Persistence of molecular excitations in metallic fullerides and their role in a possible metal to insulator transition at high temperatures

Abstract

We present 13C NMR spin-lattice relaxation measurements (1/T1) in Na2CsC60 and Rb3C60 from 10 to 700K. The large temperature range of this measurement allow to define unambiguously an increase of 1/T1T with increasing temperature, which is anomalous in a simple metallic picture, where the Korringa law predicts 1/T1T = cst. We attribute this increase to the existence of an additional relaxation channel related to singlet-triplet (ST) excitations of Jahn-Teller distorted C60^{2-} and C60^{4-}. These units are formed within the metal on very short time scales (10^{-14} sec) that do not imply static charge segregation. We show that the amplitude of the ST component depends directly on the density of states, which indicates an interplay between metallic and molecular excitations. Such an interaction is also revealed by the high temperature behavior of Na2CsC60 and CsC60, that we then discuss. A divergence between the behaviors of 1/T1, the NMR shift and the ESR susceptibility is interpreted as the result of a rapid increase of the lifetime of the charge carriers, signaling a tendency to charge localization. In our analysis, the particular stability of C60^{2n-} is then a common feature of all known metallic fullerides and allow to reconcile apparently contradicting properties of these systems.