Infrared optical conductivity and the electronic phase diagram in the organic superconductor $\kappa $-(BEDT-TTF)$_{2}$X

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 in the mid-infrared region change correlatively at $T$*=45-50K. The results suggest that the electron-molecular vibration coupling on 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 $T$*. Key words. $\kappa $-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$, optical conductivity, electron-molecular vibration coupling