Antiferroelectric Distortion with Anomalous Phonon Softening in Excitonic Insulator Ta₂NiSe₅

Ta₂NiSe₅ is proposed as a candidate substance for excitonic insulator (E.I.) because the top of the valence band is anomalous flattened below the transition temperature as shown by photoemission spectroscopy measurements (T_s =328 K)[1]. The E.I. transition of Ta₂NiSe₅ is accompanied with orthorhombic to monoclinic structural phase transition at T_s [2]. The theoretical calculation predicted that the transition is driven by exciton condensation which makes a particular phonon mode at q~0 instable[3]. However, the appropriate phonon soft mode has not been observed yet.To identify the phonon mode which plays a major role for the structural phase transition, we carried out a synchrotron inelastic X-ray scattering and diffraction experiments.
An anomalous acoustic phonon softening which was observed just above T_s indicates that the structural phase transition of Ta₂NiSe₅ is classified as the ferroelastic transition. Furthermore, an anomaly also occurs in linewidth of the particular optical phonon mode.
[1] Wakisaka, Y. et al., Phys. Rev. Lett. 103, 026402 (2009).
[2] Di Salvo, F. J. et al., J. Less Common Metals 116, 51–61 (1986).
[3] Kaneko, T.et al., Phys. Rev. B 87, 035121 (2013).