  The existence of the delocalized-localized quantum phase transition (QPT) in
the ohmic spin-boson model has been commonly recognized. While the physics in
the localized regime is relatively simple, the delocalized regime shows many
interesting behaviors. Here we reveal that in this regime there exists a novel
QPT: namely, from a phase without a bound state to a phase with a bound state,
which leads to completely different dynamical behaviors in these two phases. If
the reservoir is initially in the displaced vacuum state (i.e., the coherent
state), the spin dynamics exhibits lossless oscillation when the bound state
exists; otherwise, the oscillation will decay completely. This result is
compatible with the coherence-incoherence transition occurring in the
small-tunneling limit. Our work indicates that the QPT physics in the
spin-boson model needs further exploration.
