Electronic anisotropy, magnetic field-temperature phase diagram and their dependence on resistivity inc-axis oriented MgB2thin films

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 c 2 ∥ / H c 2 ⊥ 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 c 2 ⊥ ( T ) in untextured samples. H c 2 ∥ (0) is strongly enhanced by alloying to 39 T for the highest resistivity film, more than twice that seen in bulk samples.