High-field magnetotransport in composite conductors: Effective-medium approximation

The self-consistent effective-medium approximation (SEMA) is used to study three-dimensional random conducting composites under the influence of a strong magnetic field B, in the case where all constituents exhibit isotropic response. Asymptotic analysis is used to obtain almost closed-form results for the strong-field magnetoresistance and Wall resistance in various types of two- and three-constituent isotropic mixtures for the entire range of compositions. Numerical solutions of the SEMA equations are also obtained, in some cases, and compared with those results. In two-constituent free-electron-metal/perfect-insulator mixtures, the magnetoresistance is asymptotically proportional to \B\ at all concentrations above the percolation threshold. In three-constituent metal/insulator/superconductor mixtures a line of critical points is found, where the strong-field magnetoresistance switches abruptly from saturating to nonsaturating dependence on \B\, at a certain value of the insulator-to-superconductor concentration ratio. This transition appears to be related to the phenomenon of anisotropic percolation.