Direct Observation of Full-Gap Superconductivity and Pseudogap in Two-Dimensional Fullerides.

Alkali-fulleride superconductors with a maximum critical temperature T-c similar to 40 K exhibit a similar electronic phase diagram to that of unconventional high-T-c superconductors. Here we employ cryogenic scanning tunneling microscopy to show that trilayer K3C60 displays fully gapped strong coupling s-wave superconductivity, accompanied by a pseudogap above T-c similar to 22 K and within vortices. A precise control of the electronic correlations and potassium doping enables us to reveal that superconductivity occurs near a superconductor-Mott-insulator transition and reaches maximum at half-filling. The s-wave symmetry retains over the entire phase diagram, which, in conjunction with an abrupt decline of the superconductivity below half-filling, indicates that alkali fullerides are predominantly phonon-mediated superconductors, although the electronic correlations also come into play.