Energy gap in superconductors with A-15 structure

The effect of the fine structure of the electron density of states N(ξ) on the magnitude of the ratio 2Δ(z)Tc in narrow-gap superconductors is studied on the basis of a model with a peak in the density of states N(ξ) near the Fermi level. It was established experimentally that the drop in Tc in defective samples of Nb3Sn from 17.7 to 13.9 K is accompanied by a decrease in 2Δ(0)/Tc from 4.0 to 3.6. This result agrees well with the theoretical estimates made.