Metal-insulator transition in 2D: a role of the upper Habbard band

To explain the main features of the metal-insulator transition (MIT) in 2D we suggest a simple model taking into account strongly localized states in the band tail of 2D conductivity band with a specific emphasize of a role of doubly-occupied states (upper Hubbard band). The metallic behavior of resistance is explained as result of activation of localized electrons to conductance band leading to a suppression of non-linear screening of the disorder potential. The magnetoresistance (MR) in the critical region is related to depopulation of double occupied localized states also leading to partial suppression of the nonlinear screening. The most informative data are related to nearly activated temperature dependence of MR in strongly insulating limit (which can be in particular reached from the metallic state in high enough fields). According to our model this behavior originates due to a lowering of a position of chemical potential in the upper Hubbard band due to Zeeman splitting. We compare the theoretical predictions to the existing experimental data and demonstrate that the model explains such features of the 2D MIT as scaling behavior in the critical region, saturation of MR and H/T scaling of MR in the insulating limit. The quantitative analysis of MR in strongly insulating limit based on the model suggested leads to the values of g-factors being in good agreement with known values for localized states in corresponding materials.Comment: 18 pages, 4 PNG figure

Keijsers, Shklyarevskii and van Kempen Reply

Answer to the Comment on ``Point-Contact Study of Fast and Slow Two-Level Fluctuators in Metallic Glasses'' by Jan von Delft et al.Comment: 3 pages, no figures, accepted Phys. Rev. Letter

Zero-bias anomalies of point contact resistance due to adiabatic electron renormalization of dynamical defects

We study effect of the adiabatic electron renormalization on the parameters of the dynamical defects in the ballistic metallic point contact. The upper energy states of the ``dressed'' defect are shown to give a smaller contribution to a resistance of the contact than the lower energy ones. This holds both for the "classical" renormalization related to defect coupling with average local electron density and for the "mesoscopic" renormalization caused by the mesoscopic fluctuations of electronic density the dynamical defects are coupled with. In the case of mesoscopic renormalization one may treat the dynamical defect as coupled with Friedel oscillations originated by the other defects, both static and mobile. Such coupling lifts the energy degeneracy of the states of the dynamical defects giving different mesoscopic contribution to resistance, and provides a new model for the fluctuator as for the object originated by the electronic mesoscopic disorder rather than by the structural one. The correlation between the defect energy and the defect contribution to the resistance leads to zero-temperature and zero-bias anomalies of the point contact resistance. A comparison of these anomalies with those predicted by the Two Channel Kondo Model (TCKM) is made. It is shown, that although the proposed model is based on a completely different from TCKM physical background, it leads to a zero-bias anomalies of the point contact resistance, which are qualitatively similar to TCKM predictions.Comment: 6 pages, to be published in Phys. Rev.