Quantum criticality of the sub-ohmic spin-boson model

TL;DR

This paper investigates the quantum critical behavior of the sub-ohmic spin-boson model, revealing interacting critical points contrary to classical predictions, and highlights the role of Berry-phase effects in this quantum phase transition.

Contribution

It provides evidence that the quantum critical point is interacting, challenging the quantum-to-classical mapping, and links Berry-phase effects to the critical behavior in the model.

Findings

Quantum critical point is interacting, not mean-field.

W/T scaling observed in local dynamical susceptibility.

Berry-phase term causes quantum-to-classical mapping violation.

Abstract

We revisit the critical behavior of the sub-ohmic spin-boson model. Analysis of both the leading and subleading terms in the temperature dependence of the inverse static local spin susceptibility at the quantum critical point, calculated using a numerical renormalization-group method, provides evidence that the quantum critical point is interacting in cases where the quantum-to-classical mapping would predict mean-field behavior. The subleading term is shown to be consistent with an w/T scaling of the local dynamical susceptibility, as is the leading term. The frequency and temperature dependences of the local spin susceptibility in the strong-coupling (delocalized) regime are also presented. We attribute the violation of the quantum-to-classical mapping to a Berry-phase term in a continuum path-integral representation of the model. This effect connects the behavior discussed here with its counterparts in models with continuous spin symmetry.