Temperature Induced Transitions between Insulator, Metal, and Quantum Hall States

We report the observation of TEMPERATURE-INDUCED transitions between insulator, metal, and quantum-Hall behaviors for transport coefficients in the very dilute high mobility two-dimensional electron system in silicon. We consider the $\nu=1$ quantum Hall effect at the border of it's existence, at very low electron density. Our data show that as the temperature decreases, the extended states at $\nu=1$ (above the Fermi level at higher temperature so that the system behaves as an insulator) sink below the Fermi energy, so that the quantum Hall effect occurs. As the extended states cross the Fermi level, the conductivity has a temperature dependence characteristic of a metallic system.