  We study decoherence processes of an S = 1/2 localized spin coupled to
conduction band electrons in a metal or a semiconductor via an Ising-like
interaction. We derive master equations for the density matrix of the localized
spin, by tracing out all degrees of freedom in the conduction electron system
based on the linked-cluster-expansion technique. It is found that the
decoherence occurs more rapidly for the metallic case than for semiconducting
case.
