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

TL;DR

This study reveals anomalous Raman scattering phenomena in layered AgCrP$_2$Se$_6$, including helical modes and excitation energy-dependent intensities, driven by its unique stacking structure and electron-phonon interactions, with implications for optoelectronic applications.

Contribution

It uncovers novel Raman spectral features and their dependence on thickness and excitation energy in AgCrP$_2$Se$_6$, linking these anomalies to its structural and electronic properties.

Findings

Identification of three helical vibrational modes with large optical activity

Observation of excitation energy-dependent Raman peak intensities

Detection of apparent cooling effects up to 220 K due to Raman anomalies

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.