Bandwidth-controlled Mott transition in kappa-(BEDT-TTF)2Cu[N(CN)2]Br{x}Cl{1-x}: Optical studies of correlated carriers

TL;DR

This study investigates how varying bromine content in organic salts tunes the Mott transition, revealing the evolution from insulating to metallic states and confirming Fermi liquid behavior near the transition.

Contribution

It provides detailed optical evidence of bandwidth-controlled Mott transition and quasiparticle dynamics in kappa-(BEDT-TTF)2Cu[N(CN)2]Br{x}Cl{1-x} salts.

Findings

Energy gap develops in Cl-rich samples at low temperatures.

Spectral weight shifts into the gap with increasing Br content.

Fermi liquid behavior observed near the critical composition.

Abstract

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.