The Magnetic-field Dependence of the Gaps in a Two-band Superconductor: A Point-contact Study of MgB2 Single Crystals
R. S. Gonnelli, D. Daghero, A. Calzolari, G. A. Ummarino, Valeria Dellarocca, V. A. Stepanov, J. Jun, S. M. Kazakov, J. Karpinski
Abstract
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 were able to extract the values of the two gaps Deltasigma and Deltapi. The comparison of the resulting Deltasigma(B) and Deltapi(B) 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.
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