Twisted boundary energy and low energy excitation of the XXZ spin torus at the ferromagnetic region

We investigate the thermodynamic limit of the one-dimensional ferromagnetic XXZ model with twisted (or antiperiodic) boundary conditions. It is shown that the distribution of the Bethe roots of the inhomogeneous Bethe ansatz equations (BAEs) for the ground state as well as for the low-lying excited states satisfy the string hypothesis, although the inhomogeneous BAEs are not in the standard product form which has made the study of the corresponding thermodynamic limit nontrivial. We also obtain the twisted boundary energy induced by the nontrivial twisted boundary conditions in the thermodynamic limit. (paper)