  We investigate the quantum Zeno and anti-Zeno effects in spin bath models:
the spin-boson model and a spin-fermion model. We show that the Zeno-anti-Zeno
transition is critically controlled by the system-bath coupling parameter, the
same parameter that determines spin decoherence rate. We also discuss the
crossover in a biased system, at high temperatures, and for a nonequilibrium
spin-fermion system, manifesting the counteracting roles of electrical bias,
temperature, and magnetic field on the spin decoherence rate.
