Dephasing and pseudo-coherent quantum dynamics in super-Ohmic environments

Dephasing in quantum systems is typically the result of its interaction with environmental degrees of freedom. We investigate within a spin-boson model the influence of a super-Ohmic environment on the dynamics of a quantum two-state system. A super-Ohmic enviroment, thereby, models typical bulk phonons which are a common disturbance for solid state quantum systems as, for example, NV centers. By applying the numerically exact quasi-adiabatic path integral approach we show that for strong system-bath coupling, pseudo-coherent dynamics emerges, i.e., oscillatory dynamics at short times due to slaving of the quantum system to the bath dynamics. We extend the phase diagram known for sub-Ohmic and Ohmic environments into the super-Ohmic regime and observe a pronounced non-monotonous behaviour. Super-Ohmic purely dephasing fluctuations strongly suppress the amplitude of coherent dynamics at very short times with no subsequent further decay at later times. Nevertheless, they render the dynamics overdamped. The according phase separation line shows also a non-monotonous behaviour, very similar to the pseudo-coherent dynamics.