Small-polaron hopping in magnetic semiconductors

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Abstract
The theory of small-polaron hopping transport in magnetic semiconductors and insulators is formulated via a many-electroon generalization of the one-electron tight-binding approach utilized in standard small-polaron theory. Here the magnetic effects are taken into account from first principles by constructing the electronic wave function from linear combinations of antisymmetrized products of Wannier states involving both the itinerant electron and the valence electrons. This enables us to determine the effect of the indistinguishability of an itinerant electron from valence electrons on the transfer of an electron between two magnetic ions. To treat hopping within a magnetic lattice the magnetic environment surrounding the two sites directly involved in a hop is approximated by an effective internal magnetic field. It is found that in the paramagnetic and ferromagnetic regimes the magnetic nature of the solid only reduces the small-polaron mobility by a weakly temperature-dependent factor of the order of unity. However, below the Néel temperature in an antiferromagnet it is found that electronic transitions only occur between nondegenerate states. This results in an additional activated factor in the small-polaron mobility.

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