Insulator-metal transition in Rb_4C_60 under pressure: Jahn-Teller theory versus NMR experiments

摘要

NMR experiments on Rb_4C_60 perfomred under pressure are presented. The temperature dependence of the ~13C NMR relaxation rate T_1~(-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 Rb_4C_60 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.