Two-band superconductivity of lead probed by scanning tunneling spectroscopy

The type I superconductor lead (Pb) has been theoretically predicted to be a two-band superconductor. We use scanning tunneling spectroscopy to resolve two superconducting gaps with an energy difference of 130 μeV. Tunneling into Pb(111), Pb(110) and Pb(100) crystals reveals a strong dependence of the two coherence peak intensities on the crystal orientation. We show that this is the result of a selective tunneling into the two bands at the energy of the two coherence peaks. This is further sustained by the observation of signatures of the Fermi sheets in differential conductance maps around subsurface defects.