Phonons in the multiferroic langasite Ba$\_3$NbFe$\_3$Si$\_2$O$\_{14}$ : evidences for symmetry breaking

TL;DR

This study investigates the phonon spectrum of the multiferroic langasite Ba₃NbFe₃Si₂O₁₄ to identify symmetry breaking and polar order, revealing signatures of polar states at room temperature and additional symmetry lowering below 120K.

Contribution

It combines optical spectroscopy and ab initio calculations to provide evidence of symmetry breaking and polar order in langasite, clarifying its multiferroic properties.

Findings

Signatures of polar state present at room temperature

Additional symmetry lowering below 120 K

Emergence of softer phonon modes in THz range

Abstract

The chiral langasite Ba$\_3$NbFe$\_3$Si$\_2$O$\_{14}$ is a multiferroic compound. While its magnetic order below T$\_N$=27 K is now well characterised, its polar order is still controversial. We thus looked at the phonon spectrum and its temperature dependence to unravel possible crystal symmetry breaking. We combined optical measurements (both infrared and Raman spectroscopy) with ab initio calculations and show that signatures of a polar state are clearly present in the phonon spectrum even at room temperature. An additional symmetry lowering occurs below 120~K as seen from emergence of softer phonon modes in the THz range. These results confirm the multiferroic nature of this langasite and open new routes to understand the origin of the polar state.