Magnetic-field dependence of the gaps in a two-band superconductor: a point-contact study of MgB2 single crystals

We present the results of directional point-contact measurements in MgB2 single crystals, in magnetic fields up to 9 T parallel to the c axis. By fitting the conductance curves of our point contacts—showing clear Andreev-reflection features—with a generalized Blonder-Tinkham-Klapwijk model, we extracted the values of the two gaps Ds and Dp . The resulting Ds(B) and Dp(B) curves agree very well with the predictions of the two-band model in dirty limit when the p-band diffusivity is a few times greater than the s-band one. This result concurs with other experimental and theoretical findings in indicating that, even in the best crystals, the p band is moderately dirty.