Coherent many-body exciton in van der Waals antiferromagnet NiPS3

• Published in: Nature (London) Vol. 583; no. 7818; pp. 785 - 789
• Main Authors: Kang, Soonmin, Kim, Kangwon, Kim, Beom Hyun, Kim, Jonghyeon, Sim, Kyung Ik, Lee, Jae-Ung, Lee, Sungmin, Park, Kisoo, Yun, Seokhwan, Kim, Taehun, Nag, Abhishek, Walters, Andrew, Garcia-Fernandez, Mirian, Li, Jiemin, Chapon, Laurent, Zhou, Ke-Jin, Son, Young-Woo, Kim, Jae Hoon, Cheong, Hyeonsik, Park, Je-Geun
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 30.07.2020; Nature Publishing Group
• Subjects: 140/133; 639/301/119/997; 639/766/119/995; Antiferromagnetism; Coherence; Condensates; Energy; Excitons; Holes (electron deficiencies); Humanities and Social Sciences; Inelastic scattering; Magnets; multidisciplinary; Optical lattices; Photoluminescence; Photons; Rice; Science; Science (multidisciplinary); Spectrum analysis; X-ray scattering
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/s41586-020-2520-5

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 Néel temperature of 150 kelvin in NiPS 3 , 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.