Effects of disorder on high-temperature superconductivity in cubic MoN

Three types of calculations have been used to help us understand the effects of disorder, alloying, and nonstoichiometry on the superconducting transition temperature $T_c$ of MoN, predicted to have $T_c$=29 K in the ideal B1 structure. (1) Simple disorder broadening indicates a strong lowering of $T_c$ which, however, does not account for the very low $T_c$’s measured in ${\mathrm{MoN}}_{\mathrm{x}}$ thin films. (2) Rigid-band calculations of the alloy system ${\mathrm{Nb}}_{1\mathrm{-}\mathrm{y}}$ ${\mathrm{Mo}}_{\mathrm{y}}$ ${\mathrm{N}}_{1.0}$ reproduce the measured trend in $T_c$ for y≤0.5. (3) Taking explicit account of metal $t_{2g}$–N p bonding in a coherent-potential-approximation model of ${\mathrm{MoN}}_{\mathrm{x}}$ leads to a semiquantitative understanding of the low measured $T_c$ values in films in terms of N vacancies, despite the nominal stoichiometry in some of the films.