Observation of the order of Cu ions in the mixed-valence state of (DMe-DCNQI)2Cu by Raman scattering

Raman scattering was measured in the organic conductor (DMe-DCNQI)2Cu in order to study the metal-insulator (M-I) and reentrant phase transitions. The effects of the ordering of the Cu ions in the mixed valence state were observed in the low-frequency phonon Raman peaks of the deuterated salts as the temperature was decreased towards the M-I transition. Moreover we observed a sudden broadening of the low-frequency Raman spectrum around the reentrant transition temperature in the -d2 deuterated salt, indicating the disorder of the Cu ions in the low-temperature metallic phase. The peaks became sharp again as the temperature was decreased below about 20 K.