Quantum phase transition in the delocalized regime of the spin-boson model

TL;DR

This paper uncovers a novel quantum phase transition in the delocalized regime of the spin-boson model, from a phase without a bound state to one with a bound state, significantly affecting the system's dynamical behavior.

Contribution

It reveals a new quantum phase transition in the delocalized regime, linking bound state formation to dynamical behavior changes in the spin-boson model.

Findings

Existence of a phase with a bound state in the delocalized regime.

Lossless oscillations occur when the bound state exists.

Transition from lossless to decaying oscillations depending on the bound state presence.

Abstract

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.