On kink states of ferromagnetic chains

We study kink states of quantum, ferromagnetic, easy axis spin 1/2 chains a t zero temperature. These are produced by applying opposite magnetic fields on the two end sites of the chain. For sufficiently strong anisotropy and boundary field, we obtain estimates on the wave function of the lowest ener gy states in sectors with fixed third component of the total spin. These es timates imply that the magnetization profile has a kink structure with a we ll-defined location and a finite width. The energies of kink states in diff erent sectors are exponentially close as Long as they are not located near the boundaries. The basic tool that we use here is the principle of exponen tial focalization of eigenvectors. We illustrate the method in the simplest case of the Heisenberg XXZ model and then show how it can be generalized t o more complicated models. In the particular case of the Heisenberg XXZ mod el our results are consistent with the exact kink wave functions known for a special value of the boundary magnetic field. (C) 2000 Elsevier Science B .V. All rights reserved.