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 C-13 NMR relaxation rate T-1(-1) shows t wo activated behaviors coexisting at ambient pressure; the lowest acti vation energy disappears under pressure in favor of the emerging Korri nga law. The theory is suggested to describe the Rb4C60 as a Jahn-Tell er 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; th e other being related to electron-hole excitations through an indirect band gap.