Unconventional ferroelectric transition in the multiferroic compound TbMnO revealed by the absence of an anomaly in -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 exhibits a spontaneous electric polarization along concomitantly with a spiral spin ordering modulated along below = 28 K. We have performed inelastic X-ray scattering measurements on a single crystal of TbMnO to clarify whether phonon anomalies related to the ferroelectricity exist. We measured transverse modes, especially the Mn-O-Mn bending mode polarized along and propagating along , which we expect is most relevant to the ferroelectricity. However, no anomaly was found in the phonon dispersion below 50 meV across . The present result suggests that the mechanism of ferroelectricity in TbMnO is different from that of a conventional displacive-type ferroelectric. The weak coupling between electric polarization and lattice in TbMnO strongly suggests that the ferroelectricity is mainly derived from the spiral spin ordering.