Momentum-dependent multiple gaps in magnesium diboride probed by electron tunnelling spectroscopy.

The energy gap is the most fundamental property of a superconductor. MgB2, a superconductor discovered in 2001, exhibits two different superconducting gaps caused by the different electron-phonon interactions in two weakly interacting bands. Theoretical calculations predict that the gap values should also vary across the Fermi surface sheets of MgB2. However, until now, no such variation has been observed. It has been suggested that two gap values were sufficient to describe real MgB2 samples. Here we present an electron tunnelling spectroscopy study on MgB2/native oxide/Pb tunnel junctions that clearly shows a distribution of gap values, confirming the importance of the anisotropic electron-phonon interaction. The gap values, and their spreads found from the tunnel junction measurements, provide valuable experimental tests for various theoretical approaches to the multi-band superconductivity in MgB2.