We studied the voltage and temperature dependency of the dynamic conductance of normal metal-MgB2 junctions obtained either with the point-contact technique (with Au and Pt tips) or by making Ag-paint spots on the surface of high-quality single-crystal-like MgB2 samples. The fit of the conductance curves with the generalized BTK model gives evidence of pure s-wave gap symmetry. The temperature dependency of the gap, measured in Ag-paint junctions (dirty limit), follows the standard BCS curve with 2Delta/kTc = 3.3. In out-of-plane, high-pressure point contacts we obtained almost ideal Andreev reflection characteristics showing a single small s-wave gap Delta = 2.6 +/- 0.2 (clean limit). These results support the two-gap model of superconductivity, the presence of a modified layer at the surface of the crystals and an important and non-conventional role of the impurities in MgB2.
@article{arxiv.cond-mat/0107239,
title = {Temperature and junction-type dependency of Andreev reflection in MgB2},
author = {R. S. Gonnelli and A. Calzolari and D. Daghero and G. A. Ummarino and V. A. Stepanov and P. Fino and G. Giunchi and S. Ceresara and G. Ripamonti},
journal= {arXiv preprint arXiv:cond-mat/0107239},
year = {2009}
}