Bilayer two-dimensional electron systems formed by a thin barrier in the GaAs buffer of a standard heterostructure were investigated by magnetotransport measurements. In magnetic fields oriented parallel to the electron layers, the magnetoresistance exhibits an oscillation associated with the depopulation of the higher occupied subband and the field-induced transition into a decoupled bilayer. Shubnikov-de Haas oscillations in slightly tilted magnetic fields allow us to reconstruct the evolution of the electron concentration in the individual subbands as a function of the in-plane magnetic field. The characteristics of the system derived experimentally are in quantitative agreement with numerical self-consistent field calculations of the electronic structure.

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The resistance in an asymmetric double-well structure was measured as a function of magnetic fields oriented almost parallel to the plane of the electron 1ayer. It was shown that the shape of the magnetoresistance curves is close to the in-plane magnetic-field-dependent density of states which we obtained by self-consistent numerical calculation. The novel feature is the negative magnetoresistance observed at low magnetic fields.