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

Muon-spin-rotation and relaxation experiments have been performed on the alkali-fulleride superconductors: (Formula presented). A small fraction of the implanted muons form endohedral muonium (Formula presented) i.e., a (Formula presented) atom on the inside of the (Formula presented) 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 (Formula presented) spin-relaxation rate of (Formula presented) 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 (Formula presented) In (Formula presented) the coherence peak is strongly suppressed by a magnetic field which is well below (Formula presented) 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 (Formula presented) we find a quench rate dependence suggesting the existence of two low-temperature metallic phases, only one of which is superconducting.