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Inelastic neutron scattering study on phonon spectra in dimer-Mott insulator $$beta^{prime}$$-(BEDT-TTF)$$_2$$ICl$$_2$$

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 $$beta^{prime}$$-(BEDT-TTF)$$_2$$ICl$$_2$$ 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 ($$Q$$)-energy ($$E$$) region for the first time. Clear optical modes with the energies up to $$sim$$ 36 meV were observed together with an acoustic phonon dispersion. The intensity of the optical mode with the lowest energy $$E$$ = 4.2 meV increases at a zone boundary along the $$b$$* axis below $$T_{FE}$$ = 62 K, at which a glassy ferroelectricity appears, and it further increases all over the measured $$Q$$ range below the antiferromangetic transition temperature $$T_{rm N}$$ = 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.



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