Collective excitation of electric dipole on molecular dimer in organic   dimer-Mott insulator
  The terahertz (THz) response in 10-100 cm^-1 was investigated in an organic dimer-Mott (DM) insulator kappa-(ET)_2Cu_2(CN)_3 that exhibits a relaxor-like dielectric anomaly. 30 cm^-1 band in the optical conductivity was attributable to collective excitation of the intra-dimer electric dipoles which are formed by an electron correlation. We succeeded in observing photoinduced enhancement of this 30 cm^-1 band, reflecting the growth of the electric dipole cluster in the DM phase. Such optical responses in kappa-(ET)_2Cu_2(CN)_3 reflect instability near the boundary between the DM-ferroelectric charge ordered phases. 
K. Itoh, H. Itoh, M. Naka, S. Saito, I. Hosako, N. Yoneyama, S.   Ishihara, T. Sasaki, and S. Iwai
cond-mat.str-el