Evidence of a metal-insulator transition in orthorhombic RbC 60: A pulsed ESR study

Abstract

Continuous wave (CW) ESR of the polymeric state of RbC 60 suggests a Pauli-like susceptibility and metallic behaviour. We report pulsed ESR studies from 20 K to 300 K which show that the absorption line observed by CW ESR is homogeneous. In the temperature range 40–100 K, the corresponding electronic spin-spin and spin-lattice relaxation times are equal and are the only contribution to the absorption linewidth. This provides convincing evidence for the metallic nature of polymeric RbC 60 in which the spin relaxation is governed by electron-phonon scattering. A comparison of the resistivity vs temperature of K 3C 60 and Rb 3C 60 in the normal state, and KC 60 in the polymer state, with that of the RbC 60 ESR linewidth suggests that the metallic transport in all these alkali fullerides arises from the same mechanism. The equality of the two relaxation times no longer holds below 40 K; the spin-spin relaxation time decreases and the spin-lattice relaxation time drastically increases with decreasing temperature. These are all characteristic features of an insulating ground state.