  In the two-dimensional organic charge-transfer salts
kappa-(BEDT-TTF)2Cu[N(CN)2]Br{x}Cl{1-x} a systematic variation of the Br
content from x = 0 to 0.9 allows us to tune the Mott transition by increasing
the bandwidth. At temperatures below 50 K, an energy gap develops in the
Cl-rich samples and grows to approximately 1000 cm-1 for T -> 0. With
increasing Br concentration spectral weight shifts into the gap region and
eventually fills it up completely. As the samples with x = 0.73, 0.85 and 0.9
become metallic at low temperatures, a Drude-like response develops due to the
coherent quasiparticles. Here, the quasiparticle scattering rate shows a
omega^2 dependence and the effective mass of the carriers is enhanced in
agreement with the predictions for a Fermi liquid. These typical signatures of
strong electron-electron interactions are more pronounced for compositions
close to the critical value x_c \approx 0.7 where the metal-to-insulator
transition occurs.

Strongly Correlated Electrons