NMR evidence for Mott-Hubbard localization in (NH3)K3C60

C-13, H-1, and K-39-NMR measurements of (NH3)K3C60 unambiguously demonstrated that its magnetic properties are described as a S = 1/2 localized spin system over the entire temperature range (300 K-4.2 K). A possible antiferromagnetic structure that appeared below 45 K is compatible with the orientational order of KNH3 pairs on the octahedral (O) site. The present NMR study strongly suggests that the Mott-Hubbard localization occurs due to the removal of the C-60 t(1u)-orbital degeneracy.