Functional Bethe ansatz methods for the open XXX chain

TL;DR

This paper develops new functional equation methods, including Bethe-type equations and TBA integral equations, to analyze the spectrum of the open XXX Heisenberg spin chain with non-diagonal boundary fields, enabling efficient numerical solutions.

Contribution

It introduces two novel approaches for solving the spectral problem of the open XXX chain with non-diagonal boundaries, extending beyond traditional Bethe ansatz techniques.

Findings

Successful formulation of Bethe-type equations for the model

Mapping of functional equations to TBA-type integral equations

Numerical analysis of finite size corrections and ground state properties

Abstract

We study the spectrum of the integrable open XXX Heisenberg spin chain subject to non-diagonal boundary magnetic fields. The spectral problem for this model can be formulated in terms of functional equations obtained by separation of variables or, equivalently, from the fusion of transfer matrices. For generic boundary conditions the eigenvalues cannot be obtained from the solution of finitely many algebraic Bethe equations. Based on careful finite size studies of the analytic properties of the underlying hierarchy of transfer matrices we devise two approaches to analyze the functional equations. First we introduce a truncation method leading to Bethe type equations determining the energy spectrum of the spin chain. In a second approach the hierarchy of functional equations is mapped to an infinite system of non-linear integral equations of TBA type. The two schemes have complementary ranges of applicability and facilitate an efficient numerical analysis for a wide range of boundary parameters. Some data are presented on the finite size corrections to the energy of the state which evolves into the antiferromagnetic ground state in the limit of parallel boundary fields.