  The dynamics of charge carriers close to the Mott transition is explored
theoretically and experimentally in the quasi two-dimensional organic
charge-transfer salt $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br$_x$Cl$_{1-x}$, with
varying Br content. The frequency dependence of the conductivity deviates
significantly from simple Drude model behavior: there is a strong
redistribution of spectral weight as the Mott transition is approached and with
temperature. The effective mass of the quasiparticles increases considerably
when coming close to the insulating phase. A dynamical mean-field-theory
treatment of the relevant Hubbard model gives a good quantitative description
of the experimental data.
