paper

Continued fraction TBA and functional relations in XXZ model at root of unity

arXiv:math/9803056 · doi:10.1016/S0550-3213(98)00300-9

Abstract

Thermodynamics of the spin 1/2 XXZ model is studied in the critical regime using the quantum transfer matrix (QTM) approach. We find functional relations indexed by the Takahashi-Suzuki numbers among the fusion hierarchy of the QTM's (T-system) and their certain combinations (Y-system). By investigating analyticity of the latter, we derive a closed set of non-linear integral equations which characterize the free energy and the correlation lengths for both <sigma_j^{+} sigma_i^{-}> and <sigma_j^{z} sigma_i^{z}> at any finite temperatures. Concerning the free energy, they exactly coincide with Takahashi-Suzuki's TBA equations based on the string hypothesis. By solving the integral equations numerically the correlation lengths are determined, which agrees with the earlier results in the low temperature limit.

28pages,LaTeX2.09, 9 eps files. To be published in Nucl. Phys. B

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