Magnetic order dependent excitonic effects in monolayer NiBr2: A first-principles GW+BSE study

The excitonic effects play an important role in the optical response of two-dimensional (2D) magnetic semiconductors, which are crucial for identifying magnetic properties. Although diverse magnetic orders and corresponding appealing physical properties are reported in the emerging layered ${\text{Ni}X}_{2}$ $(X=\mathrm{Cl}, \mathrm{Br}, \mathrm{I})$, the influence of magnetic order on excitonic effects remains unrevealed. Here, we employ the many-body GW plus Bethe-Salpeter equation method to conduct a comparative investigation of the electronic structure and optical properties of monolayer ${\mathrm{NiBr}}_{2}$ in ferromagnetic (FM) and antiferromagnetic (AFM) phases, focusing particularly on the excitons. For both FM and AFM phases, strong self-energy effects on the quasiparticle band structures are found, which are spin dependent owing to the localized nature of the $3d$ orbitals. Compared to the FM monolayer, the AFM spin configuration significantly changes the optical spectra and excitonic effects, affecting aspects such as the anisotropic response, intensity and location of absorption peaks, excitonic binding energy, spatial distribution of excitonic wave functions, as well as the dark excitons. Our work provides a theoretical understanding of the optical response in different magnetic phases, which is essential for developing magneto-optical and optoelectronic devices based on 2D magnetic semiconductors.