Temperature-dependent Fermi gap opening in the c(6 x 4)-C-60/Ag(100) two-dimensional superstructure

High-resolution angle-integrated photoemission of one monolayer of C-60 che misorbed on Ag(100) shows the reversible opening of a gap at the Fermi leve l at temperatures 25 less than or equal to T less than or equal to 300 K. T he gap reaches a maximum value of approximate to 10 meV at T less than or e qual to 70 K. This finding is the first evidence of an electronic phase tra nsition in C-60 monolayers and has implications on the ongoing debate about surface superconductivity in C-60-based bulk materials.