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 $\alpha$ is sufficiently small, i.e., $\alpha <1\mathrm{}/2$; while the decoherence exhibits pure exponential decay if $\alpha >1\mathrm{}/2$.