Ground-state entanglement in the XXZ model

In this paper, we investigate spin entanglement in the XXZ model defined on a d-dimensional bipartite lattice. The concurrence, a measure of the entanglement between two spins, is analyzed. We prove rigorously that the ground-state concurrence reaches maximum at the isotropic point. For dimensionality d >= 2, the concurrence develops a cusp at the isotropic point and we attribute it to the existence of magnetic long-range order.