Anisotropy in the optical response of superconducting MgB2 films

The anisotropy of the dielectric function of c-axis oriented MgB2 thin films has been revealed by a combined study of normal-incident reflectance and spectroscopic ellipsometry. The application of this procedure enabled us to determine the complete set of optical response functions and to show that the dominant effect responsible for the anisotropy is due to interband transitions, which distinctly screen the plasma edge depending on the polarization of incident radiation. The most important transition occurs at 23,800 cm−1 and is polarized in the ab-plane. This is in good agreement with the results obtained previously for single crystals. Our results are also in a qualitative agreement with those obtained by band structure calculations.