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 systems2,3, and examples have been found more recently in optical lattices and two-dimensional materials4–9. Unlike these conventional excitons formed from extended Bloch states4–9, 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 singlet10,11, 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.
|Number of pages||5|
|Publication status||Published - 2020 Jul 30|
Bibliographical noteFunding Information:
Acknowledgements We thank F. de Groot, C. Belvin, E. Baldini and N. Gedik for discussions. The CCES was supported by the Institute for Basic Science (IBS) in Korea (grant number IBS-R009-G1). The work at Sogang University and Yonsei University was supported by National Research Foundation (NRF) grants funded by the Korean government (MSIT; grants 2019R1A2C3006189 and 2019R1I1A2A01062306), the SRC programme (vdWMRC; grant number 2017R1A5A1014862) and by a grant (2013M3A6A5073173) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science and ICT of Korea. Work at the Korea Institute for Advanced Study (KIAS) was supported by KIAS individual grants (CG031509 and CG068701). We thank the KIAS for providing computing resources (the KIAS Center for Advanced Computation Linux Cluster System) for this work. We acknowledge Diamond Light Source for time on beamline I21 under proposals SP18503 and SP18906. J.-G.P. was partially supported by the Leading Researcher Program of the National Research Foundation of Korea (grant number 2020R1A3B2079375).
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