Energy Gap in Superconducting Fullerides: Optical and Tunneling Studies

Tunneling and optical transmission studies have been performed on superconducting samples of ${\mathrm{Rb}}_3{\mathrm{C}}_{60}$. At temperatures much below the superconducting transition temperature $T_{\mathrm{c}}$, the energy gap is $2\Delta =5.2\mathrm{}\pm 0.2\mathrm{meV}$, corresponding to $2\Delta /k_{\mathrm{B}}{T}_{\mathrm{c}}=4.2\mathrm{}$. The low temperature density of states and the temperature dependence of the optical conductivity resemble BCS behavior, although there is an enhanced “normal state” contribution. The results indicate that this fulleride material is an $s$-wave superconductor, but the superconductivity cannot be described in the weak coupling limit.