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Origin of the singular Bethe ansatz solutions for the Heisenberg XXZ spin chain

Jae Dong Noh·Deok-Sun Lee·Doochul Kim
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Abstract

We investigate symmetry properties of the Bethe ansatz wave functions for the Heisenberg $XXZ$ spin chain. The $XXZ$ Hamiltonian commutes simultaneously with the shift operator $T$ and the lattice inversion operator $V$ in the space of $Ω=\pm 1$ with $Ω$ the eigenvalue of $T$. We show that the Bethe ansatz solutions with normalizable wave functions cannot be the eigenstates of $T$ and $V$ with quantum number $(Ω,Υ)=(\pm 1,\mp 1)$ where $Υ$ is the eigenvalue of $V$. Therefore the Bethe ansatz wave functions should be singular for nondegenerate eigenstates of the Hamiltonian with quantum number $(Ω,Υ)=(\pm 1,\mp 1)$. It is also shown that such states exist in any nontrivial down-spin number sector and that the number of them diverges exponentially with the chain length.

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Jae Dong Noh, Deok-Sun Lee, Doochul Kim. 2000-10-31. Origin of the singular Bethe ansatz solutions for the Heisenberg XXZ spin chain. https://doi.org/10.1016/s0378-4371(00)00450-7

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