Two-Band-Type Superconducting Instability in MgB_2 : Condensed Matter: Electronic Properties, etc.

概要

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Using the tight-binding method for the π-bands in MgB_2, the Hubbard on-site Coulomb interaction on two inequivalent boron p.-orbitals is transformed into expressions in terms of π-band operators. For scattering processes relevant to the problem in which a wave vector q is parallel to z^^^, it is found to take a relatively simple form consisting of intra-band Coulomb scattering, interband pair scattering etc. with large constant coupling constants. This allows to get a simple expression for the amplitude of interband pair scattering between two π-bands, which diverges if the interband polarization function in it becomes large enough. The latter was approximately evaluated and found to be largely enhanced in the band structure in MgB_2. These results lead to a divergent interband pair scattering, meaning two-band-type superconducting instability with enhanced T_c. Adding a subsidiary BCS attractive interaction in each band into consideration, a semi-quantitative gap equation is given, and T_c and isotope exponent are derived. The present instability is asserted to be the origin of high T_c in MgB_2.
社団法人日本物理学会の論文
2001-06-15