A Hubbard exciton fluid in a photo-doped antiferromagnetic Mott insulator

The undoped antiferromagnetic Mott insulator naturally has one charge carrier per lattice site. When it is doped with additional carriers, they are unstable to spin fluctuation-mediated Cooper pairing as well as other unconventional types of charge, spin, and orbital current ordering. Photo-excitation can produce charge carriers in the form of empty (holons) and doubly occupied (doublons) sites that may also exhibit charge instabilities. There is evidence that antiferromagnetic correlations enhance attractive interactions between holons and doublons, which can then form bound pairs known as Hubbard excitons, and that these might self-organize into an insulating Hubbard exciton fluid. However, this out-of-equilibrium phenomenon has not been detected experimentally. Here, we report the transient formation of a Hubbard exciton fluid in the antiferromagnetic Mott insulator Sr$_{2}$IrO$_{4}$ using ultrafast terahertz conductivity. Following photo-excitation, we observe rapid spectral weight transfer from a Drude metallic response to an insulating response. The latter is characterized by a finite energy peak originating from intra-excitonic transitions, whose assignment is corroborated by our numerical simulations of an extended Hubbard model. The lifetime of the peak is short, approximately one picosecond, and scales exponentially with Mott gap size, implying extremely strong coupling to magnon modes.