Coherent many-body exciton in van der Waals antiferromagnet NiPS3

Abstract

© 2020, The Author(s), under exclusive licence to Springer Nature Limited. An exciton is the bosonic quasiparticle of electron-hole pairs bound by the Coulomb interaction(1). 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 Neel 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 NiPS(3)introduces van der Waals magnets as a platform to study coherent many-body excitons. A spin-orbit-entangled exciton state in the van der Waals material NiPS(3)is observed, and found to arise from many-body states of a Zhang-Rice singlet