Ground-state instabilities and entanglement in the spin-boson model

Ground-state instabilities of the spin-boson model is studied in this work. The existence of sequential ground-state instabilities is shown analytically for arbitrary detuning in the two-spin system. In this model, extra discontinuities of concurrence (entanglement measure) are found in the finite system, which do not appear in the on-resonant model. The above results remain intact by including extra boson modes. Moreover, by including extra modes, it is found that ground-state entanglement can be obtained and enhanced even in the weak coupling regime.