INSULATOR-METAL TRANSITION IN RB4C60 UNDER PRESSURE FROM C-13-NMR

We present an NMR study of Rb4C60 performed under pressure up to 12 kb ar. The temperature dependence of the C-13 spin-lattice relaxation rat e is activated at ambient pressure and becomes Korringa-like under pre ssure. The behaviour of the relaxation is interpreted in terms of one channel due to excited intramolecular tripler states above the Jahn-Te ller ground state plus another one related to electron-hole excitation s through an indirect band gap. In spite of a finite value of the dens ity of states at the Fermi level indicated by the NMR data under press ure, no superconductivity could be detected at 15 kbar above 0.4 K.