Bandwidth-controlled Mott transition in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>κ</mml:mi><mml:mtext>−</mml:mtext><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>BEDT</mml:mi><mml:mtext>−</mml:mtext><mml:mi>TTF</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:mi mathvariant="normal">Cu</mml:mi><mml:mrow><mml:mo>[</mml:mo><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi mathvariant="normal">C</mml:mi><mml:mi mathvariant="normal">N</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:mo>]</mml:mo></mml:mrow><mml:msub><mml:mi mathvariant="normal">Br</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi mathvariant="normal">Cl</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>: Optical studies of localized charge excitations

Infrared reflection measurements of the half-filled two-dimensional organic conductors $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{C}\mathrm{N})}_{2}]{\mathrm{Br}}_{x}{\mathrm{Cl}}_{1\ensuremath{-}x}$ [BEDT-TTF denotes bis-(ethylenedithio)tetrathiafulvalene] were performed as a function of temperature $(5\phantom{\rule{0.3em}{0ex}}\mathrm{K}&lt;T&lt;300\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ 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 ${(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}$ 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.