Scanning tunneling spectroscopy in MgB2

We present extensive scanning tunneling spectroscopy measurements at low temperatures in the multiband superconductor MgB2. We find a similar behavior in single crystalline samples and in single grains, which clearly shows the partial superconducting density of states of both the pi and sigma bands of this material. The superconducting gaps corresponding to both bands are not single valued. Instead, we find a distribution of superconducting gaps centered around 1.9 and 7.5 mV, corresponding respectively to each set of bands. Interband scattering effects, leading to a single gap structure at 4 mV and a smaller critical temperature can be observed in some locations on the surface. S-S junctions formed by pieces of MgB2 attached to the tip clearly show the subharmonic gap structure associated with this type of junctions. We discuss future developments and possible new effects associated with the multiband nature of superconductivity in this compound. (C) 2002 Elsevier Science B.V. All rights reserved.