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

We present 1 3 C nuclear magnetic resonance (NMR) spin-lattice relaxation measurements (1/T 1 ) in Na 2 CsC 6 0 and Rb 3 C 6 0 from 10 to 700 K. The large temperature range of this measurement allows to define unambiguously an increase of 1/T 1 T with increasing temperature, which is anomalous in a simple metallic picture, where the Korringa law predicts 1/T 1 T=cst. From the analogy with the relaxation data in Na 2 C 6 0 and K 4 C 6 0 , we suggest that this increase is associated with the existence of an additional relaxation channel related to singlet-triplet (ST) excitations of Jahn-Teller distorted C 6 0 2- and C 6 0 4-. The amplitude of the ST component is found to depend directly on the density of states, which indicates an interplay between metallic and molecular excitations. We propose a phenomenological model to describe the correlation between the two phenomena. The C 6 0 2- and C 6 0 4- would be formed within the metal on very short time scales (10 - 1 4 s) that do not imply static charge segregation. The interaction between metallic and molecular properties is also revealed by the high-temperature behavior of Na 2 CsC 6 0 and CsC 6 0 , which we then discuss. A divergence between the behaviors of 1/T 1 , the NMR shift, and the electron-spin resonance 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 C 6 0 2n- is then a common feature of all known metallic fullerides and allows to reconcile apparently contradicting properties of these systems.