Electronic transport at low temperature below the field-driven superconductor–insulator transition in thin a-Mo xSi 1− x films

Abstract

We have shown so far that thin amorphous (a-)Mo x Si 1− x films exhibit the field-driven superconductor–insulator transition (SIT) at zero temperature ( T = 0), while the existence of a metallic quantum–vortex–liquid at T = 0 has been reported in several thin amorphous films. Here we reexamine the possibility of the metallic phase by measuring the T dependence of resistance R( T) for the thin a-Mo x Si 1− x films with various transition temperatures T c0 in fields B below the critical field of the “SIT”. We find that the value of T c0 dominates R( T) at low T. For films with T c0 larger than 1.0 K, the activation energy U derived from the slope of the Arrhenius plot of R is constant over the whole T region, while for films with T c0 < 1.0 K, U exhibits a discontinuous decrease below about 0.1 K; however, U remains constant and positive down to the lowest T. All of the data are consistent with the picture of the field-driven SIT.