Coherence peak and superconducting energy gap inRb3C60observed by muon spin relaxation

Muon spin relaxation resulting from spin exchange scattering of endohedral muonium (${\mathrm{\mu }}^{+}$ ${\mathit{e}}^{\mathrm{-}}$) with thermal electronic excitations has been observed in the fullerene superconductor ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$. The temperature dependence of ${\mathit{T}}_1^{\mathrm{-}1}$ shows a coherence peak just below ${\mathit{T}}_{\mathit{c}}$ and can be fit to the conventional Hebel-Slichter theory for spin relaxation in a superconductor with a broadened BCS density of states. The average energy gap for electronic excitations, Δ/${\mathit{k}}_{\mathit{B}}$=53(4) K or 2Δ/${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$=3.6(3), is consistent with the BCS weak coupling limit.