Cyclotron effective mass in wide parabolic quantum wells

Cyclotron-resonance measurements are reported for wide (100 300 nm) modulation-doped AlxGa1-xAs graded parabolic quantum wells for electron areal densities NS=109/cm2 2.5×1011/cm2. A clear dependence of the cyclotron frequency on NS is observed in the extreme quantum limit which is understood in terms of alloy effects. Self-consistent calculations that include the x dependence of the local effective mass and exchange-correlation effects in a local-density approximation are in quantitative agreement with the measurements for high densities. At low densities a pinning of the cyclotron frequency is observed that is not predicted by the model.