Exciton dynamics and exciton-phonon coupling in bulk and thin flakes of layered van der Waals antiferromagnet Ni$_2$P$_2$S$_6$

TL;DR

This study investigates exciton-phonon interactions and exciton dynamics in layered Ni2P2S6, revealing temperature and thickness-dependent properties that are promising for tunable optoelectronic applications.

Contribution

It provides the first comprehensive analysis of Zhang-Rice exciton behavior and exciton-phonon coupling in Ni2P2S6, highlighting their dimensional sensitivity and anisotropic optical properties.

Findings

Distinct phonon sidebands indicate exciton-phonon hybridization.

Strong optical anisotropy with ~40% polarization degree at 4 K.

Phonon sidebands' temperature and thickness dependence demonstrate tunability.

Abstract

The Zhang-Rice (ZR) singlet is an intriguing quantum state offering potential to realize a spin-orbit-entangled bosonic quasiparticle, which gives rise to the Zhang-Rice exciton. Its formation is attributed to the correlation between a localized d-orbital of a transition metal and the p-orbitals of the neighbouring ligands. The layered two-dimensional (2D) antiferromagnetic Ni2P2S6 system provide an excellent platform to probe the ZR exciton dynamics along with the role of exciton-phonon coupling. Here, we present a comprehensive study of ZR exciton and coupling with the phonons in bulk and few-layered single crystals of Ni2P2S6 using temperature, polarization and power-dependent photoluminescence (PL) spectroscopy. At cryogenic temperatures, the PL spectra reveal distinct phonon sidebands spaced by an energy difference of nearly 117 cm-1, indicative of exciton-phonon hybridization. Polarization-resolved measurements demonstrate a strong optical anisotropy, with a linear polarization degree of ~ 40 % at 4 K. Excitation power variation highlights linear scaling of PL intensity in the low-power regime, followed by spectral deformation at higher powers attributed to the phonon-assisted recombination and exciton saturation effects. ZR exciton and phonon side bands survival temperature decreases with decreasing flake thickness suggesting their tunability. The emergence and suppression of phonon sidebands with temperature and flake thickness emphasize dimensional sensitivity and coherence limits of excitonic states. Our findings position Ni2P2S6 as a promising candidate for tunable and anisotropic optoelectronic applications, while offering insight into quasiparticle interactions in 2D magnetic systems.