Failure of the displaced-squeezed state for spin-boson models in the thermodynamic limit

Abstract

We present an analysis of a variational coherent-squeezed state that has been discussed in the literature as a potential ground state for the spin-boson model. We show that when the system-size scaling of the spin-bath coupling is included properly, all squeezing effects and non-universal physics vanish in the thermodynamic limit. We also present finite-size corrections to the renormalisation of the spin's coherence, showing that squeezing effects are also absent to leading order in the inverse bath-size.