Anharmonic phonon coupling enabled by local inversion symmetry breaking at domain walls in ferroelastics

TL;DR

This study reveals local anharmonic phonon coupling at domain walls in ferroelastic LaAlO$_3$, enabled by inversion symmetry breaking, using advanced two-dimensional Raman-terahertz spectroscopy.

Contribution

It provides direct experimental evidence of local anharmonic phonon coupling at domain walls due to symmetry breaking, a novel insight into ferroelastic lattice dynamics.

Findings

Observation of cross-peaks indicating anharmonic coupling in 2D spectra

Identification of local inversion symmetry breaking at domain walls

Detection of weak anharmonic signals using Raman-terahertz spectroscopy

Abstract

In ferroelastic materials, spontaneous symmetry breaking leads to the formation of twin domains. Although the bulk crystal typically remains centrosymmetric, inversion symmetry can be locally broken at the domain walls, potentially changing phonon selection rules and enabling local anharmonic phonon coupling. Here we report direct evidence of such anharmonic coupling in ferroelastic LaAlO$_3$ using two-dimensional Raman-terahertz spectroscopy. We attribute the cross-peaks observed in the two-dimensional spectra to both mechanical and electrical anharmonicity between the $A_{1g}$ Raman-active phonon and the $E_g$ phonon, which acquires finite infrared activity through local inversion symmetry breaking at ferroelastic domain walls. These findings provide new insight into the complex lattice dynamics of ferroelastic materials and highlight the potential of two-dimensional Raman-terahertz spectroscopy to uncover subtle symmetry breaking through the detection of intrinsically weak anharmonic signals.