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

TL;DR

This study investigates how superconductivity in thin $ ext{MoGe}$ films weakens with decreasing thickness, revealing a consistent energy gap and novel optical features despite reduced transition temperatures.

Contribution

It provides the first observation of a peak in optical mass renormalization at twice the energy gap in thin $ ext{MoGe}$ films, expanding understanding of their superconducting properties.

Findings

Energy gap ratio $2 ext{Δ}_0/k_B T_c$ is slightly above BCS value.

Optical scattering rate peak is smeared in thinner films.

First observation of a peak in optical mass renormalization at $2 ext{Δ}_0$.

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 $\Omega/\Box$. 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 $2\Delta_0/k_BT_{c} = 3.8 \pm 0.1$ 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.