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arXiv:2409.19775

R-matrix for the XX spin chain in a staggered magnetic field

P. N. Bibikov

Published Sep 29, 2024• 0 citations
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Abstract

We suggest the elliptic $16\times16$ R-matrix for the XX spin chain in a staggered magnetic field. This integrable model is a special case of the more general one, studied long ago within the alternative approach by V. M. Kontorovich and V. M. Tsukernik. The Yang-Baxter equation is verified with the full rigorous only in the two special degenerate trigonometric cases. For the general one, studying the first terms of the Taylor expansions for the $R$-matrix entries, we give the strong evidence for implementation of the Yang-Baxter equation.

DOI
10.48550/arxiv.2409.19775

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We suggest the elliptic 16×1616\times1616×16 R-matrix for the XX spin chain in a staggered magnetic field. This integrable model is a special case of the more general one, studied long ago within the alternative approach by V. M. Kontorovich and V. M. Tsukernik. The Yang-Baxter equation is verified with the full rigorous only in the two special degenerate trigonometric cases. For the general one, studying the first terms of the Taylor expansions for the RRR-matrix entries, we give the strong evidence for implementation of the Yang-Baxter equation.
It is pointed, that the 16×1616\times1616×16 Hamiltonian density matrix, corresponding to XX spin chain in a staggered magnetic field, satisfies the Reshetikhin condition as well, as the two higher ones. All of them are necessary for the existence of the corresponding RRR-matrix. At the special imaginary value of the staggered field, the RRR-matrix is obtained explicitly. It is shown that the corresponding Hamiltonian has purely imaginary spectrum.
Matrix representations of BCn{\rm BC}_nBCn​ associative Yang--Baxter equations and related long-range spin chains
We construct a matrix representation of the algebra defined by the associative Yang--Baxter equations of BCn{\rm BC}_nBCn​ type. In the elliptic case, two sets of generators are represented in terms of the Baxter--Belavin quantum RRR-matrix in the fundamental representation of GLN{\rm GL}_NGLN​, while the remaining generators are represented by the elliptic KKK-matrix found by Komori and Hikami. We revisit its derivation using the Felder--Pasquier procedure and correct the unitarity relation for odd NNN. We then use these matrix representations to construct examples of long-range quantum spin chains from RRR-matrix-valued Lax pairs for the classical Calogero--Inozemtsev system associated with the BCn{\rm BC}_nBCn​ root system. We also construct RRR-matrix-valued Lax pairs for Calogero--Moser models in an external field, leading to Haldane--Shastry and Polychronakos--Frahm chains with modified spin positions.
An elliptic generalization of the D2(2)D^{(2)}_{2}D2(2)​ Spin chain
An integrable elliptic generalization of the closed D2(2)D^{(2)}_2D2(2)​ quantum spin chain is constructed. Taking the trigonometric limit, we have shown that the corresponding R-matrix reduces to that of the D2(2)D^{(2)}_2D2(2)​ one, up to a similarity transformation. Basing on the very fact of the factorization for the R-matrix and the corresponding transfer matrix which indeed ensures the integrability of the model, we give the exact solution of the model which can be expressed in terms of a homogeneous T-Q relation.
In this paper we discuss R-matrix-valued Lax pairs for slN Calogero–Moser model and their relation to integrable quantum long-range spin chains of the Haldane–Shastry–Inozemtsev type. First, we construct the R-matrix-valued Lax pairs for the third flow of the classical Calogero–Moser model. Then we notice that the scalar parts (in the auxiliary space) of the M-matrices corresponding to the second and third flows have form of special spin exchange operators. The freezing trick restricts them to quantum Hamiltonians of long-range spin chains. We show that for a special choice of the R-matrix these Hamiltonians reproduce those for the Inozemtsev chain. In the general case related to the Baxter's elliptic R-matrix we obtain a natural anisotropic extension of the Inozemtsev chain. Commutativity of the Hamiltonians is verified numerically. Trigonometric limits lead to the Haldane–Shastry chains and their anisotropic generalizations.