Equivalence of TBA and QTM

Abstract

The traditional thermodynamic Bethe ansatz (TBA) equations for the XXZ model at $|Δ|\ge 1$ are derived within the quantum transfer matrix (QTM) method. This provides further evidence of the equivalence of both methods. Most importantly, we derive an integral equation for the free energy formulated for just one unknown function. This integral equation is different in physical and mathematical aspects from the established ones. The single integral equation is analytically continued to the regime $|Δ|<1$.