Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S23: Superconductivity and Its CompetitorsInvited
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Sponsoring Units: DCMP Chair: Ilya Eremin, Ruhr Univ Bochum Room: New Orleans Theater B |
Thursday, March 16, 2017 11:15AM - 11:51AM |
S23.00001: High Tc in monolayers and intercalates of FeSe: role of incipient bands and orbital selectivity Invited Speaker: Peter Hirschfeld High temperature superconductivity in monolayers and some intercalates of FeSe is puzzling because it appears to contradict the usual spin fluctuation pairing scenario for Fe-based superconductors, where the repulsive interactions between hole and electron Fermi pockets lead to strong pairing. In this picture, however, $T_c$ should be negligible in these Fe-chalcogenide systems since they are known to be missing $\Gamma$-centered hole pockets. I discuss two possible modifications of the conventional spin-fluctuation approach that may be relevant to this paradox. First, I consider the hitherto neglected role of bands away from the Fermi level (``incipient")[1], and argue that the incipient band contributes significantly to spin-fluctuation pairing in the strong coupling limit where the system is close to a magnetic instability [2]. As the incipient band extremum (or doping) is tuned, the competition between the paramagnon pairing bandwidth and interaction can lead to high $T_c$ with a dome-like dependence. In this context I further discuss the role of phonons and impurities [3]. Finally, I consider the effect of orbitally selective electronic correlations, which can strongly affect the anisotropy of the gap functions observed [4]. \vskip .4cm 1. X. Chen, S. Maiti, A. Linscheid, and P. J. Hirschfeld, Phys. Rev. B 92, 224514 (2015). \vskip .03cm 2. A. Linscheid, S. Maiti, Y. Wang, S. Johnston and P. J. Hirschfeld, Phys. Rev. Lett. 117, 077003 (2016). \vskip .03cm 3. X. Chen, V. Mishra and P.J. Hirschfeld, Phys. Rev. B 94, 054524 (2016). \vskip .03cm 4. A. Kreisel, P. O. Sprau, A. Kostin, J. C. Davis, B. M. Andersen, P. J. Hirschfeld, arXiv:1611.02643. [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:27PM |
S23.00002: Interplay between magnetism, superconductivity, and orbital order in iron-based superconductors -- parquet renormalization group study Invited Speaker: Andrey Chubukov Magnetism and nematic order are the two non-superconducting orders observed in iron-based superconductors. To elucidate the interplay between them and ultimately unveil the pairing mechanism, several models have been investigated. In models with quenched orbital degrees of freedom, magnetic fluctuations promote stripe magnetism which induces orbital order. In models with quenched spin degrees of freedom, charge fluctuations promote spontaneous orbital order which induces stripe magnetism. I will talk about renormalization group (RG) approach, in which we treat magnetic and orbital fluctuations on equal footing. The new element in the approach, compared to earlier works, is the inclusion of the orbital character of the low-energy electronic states into analysis. I will analyze the RG flow of the couplings and argue that the same magnetic fluctuations, which are known to promote $s^{+-}$ superconductivity, also promote an attraction in the orbital channel, even if the bare orbital interaction is repulsive. I analyze the RG flow of the susceptibilities and show that, if all Fermi pockets are small, the system first develops a spontaneous orbital order, then $s^{+-}$ superconductivity, while a magnetic order does not develop down to $T=0$. I will argue that this scenario applies to FeSe. In systems with larger pockets, such as BaFe$_{2}$As$_{2}$ and LaFeAsO, the situation is different and the leading instability is either towards a spin-density-wave or superconductivity. I argue that in this situation nematic order is caused by composite spin fluctuations and is vestigial to stripe magnetism. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 1:03PM |
