Anisotropy in c-oriented MgB2 thin films grown by pulsed laser deposition

The electronic anisotropy in MgB2 is still a not completely clear topic; high quality c-oriented films are suitable systems to investigate this property. In this work we present our results on MgB2 superconducting thin films grown on MgO and sapphire substrates. The films are deposited in high vacuum, at room temperature, by laser ablation, starting from two different targets: pure boron and stoichiometric MgB2. In both cases, to obtain and crystallize the superconducting phase, an ex situ annealing in magnesium vapor is needed. The films were characterized by synchrotron radiation diffraction measurements; the films turned out to be strongly c-oriented, with the c-axis perpendicular to the film surface and an influence of the substrate on crystallographic parameters is observed. Resisivity measurements with the magnetic field perpendicular and parallel directions to the film surface evidenced an anisotropic upper critical field behavior. The H-c2 ratios (eta) resulted in the range 1.2-1.8; this difference will be discussed also in comparison with the literature data.