11. We have realized 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 were able to extract the values of the two gaps D_s and D_p. The comparison of the resulting D_s and D_p curves to the theoretical predictions of the two-band model in dirty limit, recently appeared in literature, allows the first direct test of this model and gives a clear and quantitative proof that the pi band, even in the best single crystals, is in the moderate dirty limit.

D. Daghero, R.S. Gonnelli, G.A. Ummarino, V.A. Stepanov, J. Jun, S.M. Kazakov, J. Karpinski
“Point-contact spectroscopy in MgB₂ single crystal in magnetic field”, Physica C, 385, 255 – 263, (2003).

R.S. Gonnelli, D. Daghero, A. Calzolari, G.A. Ummarino, Valeria Dellarocca, V.A. Stepanov, J. Jun, S.M. Kazakov, J. Karpinski
“The magnetic-field dependence of the gaps in a two-band superconductor: a point-contact study of MgB₂ single crystals”, submitted, (cond-mat/0308152).