Paired Zhang-Rice-Singlets theory for excitons in NiPS₃

A sharp photoluminescence (PL) peak related to Zhang-Rice singlet (ZRS) excitons was observed in NiPS₃ below its Neel temperature [1]. To solve several puzzles on exciton physics in NiPS₃ [1], we propose a paired ZRS-excitons (PZRS) theory. We show that a ZRS exciton can propagate in a one-dimensional (1-D) ferromagnetic (FM) chain of the zigzag antiferromagnetic background and that their pair formation is favored over isolated ones when two ZRS excitons are on the nearest neighboring sites along the FM chain. Three notable puzzles resolved from our new proposal are as follows. First, unlike the isolated dark ZRS, the PZRS state has a well-defined optical transition channel that preserves spin value and changes electric polarization [1,2]. Second, the small energy scale measured for the coherent excitons [1,2] could be an emergent energy scale from the binding energy between two ZRS excitons. Third, the hypersensitive photoluminescence spectra for a small pressure [2] and a minute Cd doping [3] can be attributed to its fragile nature owing to its small net binding energy.



[1] S. Kang, K. Kim, B. H. Kim, J. Kim et al., Nature 583, 785 (2020)

[2] K.-X. Zhang et al., in preparation.

[3] J. Kim et al., Nano Lett. 23, 10189 (2023)