Exact Bethe quantum numbers of the massive XXZ chain in the two down-spin sector

Abstract

Every solution of the Bethe ansatz equations (BAE) is characterized by a set of quantum numbers called the Bethe quantum numbers, which are fundamental for evaluating it numerically. We rigorously derive the Bethe quantum numbers for the real solutions of the spin-1/2 massive XXZ spin chain in the two down-spin sector, assuming the existence of solutions to some form of BAE. In the sector the quantum numbers $J_1$ and $J_2$ were derived for complex solutions, but not for real solutions. We show the exact results in the sector as follows. (\si) When two Bethe quantum numbers are different, i.e., for $J_1 \ne J_2$, we introduce a graphical method, which we call a contour method, for deriving the solution of BAE to a given set of Bethe quantum numbers. By the method, we can readily show the existence and the uniqueness of the solution. (\sii) When two Bethe quantum numbers are equal, i.e. for $J_1 = J_2$, we derive the criteria for the collapse of two-strings and the emergence of an extra two-string by an analytic method. (\siii) We obtain the number of real solutions, which depends on the site number $N$ and the XXZ anisotropy parameter $\zeta$. (\siv) We derive all infinite-valued solutions of BAE for the XXX spin chain in the two down-spin sector through the XXX limit. (\sv) We explicitly show the completeness of the Bethe ansatz in terms of the Bethe quantum numbers.