Nonsaturating magnetoresistance and Hall coefficient reversal in a model composite semiconductor

We calculate the transverse magnetoresistance (TMR) and Hall coefficient R-H of a three-dimensional composite medium with both positive and negative charge carriers, using the effective-medium approximation. The composite is assumed to be composed of two types of small crystallites, in which the charge carriers are either all electrons or all holes. The conductivity tensors of the two components are assumed to be of the standard free-electron form. At all nonzero concentrations, the composite is found to have a large, nonsaturating TMR. For a given magnetic field h, the TMR is a maximum at the concentration p(*) where R-H changes sign; at this concentration, the TMR may be a factor of hundreds or thousands for realistic magnetic field. We discuss the relevance of these results to recent experiments on silver chalcogenide semiconductors, where similar behavior has been reported as a function of pressure.