The unusual thickness dependence of superconductivity in $\alpha$-MoGe thin films

Abstract

Thin films of $\alpha$-MoGe show progressively reduced $T_c$'s as the thickness is decreased below 30 nm and the sheet resistance exceeds 100 $\Omega /\square$. We have performed far-infrared transmission and reflection measurements for a set of $\alpha$-MoGe films to characterize this weakened superconducting state. Our results show the presence of an energy gap with ratio $2{\Delta }_0/k_B{T}_c=3.8\pm 0.1$ in all films studied, slightly higher than the BCS value, even though the transition temperatures decrease significantly as film thickness is reduced. The material properties follow BCS-Eliashberg theory with a large residual scattering rate except that the coherence peak seen in the optical scattering rate is found to be strongly smeared out in the thinner superconducting samples. A peak in the optical mass renormalization at $2{\Delta }_0$ is predicted and observed for the first time.

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