Infrared and Raman Studies of the Charge-Ordering Phase Transition at〜170K in the Quarter-Filled Organic Conductor, β"-(ET)(TCNQ)(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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We present the optical conductivity, infrared- and Raman-active charge-sensitive phonon modes, and the x-ray diffraction superlattice spots of β"-(ET)(TCNQ) [ET=bis (ethylenedithio) tetrathiafulvalene, TCNQ=7,7,8,8-tetracyanoquino-dimethane], which is characterized as a quarter-filled narrow band system. Above 〜170K, we found weak superlattice spots and splitting of the C=C and C-S stretching modes of ET. These results indicate the charge-ordering state in the ET layer. As the temperature is decreased, the superlattice spots abruptly disappeared at 〜170K and the split modes continuously merged into a single band below 〜170K. Concomitantly, the spectral weight of the optical conductivity ascribed to the ET layer significantly shifts toward lower energy. Through the analyses of these experimental results, we propose the view that the incoherent conducting electron in the ET layer shows a crossover behavior into a coherent Fermi liquid state below the structural phase transition at 〜170K.
社団法人日本物理学会の論文
2006-07-15