Low temperature transport in an electron-hole bilayer : search for excitonic phases and collective modes

Recently it has been possible to design independently contacted electron-hole bilayers (EHBL) with densities < 5e10/cm2 and a separation of 10-20 nm in a GaAs/AlGaAs system. Simple estimates show that in these EHBLs the interlayer interaction is stronger than the   intralayer interactions. Excitonic superfluidity in such EHBLs with a spatially separated layer of electrons and holes was first predicted almost forty years  ago. Since then theoretical works have indicated the possibility of a very rich phase diagram, consisting of superfluid phases, charge density waves, Wigner crystals and a BCS-BEC crossover. However this system is difficult to fabricate in practice. Only in last 2-3 years reliable methods to fabricate such devices have been found. Very recent experiments have revealed novel features in the interlayer scattering (Coulomb drag) below ~1K. The coulomb drag shows strong non-monotonic deviations from a ~T2 behaviour expected for Fermi-liquids at low temperatures. In addition, experimental data available so far indicate that the system 
somehow violates Onsager's reciprocity theorem for four terminal measurement of resistances. Simultaneously an unexpected insulating behaviour in the single layer resistances (at a highly "metallic" regime with k_{F}l > 500) also appears in both layers inspite of electron mobilities above 1e6cm2/Vs and hole mobilities above 1e5cm2/Vs. The experimental results may indicate a competition between an excitonic ground state and a density modulated phase.