Two-spin entanglement distribution near factorized states

We study the two-spin entanglement distribution along the infinite S = 1/ 2 chain described by the XY model in a transverse field; closed analytical expressions are derived for the one-tangle and the concurrences C-r, with r being the distance between the two possibly entangled spins, for the values of the Hamiltonian parameters close to those corresponding to factorized ground states. The total amount of the entanglement, the fraction of such entanglement which is stored in pairwise entanglement and the way such fraction distributes along the chain are discussed, with attention focused on the dependence on the anisotropy of the exchange interaction. Near factorization a characteristic length scale naturally emerges in the system, which is specifically related with the entanglement properties and diverges at the critical point of the fully isotropic model. In general, we find that anisotropy rules a complex behavior of the entanglement properties, which results in the fact that more isotropic models, despite being characterized by a larger amount of the total entanglement, present a smaller fraction of the pairwise entanglement: the latter, in turn, is more evenly distributed along the chain, to the extent that, in the fully isotropic model at the critical field, the concurrences do not depend on r.