Magnetic Field-Tunable Repulsive Exciton-Exciton Interaction in the van der Waals Antiferromagnet NiPS$_3$

Abstract

Two ultra-narrow absorption peaks around 1.5 eV, which are widely believed to originate from a transition from a spin-orbital entangled triplet to a singlet state, in the two-dimensional van der Waals crystal NiPS$_3$, have attracted considerable attention because of their pronounced spin-dependent character. An interesting question is whether ultrahigh magnetic fields modify an interaction-driven hybridization between those two peaks. In this work we perform systematic magneto-optical measurements of NiPS$_3$ in pulsed magnetic fields of up to 178 T and observe a pronounced mutual repulsion between the two sharp exciton peaks accompanied by a redistribution of oscillator strength, while the band edge shows no detectable field-induced shift within our experimental resolution. We construct a minimal two-level interaction model and compare it semi-quantitatively with the experimental data. Our results reveal a magnetic-field-tunable exciton-exciton coupling as the dominant high-field response of NiPS$_3$, and clarify this material as a new experimental platform for exploring strongly correlated exciton physics in magnetic van der Waals insulators.