Experimentally Distinguishing Disorder from Electron Correlation

3 EXPERIMENTALLY DISTINGUISHING DISORDER FROM ELECTRON CORRELATION 

The main effect of both types of electron localization, of course, is a crossover from metallic to nonmetalhc behavior (a M-NM transition). Nevertheless, it would be very beneficial to have a method of experimentally distinguishing between the effects of electron-electron Coulomb repulsion and disorder. In cases where only one or the other type of localization is present this task is relatively simpler. The Anderson transition, for example, is predicted to be continuous. That is, the zero-temperature electrical conductivity should drop to zero continuously as the impurity concentration is increased. By contrast, Mott predicted that electron-correlation effects lead to a first order, or discontinuous transition. The conductivity should show a discontinuous drop to zero with increasing impurity concentration. Unfortunately, experimental verification of a true first order Mott transition remains elusive. 

Disorder and correlation are often both present in a system. One then has the more difficult task of ascertaining which is the dominant electron localizing mechanism. As might be expected, the most useful experimental approaches to this problem involve 

This is the expression for the exchange interaction between localized magnetic moments. In Eq. 7.10, J is the exchange integral, given by  

Systems exhibiting both strong disorder and electron correlation, so-caUed disordered Mott-Hubbard insulators, are difficult to evaluate. The description of electronic states in the presence of both disorder and correlation is still an unresolved issue in condensed matter physics. Whether disorder or the correlation is the predominant factor in controlling transport properties in a material depends on a complex 

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