An unexpected conducting phase in two dimensions

Within the scaling theory of localization for noninteracting electrons, all two dimensional systems are insulating in the absence of a magnetic field when examined at sufficiently large length scales, or in the limit of zero temperature. Contrary to this two-decades-old expectation, recent experiments have shown there exists a conducting phase at low temperatures for low electron (hole) densities, a regime where electron-electron interaction energies are large compared to the Fermi energy. We briefly review experiments in silicon MOSFETs that provide evidence of an apparent conductor-insulator transition in two dimensions.