Point-contact spectroscopy in MgB2 single crystals in magnetic field

We present the results of a spectroscopic study of state-of-the-art MgB2 single crystals, carried out by using a modified point-contact technique. The use of single crystals allowed us to obtain point contacts with current injection either parallel or perpendicular to the ab-planes. The effect of magnetic fields up to 9 T on the conductance spectra of these contacts is here thoroughly studied, for both parallel and perpendicular to the ab-planes. The complete thermal evolution of the upper critical field of the π band is determined for the first time, and quantitative information about the upper critical field of the σ band and its anisotropy is obtained. Finally, by exploiting the different effect of a magnetic field applied parallel to the ab-planes on the two band systems, the partial contributions of the σ and π bands to the total conductance are obtained separately. Fitting each of them with the standard BTK model yields a great reduction of the uncertainty on Δσ and Δπ, whose complete temperature dependence is obtained with the greatest accuracy.