Coherent many-body exciton in van der Waals antiferromagnet NiPS₃

Excitons are the composite particles of an electron-hole pair and rarely observed for many-body states with strong correlation. We have found that a rare spin-orbital entangled exciton state appears below T_N=150 K in NiPS₃, an antiferromagnetic (AFM) van der Waals (vdW) material. It arises intrinsically from the archetypal many-body states of Zhang-Rice singlet (ZRS) and reach a coherent state assisted by the AFM order. By combining three spectroscopic tools: resonant inelastic X-ray scattering, photoluminescence, and optical absorption, we demonstrate that this exciton not only exists in NiPS₃ but also becomes extremely narrow below 50 K. Using a configuration-interaction theory, we identified the coherent excitonic excitations as arising from a Zhang-Rice triplet to ZRS transition, i.e., a spin-orbital entangled exciton. This discovery of the spin-orbital entangled exciton in AFM vdW NiPS₃ opens up a new window of novel opportunities by allowing the manipulation of the new quantum state with real material.