Why a Non-Chiral Crystal's Response to Light Differs by Up to About 1,000-Fold: Mechanism Clarified (Institute of Science Tokyo)

Explanation

On July 31, 2026, a research team led by Professor Takuya Sato of the Department of Physics, School of Science, Institute of Science Tokyo, announced the cause of a phenomenon in which, despite being a non-chiral crystal, the intensity of the returned light differs depending on the combination of circularly polarized light used.

The research team focused on nickel titanate (NiTiO₃), a crystal that retains central symmetry. Although the atoms themselves are not twisted, the arrangement of atoms has a rotational distortion in a consistent direction around a certain axis—a kind of small "twist," so to speak. This state is called "ferroaxial order," and it is distinct from chirality, the property of handedness.

When this crystal was examined using Raman spectroscopy with circularly polarized light, the intensity of the returned light varied clearly depending on the combination of circular polarization used, and the difference reached about 1,000 times that known for Raman optical activity in typical chiral molecules. Analysis revealed that ferroaxial order creates a difference in rotational direction in the coupling between atomic vibration and electronic states. Measurements of the front and back surfaces, among other checks, also confirmed that the difference originates from the crystal itself.

This achievement demonstrates that a hidden "direction of rotation" can be read out using light, and it is expected to contribute to the search for materials with ferroaxial order.

Published paper: DOI 10.1103/wrv8-4f7k