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.Comment: 4 pages, 2 figure

Investigations of the Fermi surface of a new organic metal: (BEDT-TTF)

The Fermi surface (FS) of the new organic metal (BEDT-TTF)4[ Ni(dto)$_{2}]$ has been investigated by de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) experiments. In both quantum oscillations with two different frequencies $F_{\alpha}$ = 634 T and $F_{\beta}$ = 4245 T are observed. These results confirm the calculated FS, which consists of one-dimensional and two-dimensional parts separated by a small energy gap. It is shown that the temperature and field dependence of the oscillation amplitudes can be well described by standard Lifshitz-Kosevich theory considering the magnetic breakdown by the so-called coupled network model. Beatings of the oscillation amplitudes are observed by field-dependent dHvA and SdH experiments. They can be explained by a slightly warped FS showing that this material represents a quasi–two-dimensional electronic system. The observed beating nodes in the dHvA and SdH signals appear at different magnetic fields. This fact is discussed in terms of additional scattering mechanisms