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

TL;DR

This paper demonstrates that the previously proposed squeezed states do not serve as valid ground states for the spin-boson model in the thermodynamic limit, due to the vanishing of squeezing effects when proper system-size scaling is considered.

Contribution

It clarifies the limitations of variational squeezed states as ground states for the spin-boson model by analyzing their behavior in the thermodynamic limit with correct scaling.

Findings

Squeezing effects vanish in the thermodynamic limit when proper scaling is included.

Finite-size corrections show squeezing effects are absent to leading order.

The variational approach using squeezed states is not valid 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.