Decoherence processes of a quantum two-level system coupledto a fermionic environment

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.