Upper critical fields in as-grown MgB2 films prepared by ultra-high-vacuum MBE

We report on the upper critical fields (Hc2's) of as-grown MgB2 thin films deposited on the epitaxial Ti buffer layer on c-plane ZnO substrates by using a molecular beam epitaxy (MBE) apparatus. The Hc2 was estimated from the magnetoresistance measurements under the pulsed magnetic field up to 37 T. Hc2(T) for both H ∥ ab-plane and H ∥ c-axis were measured to obtain the anisotropic superconducting properties. The results are successfully analyzed with the Gurevich theory of dirty two-gap superconductivity with a cleaner π band case.