Inelastic X-ray scattering study of phonons in the multiferroic compound TbMnO

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*

An inelastic X-ray scattering study on a single crystal of TbMnO was performed to reveal whether phonon anomaly related to the appearance of the ferroelectricity exists. We have measured the transverse modes, especially the Mn-O-Mn bending mode, polarized along the axis propagating along the axis, because these modes are expected to be most relevant to the spontaneous electric polarization along the axis concomitantly with the spiral spin ordering modulated along the axis. However, no anomaly in phonon dispersions was observed even below the ferroelectric transition temperature below 50 meV. The present result suggests that the mechanism of ferroelectric transition in MnO is different from that of a conventional displacive-type ferroelectric.