Josephson effect and Andreev reflection measurements in MgB2 break- junction and point- contact heterostructures

We present the first observation of a nonhysteretic dc and ac Josephson effect in break junctions obtained at 4.2 K in high-quality, high-density polycrystalline metallic MgB2 samples recently reported by us (1). Shapiro steps spaced by the ideal dV value were observed by microwave irradiation and we obtained the temperature dependence of the dc Josephson current as well as the dependence on microwave power of the step heights. Besides giving direct proof of the existence of pairs with charge 2e in MgB2 and of the SNS long weak-link character of these junctions, these results provide information on the energy gap. Josephson data are consistent with a dirty-limit gap Delta~4 meV with Tc~27 K in agreement with the two-band model of superconductivity in MgB2. In different experiments we studied the behaviour in voltage and temperature of the dynamic conductance of normal metal-MgB2 junctions obtained either by using the point-contact technique (with Au and Pt tips) or by making a spot of Ag paint on the sample surface. The conductance curves presented clear Andreev reflection features and were fitted with the generalized BTK model giving evidence of pure s-wave gap symmetry and of a temperature dependency of the gap in dirty limit which follows the standard BCS curve. Out-of-plane, high-pressure point-contact junctions showed almost ideal Andreev reflection characteristics from which the small 3D gap in the clean limit Delta3D = 2.6 ± 0.2 meV and its temperature dependency were obtained, once more in agreement with the two-band model for MgB2. (1) R.S. Gonnelli et al., Phys.Rev. Lett. 87, 097001 (2001