Muon spin relaxation studies of superconductivity in a crystalline array of weakly coupled metal nanoparticles

We report muon-spin-relaxation studies in weak transverse fields of the superconductivity in the metal cluster compound, Ga-84[N(SiMe3)(2)](20)-Li6Br2(thf)(20)center dot 2 toluene. The temperature and field dependence of the muon-spin-relaxation rate and Knight shift clearly evidence type II bulk superconductivity below T-c approximate to 7.8 K, with B-c1 approximate to 0.06 T, B-c2 approximate to 0.26 T, kappa similar to 2, and weak flux pinning. The data are well described by the s-wave BCS model with weak electron-phonon coupling in the clean limit. A qualitative explanation for the conduction mechanism in this novel type of narrow-band superconductor is presented.