Quantum phase transitions of the spin-boson model within multi-coherent-states

TL;DR

This paper employs a variational multi-coherent-state approach to study quantum phase transitions in the spin-boson model, revealing Gaussian critical behavior and the breakdown of quantum-to-classical mapping in certain regimes.

Contribution

It introduces a novel variational method with asymmetric parameters for the spin-boson model, providing accurate critical coupling estimates without artificial approximations.

Findings

Gaussian critical behavior in the sub-Ohmic regime

Breakdown of quantum-to-classical mapping for 1/2<s<1

Steep entanglement entropy jumps at critical points

Abstract

A variational approach based on the multi-coherent-state ansatz with asymmetric parameters is employed to study the ground state of the spin-boson model. Without any artificial approximations except for the finite number of the coherent states, we find the robust Gaussian critical behavior in the whole sub-Ohmic bath regime. The converged critical coupling strength can be estimated with the $1/N$ scaling, where $N $ is the number of the coherent states. It is strongly demonstrated the breakdown of the well-known quantum-to-classical mapping for $1/2<s<1$. In addition, the entanglement entropy displays more steep jump around the critical points for the Ohmic bath than the sub-Ohmic bath.