Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N2: Invited Session: Electron Matter in FE-Based Superconductors |
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Sponsoring Units: DCMP Chair: Zhi-Xun Shen, Stanford University Room: Ballroom II |
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N2.00001: Pairing mechanism and gap symmetry in Fe-based superconductors with only electron or only hole pockets Invited Speaker: Andrey Chubukov The pairing in moderately doped Fe-pnictides and Fe-chalcogenides is generally understood as being due to magnetically enhanced interaction between hole and electron pockets. Recently, however, superconductivity has been observed in AFe$_{2}$Se$_{2}$ (A $=$ K, Rb, Cs), which contain only electron pockets, and in KFe$_{2}$As$_{2}$, which contains only hole pockets. In the talk, I review different (and sometimes conflicting) scenarios for the pairing in these systems and propose my own. I argue that the pairing condensate in systems with only electron pockets necessary contains not only a conventional intra-pocket component, but also inter-pocket component, made of two fermions belonging to different electron pockets. I analyze the interplay between intra-pocket and inter-pocket pairing depending on the ellipticity of electron pockets and the strength of their hybridization and show that with increasing hybridization the system undergoes a transition from a d-wave state to an s$^{+-}$ state, in which the gap changes sign between hybridized pockets. This s$^{+-}$ state has the full gap and at the same time supports spin resonance, in agreement with the data. Near the boundary between d and s$^{+-}$ states the system develops s$+$id state which breaks time-reversal symmetry. For systems with only hole pockets, I argue for s$^{+-}$ state in which the gap changes sign between hole pockets. I show that this state is qualitatively different from s$^{\mathrm{+-}}$ state when both hole and electron pockets are present. I further show that the transition from one s-wave state to the other involves highly unusual s$+$is state which again breaks time reversal symmetry. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N2.00002: Atomic-scale Visualization of Electronic Nematicity and Cooper Pairing in Iron-based Superconductors Invited Speaker: Milan P. Allan The mechanism of high-temperature superconductivity in the relatively novel iron-based high-T$_c$ superconductors is unresolved, both in terms of how the phases evolve with doping, and in terms of the actual Cooper pairing process. To explore these issues, we used spectroscopic-imaging scanning tunneling microscopy to study the electronic structure of CaFe$_2$As$_2$ in the antiferromagnetic-orthorhombic `parent' state from which the superconductivity emerges. We discovered and visualized the now widely studied electronic `nematicity' of this phase, whose suppression is associated with the emergence of superconductivity (\emph{Science} 327, 181, 2010). As subsequent transport experiments discovered a related anisotropic conductance which increases with dopant concentration, the interplay between the electronic structure surrounding each dopant atom, quasiparticle scattering therefrom, and the transport nematicity has become a pivotal focus of research. We find that substituting Co for Fe atoms in underdoped Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ generates a dense population of identical and strongly anisotropic impurity states that are distributed randomly but aligned with the antiferromagnetic $a$-axis. We also demonstrate, by imaging their surrounding interference patterns, that these impurity states scatter quasiparticles and thus influence transport in a highly anisotropic manner (M.P. Allan et al., 2013). Next, we studied the momentum dependence of the energy gaps of iron-based superconductivity, now focusing on LiFeAs. If strong electron-electron interactions mediate the Cooper pairing, then momentum-space anisotropic superconducting energy gaps $\Delta_i(k)$ were predicted by multiple techniques to appear on the different electronic bands $i$. We introduced intraband Bogoliubov quasiparticle scattering interference (QPI) techniques for the determination of anisotropic energy gaps to test these hypotheses and discovered the anisotropy, magnitude, and relative orientations of the energy gaps on multiple bands (\emph{Science} 336, 563 (2012)). Finally, the electron-electron interactions generating Cooper pairing are often conjectured to involve bosonic spin fluctuations generated by interband scattering of electrons. We explore the STM signatures of both the interband scattering and the electron-boson coupling self-energy in LiFeAs, and detect the signatures of the electron-boson coupling (M.P. Allan et al., in preparation). [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N2.00003: Spin dynamics in electron and hole-doped iron pnictide superconductors Invited Speaker: Pengcheng Dai |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N2.00004: ARPES studies of the superconducting gap symmetry of Fe-based superconductors Invited Speaker: Pierre Richard The superconducting gap is the fundamental parameter that characterizes the superconducting state, and its symmetry is a direct consequence of the mechanism responsible for Cooper pairing. Here I discuss about angle-resolved photoemission spectroscopy measurements of the superconducting gap in the Fe-based high-temperature superconductors. I show that the superconducting gap is Fermi surface dependent and nodeless with small anisotropy, or more precisely, a function of momentum. I show that while this observation is inconsistent with weak coupling approaches for superconductivity in these materials, it is well supported by strong coupling models and global superconducting gaps. I also stress the importance of scattering and the lifetime of quasiparticles in evaluation the superconducting gap by angle-resolved photoemission spectroscopy and other experimental techniques. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 2:15PM |
N2.00005: Effects of disordered substitutions and vacancies in Fe based superconductors from first principles Invited Speaker: Tom Berlijn Most Fe pnictide and selenide superconductors are created by chemical substitution which inevitably introduces disorder. The relationship between nominal chemical valence, doping, and quasiparticle spectral weight appears to be quite complex. Using a recently developed Wannier function based first principles method for disordered systems [1], we compute the configuration-averaged spectral function $\langle A(k,\omega)\rangle$ of Fe based superconductors containing disordered substitutions and vacancies. In the transition metal doped Ba(Fe$_{1-x}$M$_x$)$_2$As$_2$ with M=Co/Zn we find[2] a loss of coherent carrier spectral weight. For the case of disordered Fe and K vacancies in K$_{0.8}$Fe$_{1.6}$Se$_2$ we find a disorder induced effective doping to give rise to enlarged electron pockets without adding electrons to the system. For the case of Ru substitutions in Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ we find[4] a cancelation between on- and off-site disorder to give rise to a surprising protection of the Fermi surface.\\[4pt] [1] T. Berlijn, D. Volja and W. Ku, PRL 106, 077005 (2011)\\[0pt] [2] T. Berlijn, C.-H. Lin, W. Garber and W. Ku, PRL 108, 207003 (2012)\\[0pt] [3] T. Berlijn, P. J. Hirschfeld and W. Ku, PRL 109, 147003 (2012)\\[0pt] [4] L. Wang, T. Berlijn, Y. Wang, C.-H. Lin, P. J. Hirschfeld and W. Ku, arXiv:1209.3001 [Preview Abstract] |
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