Ferroaxial phonons in chiral and polar NiCo₂TeO₆

Perfect circular dichroism has been observed in Raman scattering spectra 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 of the crystal. These two symmetry operations are equivalent to the ferroaxial order (C⋅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, who’s preferable -vector direction is determined by the sign of C⋅P. The combination of Raman scattering and the polarization rotation of transmitted linearly polarized white light allows for identification of the direction of electric polarization P in mono domain chiral crystals.