Metal-insulator transition in two dimensions: Role of the upper Hubbard band

To explain the main features of the metal-insulator transition (MIT) in a two-dimensional (2D) electron system, we suggest a simple model, taking into account strongly localized states in the tail of a 2D conductivity band with a specific emphasis on the role of doubly occupied states [the upper Hubbard band (UHB)]. The metallic behavior of the resistance is explained as result of the activation of localized electrons to a conductance band, leading to a suppression of the nonlinear screening of the disorder potential. The magnetoresistance (MR) in the critical region is related to depopulation of double occupied localized states, also leading to a partial suppression of the nonlinear screening. The most informative data are related to a nearly activated temperature dependence of MR in the strongly insulating limit (which can, in particular, be reached from the metallic state in high enough fields). According to our model, this behavior originates in a lowering of the chemical potential in the UHB due to Zeeman splitting. We compare theoretical predictions with the existing experimental data, and demonstrate that the model explains such features of the 2D MIT as the scaling behavior in the critical region, the saturation of the MR and the H/T scaling of the MR in the insulating limit. The quantitative analysis of the MR in strongly insulating limit based on our model leads to values of the g factors in good agreement with the known values in the localized states in the same materials.