8. A spectroscopic study was carried out in MgB₂ just after the discovery of its superconducting properties (January 2001). The samples were good-quality, very hard polycrystals produced by EDISON SpA. Point-contact measurements on these samples showed an unexpected dependence of the conductance curves (all featuring Andreev-reflection structures) on the pressure applied by the tip on the sample surface. This speaks in favour of the existence of a degraded layer on the surface, where the material is in dirty limit (i.e. the interband scattering is strong). We were able to follow the temperature dependence of the gap in dirty limit measured on top of this layer. Tunneling measurements in break junctions gave results in complete agreement with these findings.

R.S. Gonnelli, A. Calzolari, D. Daghero, G.A. Ummarino, V.A. Stepanov, P. Fino, G. Giunchi, S. Ceresara, G. Ripamonti
“Temperature and junction-type dependency of Andreev reflection in MgB₂”, Journal of Physics and Chemistry of Solids, 63/12, 2319-2323, (2002).
R.S. Gonnelli, G.A. Ummarino, D. Daghero, A. Calzolari, V.A. Stepanov
“Gap measurements in MgB₂ break-junction and point-contact heterostructures: test of the two-band models”, International Journal of Modern Physics B, 16, 1553-1561, (2002).

Finally, we performed the first measurements of ac and dc Josephson effect in break junctions of MgB₂. By irradiating the junctions with microwaves we observed clear Shapiro steps spaced by the ideal DV value. We obtained the temperature ependence of the dc Josephson current and the dependence of the height of the steps on the microwave power. These results directly prove the existence of pairs with charge 2e in MgB₂ and give evidence of the superconductor-normal metal-superconductor weak link character of these junctions.

R.S. Gonnelli, A. Calzolari, D. Daghero, G.A. Ummarino, V.A. Stepanov, G. Giunchi, S. Ceresara, G. Ripamonti
“Josephson effect in MgB₂ break junctions”, Phys. Rev. Lett. 87, 097001 (2001).