Key points are not available for this paper at this time.
A two-band model of superconductivity is investigated. One band is treated in line with BCS (the cut of the interaction outside ℏω of the Fermi energy). In the other band the interaction is assumed to be repulsive. The superconducting transition temperature is always raised over that of the single attractive band, when an interaction between the bands are introduced. The increase of the temperature is large when the density of states of the repulsive band at the Fermi surface is large. The isotope effect vanishes when the interband interaction is large. This model can account for the superconducting properties of the V3M series and Nb3Sn. It is argued that superconducting non-transition metals belong to single-band superconductors, whereas the superconductivity of transition metals is largely due to their overlapping bands, especially in lanthanum.
Jun Kondo (1963) studied this question.