TEMPERATURE-INDUCED INSULATOR-METAL-QHE TRANSITIONS

We report the observation of temperature induced transitions between i nsulator, metal, and quantum Hall states in the very dilute high mobil ity two-dimensional electron system in silicon at a magnetic field cor responding to Landau level filling factor v = 1. Our data show that as the temperature decreases, the extended states at v = 1 (above the Fe rmi level at higher temperature so that the system behaves as an insul ator) sink below the Fermi energy, so that the quantum Hall effect occ urs. As the extended states cross the Fermi level, the conductivity ha s a temperature dependence characteristic of a metallic system.