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 B parallel and perpendicular to the ab planes. The complete thermal evolution of the upper critical field of the pi band is determined for the first time, and quantitative information about the upper critical field of the sigma 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 sigma and pi 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 Delta(sigma) and Delta(pi), whose complete temperature dependence is obtained with the greatest accuracy. (C) 2002 Elsevier Science B.V. All rights reserved.