Detection by NMR of a "local spin-gap" in quenched CsC60

TL;DR

This study uses NMR to reveal a local spin-gap in quenched CsC60, contrasting previous metallic assumptions, by detecting localized spin-singlets on some C60 molecules.

Contribution

It provides the first local-scale NMR evidence of a spin-gap in quenched CsC60, identifying localized spin-singlets as the origin.

Findings

Partial spin-gap opens below 50 K

Two inequivalent Cs sites detected by NMR

Localized spin-singlets on some C60 molecules

Abstract

We present a 13C and 133Cs NMR investigation of the CsC60 cubic quenched phase. Previous ESR measurements suggest that this phase is metallic, but NMR reveals contrasting electronic behavior on the local scale. The 13C spin-lattice relaxation time (T1) exhibits a typical metallic behavior down to 50 K, but indicates that a partial spin-gap opens for T<50 K. Unexpectedly, 133Cs NMR shows that there are two inequivalent Cs sites. For one of these sites, the NMR shift and (T1T)^{-1} 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 C60 molecules.