Relationship between the density of states and the superconducting transition temperature in A-15 compounds

Data by Wiesmann et al. on the temperature coefficient of the upper critical field, dHc2(T)dT, and the normal-state resistivity ρ0 near the superconducting transition temperature Tc are analyzed to estimate the bare density of states N(EF) as a function of structural disorder and Tc in various α-particle irradiated samples of A-15 Nb3Ge and Nb3Sn. By taking into account the change in the electron-phonon coupling strength with Tc and the effect of mass enhancement arising from the electron-phonon interaction, the results of the new analysis indicate that (i) the drop in N(EF) for Nb3Ge is relatively small as Tc changes from ∼ 21 to ∼ 4 K, in agreement with a recent low-temperature specific-heat measurement; (ii) the value of N(EF) in Nb3Sn decreases by a factor of ∼ 4 as Tc varies from ∼ 18 to ∼ 3 K. The implication of these findings is discussed in terms of the origin of high-Tc superconductivity in A-15 compounds. © 1978 The American Physical Society.