Systematic characterization of upper critical fields for MgB2 thin films by means of the two-band superconducting theory
We present experimental results for the upper critical fields Hc2 of various MgB2 thin films prepared by molecular beam epitaxy, multiple-targets sputtering, and co-evaporation deposition. Experimental data for the Hc2(T) are successfully analyzed by applying the Gurevich theory of dirty two-band superconductivity in the case of Dπ/Dσ>1, where Dπ and Dσ are the intraband electron diffusivities for the π and σ bands, respectively. We find that the parameters obtained from the analysis are strongly correlated to the superconducting transition temperature Tc of the films. We also discuss the anomalous narrowing of the transition width at intermediate temperatures confirmed by the magnetoresistance measurements.