Chiral Phonons and Anomalous Excitation-Energy-Dependent Raman Intensities in Layered AgCrP₂Se₆.

Abstract

Structural anisotropy in layered two-dimensional materials can lead to highly anisotropic optical absorption which, in turn, can profoundly affect their phonon modes. These effects include lattice orientation-dependent and excitation energy-dependent mode intensities that can enable next-generation phononic and optoelectronic applications. Here, we report anomalous Raman spectra in single-crystalline AgCrP₂Se₆, a layered antiferromagnetic material. Density functional theory calculations and experimental measurements reveal several features in the Raman spectra of bulk and exfoliated AgCrP₂Se₆ crystals including three chiral phonon modes. These modes exhibit large Raman optical activities (circular intensity differences) in bulk AgCrP₂Se₆, which progressively decrease with thickness. We also observe strong excitation-energy-dependent peak intensities as well as a decrease in anti-Stokes peak intensities at room temperature with increasing excitation energy, resulting in an apparent cooling by up to 220 K. All of these anomalies in bulk and exfoliated flakes are attributed to 1) the ABC layer stacking structure of AgCrP₂Se₆ and 2) the more constrained metal ion environment in the Se-bounded octahedral cage, causing hybridization between the Se and Ag/Cr electron densities and resulting in charge transfer that strongly affects the electron-phonon coupling. Consequently, this work positions AgCrP₂Se₆ as an exciting two-dimensional material for optical and phononic applications.