Bandwidth-controlled Mott transition in κ-(BEDT-TTF)2Cu[N(CN)2]BrxCl1-x:: Optical studies of localized charge excitations

Infrared reflection measurements of the half-filled two-dimensional organic conductors kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]BrxCl1-x [BEDT-TTF denotes bis-(ethylenedithio)tetrathiafulvalene] were performed as a function of temperature (5 K < T < 300 K) and Br substitution (x=0%, 40%, 73%, 85%, and 90%) in order to study the metal-insulator transition. We can distinguish absorption processes due to itinerant and localized charge carriers. The broad midinfrared absorption has two contributions: transitions between the two Hubbard bands and intradimer excitations from the charges localized on the (BEDT-TTF)(2) dimer. Since the latter couple to intramolecular vibrations of BEDT-TTF, the analysis of both electronic and vibrational features provides a tool to disentangle these contributions and to follow their temperature and electronic-correlation dependence. Calculations based on the cluster model support our interpretation.