Coherent many-body exciton in van der Waals antiferromagnet NiPS₃

An exciton is the bosonic quasiparticle of electron–hole pairs bound by the Coulomb interaction¹. Bose–Einstein condensation of this exciton state has long been the subject of speculation in various model systems^2,3, and examples have been found more recently in optical lattices and two-dimensional materials^4–9. Unlike these conventional excitons formed from extended Bloch states^4–9, excitonic bound states from intrinsically many-body localized states are rare. Here we show that a spin–orbit-entangled exciton state appears below the Néel temperature of 150 kelvin in NiPS₃, an antiferromagnetic van der Waals material. It arises intrinsically from the archetypal many-body states of the Zhang–Rice singlet^10,11, and reaches a coherent state assisted by the antiferromagnetic order. Using configuration-interaction theory, we determine the origin of the coherent excitonic excitation to be a transition from a Zhang–Rice triplet to a Zhang–Rice singlet. We combine three spectroscopic tools—resonant inelastic X-ray scattering, photoluminescence and optical absorption—to characterize the exciton and to demonstrate an extremely narrow excitonic linewidth below 50 kelvin. The discovery of the spin–orbit-entangled exciton in antiferromagnetic NiPS₃ introduces van der Waals magnets as a platform to study coherent many-body excitons.