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(-1) as the temperature is reduced belo w 150 K. Below this frequency a narrow Drude-like response is observed whic h accounts for the metallic behavior. Since de Haas-van Alphen oscillations at low temperatures confirm band structure calculations of bands crossing the Fermi energy, we assign the observed behavior to a two-dimensional meta llic state in the proximity of a correlation induced metal-insulator transi tion.