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arXiv:2108.07580 · SciPost Physics

Algebraic Bethe Ansatz for spinor R-matrices

Vidas Regelskis

Published Feb 17, 2022• 1 citation
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Abstract

We present a supermatrix realisation of q q -deformed spinor-spinor and spinor-vector R R -matrices. These R R -matrices are then used to construct transfer matrices for U_{q^2}(\mathfrak{so}_{2n+1}) U q 2 ( 𝔰 𝔬 2 n + 1 ) - and U_{q}(\mathfrak{so}_{2n+2}) U q ( 𝔰 𝔬 2 n + 2 ) -symmetric closed spin chains. Their eigenvectors and eigenvalues are computed.

DOI
10.21468/scipostphys.12.2.067

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We have recently constructed a large class of open quantum spin chains which have quantum-algebra symmetry and which are integrable. We show here that these models can be exactly solved using a generalization of the analytical Bethe Ansatz (BA) method. In particular, we determine in this way the spectrum of the transfer matrices of the Uq[su(2)]-invariant spin chains associated with A1(1) and A2(2) in the fundamental representation. The quantum-algebra invariance of these models plays an essential role in obtaining these results. The BA equations for these open chains are “doubled” with respect to the BA equations for the corresponding closed chains.
We present a nested algebraic Bethe ansatz for one-dimensional so2n- and sp2n-symmetric open spin chains with diagonal boundary conditions. The monodromy matrix of these spin chains satisfies the defining relations on the extended twisted Yangians Xρ(so2n,so2nρ)tw and Xρ(sp2n,sp2nρ)tw, respectively. We use a generalisation of the De Vega and Karowski approach allowing us to relate the spectral problem of so2n- or sp2n-symmetric open spin chain to that of gln-symmetric open spin chain studied by Belliard and Ragoucy. We explicitly derive the structure of Bethe vectors, their eigenvalues and the nested Bethe equations. We also provide a proof of Belliard and Ragoucy's trace formula for Bethe vectors of gln-symmetric open spin chains.
We present an 'algebraic treatment' of the analytical Bethe ansatz. For this purpose, we introduce abstract monodromy and transfer matrices which provide an algebraic framework for the analytical Bethe ansatz. It allows us to deal with a generic gl ( )-spin chain possessing on each site an arbitrary gl ( )-representation. For open spin chains, we use the classification of the reflection matrices to treat all the diagonal boundary cases. As a result, we obtain the Bethe equations in their full generality for closed and open spin chains. The classifications of finite-dimensional irreducible representations for the Yangian (closed spin chains) and for the reflection algebras (open spin chains) are directly linked to the calculation of the transfer matrix eigenvalues. The local Hamiltonian associated with a given integrable spin chain needs to be computed case by case. A general formula is still lacking at that point. As examples, we recover the usual Hamiltonian and Bethe equations for closed and open spin chains; we treat the alternating spin chains and the closed spin chain with an impurity. We also compute the Hamiltonian and Bethe equations for the open spin chain model used in the context of large- N QCD.