Anomalous Raman scattering in layered AgCrP$_2$Se$_6$: Helical modes and excitation energy-dependent intensities

Abstract: Structural anisotropy in layered two-dimensional materials can lead to highly anisotropic optical absorption which, in turn, can profoundly a^ect their phonon modes. These e^ects include lattice orientation-dependent and excitation energy-dependent mode intensities that can enable new phononic and optoelectronic applications. Here, we report anomalous Raman spectra in single-crystalline AgCrP$_2$Se$_6$, a layered antiferromagnetic material. Density functional theory calculations and experimental measurements reveal several unique features in the Raman spectra of bulk and exfoliated AgCrP$_2$Se$_6$ crystals including three helical vibrational modes. These modes exhibit large Raman optical activities (circular intensity di^erences) in bulk AgCrP$_2$Se$_6$, 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 the unique ABC layer stacking structure of AgCrP$_2$Se$_6$ and to the smaller unit cell volume that causes hybridization between the Se and Ag/Cr electron densities, resulting in charge transfer and strongly a^ecting the electron-phonon coupling. This work thus positions AgCrP$_2$Se$_6$ as an exciting new 2D material for optical and phononic applications.