Anomalous peaks in the Shubnikov-de Haas spectra of a top gated AlGaAsGaAs heterostructure

We have measured Shubnikov-de Haas oscillations on a top-gated AlGaAsGaAs heterostructure. The sample is mesa etched into a Hall bar configuration and part of the Hall bar is covered with a Schottky gate. We observe anomalous peaks in the longitudinal resistivity. The strengths and positions of the anomalous peaks depend on the applied gate voltage. A model explaining the experimental results is presented. Using the model we determine the energy shift of the Landau levels as a function of gate voltage.