Two gap signature in magnesium diboride

Scanning Tunneling Spectroscopy is used at low temperature in an extensive study of the recently discovered superconducting compound MgB2. Both powder- and crystal-based samples show a very unusual double-gap structure in the tunneling conductance spectra. The spectral weights of the gaps may vary from one junction to another but their energy positions remain constant. Both gaps vanish at the critical temperature of the bulk material. These findings confirm the existence of two-gap superconductivity in MgB2, which opens a new class of multiple-gap superconductors.