We present a ${}^{13}$C and ${}^{133}$Cs NMR investigation of the Cs${\mathrm{C}}_{60}$ cubic quenched phase. Previous ESR measurements suggest that this phase is metallic, but NMR reveals contrasting electronic behavior on the local scale. The ${}^{13}$C spin-lattice relaxation time $\left(T_1\right)$ exhibits a typical metallic behavior down to 50 K, but indicates that a partial spin gap opens for $T<\mathrm{}50\mathrm{}\mathrm{K}$. Unexpectedly, ${}^{133}$Cs NMR shows that there are two inequivalent Cs sites. For one of these sites, the NMR shift and $(T_{1}T{)}^{-1\mathrm{}}$ follow an activated law, confirming the existence of a spin gap. We ascribe this spin gap to the occurrence of localized spin singlets on a small fraction of the ${\mathrm{C}}_{60}$ molecules.

• Received 11 September 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.2131