Pr. Yasutami Takada, Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan

Jeudi 6 Fevrier 2014, 14h
Amphi Holweck, Esc C, 1ème etage

Mechanism of Superconductivity in Polar Crystals

Yasutami Takada
Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
E-mail : takada (arobase) issp.u-tokyo.ac.jp

In order to make a reliable discussion on the mechanisms of superconductivity, it is required, in the first place, to establish a firm and general theoretical framework to calculate the transition temperature Tc starting directly from the first-principles Hamiltonian (or at least a microscopic model Hamiltonian). Thus in my talk, I begin with a critical review of the theories for quantitative calculations of Tc from first principles. In view of the importance to avoid employing the phenomenological parameters such as the Coulomb pseudopotential mu^* in describing the effect of the Coulomb repulsion on the Cooper-pair formation, I focus on the G0W0 approximation to the exact theory in the Green’s-function approach as well as the density functional theory for superconductors (SCDFT) with paying special attention to the surprising similarity in the gap equation obtained in each theory, implying a deep implicit interconnection between the two theories, in spite of the large difference in their basic theoretical frameworks. Then, by employing the framework in the G0W0 approximation, I discuss the general behavior of Tc in polar crystals in which a plasmon-polar LO phonon coupled mechanism of superconductivity is considered to work. Finally I successfully apply the theory to the models representing n-type doped SrTiO3 (STO) and graphite intercalation compounds (GIC) to clarify, for example, the reason why Tc is enhanced by a hundred times in CaC6 compared to KC8. Some discussions including the maximum Tc to be expected in GIC and possible reasons on the difference in Tc for STO between my old theory and the recent experiment by Behnia’s group for the electron density as low as 1018 cm-3 or less.

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