Polaron absorption in infrared spectrum of organic superconductor kappa-(BEDT-TTF)(2) Cu(NCS)(2)

We present the infrared properties of kappa-(BEDT-TTF)(2) Cu(NCS)(2) for E parallel to b and E parallel to c, aiming at elucidating the mechanism of c harge conduction from high-T insulating to low-T metallic behavior. The opt ical conductivity at high T can be well explained by self-trapped small pol arons and their photoionization. At low T, coherent large polarons are resp onsible for the metallic DC transport and low-omega Drude response. The ins ulator-metal transition can be understood as a crossover from localized sma ll polaron to coherent large polaron behavior. (C) 2000 Elsevier Science B. V. All rights reserved.