Electronic structure of a persistent excitonic fluid far beyond the Mott limit
Excitons are conventionally expected to destabilize at high photoexcitation densities, as screening weakens the electron-hole attraction. Recent studies, however, challenge this paradigm, suggesting that excitons generated by strong optical pumping can remain stable even beyond the onset of population inversion. Here, using time- and angle-resolved photoelectron spectroscopy on 2D hybrid perovskites, we provide direct evidence for such a high-density excitonic state, characterized by a distinctive Mexican-hat dispersion. Remarkably, this excitonic band reconstruction endures for orders of magnitude longer than the optical coherence time, revealing a population-sustained electronic structure that survives well into the incoherent regime. The excitonic resonance also persists far beyond optical saturation, at densities where exciton wavefunctions strongly overlap. Our findings reveal a long-lived, high-density quantum fluid of excitons with characteristics reminiscent of a BCS condensate, an electronic structure markedly different from that of the dilute limit.