Bandwidth-controlled Mott transition in κ−( BEDT−TTF)2Cu [N(CN)2]BrxCl1−x : Optical studies of localized charge excitations
Résumé
Infrared reflection measurements of the half-filled two-dimensional organic conductors
kappa-(BEDT-TTF)2Cu[N(CN)2]BrxCl1−x (BEDT-TTF denotes bis-(ethylenedithio)tetrathiafulvalene] were performed
as a function of temperature (5 K300 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.