Infrared optical conductivity and the electronic phase diagram in the organic superconductor κ-(BEDT-TTF)₂X

The polarized optical reflectance spectra of kappa-(BEDT-TTF)(2)Cu(NCS)(2) and its deuterated one are measured in the infrared region. Both the specific molecular vibration mode nu(3)(a(g)) of the BEDT-TTF molecule and the optical conductivity hump ill the mid-infrared region change correlatively at T*=45-50K. The results suggest that the electron-molecular vibration coupling oil the nu(3)(a(g)) mode becomes weak due to the enhancement of the itinerant nature of the carriers on the BEDT-TTF dimers below T*. The present observations demonstrate that two different metallic states are regarded as a correlated good metal below T* and a half filling bad metal above T*.