Anisotropy and critical current density of MgB/sub 2/ thin films grown in-situ by molecular beam epitaxy

We report transport properties of superconducting MgB/sub 2/ thin films in-situ grown by molecular beam epitaxy. The MgB/sub 2/ films show a superconducting transition at 34.5 K with (Delta)T/sub c/<1 K. We measure the in-plane electrical resistivity of the films in magnetic field to 8 T and estimate the upper critical field H/sub c2//sup .../(0)~32 T for field oriented along the c-axis and H/sub c2//sup .../(0)~35 T in the plane of the film. We find the zero-temperature coherence lengths (xi)/sub c/(0)~31 A and (xi)/sub ab/(0)~36 A, indicating the field anisotropy ratio is 1.2, comparable with reported in-situ epitaxial thin films, but less than single crystals. The calculated electronic mean free path l=25 A is smaller than the coherence length, which places our films in the dirty limit. Estimates of the critical current density, J/sub c/, using magnetic field hysteresis loops and the Bean critical state model give nominal critical current densities on the order of 10/sup 6/ A/cm/sup 2/ at 15 K and self-field.