Nonlinear microwave response of Nb/sub 3/Sn films: a case study of granular superconductors

We have investigated the microwave magnetic field dependent surface resistance, R/sub s/(B/sub s/), of Nb/sub 3/Sn films of different film thickness and on different substrates at 19 GHz. The film thickness controls the grain size of the polycrystalline films. It provides a tool to study the role of grain boundaries on the nonlinear microwave response of granular superconductors. We find a non-monotonic dependence of the onset field, B/sub on/, of nonlinear surface resistance on film thickness, with a maximum of 25 mT at 1.2 /spl mu/m. This behaviour reveals that magnetic losses at grain boundaries dominate in thin films, and microwave heating in thick films. The universal scaling behaviour of B/sub on/ with temperature, its dependence on the thermal conductivity of the substrate, and the good agreement between the quadratic field dependence of R/sub s/ and a weakly-coupled grain model indicate heating at grain boundaries as a possible mechanism. The correlation between the nonlinear microwave response and microstructure of the granular films indicates possible improvements and intrinsic limitations.