The center-of-mass ͑CM͒ dispersion of spatially indirect excitons ͑IE's͒ in biased GaAs/Al 0.35 Ga 0.65 As double quantum wells is determined by photoluminescence ͑PL͒ spectroscopy in an in-plane magnetic field B. The field rigidly shifts the IE dispersion in k space by an amount proportional to both B and the electron-hole separation. The PL emission arises from IE's with zero total momentum, corresponding to finite CM velocity, which allows the direct measurement of the IE CM dispersion. The observed PL energy increases quadratically in B, corresponding to the IE kinetic energy, whereas the PL intensity follows the thermal occupation of the optically active IE states and decreases as a Gaussian function of B.

We observed a new discontinuity and bistability in the tunnel current of a 12-nm single-barrier GaAs/AlAs p-i-n heterostructure where a system of spatially separated two-dimensional electron and hole (e-h) layers of equal and tunable density is realized. Both features appear at TՇ300 mK and are substantially enhanced in a magnetic field Bտ10 T perpendicular to the layers. They correspond to a discontinuity in the e-h density and in the phase of the current magneto-oscillations, which we suggest to arise from a transition between indirect excitons and the uncoupled e-h gases. ͓S0163-1829͑99͒12343-9͔

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