Decoherent dynamics of a two-level system coupled to a sea of spins

The dynamics of a two-level system which is coupled to an environmental sea of infinitely many spin-1/2 particles is investigated by use of the resolvent operator approach. Only at zero temperature does this spin-spin-bath model exhibit identical behavior as the more familiar spin-boson model. It is found that increasing temperature favors coherent dynamics. At high temperatures, the spin-spin-bath model for an Ohmic spectral density sustains a coherent dynamics if the dissipation coefficient a is sufficiently small, i.e., alpha < 1/2; while the decoherence exhibits pure exponential decay if alpha > 1/2.