BCS-like Density of States in SuperconductingA3C60Surfaces

Abstract

We report on an ultrahigh resolution photoemission study on the topmost molecular layer of ${\mathrm{K}}_3{\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3{\mathrm{C}}_{60}$ films below and above the superconducting transition temperature $T_C$. We observed not only clear evidence for the opening of the superconducting gap, but also a modification in the photoemission line shape consistent with a change from a normal-metallic to a BCS-like density of states, including the formation of a condensation peak. The data can be accurately modeled by a BCS-type function with a gap $\Delta$ derived from $T_C$ in the weak-coupling limit ( $2\Delta /k_B{T}_C=3.53\mathrm{}$).