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

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 ΔT_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_c2^⊥(0)∼32 T for field oriented along the c-axis and H_c2^||(0)∼35 T in the plane of the film. We find the zero-temperature coherence lengths ξ_c(0)∼31 Å and ξ_ab(0)∼36 Å, 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 Å is smaller than the coherence length, which places our films in the dirty limit. Estimates of the critical current density, J_c, 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.