Insulator-metal transition in Rb4C60 under pressure: Jahn-Teller theory versus NMR experiments

NMR experiments on Rb4C60 performed under pressure are presented. The temperature dependence of the 13C NMR relaxation rate T1−1 shows two activated behaviors coexisting at ambient pressure; the lowest activation energy disappears under pressure in favor of the emerging Korringa law. The theory is suggested to describe the Rb4C60 as a Jahn-Teller crystal with corresponding consequences for electronic band spectra and for self-trapped states. Experiments are interpreted in terms of two relaxation channels: one due to intramolecular triplet exitons; the other being related to electron-hole excitations through an indirect band gap.