Superconductivity Driven by the Interband Coulomb Interaction and Implications for the Superconducting Mechanism of MgB2

摘要

Superconducting mechanism mediated by interband exchange Coulomb repulsion isexamined in an extended two-band Hubbard models with a wide band crossing theFermi level and coexisting with a narrower band located at moderately lowerenergy. We apply newly developed path-integral renormalization group method toreliably calculate pairing correlations. The correlation shows markedenhancement at moderate amplitudes of the exchange Coulomb repulsion takensmaller than the on-site repulsion for the narrower band. The pairing symmetryis s-wave while it has unconventional phases with the opposite sign between theorder parameters on the two bands, in agreement with the mean-field prediction.Since the band structure of recently discovered superconductor MgB$_2$ sharesbasic similarities with our model, we propose that the present results providea relevant clue for the understanding of the superconducting mechanism inMgB$_2$ as well as in this class of multi-band materials with good metallicconduction in the normal state.