Exact asymptotic form of the exchange interactions between shallow centers in doped semiconductors

The method developed in L. P. Gor'kov and L. P. Pitaevskii, Sov. Phys. Dokl. 8, 788 (1964) and C. Herring and M. Flicker, Phys. Rev. 134, A362 (1964) to calculate the asymptotic form of exchange interactions between hydrogen atoms in the ground state is extended to excited states. The approach is then applied to shallow centers in semiconductors. The problem of the asymptotic dependence of the exchange interactions in semiconductors is complicated by the multiple degeneracy of the ground state of an impurity (donor or acceptor) center in valley or band indices, crystalline anisotropy, and strong spin-orbital interactions, especially for acceptor centers in III-V and II-VI group semiconductors. The properties of two coupled centers in the dilute limit can be accessed experimentally, and knowledge of the exact asymptotic expressions, in addition to being of fundamental interest, must be very helpful for numerical calculations and for interpolation of exchange forces in the case of intermediate concentrations. Our main conclusion concerns the sign of the magnetic interaction-the ground state of a pair is always nonmagnetic. The behavior of the exchange interactions in applied magnetic fields is also discussed.