  Electrons and holes in a semiconductor form hydrogen-atom-like bound states,
called excitons. At high electron-hole densities the attractive Coulomb force
becomes screened and excitons can no longer exist. Bardeen-Cooper-Schrieffer
theory predicts that at such high densities co-operative many-body effects can
at low temperatures induce a bound state, an electron-hole Cooper pair,
comparable to an electron-electron Cooper pair in a superconductor. Here we
report the first observation of preformed electron-hole Cooper pairs in a
semiconductor. By measuring stimulated emission from a dense electron-hole gas
in ZnO, we have explored both the crossover from the electron-hole plasma to
the preformed Cooper-pair regime, and the crossover from the exciton to the
preformed Cooper-pair regime.
