Insulator-metal transition in Rb4C60 under pressure from 13C-NMR

Abstract

We present an NMR study of Rb 4 C 60 performed under pressure up to 12 kbar. The temperature dependence of the 13 C spin-lattice relaxation rate is activated at ambient pressure and becomes Korringa-like under pressure. The behaviour of the relaxation is interpreted in terms of one channel due to excited intramolecular triplet states above the Jahn-Teller ground state plus another one related to electron-hole excitations through an indirect band gap. In spite of a finite value of the density of states at the Fermi level indicated by the NMR data under pressure, no superconductivity could be detected at 15 kbar above 0.4 K.