Sound behavior near the Lifshitz point in proper ferroelectrics

TL;DR

This study investigates phonon interactions and elastic properties near the Lifshitz point in proper ferroelectrics, revealing softening and attenuation phenomena linked to optic-acoustic phonon coupling and order parameter fluctuations.

Contribution

It provides new insights into phonon behavior and elastic softening near the Lifshitz point in proper ferroelectrics using Brillouin scattering and ultrasonic techniques.

Findings

Elastic softening of hypersound velocity near the Lifshitz point

Increased ultrasound attenuation in the incommensurate phase

Temperature dependence described by Landau-Khalatnikov model

Abstract

The interaction between soft optic and acoustic phonons was investigated for Sn2P2(Se0.28S0.72)6 proper uniaxial ferroelectrics by Brillouin scattering and ultrasonic pulse-echo techniques. The elastic softening of hypersound velocity of transverse acoustic phonons and for both longitudinal and transverse ultrasound waves which propagate near direction of the modulation wave vector (in the incommensurate phase at x>xLP) was found at cooling to the Lifshitz point in the paraelectric phase. The strong increase of the ultrasound attenuation have also been observed. Such phenomena are related to the linear interaction of the soft optic and acoustic branches in the region of relatively short-range hypersound waves and to the strongly developed long-range order parameter fluctuations in the ultrasound frequency range. The hypersound velocity temperature dependence was described within the Landau-Khalatnikov approximation for the ferroelectric phase.