Magnetic field induced transitions between quantized hall and insulator states in a dilute 2D electron gas

We report on the observation of a new phenomenon: a sequence of magnetic field induced transitions between well defined quantum Hall effect states, with a Hall resistance quantized as integer fractions of h/e² and a vanishingly small longitudinal resistance, and insulator states with longitudinal resistance exceeding 2×10⁹ Ω. This phenomenon is observed in extremely high mobility Si MOSFETs, in a range of electron concentrations corresponding to a dilute 2D electron gas in or near an activated electronic transport regime. We attribute this effect to a modulation of the metal-insulator transition by the quantum Hall effect or to the formation of a pinned Wigner solid.