Exciton pairs coupled via long-living phonons and their superfluorescent markers

A system of several Wannier-Mott excitons interacting with phonons in a bulk material is considered. We show that strong exciton-phonon coupling causes the formation of a coherent two-exciton state---the exciton pair. Unlike the biexcitons, where the photons play the role of force carrier, the exciton pair is formed via entanglement with the long-living phonon mode: (i) The essentially multiparticle theory requires excitons (cobosons composed of an electron and a hole) to satisfy the mixed Bose-Fermi statistics; and (ii) this allows us to formulate a system of nonlinear dynamic equations, using the multiconfiguration Hartree method applied to the Fr"ohlich Hamiltonian. The system of equations possesses a stationary solution, which, for the case of a single exciton, describes the excitonic polaron and corresponds to the exciton pair in the two-exciton case. We also compare the fluorescent spectra of exciton polarons and exciton pairs estimated from our theory with those observed in experiments on room-temperature superfluorescence (collective emission of fluorescent light) in hybrid perovskites to give an additional insight into the superfluorescence phenomenon.