Landau quantization, Localization, and Insulator-quantum Hall Transition at Low Fields

We have performed a magnetotransport study on a two-dimensional gated GaAs electron system containing self-assembled InAs quantum dots. In our system Shubnikov-de Haas (SdH) oscillations are induced by Landau quantization in the low- field insulator, and the system undergoes a direct insulator- quantum Hall ($\nu=4$) transition as the magnetic field is increased. The low-field Landau quantization, in fact, is governed by the SdH theory and can modulate the density of states without causing the formation of a quantum Hall liquid in our system. While the expected property $\rho_{xy} \sim \rho_ {xx}$ may not be valid at direct insulator-quantum Hall transitions, we find that such transitions do occur as the product $\mu B \sim 1$ and hence well-separated Landau bands exist in the energy spectrum.