Phonon coupling to dynamic short-range polar order in a relaxor ferroelectric near the morphotropic phase boundary

TL;DR

This study uses neutron scattering to explore how phonons in a relaxor ferroelectric near the morphotropic phase boundary are affected by electric fields, revealing anisotropic coupling between lattice dynamics and short-range polar order.

Contribution

It demonstrates the anisotropic response of phonons to electric fields and links lattice dynamics to dynamic short-range polar correlations in a relaxor ferroelectric.

Findings

Phonons polarized parallel to the electric field weaken and shift to higher energy.

Phonons polarized perpendicular to the electric field show no change.

Lattice dynamics are coupled to dynamic short-range polar correlations.

Abstract

We report neutron inelastic scattering experiments on single crystal PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$ doped with 32\% PbTiO$_{3}$, a relaxor ferroelectric that lies close to the morphotropic phase boundary. When cooled under an electric field $\mathbf{E} \parallel$ [001] into tetragonal and monoclinic phases, the scattering cross section from transverse acoustic (TA) phonons polarized parallel to $\mathbf{E}$ weakens and shifts to higher energy relative to that under zero-field-cooled conditions. Likewise, the scattering cross section from transverse optic (TO) phonons polarized parallel to $\mathbf{E}$ weakens for energy transfers $4 \leq \hbar \omega \leq 9$ meV. However, TA and TO phonons polarized perpendicular to $\mathbf{E}$ show no change. This anisotropic field response is similar to that of the diffuse scattering cross section, which, as previously reported, is suppressed when polarized parallel to $\mathbf{E}$, but not when polarized perpendicular to $\mathbf{E}$. Our findings suggest that the lattice dynamics and dynamic short-range polar correlations that give rise to the diffuse scattering are coupled.