Raman interrogation of the ferroelectric phase transition in polar metal LiOsO₃.

Abstract

Ferroelectric (FE) distortions in a metallic material were believed to be experimentally inaccessible because itinerant electrons would screen the long-range Coulomb interactions that favor a polar structure. It has been suggested by Anderson and Blount [P. W. Anderson, E. I. Blount, <i>Phys. Rev. Lett.</i> 14, 217-219 (1965)] that a transition from paraelectric phase to FE phase is possible for a metal if, in the paraelectric phase, the electrons at the Fermi level are decoupled from the soft transverse optical phonons, which lead to ferroelectricity. Here, using Raman spectroscopy combined with magnetotransport measurements on a recently discovered FE metal LiOsO₃, we demonstrate active interplay of itinerant electrons and the FE order: Itinerant electrons cause strong renormalization of the FE order parameter, leading to a more gradual transition in LiOsO₃ than typical insulating FEs. In return, the FE order enhances the anisotropy of charge transport between parallel and perpendicular to the polarization direction. The temperature-dependent evolution of Raman active in-plane ³E_g phonon, which strongly couples to the polar-active out-of-the-plane A_2u phonon mode in the high-temperature paraelectric state, exhibits a deviation in Raman shift from the expectation of the pseudospin-phonon model that is widely used to model many insulating FEs. The Curie-Weiss temperature (θ ≈ 97 K) obtained from the optical susceptibility is substantially lower than <i>T</i> _s, suggesting a strong suppression of FE fluctuations. Both line width and Fano line shape of ³E_g Raman mode exhibit a strong electron-phonon coupling in the high-temperature paraelectric phase, which disappears in the FE phase, challenging Anderson/Blount's proposal for the formation of FE metals.