Unconventional ferroelectric transition in the multiferroic compound TbMnO3 revealed by the absence of an anomaly in c-polarized phonon dispersion.
TbMnO(3) exhibits a spontaneous electric polarization along c concomitantly with a spiral spin ordering modulated along b below T_{C} = 28 K. We have performed inelastic x-ray scattering measurements on a single crystal of TbMnO(3) to clarify whether phonon anomalies related to the ferroelectricity exist. We measured transverse modes, especially the Mn-O-Mn bending mode polarized along c and propagating along b, which we expect is most relevant to the ferroelectricity. However, no anomaly was found in the phonon dispersion below 50 meV across T_{C}. The present result suggests that the mechanism of ferroelectricity in TbMnO(3) is different from that of a conventional displacive-type ferroelectric. The weak coupling between electric polarization and lattice in TbMnO(3) strongly suggests that the ferroelectricity is mainly derived from the spiral spin ordering.
Kajimoto
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Neutron Technology Research & Development Center, Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan. ryoichi.kajimoto@j-parc.jp
Sagayama
H
H
Sasai
K
K
Fukuda
T
T
Tsutsui
S
S
Arima
T
T
Hirota
K
K
Mitsui
Y
Y
Yoshizawa
H
H
Baron
A Q R
AQ
Yamasaki
Y
Y
Tokura
Y
Y