Evidence of charge ordering in the electronic spectra of two-dimensional organic conductors

A comparative experimental study of the low-energy optical properties of different two-dimensional organic conductors is presented; this includes two-dimensional metals with 12, 14, and 15 filling of the conduction band. In 12-filled compounds indications of the metal-to-insulator transition are observed for U close to some critical value. Also systems with 14-filled bands cross over to a charge-ordered insulator for large intersite Coulomb repulsion V; but even close to this quantum-phase transitions, fluctuations become evident in the optical conductivity. For less commensurate filling factors, like 15, the influence of charge order on the electronic properties decreases and the Drude contribution becomes more pronounced