Localization and interaction effects in weakly disordered two-dimensional systems

The question of whether a two-dimensional metal can exist is discussed. The theoretical and experimental position is reviewed. Experimentally, the conductivity of a two-dimensional system (such as inversion layers or thin film metals) shows the following nonmetallic behavior: (i) logarithmic decrease of the conductivity as T → 0. (ii) negative magnetoresistance, (iii) logarithmic decrease of the Hall coefficient as T → 0, and (iv) nonohmic conductivity, increasing logarithmically with electric field.The nonmetallic behavior of the conductivity is discussed in view of two recent theoretical approaches, (i) The localization approach, which predicts that all electronic states in a two-dimensional system are localized, and (ii) the interaction approach which predicts a logarithmic decrease in the density of states as the temperature is decreased.Evidence for both theories is presented.