagnetic order. Our results reveal that magnetism and superconductivity in iron chalcogenides may have different orbital origin, as Pauli susceptibilities of different orbitals evolves differently as a function of concentration of excess Fe and height of chalcogen atom measured from iron plane. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G37.00010: Fluctuation of Valley Density Wave in Iron Pnictides Jian Kang, Zlatko Tesanovic We studied the fluctuations within the U(n)*U(n) [1] theory, which was developed to explain the magnetic and structural transitions in the parent compound of iron pnictides. The self-energy of the fermion contains singularity in low energy scale. It behaves similar to marginal Fermi liquid theory and depends on n. The optical conductivity and spin lattice relaxation time are calculated and compared with some experiment on ``pseudogap'' in iron pnictides. More experiments are proposed to provide a direct view our U(4)*U(4) theory being assembled as one moves from low to high energies. \\[4pt] [1] J. Kang and Z. Tesanovic, Phys. Rev. B 83, 020505(R) (2011). [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 2:15PM |
G37.00011: Superconducting energy gap and nodes in the doped BaFe2As2 system Invited Speaker: Shik Shin Angle resolved photoemission spectroscopy (ARPES) is very powerful to know the solid state properties. We have developed low-temperature high-resolution laser-based ARPES system and recently achieved the highest energy resolution of $\sim$ 100 $\mu $eV and the lowest sample temperature of $\sim$ 1.0 K. We would like to show our recent results of superconducting-gap measurements on the iron-based superconductors by laser-ARPES, mainly for Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ [1-3]. Little Fermi-surface dependent superconducting gap sizes are found for the Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ that has the maximum Tc around 40K [1]. Inter-orbital interaction is important as well as intra-orbital interaction. On the other hand, KFe$_{2}$As$_{2}$ is an extremely hole-doped compound in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ system and no longer has electron Fermi surfaces. Regardless of this, KFe$_{2}$As$_{2}$ still exhibits superconductivity with Tc of 3.4 K and the existence of nodes in its superconducting gap has been suggested by the several transport measurements. Our ultrahigh-resolution laser ARPES [2] unveils that KFe$_{2}$As$_{2}$ is a nodal s-wave superconductor with highly unusual FS-selective multi-gap structure: a nodeless gap on the inner FS, an unconventional gap with octet-line nodes on the middle FS, and an almost-zero gap on the outer FS. This gap structure may arise from the frustration between competing pairing interactions on the hole FSs causing the eightfold sign reversal. Our results suggest that the A1g superconducting symmetry is universal in iron-pnictides, in spite of the variety of gap functions.\\[4pt] [1] Shimojima \textit{et al.}, Science \textbf{332} (2011) 564.\\[0pt] [2] Okazaki \textit{et al.}, Science \textbf{337} 1314 (2012).\\[0pt] [3] Malaeb \textit{et al}., Phys. Rev.B\textbf{86} (2012) 165117. [Preview Abstract] |
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