Is there a true metallic state in two dimensions?

Full text: There has been intense international debate about the possible existence of a metallic state and hence metal-insulator in two dimensional (2D) systems. In 1979 powerful theoretical arguments known as scaling theory stated that all states in 2D systems are localised at T=0. For the last two decades it was generally accepted that whilst a 2D system might appear conducting at high temperatures, localisation effects would take over as it is cooled to the absolute zero of temperature, giving rise to insulating behaviour. In 1994 this theoretical understanding of conduction in 2D systems was challenged with strong evidence for a metallic state in high quality silicon metal oxide field effect transistors (MOSFETs). Since then 2D metallic behaviour has been observed in a number of different materials including p-type SiGe and both p and n-type GaAs. Despite these works there is still no theoretical understanding of the nature of the metallic behaviour. In this presentation I will describe recent electrical transport experiments of high quality 2D hole gases in GaAs in which metallic behaviour has been observed previously. By increasing the carrier density of the sample a transition from insulating behaviour at low densities to metallic behaviour occurs. Magnetotransport studies are presented close to and either side of the apparent metal-insulator transition. These results demonstrate that both phase coherence effects and weak electron-electron interactions give a localising correction to the conductivity that increases as T→0. The implications of this on the possibility of a true metallic state in 2D will be discussed