Unusual thickness dependence of the superconducting transition of α-MoGe thin films

Thin films of α-MoGe show progressively reduced T_c ’s as the thickness is decreased below 30 nm and the sheet resistance exceeds 100Ω/◻ . We have performed far-infrared transmission and reflection measurements for a set of α-MoGe films to characterize this weakened superconducting state. Our results show the presence of an energy gap with ratio 2Δ₀/k_BT_c=3.8±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 but the expected coherence peak seen in the optical scattering rate is lost in the thinner samples. This result cannot be explained within conventional theories.