Abstract
Abstract Superconducting phase transition in narrow band metal described by Hubbard Hamiltonian with negative U-centers is studied within the mean field approximation. Unlike in the BCS theory of superconductivity, the gap to critical temperature ratio, Δ o /T c , proves to be greater than 2 and is weakly dependent on the occupancy ν (the number of electrons per lattice site). The jump in the specific heat at T c equals ΔC s / C n =5.25/(1+4 ν ) at ν ⪡ 1. Critical temperature diminishes, at even value of ν, which may serve as an explanation of the behavior observed in transition-metal superconductors (Matthias rule).
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