Helical vibrational modes and anomalous excitation energy-dependent intensities in the Raman spectra of layered AgCrP₂Se₆

Structural anisotropy in layered two-dimensional (2D) materials can lead to highly anisotropic optical absorption, enabling new ways to tune light-matter coupling through the excitation, detection and control of light along various crystallographic axes. These anisotropies affect the materials’ phonon modes profoundly, with lattice orientation-dependent as well as excitation energy-dependent vibrational mode intensities. Here, we report anomalous Raman spectra in single-crystalline AgCrP₂Se₆, which is a layered material antiferromagnetic material (T_N ~42K)¹. Density functional theory calculations and experimental measurements revealed several unique features in the Raman spectra of bulk and exfoliated AgCrP₂Se₆ crystals including three helical vibrational modes. These modes exhibit large Raman optical activities (circular intensity differences) in bulk AgCrP₂Se₆, which progressively decrease with thickness. We also observed variations in peak intensities in the bulk crystal and exfoliated flakes, 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 are attributed to the unique ABC layer stacking structure of AgCrP₂Se₆ and to the smaller unit cell volume that causes hybridization between the Se and Ag/Cr electron densities and consequently charge transfer and variations in electron-phonon coupling.