Strong electron-phonon coupling and multiband effects in the superconducting $\beta$-phase Mo$_{1-x}$Re$_x$ alloys

Abstract

Superconducting transition temperature $T_C$ of some of the cubic $\beta$-phase Mo$_{1-x}$Re$_x$ alloys with x > 0.10 is an order of magnitude higher than that in the elements Mo and Re. We investigate this rather enigmatic issue of the enhanced superconductivity with the help of experimental studies of the temperature dependent electrical resistivity ($\rho$(T)) and heat capacity (C$_P$(T)), as well as the theoretical estimation of electronic density of states (DOS) using band structure calculations. The $\rho$(T) in the normal state of the Mo$_{1-x}$Re$_x$ alloys with x > 0.15 is distinctly different from that of Mo and the alloys with x < 0.10. We have also observed that the Sommerfeld coefficient of electronic heat capacity $\gamma$, superconducting transition temperature $T_C$ and the DOS at the Fermi level show an abrupt change above x > 0.10. The analysis of these results indicates that the value of electron-phonon coupling constant {\lambda}ep required to explain the $T_C$ of the alloys with x > 0.10 is much higher than that estimated from $\gamma$. On the other hand the analysis of the results of the $\rho$(T) reveals the presence of phonon assisted inter-band s-d scattering in this composition range. We argue that a strong electron-phonon coupling arising due to the multiband effects is responsible for the enhanced $T_C$ in the $\beta$-phase Mo$_{1-x}$Re$_x$ alloys with x > 0.10.