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Unconventional ferroelectric transition in the multiferroic compound TbMnO$$_{3}$$ revealed by the absence of an anomaly in $$c$$-polarized phonon dispersion

Kajimoto, Ryoichi  ; Sagayama, Hajime*; Sasai, Kenzo*; Fukuda, Tatsuo; Tsutsui, Satoshi*; Arima, Takahisa*; Hirota, Kazuma*; Mitsui, Yukari*; Yoshizawa, Hideki*; Baron, A. Q. R.*; Yamazaki, Yuichi*; Tokura, Yoshinori*

TbMnO$$_{3}$$ exhibits a spontaneous electric polarization along $$mathbf{c}$$ concomitantly with a spiral spin ordering modulated along $$mathbf{b}$$ below $$T$$$$_{rm c}$$ = 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 $$mathbf{c}$$ and propagating along $$mathbf{b}$$, which we expect is most relevant to the ferroelectricity. However, no anomaly was found in the phonon dispersion below 50 meV across $$T$$$$_{rm c}$$. 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.

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Category:Physics, Multidisciplinary

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