By using tunneling experiments using thin film tunnel junctions, electron-coupled phonon spectra were determined showing that superconductivity in MgB2 was phonon-mediated. In a further analysis, which involved first-principles calculations, the strongest feature in these spectra could be traced back to the key quantity of 2-band superconductivity, the interband pairing interaction. For phonons, this interaction was quite selective. It involved mainly low-energy optical phonon modes, in which the B atoms moved perpendicularly to the B planes.

Observation of Interband Pairing Interaction in a Two-Band Superconductor - MgB2. J.Geerk, R.Schneider, G.Linker, A.G.Zaitsev, R.Heid, K.P.Bohnen, H.Lohneysen: Physical Review Letters, 2005, 94[22], 227005 (3pp)