Superfluidity in asymmetric electron-hole systems

The pairing in a system of electrons and holes in two spatially separated parallel planes is studied in the case of electron-hole asymmetry caused by the difference in the carriers masses and their chemical potentials. It is found that the superfluid state is possible in a wide range of the asymmetry parameters, because the asymmetries can effectively compensate each other. It is predicted that the system may exhibit two critical temperatures for some asymmetry parameters. The lower critical temperature corresponds to the superfluid transition induced by thermal fluctuations. In the asymmetric system a coexistence of the normal and superfluid states is possible even at zero temperature.