Correlation gap in the optical spectra of the two-dimensional organic   metal (BEDT-TTF)_4[Ni(dto)_2]
  Optical reflection measurements within the highly conducting (a,b)-plane of the organic metal (BEDT-TTF)_4[Ni(dto)_2] reveal the gradual development of a sharp feature at around 200 cm as the temperature is reduced below 150 K. Below this frequency a narrow Drude-like response is observed which accounts for the metallic behavior. Since de Haas-von Alphen oscillations at low temperatures confirm band structure calculations of bands crossing the Fermi energy, we assign the observed behavior to a two-dimensional metallic state in the proximity of a correlation induced metal-insulator transition. 
P. Haas, E. Griesshaber, B. Gorshunov, D. Schweitzer, M. Dressel, and   F.F. Assaad (U Stuttgart)