Quasiparticles at the verge of localization near the Mott metal-insulator transition in a two-dimensional material.

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 good quantitative description of experimental data.