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

We present the infrared properties of kappa-(BEDT-TTF)(2)Cu(NCS)(2) for E p arallel to b and E parallel to c focusing on the evolution of charge conduc tion from high-T insulating to low-T metallic behaviour. Anomalous absorpti on manifested in a broad peak in the optical conductivity at high temperatu re 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 an d incoherent bands of large polarons. The insulator-metal transition can be understood as a crossover from localized small-polaron to coherent large-p olaron behaviour.