The interplay between disorder and spin polarization in a GaMnAs thin layer results into spin-polarized impurity hole bands. A figure of merit is defined to label the nature of the sample as metallic or nonmetallic. It is shown that samples with the highest figures of merit have a ratio between the extended hole density and the Mn concentration near 0.2, in agreement with the ratio of 0.1–0.25 known to occur among samples produced with the highest Curie temperatures. Both the nonmetal-to-metal and the metal-to-nonmetal transitions experimentally observed in the ferromagnetic regime are obtained as the Mn concentration increases.

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