Bandwidth-controlled Mott transition in kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]BrxCl1-x: Optical studies of correlated carriers.
Résumé
In the two-dimensional organic charge-transfer salts κ-(BEDT-TTF)2Cu[N(CN)2]BrxCl1−x a systematic variation in 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-type response develops due to the coherent quasiparticles. Here, the quasiparticle scattering rate shows a ω2 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 xc≈0.7, where the metal-to-insulator transition occurs.