Phase diagram of a two-dimensional liquid in GaAs/AlxGa1-xAs biased doublequantum wells

Photoluminescence (PL) and PL excitation (PLE) measurements have been perfo rmed in GaAs/AlxGa1-xAs biased double quantum well heterostructures. The re combination of electrons, e, with holes, h, located in the same or in two a djacent wells, has been investigated for different exciting power densities , P, and temperatures, T. For increasing P or decreasing T, a sharp transit ion from two gases of photoexcited electrons and holes, spatially separated and confined in the two wells, to two two-dimensional (2D) liquids has bee n observed. The gas-to-2D-liquid transition is evidenced by a strong screen ing of applied biases and by major changes in the optical spectra. The phas e diagram in the (P,T) plane of the e-h system has been determined. Time-re solved FL, cw FL, and PLE in the presence of a magnetic field normal to the quantum wells support the presence of e- and h-liquid phases in the two we lls with a critical density equal to 8.8 x 10(10) cm(-2) and a binding ener gy of 2.5 meV.