Point-contact measurements were performed on polycrystalline MgB2 samples using a Cu counter electrode. The current–voltage (I–V) characteristics revealed Andreev-reflection structures from the ballistic transport across the normal-metal/superconductor interface. The spectra contain two different energy gaps which close near the same bulk critical temperature. These results support the multiband model of superconductivity for MgB2. Point-contact experiments in a magnetic field showed that the small-gap structure disappears in fields of 1–2T whereas the large-gap structure was suppressed in larger fields going up to around 15T. Nonlinearities in the current–voltage characteristics contain information on the energy dependence of the electron–phonon interaction.

Andreev-Reflection Study in MgB2. P.Szabó, P.Samuely, J.Kacmarcik, A.G.M.Jansen, T.Klein, J.Marcus, C.Marcenat: Superconductor Science and Technology, 2003, 16, 162-6