The microscopic nature of localization in the quantum Hall effect

Using a scanning single electron transistor we probe the individual localized states in the integer quantum Hall regime. We find that the observed states are utterly different than those predicted by the single-particle theory and are mainly determined by Coulomb interactions. They appear only when quantization of kinetic energy limits the screening ability of electrons. Consequently, the QH effect becomes more diverse acquiring new regimes and phase transitions, absent in the single-particle framework. Our experiments suggest a unified picture of localization within the QH phenomena, in which the validity of the single-particle model is constrained only to the limit of strong disorder.