  Alkali-fulleride superconductors with a maximum critical temperature Tc of 40
K exhibit similar electronic phase diagram with unconventional high-Tc
superconductors where the superconductivity resides proximate to a magnetic
Mott-insulating state. However, distinct from cuprate compounds, which
superconduct through two-dimensional (2D) CuO2 planes, alkali fullerides are
attributed to the three-dimensional (3D) members of high-Tc family. Here, we
employ scanning tunneling microscopy to show that trilayer K3C60 displays fully
gapped strong coupling s-wave superconductivity that coexists spatially with a
cuprate-like pseudogap state above Tc = 22 K and within vortices. A precise
control of electronic correlations and doping reveals 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 superconductivity below half-filling,
demonstrates that alkali fullerides are predominantly phonon-mediated
superconductors, although the multiorbital electronic correlations also come
into play.
