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Resonating mean-field theoretical approach to two-gap superconductivity

Authors: S. Nishiyama, J. da Providencia, C. Providencia, H. Ohnishi

Ref.: Adv.Stud.Theor.Phys. 4, 283-303 (2010)

Abstract: We have developed the resonating mean-field theories (Res-MFTs) both for normal and superconducting fermion systems with large quantum fluctuations. We have successfully applied them to descriptions of exact solvable Lipkin model in nuclei, deformed-shape coexistence in superconducting nuclei and low-energy light-mesons mass spectra in hadrons. To show their effectiveness, we also apply the Res-MFT to a superconducting metal to describe one of current and hot topics, the two-gap superconductivity. A state with large quantum fluctuations is approximated by superposition of two MF wave functions (WFs) composed of non-orthogonal Hartree-Bogoliubov (HB) WFs with different correlation structures. We make a direct optimization of the Res-HB ground-state energy with respect to the energy gaps at T =0. Adopting a suitable chemical potential, the two-gap superconductivity in MgB2 is well described by the Res-HB approximation. The Res-HB ground state generated with HB WFs is expected to explain almost the value of the ground-state correlation energy in all the correlation regimes.