Superconductivity in Mo-Re and Nb-IrσPhases

Superconducting transition temperatures and low-temperature specific-heat capacities were measured for well-characterized Nb-Ir and Mo-Re $\sigma$-phase alloys. The transition temperatures $T_c$, and the electronic specific-heat-capacity coefficients $\gamma$ were found to vary only slowly with composition in these alloys. By considering data from the literature on other $\sigma$-phase alloys as well as our own data, it was concluded that no clear correlation could be made between $T_c$ and $\gamma$ [and therefore the electronic density of states at the Fermi level $N_{\mathrm{bs}}\mathrm{}\left(0\right)\mathrm{}$]. Good correlation was obtained, however, between McMillan's electron-phonon coupling constant $\lambda$ and $T_c$. This suggests that the phonon spectrum may be more important than the electronic density of states at the Fermi level in controlling $T_c$ for $\sigma$ phases. The high value of $T_c$ (9. 8 °K) reported in the literature for the Nb-40 at.% Ir $\sigma$ phase is believed to be due to small quantities of a second superconducting phase.