New superconducting phase in a two-component Fermion system

We calculate the phase diagram of a doped electron-hole system. A slight im balance between the numbers of electrons N-e and holes N-h < N-e leads to a new phase, where two superconducting orders (e-h and e-e) co-exist. The e- e order turns out to be strongly enhanced by a mechanism which is different from that in conventional doped excitonic insulators. The new phase should be observable in electron-hole systems created by photoexcitation in doped semiconductors at low temperatures. (C) 2000 Elsevier Science B.V. All rig hts reserved.