Improved Silbey-Harris polaron ansatz for the spin-boson model

TL;DR

This paper enhances the Silbey-Harris polaron ansatz for the spin-boson model by incorporating orthogonal displaced Fock states, leading to highly accurate ground state and phase transition results across different bath types.

Contribution

The authors introduce an improved ansatz that converges rapidly and provides near-exact results for the spin-boson model, especially in the sub-Ohmic regime.

Findings

Rapid convergence of ground state results with finite displaced Fock states

Accurate critical coupling strengths for quantum phase transition

Converged magnetization in biased spin-boson model

Abstract

In this paper, the well-known Silbey-Harris (SH) polaron ansatz for the spin-boson model is improved by adding orthogonal displaced Fock states. The obtained results for the ground state in all baths converge very quickly within finite displaced Fock states and corresponding SH results are corrected considerably. Especially for the sub-Ohmic spin-boson model, the converging results are obtained for 0 < s < 1/2 in the fourth-order correction and very accurate critical coupling strengths of the quantum phase transition are achieved. Converging magnetization in the biased spin-boson model is also arrived at. Since the present improved SH ansatz can yield very accurate, even almost exact results, it should have wide applications and extensions in various spin-boson model and related fields.