Ferroaxial phonons in chiral and polar NiCo2TeO6

TL;DR

This paper reports the observation of perfect circular dichroism in Raman scattering by ferroaxial phonons in chiral NiCo2TeO6 crystals, revealing non-reciprocal phonon propagation linked to the material's symmetry and polarization.

Contribution

It introduces the concept of ferroaxial phonons in chiral crystals and demonstrates their role in non-reciprocal optical phenomena and polarization identification.

Findings

Observation of perfect circular dichroism in Raman scattering

Identification of ferroaxial phonons linked to chiral symmetry

Non-reciprocal propagation of optical phonons

Abstract

Perfect circular dichroism has been observed in the Raman scattering by the optical phonons in single chiral domain NiCo2TeO6 crystals. The selection rules for the optical phonons are determined by the combination of the chiral structure C and the electric polarization P along the c-axis. These two symmetry operations are equivalent to the ferroaxial order (C dot P) = A, so the observed optical phonons are referred to as "ferroaxial". For a given Raman scattering geometry the observed effect may also be described as a complete non-reciprocal propagation of the optical phonons, whose preferable vector direction is determined by the sign of C dot P. The combination of Raman scattering and polarization plane rotation of the transmitted white light allows for identification of the direction of electric polarization P in mono domain chiral crystals.