  The presence of low-symmetry impurities or defect complexes in the
zinc-blende direct-gap semiconductors (e.g. interstitials, DX-centers) results
in a novel spin-orbit term in the effective Hamiltonian for the conduction
band. The new extrinsic spin-orbit interaction is proportional to the matrix
element of the defect potential between the conduction and the valence bands.
Because this interaction arises already in the first order of the expansion of
the effective Hamiltonian in powers of Uext/Eg << 1 (where Uext is the
pseudopotential of an interstitial atom, and Eg is the band gap), its
contribution to the spin relaxation rate may exceed that of the previously
studied extrinsic contributions, even for moderate concentrations of
impurities.
