Evidence for spin correlation-driven exciton formation in iridate Mott antiferromagnets

Coulomb-bound excitons in rigid band semiconductors have been extensively studied for their influence on the optical and optoelectronic properties of semiconductor devices, as well as for testing fundamental quantum many-body phenomena. However, excitons in strongly correlated two-dimensional antiferromagnetic Mott-Hubbard insulators, which are predicted to feature a spin-driven binding force, are still relatively under-explored. Here, we report pump-probe time-domain THz spectroscopy measurements on the Ruddlesden-Popper iridate Mott antiferromagnets Sr₂IrO₄ and Sr₃Ir₂O₇, revealing signatures of transient Hubbard excitons. Moreover, by monitoring the behavior of intra-excitonic transitions across their antiferromagnetic ordering temperatures, we observe that the existence of these Hubbard excitons depends critically on the in-plane antiferromagnetic correlations. Our findings provide experimental evidence of a spin-based exciton binding mechanism in Mott antiferromagnets, possibly leading to new directions for exciton creation and control.