Electronic anisotropy, magnetic field-temperature phase diagram and their dependence on resistivity in c-axis oriented MgB2 thin films

An important predicted, but so far uncharacterized, property of the new sup erconductor MgB2 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(parallel to)/H-c2(perpendic ular to) is 1.8 to 2.0, the ratio increasing with higher resistivity. Measu rements of the magnetic field-temperature phase diagram show that flux pinn ing disappears at H+ approximate to 0.8H(c2)(perpendicular to)(T) in untext ured samples. H-c2(parallel to)(0) is strongly enhanced by alloying to 39 T for the highest resistivity film, more than twice that seen in bulk sample s.