In the field of spin relaxation in semiconductors, the publication of very long spin lifetimes by Kikkawa and Awschalom has generated an important activity. Experimentally, it has been observed thet the dopant-dependent lifetime assumes a maximum close to the dopant-density that corresponds to the Mott metal-insulator transition. This points towards a non-trivial relation between spin relaxation and charge transport. Important mechanisms for the spin relaxation in the insulating or the metallic regime have been proposed. However, none of the known theories can be applied close to the transition.
We have worked out a model for the impurity band taking spin-orbit coupling into account. This leads to a tight-binding model with spin-dependent hopping matrix elements. The hopping elements with spin flip are small but non-zero and lead to spin relaxation with a long spin lifetime. The order of magnitude of the estimated lifetime agrees with experimental findings.
In particular we show that the Dresselhaus spin-orbit coupling dominates the spin relaxation in la large class of systems with zincblende structure.