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arXiv:1910.05127 · Nuclear Physics B

Completing the solution for the OSp(1|2) spin chain

Rafael I. Nepomechie

Published Dec 12, 2019• 2 citations
arXivHTMLPDF

Abstract

The periodic OSp(1|2) quantum spin chain has both a graded and a non-graded version. Naively, the Bethe ansatz solution for the non-graded version does not account for the complete spectrum of the transfer matrix, and we propose a simple mechanism for achieving completeness. In contrast, for the graded version, this issue does not arise. We also clarify the symmetries of both versions of the model, and we show how these symmetries are manifested in the degeneracies and multiplicities of the transfer-matrix spectrum. While the graded version has OSp(1|2) symmetry, the non-graded version has only SU(2) symmetry. Moreover, we obtain conditions for selecting the physical singular solutions of the Bethe equations. This analysis solves a lasting controversy over signs in the Bethe equations.

DOI
10.1016/j.nuclphysb.2019.114887

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The periodic OSp(1|2) quantum spin chain has both a graded and a non-graded version. Naively, the Bethe ansatz solution for the non-graded version does not account for the complete spectrum of the transfer matrix, and we propose a simple mechanism for achieving completeness. In contrast, for the graded version, this issue does not arise. We also clarify the symmetries of both versions of the model, and we show how these symmetries are manifested in the degeneracies and multiplicities of the transfer-matrix spectrum. While the graded version has OSp(1|2) symmetry, the non-graded version has only SU(2) symmetry. Moreover, we obtain conditions for selecting the physical singular solutions of the Bethe equations. This analysis solves a lasting controversy over signs in the Bethe equations.
We consider the Temperley–Lieb (TL) open quantum spin chain with “free” boundary conditions associated with the spin- s representation of quantum-deformed s l ( 2 ) . We construct the transfer matrix, and determine its eigenvalues and the corresponding Bethe equations using analytical Bethe ansatz. We show that the transfer matrix has quantum group symmetry, and we propose explicit formulas for the number of solutions of the Bethe equations and the degeneracies of the transfer-matrix eigenvalues. We propose an algebraic Bethe ansatz construction of the off-shell Bethe states, and we conjecture that the on-shell Bethe states are highest-weight states of the quantum group. We also propose a determinant formula for the scalar product between an off-shell Bethe state and its on-shell dual, as well as for the square of the norm. We find that all of these results, except for the degeneracies and a constant factor in the scalar product, are universal in the sense that they do not depend on the value of the spin. In an appendix, we briefly consider the closed TL spin chain with periodic boundary conditions, and show how a previously-proposed solution can be improved so as to obtain the complete (albeit non-universal) spectrum.
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Abstract Two branches of integrable open quantum-group invariant quantum spin chains are known. For one branch ( ), a complete Bethe ansatz solution has been proposed. However, the other branch ( ) has so far resisted solution. In an effort to address this problem, we consider here the simplest case n = 1. We propose a Bethe ansatz solution, which however is not complete, as it describes only the transfer-matrix eigenvalues with odd degeneracy. We also consider a proposal for the missing eigenvalues.