Nonlinear response in overlapping and separated Landau levels of GaAs quantum wells

We have studied magnetotransport properties of a high-mobility two-dimensional electron system subject to weak electric fields. At low magnetic field B, the differential resistivity acquires a correction δrλ2j2/B2, where λ is the Dingle factor and j is the current density, in agreement with theoretical predictions. At higher magnetic fields, however, δr becomes B independent, δrj2. While the observed change in behavior can be attributed to a crossover from overlapping to separated Landau levels, full understanding of this behavior remains a subject of future theories.