Critical flux pinning and enhanced upper critical field in magnesium diboride films

We have conducted pulsed transport measurements on c-axis-oriented magnesium diboride films over the entire relevant ranges of magnetic field 0HHc2 (where Hc2 is the upper critical field) and current density 0jjd (where jd~10⁷A/cm² is the depairing current density). The intrinsic disorder of the films combined with the large coherence length and three dimensionality, compared to cuprate superconductors, results in a sixfold enhancement of Hc2 (to 17±3T) and raises the depinning current density jc to within an order of magnitude of jd. The current-voltage response has an unusually steep nonlinear shape at all fields, resulting from a combination of depinning and pair breaking, and has no trace of an Ohmic free-flux-flow regime.