MUON-SPIN-RELAXATION STUDIES OF THE ALKALI-FULLERIDE SUPERCONDUCTORS

Muon-spin-rotation and relaxation experiments have been performed on t he alkali-fulleride superconductors: Rb3C60, K3C60, Na2CsC60. A small fraction of the implanted muons form endohedral muonium (Mu@C-60) i.e. , a mu(+)e(-) atom on the inside of the 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 Mu@C-60 in a longi tudinal field exhibits a Korringa-like temperature dependence in the n ormal state, and a small Hebel-Slichter coherence peak followed by act ivated temperature dependence well below T-c. In Rb3C60 the coherence peak is strongly suppressed by a magnetic field which is well below H- c2. From the activated temperature dependence we obtain estimates of t he superconducting energy gap. In addition, spin precession measuremen ts of diamagnetic muons in the vortex state are used to estimate the m agnetic penetration depth. In Na2CsC60, we find a quench rate dependen ce suggesting the existence of two low-temperature metallic phases, on ly one of which is superconducting. [S0163-1829(98)03725-4].