Electron localization due to disorder

In amorphous Si, many of the localized states in the mobility gap (especially those far from the mobihty edges, near the middle of the gap) are related to defects, such as the danghng bond and fioahng bond sites menhoned above. In this case, it is easy to understand the origin of locahzation, since these defect-related states 

In order for the perturbation expansion to converge, we must have 

Anderson localization is only one possible way of inducing a transition from a localized to a delocalized state. Since the localized state is associated with insulating behavior and the delocalized state is associated with metallic behavior, this transition is also referred to as the metal-insulator transition. In the case of Anderson localization this is purely a consequence of disorder. The metal-insulator transition can be observed in other physical situations as well, but is due to more subtle many-body effects, such as correlations between electrons depending on the precise nature of the transition, these situations are referred to as the Mott transition or the Wigner crystallization. 

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Transport and magnetic properties of complex TCNQ salts with symmetric and asymmetric bipyridine donas are presented. The salt with symmetric donor is a small bandgap semiconductor with large anisotropy in the conductivity, the susceptibility indicates strong electron correlations. The asymmetric donor leads to a material in which both the electric and magnetic properties can be discussed by electron localization due to disorder. 

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