Coherence peak and superconducting energy gap in Rb3C60 observed by muon spin relaxation.

Abstract

Muon spin relaxation resulting from spin exchange scattering of endohedral muonium (${\mathrm{\ensuremath{\mu}}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$) with thermal electronic excitations has been observed in the fullerene superconductor ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$. The temperature dependence of ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ 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, \ensuremath{\Delta}/${\mathit{k}}_{\mathit{B}}$=53(4) K or 2\ensuremath{\Delta}/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$=3.6(3), is consistent with the BCS weak coupling limit.