A research team led by Prof. SHENG Zhigao from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences has discovered mirror symmetry-triggered chiral phonon behavior and a giant magneto-optical modulation in the two-dimensional material AgCrP₂S₆ with space and time symmetries.

The study, published in Advanced Functional Materials, highlights the importance of mirror symmetry in inducing chiral phonon behavior and the role of the magnetic fields in modulating the magneto-optical Raman effect of phonons.

Chiral phonons are associated with phenomena such as inter-valley scattering and topological states; however, they have primarily been observed in materials that break inversion or time-reversal symmetry. Their behavior in systems that preserve both symmetries remains poorly understood. Past magneto-optical Raman studies have primarily focused on achiral phonons, leaving their magnetic response unclear.

In this study, using a magneto-optical Raman measurement system, the researchers observed that the B_g⁴ mode of AgCrP₂S₆ exhibits chiral phonon behavior. This is the first observation of such behavior in a system with both P and T symmetries. Symmetry analysis suggests that this behavior arises from a half-wave plate-like effect under mirror symmetry.

Magneto-Raman spectroscopy further revealed an exceptionally strong field-dependent modulation. As the field strength increased, the degree of linear polarization of the A_g⁵ mode significantly decreased. The B_g⁴ mode exhibited a pronounced change, while the A_g⁵ mode responded weakly in the degree of circular polarization, underscoring the distinct and highly mode-dependent nature of the magneto-optical behavior.

"These results demonstrate that mirror symmetry plays a pivotal role in generating chiral phonons and reveal the efficiency and sensitivity with which magnetic fields can modulate Raman signals," said Prof. SHENG Zhigao. "This makes AgCrP₂S₆ a very promising material for future magneto-optical devices."

a. Schematic illustration of the magneto-optical Raman experiment; b. Observation of chiral phonon behavior in the Bg4 mode; c. Magneto-optical Raman effect (linear polarization); d. magneto-optical Raman effect (circular polarization). (Image by LI Bolin)