Temperature-induced transitions between insulator, metal, and quantum Hall states in a two-dimensional electron system

We report temperature-induced transitions between insulating, metallic, and quantum Hall states in a very dilute, high-mobility two-dimensional electron system in silicon at a magnetic field corresponding to Landau level filling factor nu =1. Our data show that at high temperatures, the extended states at nu =1 are above the Fermi energy so that the system is insulating; with decreasing temperature they sink below the Fermi energy, so that the quantum Hall effect occurs. As the extended states cross the Fermi energy, the conductivity has a temperature dependence characteristic of a metallic system.