Electronic anisotropy, magnetic field-temperature phase diagram and their dependence on resistivity in c-axis oriented MgB_2 thin films

摘要

An important predicted, but so far uncharacterized, property of the new superconductor MgB_2 is electronic anisotropy arising from its layered crystal structure. Here we report on three c-axis oriented thin films, showing that the upper critical field anisotropy ratio H_(c2)~(||)/ H_(c2)~(perpendicular) is 1.8 to 2.0, the ratio increasing with higher resistivity. Measurements of the magnetic field-temperature phase diagram show that flux pinning disappears at H≈0.8 H_(c2)~( perpendicular) (T) in untextured samples. H_(c2)~(||) (0) is strongly enhanced by alloying to 39 T for the highest resistivity film, more than twice that seen in bulk samples.