Inelastic neutron scattering study on phonon spectra in dimer-Mott insulator -(BEDT-TTF)ICl

Kawamura, Seiko   ; Matsuura, Masato*; Iguchi, Satoshi*; Sasaki, Takahiko*; Taniguchi, Hiromi*; Kubota, Aiko*; Sato, Kazuhiko*; Inamura, Yasuhiro  ; Kikuchi, Tatsuya  ; Kawakita, Yukinobu  ; Nakajima, Kenji  

A molecular dimer-Mott insulator -(BEDT-TTF)ICl is a candidate for a new type of ferroelectric system which shows a relaxor-like dielectric response. We performed inelastic neutron scattering measurements on this system by using a chopper specrometer, and observed phonon spectra in a wide wave vector ()-energy () region for the first time. Clear optical modes with the energies up to 36 meV were observed together with an acoustic phonon dispersion. The intensity of the optical mode with the lowest energy = 4.2 meV increases at a zone boundary along the * axis below = 62 K, at which a glassy ferroelectricity appears, and it further increases all over the measured range below the antiferromangetic transition temperature = 22 K. The results suggest that the lattice dynamics characterized by this optical phonon mode is significantly coupled with strongly correlated charge and spin degrees of freedom.