Photoinduced Hund excitons in the breakdown of a two-orbital Mott insulator

We study the photoinduced breakdown of a two-orbital Mott insulator and resulting metallic state. Using time-dependent density matrix renormalization group, we scrutinize the real-time dynamics of the half-filled two-orbital Hubbard model interacting with a resonant radiation field pulse. The breakdown, caused by production of doublon-holon pairs, is enhanced by Hund's exchange, which dynamically activates large orbital fluctuations. The melting of the Mott insulator is accompanied by a high- to low-spin transition with a concomitant reduction of antiferromagnetic spin fluctuations. Most notably, the overall time response is driven by the photogeneration of excitons with orbital character that are stabilized by Hund's coupling. Such Hund excitons, which can form a condensate, correspond to bound doublon-holon pairs with spin-singlet and orbital-triplet quantum numbers. Hund excitons are not directly bound by Coulomb or Hubbard-$V$ interactions, as other excitons, but due to Hund's exchange. We study exciton properties such as bandwidth, binding potential, and size within a semiclassical approach. The photometallic state, which results from a coexistence of Hund excitons and doublon-holon plasma, differs with the equilibrium concept of Hund's metal used in multiorbital physics.