Low-temperature spectrum of correlation lengths of the XXZ chain in the antiferromagnetic massive regime

TL;DR

This paper analyzes the low-temperature behavior of correlation lengths in the antiferromagnetic massive regime of the XXZ chain, revealing different structures of Bethe Ansatz equations depending on the magnetic field.

Contribution

It provides a detailed analysis of the nonlinear integral equations and their solutions in the low-temperature limit, highlighting the distinct behaviors under zero and non-zero magnetic fields.

Findings

At zero magnetic field, complex excitation parameters are determined by hole parameters on a line segment.

At non-zero magnetic field, parameters can be interpreted as particles and holes on two curves as temperature approaches zero.

The structure of Bethe Ansatz equations changes significantly with the magnetic field.

Abstract

We consider the spectrum of correlation lengths of the spin-$\frac{1}{2}$ XXZ chain in the antiferromagnetic massive regime. These are given as ratios of eigenvalues of the quantum transfer matrix of the model. The eigenvalues are determined by integrals over certain auxiliary functions and by their zeros. The auxiliary functions satisfy nonlinear integral equations. We analyse these nonlinear integral equations in the low-temperature limit. In this limit we can determine the auxiliary functions and the expressions for the eigenvalues as functions of a finite number of parameters which satisfy finite sets of algebraic equations, the so-called higher-level Bethe Ansatz equations. The behaviour of these equations, if we send the temperature $T$ to zero, is different for zero and non-zero magnetic field $h$. If $h$ is zero the situation is much like in the case of the usual transfer matrix. Non-trivial higher-level Bethe Ansatz equations remain which determine certain complex excitation parameters as functions of hole parameters which are free on a line segment in the complex plane. If $h$ is non-zero, on the other hand, a remarkable restructuring occurs, and all parameters which enter the description of the quantum transfer matrix eigenvalues can be interpreted entirely in terms of particles and holes which are freely located on two curves when $T$ goes to zero.