Optical evidence of metal-insulator phase transition in alkali metal doped fullerenes

We have investigated the complete electrodynamic response of the newly discovered alkali fulleride linear polymers A1C60. We find clear evidence for a metal-insulator phase transition at low temperatures in Rb1C60 and Cs1C60, while the K-compound remains metallic at all temperatures. The possibility of a broken symmetry ground state due to the formation of a spin-density-wave (SDW) for the Rb and Cs compound, as suggested by the magnetic properties, is discussed.