  We report an anomalous decoherence phenomenon of a quantum dissipative system
in the framework of a stochastic decoupling scheme along with a hierarchical
equations-of-motion formalism without the usual Born-Markov or weak coupling
approximations. It is found that the decoherence of a two-qubit spin-boson
model can be reduced by increasing the bath temperature in strong-coupling
regimes. For the weak-coupling situation, we find that the bath temperature may
enhance the decoherence. This result is contrary to the common recognition that
a higher bath temperature always induces a more severe decoherence and suggests
that a decoherence dynamical transition occurs in this two-qubit spin-boson
model. We also demonstrate that the critical transition point can be
characterized by the behavior of the frequency spectrum of the quantum
coherence indicator.
