Temperature variation of critical magnetic field and electronic characteristics of superconducting alloys with A15 structure

This paper reports on measurements of temperature variation of the upper critical magnetic field Hc2 that are performed for Nb3Sn films with varying degrees of disorder. An anomalous variation of dHc2/dT (where T ∼ Tc) as a function of film resistivity ρ is discussed. Electronic characteristics and parameters of electron-phonon interaction of specimens of Nb3Sn and Nb3Ge with varying degrees of disorder are determined. A decrease in electron state density at the Fermi level by a factor of four is found for Nb3Sn, by a factor of two for Nb3Ge as ρ increases to 150 μ Ohms · cm. The results are explained by means of a model of electron structure with a peak of state density