Electron subbands in a double quantum well in a quantizing magnetic field

We employ magnetocapacitance and far-infrared spectroscopy techniques to study the spectrum of the double-layer electron system in a parabolic quantum well with a narrow tunnel barrier in the center. For gate-bias-controlled asymmetric electron density distributions in this soft two-subband system we observe both individual subband gaps and double-layer gaps at integer filling factor v. The bilayer gaps are shown to be either trivially common for two subbands or caused by the wavefunction reconstruction in growth direction as induced by intersubband electron transfer at a normal to the interface magnetic field. In the latter case the observed gaps at v = 1 and v = 2 are described within a simple model for the modified bilayer spectrum. [S0163-1829(99)11319-5].