Signatures of quasi-bound Hubbard excitons in La₂CuO₄

In an antiferromagnetic Mott insulator, photo-excited holes and electrons are theorized to bind into Hubbard excitons (HEs) through an attractive force mediated by spin exchange interactions. Recently, evidence of HEs was reported in iridate Mott antiferromagnets using terahertz (THz) intra-excitonic spectroscopy. Here, we leverage time-resolved time-domain THz spectroscopy (tr-TDTS) to study the antiferromagnetic Mott insulator La₂CuO₄, a parent compound of high-Tc superconductors. We observe signatures of quasi-bound HEs, which emerge only above a specific excitation photon energy threshold. Our results provide an estimate for the HE binding energy that is consistent with theoretical predictions and offer new insights into the optical spectrum of La₂CuO₄.