Fermi edge singularity in ultracold electron-hole plasma*

We study ultracold neutral electron-hole plasma in separated electron and hole layers in GaAs/AlGaAs coupled quantum well heterostructures. The long electron-hole recombination lifetimes, due to the spatial separation between the electron and hole layers, allows the plasma to cool to low temperatures. We observe a strong enhancement of the photoluminescence intensity at the Fermi energy of the ultracold plasma, evidence of the emergence of the excitonic Fermi edge singularity. This enhancement vanishes with increasing temperature. We control the electron-hole density by the laser excitation power, and with the increasing electron-hole density, observe a transition from the ultracold gas of spatially indirect excitons, the hydrogen-like bound pairs of separated electrons and holes, at low electron-hole densities to the ultracold plasma with the Fermi edge singularity at high electron-hole densities.