Fermi edge singularity in neutral electron–hole system

In neutral dense electron–hole systems at low temperatures, theory predicted Cooper-pair-like excitons exist at the Fermi energy and form a Bardeen–Cooper–Schrieffer-like condensate. Optical excitations create electron–hole systems with the density controlled via the excitation power. However, the intense optical excitations required to achieve high densities cause substantial heating that prevents the realization of simultaneously dense and cold electron–hole systems in conventional semiconductors. Here we show that the separation of electron and hole layers enables the realization of a simultaneously dense and cold electron–hole system. We find a strong enhancement of photoluminescence intensity at the Fermi energy of the neutral dense ultracold electron–hole system that demonstrates the emergence of an excitonic Fermi edge singularity due to the formation of Cooper-pair-like excitons at the Fermi energy. Our measurements also show a crossover from the hydrogen-like excitons to the Cooper-pair-like excitons with increasing density, consistent with the theoretical prediction of a smooth transition.