  We study a generic spin-fermion model, where a two-level system (spin) is
coupled to two metallic leads with different chemical potentials, in the
presence of monochromatic driving fields. The real-time dynamics of the system
is simulated beyond the Markovian limit by an iterative numerically exact
influence functional path integral method. Our results show that although both
system-bath coupling and chemical potential difference contribute to
dissipation, their effects are distinct. In particular, under certain drivings
the asymptotic Floquet states of the system exhibit robustness against a range
of system-bath coupling strength: the asymptotic behaviors of the system are
insensitive to different system-bath coupling strength, while they are highly
tunable by the chemical potential difference of baths. Further simulations show
that such robustness may be essentially a result of the interplay between
driving, bath electronic structure and system-bath coupling. Therefore the
robustness could break down depending on the characteristics of the interplay.
In addition, under fast linearly polarized driving the quantum stochastic
resonance is demonstrated that stronger system-bath coupling (stronger
dissipation) enhances rather than suppresses the amplitude of coherent
oscillations of the system.
