Investigating spin-entangled excitons in NiPS3 through many-body perturbation theory.

In magnetic materials, excitons can couple to spin degrees of freedom, leading to unique magneto-optical properties. In layered transition metal phosphorous trichalcogenides such as NiPS₃, strong electronic correlations and spin-orbit coupling (SOC) introduce additional complexity in determining the nature of excitonic states. NiPS₃ exhibits a sharp excitonic emission at ~1.47 eV, the origin of which remains debated. In this study, we systematically investigate the excitonic properties of NiPS₃ using density functional theory (DFT) and many-body perturbation theory within the GW/BSE approximation to understand the nature of the correlated spin-excitonic state. We compare the predicted optical spectrum for GW/BSE with different DFT starting points, including local DFT and DFT+U, and investigate the role of self-consistency and spin-flip excitations. In agreement with previous studies, we find the sharp excitonic peak to be of charge-transfer character. Additionally, we determined that allowing for spin-flip excitations within a spinor formalism led to improved agreement with experiment, particularly in capturing the observed excitonic peak splitting that has been attributed to either magnon or phonon coupling to the excitonic state.