Optical properties of the A1C60 (A = K, Rb and Cs) linear polymer fullerides

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 for the Rb and Cs compounds, as suggested by the magnetic properties, is discussed.