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 $\ensuremath{\nu}.$ The bilayer gaps are shown to be either trivially common for two subbands or caused by the wave function 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 $\ensuremath{\nu}=1$ and $\ensuremath{\nu}=2$ are described within a simple model for the modified bilayer spectrum.