J. Phys. IV France 114 (2004) 321
Infrared optical conductivity and the electronic phase diagram in the organic superconductor -(BEDT-TTF) 2XI. Ito1, T. Sasaki1, N. Yoneyama1, N. Kobayashi1, N. Hanasaki2, H. Tajima2, T. Ito3 and Y. Iwasa1
1 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2 Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan
3 JAIST Hokuriku, Ishikawa 277-8581, Japan
The polarized optical reflectance spectra of -(BEDT-TTF) 2Cu(NCS) 2 and its deuterated one are measured in the infrared region. Both the specific molecular vibration mode 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 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. -(BEDT-TTF) 2Cu(NCS) 2, optical conductivity, electron-molecular vibration coupling.
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