Chiral Phonons and Helical Polarization in ZrOS

Chirality is signified by the lack of parity. Recent work has shown that chiral crystals can host non-trivial topological electronic phases such as chiral fermions¹ and guarantee optically active quantum states such as chiral phonons² without an axis for parity. Further, chiral phonons demonstrate a pathway to entangle single photons³. Hence, chiral crystals provide a novel platform to explore topology and entanglement in condensed matter systems.

In this work⁴, we synthesized ZrOS crystals in the chiral P2₁3 space group with varying enantiomeric compositions from left- to right-handedness. Single crystal X-ray diffraction revealed a tetrahedral anion-ordered dipolar structure with anisotropic electron density. Electrical polarization is intimately tied to the right- or left-handed 3₁ or 3₂ helical axes and manifests as polarization helices. The optical activity of ZrOS is dictated by a pseudo-angular momentum Raman splitting near the Γ-point in the Brillouin zone, which is consistent with the first-principles calculations of the chiral phonons in ZrOS. Moreover, left- and right-handed ZrOS enantiomers can be coherently twinned by a stacking fault in the pseudo-FCC stacking sequence. As-grown ZrOS crystals commonly adopt an unequal amount of left and right-handed domains. We demonstrate the spatial imaging of the domain handedness of ZrOS crystals using noncontact and non-destructive Raman measurements. This work sets up ZrOS as a model chiral crystal to explore novel physical phenomena.