Influence of electronic correlations and band-filling on the charge-ordering effects in two-dimensional organic conductors investigated by infrared spectroscopy

A comparative study of the low-energy optical properties of the 1/4-filled two-dimensional organic conductors alpha-(BEDT-TTF)(2)MHg(SCN)(4) (M = K, NH4) and the 1/5-filled salt beta"-(BEDO-TTF)(5)[CsHg(SCN)(4)](2) is presented. The superconducting alpha-(BEDT-TTF)(2)NH4Hg(SCN)(4) exhibits metallic-like optical properties down to 4 K. This is in contrast to the non-superconducting salt alpha-(BEDT-TTF)(2)KHg(SCN)(4) where a strong feature appears at frequencies of about 200 cm(-1) and temperatures below 200 K which indicates the opening of a pseudogap. In the anisotropic and poor metal beta"-(BEDO-TTF)(5)[CsHg(SCN)(4)]) We find a similar pseudogap in the optical spectra around 300 cm(-1) up to room temperature and the presence of a strong Drude-like contribution. Based on exact diagonalisation calculations of the optical conductivity on an extended Hubbard model at quarter-filling we argue that the proximity of the alpha-phase BEDT-TTF salts to a charge-ordering transition is responsible for the observed pseudogap. Our proposed scenario suggests that the different ground states, including superconductivity, are a consequence of the fluctuations associated with short-range charge ordering which builds up close to the quantum phase transition. In a 1/5-filled system similar features like the short-range checkerboard charge fluctuations are expected, but the Drude contribution is always present because the charge ordered state is strongly modified by the presence of mobile empty sites