Superconducting thin films of MgB2 were prepared on MgO(001) substrates by molecular beam epitaxy under the co-evaporation conditions of low deposition rate and ultra-high vacuum. A superconducting transition with an onset temperature of 31.2K was confirmed by both transport and magnetization measurements. The upper critical fields were obtained from measurements of the field dependence of the resistivity. It was estimated that the upper critical field at 0K was more than 15T. The upper critical field anisotropy ratio, HC2,ab(0)/HC2,C(0), was estimated to be 1.78 from the magnetic field–temperature phase diagram for as-grown MgB2 thin films.

Upper Critical Field of As-Grown MgB2 Thin Films by Molecular Beam Epitaxy. Y.Harada, M.Udsuka, T.Takahashi, Y.Nakanishi, M.Yoshizawa: Physica B, 2005, 359-361, 466-8