Electronic properties of correlated metals in the vicinity of a charge-order transition:: Optical spectroscopy of α-(BEDT-TTF)2MHg(SCN)4 (M=NH4, Rb, Tl)

The infrared spectra of the quasi-two-dimensional organic conductors alpha-(BEDT-TTF)(2)MHg(SCN)(4) (M=NH4, Rb, Tl) were measured in the range from 50 to 7000 cm(-1) down to low temperatures in order to explore the influence of electronic correlations in quarter-filled metals. The interpretation of electronic spectra was confirmed by measurements of pressure dependent reflectance of alpha-(BEDT-TTF)(2)KHg(SCN)(4) at T=300 K. The signatures of charge order fluctuations become more pronounced when going from the NH4 salt to Rb and further to Tl compounds. On reducing the temperature, the metallic character of the optical response in the NH4 and Rb salts increases, and the effective mass diminishes. For the Tl compound, clear signatures of charge order are found albeit the metallic properties still dominate. From the temperature dependence of the electronic scattering rate the crossover temperature is estimated below which the coherent charge carriers response sets in. The observations are in excellent agreement with recent theoretical predictions for a quarter-filled metallic system close to charge order.