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Phonon modes in a molecular dimer-Mott insulator $$beta^{prime}$$-(BEDT-TTF)$$_2$$ICl$$_2$$ studied by inelastic neutron scattering

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

Recently it has been reported that a molecular dimer-Mott insulator $$beta^{prime}$$-(BEDT-TTF)$$_2$$ICl$$_2$$ is a candidate of a new type of ferroelectric system which shows a relaxor-like dielectric response. The system exhibits a glassy ferroelectric state below $$T_c$$ = 62 K and long-range antiferromagnetic order below $$T_{rm N}$$ = 22 K. We have performed inelastic neutron scattering experiments on this system by using a chopper spectrometer AMATERAS installed at BL14 in MLF. The phonon intensity at $$sim$$18 and $$sim$$21 meV decreases with lowering temperature, while that at $$sim$$13.5 meV increases. We have already found that a period-doubled optical phonon mode at 4.2 meV is enhanced below around $$T_c$$ and then is suppressed below $$T_{rm N}$$, indicating significant relationship to both charge and spin dynamics. In sharp contrast to this low-energy mode, the intensity of the optical mode at $$sim$$18 meV decreases below $$T_c$$ to reach a constant value at a low temperature.



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