The excitons are traditionally bound via the long-range Coulomb interaction, akin to an electron bound to a proton in a hydrogen. To date, however, most discoveries related to excitons have been limited to weakly correlated rigid band III-V semiconductor and transition metal dichalcogenides, where electron-electron correlations are not significant. In contrast, far less is understood about excitons in strongly correlated materials. As an archetypal example, excitons in the Hubbard model are predicted bind via the spin exchange interaction. Thus far, there has been no smoking-gun experimental signatures for these so-called Hubbard excitons and their spin-based binding mechanism. Using THz intra-excitonic spectroscopy, we demonstrate the unambiguous existence of a photo-induced Hubbard excitonic gas in the relativistic antiferromagnetic Mott insulator Sr2IrO4. By tracking the central energy of the excitonic mode as a function of the antiferromagnetic correlation strength, we establish the presence of the spin-exchange binding mechanism. Our results demonstrate an excitonic mode that is bound by an interaction other than the Coulomb interaction, paving the path to engineer excitonic states not accessible in traditional rigid band insulators.
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