Muon-spin-relaxation studies of the alkali-fulleride superconductors

Muon-spin-rotation and relaxation experiments have been performed on the alkali-fulleride superconductors: ${\mathrm{Rb}}_3{\mathrm{C}}_{60},$ ${\mathrm{K}}_3{\mathrm{C}}_{60},$ ${\mathrm{Na}}_2{\mathrm{CsC}}_{60}$. A small fraction of the implanted muons form endohedral muonium $\left({\mathrm{Mu}\mathrm{@}\mathrm{C}}_{60}\right),$ i.e., a ${\mu }^{+}{e}^{-}$ atom on the inside of the ${\mathrm{C}}_{60}$ cage. The presence of the unpaired electron on the muon greatly enhances the sensitivity of the muon to scattering from electronic excitations and the resulting muon-spin relaxation. The ${1/T}_1$ spin-relaxation rate of ${\mathrm{Mu}\mathrm{@}\mathrm{C}}_{60}$ in a longitudinal field exhibits a Korringa-like temperature dependence in the normal state, and a small Hebel-Slichter coherence peak followed by activated temperature dependence well below $T_c.$ In ${\mathrm{Rb}}_3{\mathrm{C}}_{60}$ the coherence peak is strongly suppressed by a magnetic field which is well below $H_{c2\mathrm{}}.$ From the activated temperature dependence we obtain estimates of the superconducting energy gap. In addition, spin precession measurements of diamagnetic muons in the vortex state are used to estimate the magnetic penetration depth. In ${\mathrm{Na}}_2{\mathrm{CsC}}_{60},$ we find a quench rate dependence suggesting the existence of two low-temperature metallic phases, only one of which is superconducting.