The band gap of solid ${\mathrm{C}}_{60}$ is found to be 2.3±0.1 eV. The on-site molecular ${\mathrm{C}}_{60}$ Coulomb interaction (U) as determined from the KVV ${\mathrm{C}}_{60}$ Auger spectrum is found to be 1.6±0.2 eV. This value of U is shown to lead to Frenkel-type molecular excitons in the 1.5–2 eV range. These results lead us to suggest that doped ${\mathrm{C}}_{60}$ should be considered as a highly correlated system with U/W comparable to that in high-${\mathit{T}}_{\mathit{c}}$ cuprates and that stoichiometric ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ is a Mott-Hubbard insulator with a gap of about 0.7 eV.

• Received 11 March 1992

DOI:https://doi.org/10.1103/PhysRevLett.68.3924