INFLUENCE OF THE EXCHANGE INTERACTION ON THE POPULATION PROCESS OF THE UPPER SUBBAND

Mudulation-doped AlxGa1-x AsGaAs quantum wells (QW) have been prepared where the total carrier density Ns and thus the occupation of the upper subband can be tuned via front gate voltage Vg in a controlled and reproducible way. We performed magneto-capacitance versus voltage (MCV) measurements at very low magnetic fields (B< 1.5 T) and observed well pronounced minima originated from the magnetic-field-dependent density of states in both occupied subbands. Around the onset voltage of the upper subband, Vg=V(1) the interplay between Landau levels of both subbands is strongest leading to a double minimum structure in our MCV measurements. With a self-consistent calculation of the subband structure we can nicely explain this observation by modelling the total density of states and the resulting capacitance in a magnetic field. However, for a detailed comparison between experiment and theory it is necessary to adequately model the exchange interaction to the situation of two occupied subbands. We discuss this interesting question by means of a model calculation. © 1990.