S23.00003: Metallic quantum critical points with finite BCS couplings Invited Speaker: Srinivas Raghu The problem of superconductivity near quantum critical points (QCPs) remains a central topic of modern condensed matter physics. In such systems, there is a competition between the enhanced pairing tendency due to the presence of long-range attractive interactions near criticality, and the suppression of superconductivity due to the destruction of Landau quasiparticles. I will describe some recent work that addresses these competing effects in the context of a solvable model of a metallic quantum critical point. I will show that the two effects - namely the enhanced pairing and the destruction of Landau quasiparticles - can offset one another, resulting in stable "naked" quantum critical points without superconductivity. However, the resulting quantum critical metal exhibits strong superconducting fluctuations on all length scales. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:39PM |
S23.00004: BCS-BEC crossover in FeSe with small Fermi energies Invited Speaker: Shigeru Kasahara The BCS-BEC crossover bridges the two important theories of bound particles (Bardeen-Cooper-Schrieffer theory and Bose-Einstein condensation) in a unified picture with the ratio of the attractive interaction to the Fermi energy as a tuning parameter. A key issue is to understand the intermediate regime, where new states of matter may emerge. It has been shown that the Fermi energies of FeSe, the simplest iron-based superconductor with $T_c \sim 9$\,K, are extremely small, with the result that this system is located at the verge of a BCS-BEC crossover [1-4]. Here we discuss possible preformed pairs and a new high-field superconducting phase emergent in FeSe associated with the crossover. Our highly sensitive torque magnetometry probes presence of giant superconducting fluctuations below $T \sim 2T_c$, which is much different from the standard Gaussian fluctuation theories. Resistivity, Hall effect, Seebeck and Nernst coefficients all exhibit anomalies in the same temperature regime. Strikingly, thermal conductivity measurements in the superconducting state give evidence for a distinct phase boundary below the upper critical field, suggesting that the Zeeman splitting comparable to the Fermi energies leads to a strong modification of the quasiparticle structure. The observation of this field-induced phase appears to provide insights into previously poorly understood aspects of the highly spin-polarized Fermi liquid in the BCS-BEC crossover regime. \\ [1] S. Kasahara {\it et al}., Proc. Natl. Acad. Sci. USA {\bf 111}, 16309 (2014).\\ [2] T. Terashima {\it et al}., Phys. Rev. B {\bf 90}, 144517 (2014).\\ [3] S. Kasahara {\it et al}., Nat. Commun. {\bf 7}, 12843 (2016). \\ [4] T. Watashige {\it et al}., arXiv:1607.03256. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 2:15PM |
S23.00005: Superconductivity and ferroelectricity in calcium-substituted-oxygen-reduced strontium titanate Invited Speaker: Kamran Behnia Strontium titanate (SrTiO3) becomes a metal upon removal of a tiny fraction of its oxygen atoms. The metal goes through a superconducting instability in spite of its tiny Fermi energy, an order of magnitude small then than the Debye energy [1]. The resistivity is quadratic in temperature, a hallmark of electron-electron scattering. The T-square behavior persists in the single-band dilute limit despite the absence of the two known mechanisms for generating it [2]. Substituting strontium with calcium gives rise to a percolative ferroelectric order, which coexists with metallicity and its superconducting instability in a narrow range of doping. Upon further doping, the ferroelectric-like order is destroyed by a quantum phase transition at a critical doping level at which the Friedel oscillations generated by neighboring dipoles interfere destructively. In the vicinity of this quantum phase transition, the superconducting critical temperature is enhanced by calcium substitution. We will discuss possible origins of this enhancement caused by calcium substitution [3]. 1) X. Lin, Z. Zhu, B. Fauqu\'{e} and K. Behnia, Phys. Rev. X 3, 021002 (2013). 2) X. Lin, B. Fauqu\'{e} and K. Behnia, Science 349, 945 (2015). 3) C. W. Rischau, X. Lin, C. P. Grams, D. Finck, S. Harms, J. Engelmayer, T. Lorenz, Y. Gallais, B Fauqu\'{e}, J. Hemberger and K. Behnia, to be published (2016). [Preview Abstract] |
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