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 Δ_σ and Δ_π. The resulting Δ_σ(B) and Δ_π(B) curves agree very well with the predictions of the two-band model in dirty limit when the π-band diffusivity is a few times greater than the σ-band one. This result concurs with other experimental and theoretical findings in indicating that, even in the best crystals, the π band is moderately dirty.