Anisotropy and critical current density of MgB2 thin films grown in-situ by molecular beam epitaxy

摘要

We report transport properties of superconducting MgB2 thin films in-situ grown by molecular beam epitaxy. The MgB2 films show a superconducting transition at 34.5 K with ΔTc<1 K. We measure the in-plane electrical resistivity of the films in magnetic field to 8 T and estimate the upper critical field Hc2⊥(0)～32 T for field oriented along the c-axis and Hc2||(0)～35 T in the plane of the film. We find the zero-temperature coherence lengths ξc(0)～31 ? and ξab(0)～36 ?, 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 ? is smaller than the coherence length, which places our films in the dirty limit. Estimates of the critical current density, Jc, using magnetic field hysteresis loops and the Bean critical state model give nominal critical current densities on the order of 106 A/cm2 at 15 K and self-field.