Muon Spin Relaxation Studies of Superconductivity in a Crystalline Array of Weakly Coupled Metal Nanoparticles

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We report muon-spin-relaxation studies in weak transverse fields of the superconductivity in the metal cluster compound, ${Ga}₈₄[N({SiMe}₃{)}₂{]}₂₀{{{-}}}{Li}₆{Br}₂(thf{)}₂₀·{}2 {toluene}$. The temperature and field dependence of the muon-spin-relaxation rate and Knight shift clearly evidence type II bulk superconductivity below ${T}c{≈}7.8 K$, with ${B}c1{≈}0.06 T$, ${B}c2{≈}0.26 T$, ${κ}{~}2$, 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.