Far-infrared study of the insulator-metal transition in kappa-(BEDT-TTF)2Cu(NCS)2: evidence for polaron absorption

We present the infrared properties of -(BEDT-TTF) 2 Cu(NCS) 2 for E b and E c focusing on the evolution of charge conduction from high- T insulating to low- T metallic behaviour. Anomalous absorption manifested in a broad peak in the optical conductivity at high temperature is interpreted in terms of photon-assisted hopping of small polarons. A sharp Drude peak that develops at low temperature, together with large mid-infrared spectral weight in the conductivity, are attributed to coherent and incoherent bands of large polarons. The insulator-metal transition can be understood as a crossover from localized small-polaron to coherent large-polaron behaviour.