Effective Bethe Ansatz for Spin-1 Non-integrable Models

TL;DR

This paper validates the Effective Bethe Ansatz (EBA), a variational method deforming Bethe wavefunctions, for approximating non-integrable spin-1 chains, showing it accurately captures low-energy physics near integrability.

Contribution

It systematically evaluates EBA's accuracy for the spin-1 bilinear-biquadratic chain, establishing it as a reliable semi-analytical tool for non-integrable quantum spin systems.

Findings

EBA accurately describes ground and excited states near integrability.

Fidelity decreases controllably with increased perturbation.

EBA captures finite-size effects and signals phase transitions.

Abstract

This work presents a comprehensive benchmark and validation of a recently proposed method called Effective Bethe Ansatz (EBA). It is a variational method that deforms the exact Bethe wavefunctions of one-dimensional spin chains at integrable points to approximate non-integrable systems. We apply this method to the non-integrable regime of the spin-1 bilinear-biquadratic chain. By performing EBA method starting from the two integrable endpoints, the Takhtajan-Babujian point and the Lai-Sutherland point, we systematically evaluate the accuracy of the EBA for the ground state and first excited state. Our validation is based on a direct comparison with exact diagonalization, assessing energy, fidelity, and entanglement entropy. The results confirm that the EBA provides a physically accurate description near integrability, with fidelity decreasing controllably as the perturbation increases. The method successfully captures key finite-size effects, such as level crossings, manifested as sharp drops in fidelity, and provides a probe to potential phase transitions. This study establishes the EBA as a reliable and efficient semi-analytical tool, clarifying its scope and limitations for studying low-energy physics in non-integrable quantum spin chains.