Upper critical field of as-grown MgB2 thin films by molecular beam epitaxy

Superconducting thin films of magnesium diboride (MgB2) were prepared on MgO(001) substrate by molecular beam epitaxy in the co-evaporation conditions of low deposition rate and ultra-high vacuum. A superconducting transition with the onset temperature of 31.2K was confirmed by both transport and magnetization measurements. The upper critical fields are obtained from measurement 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