Derivation of matrix product states for the Heisenberg spin chain with open boundary conditions

TL;DR

This paper derives an exact matrix product state representation for the Bethe-ansatz states of the open boundary Heisenberg spin chain, revealing insights into boundary effects and translational invariance in integrable models.

Contribution

It introduces a novel matrix product state construction for the open boundary Heisenberg chain using algebraic Bethe ansatz, applicable to a broad class of integrable models.

Findings

Matrix product states for open boundary conditions are derived.

Auxiliary matrices are larger than in periodic cases, reflecting doubled spin-flip sectors.

Matrices do not depend on spatial position, indicating translational invariance.

Abstract

Using the algebraic Bethe ansatz, we derive a matrix product representation of the exact Bethe-ansatz states of the six-vertex Heisenberg chain (either XXX or XXZ and spin-$\frac{1}{2}$) with open boundary conditions. In this representation, the components of the Bethe eigenstates are expressed as traces of products of matrices which act on a tensor product of auxiliary spaces. As compared to the matrix product states of the same Heisenberg chain but with periodic boundary conditions, the dimension of the exact auxiliary matrices is enlarged as if the conserved number of spin-flips considered would have been doubled. This result is generic for any non-nested integrable model, as is clear from our derivation and we further show by providing an additional example of the same matrix product state construction for a well known model of a gas of interacting bosons. Counterintuitively, the matrices do not depend on the spatial coordinate despite the open boundaries and thus suggest generic ways of exploiting (emergent) translational invariance both for finite size and in the thermodynamic limit.