Response of finite spin-S Heisenberg chains to local perturbations

We consider the properties of finite isotropic antiferromagnetic Heisenberg chains with S=1/2,1,3/2 spins when a weak magnetic field is applied on a f ew sites, using White's density-matrix renormalization-group method. For th e S=l chain there exists only one length scale in the system which determin es the behavior of the one- and two-point correlation functions both around the local perturbation and near the free boundary. For the critical half-o dd-integer spin cases the exponent of the spin-spin correlation function wa s found to be eta =1 and the exponent of the decay of the site magnetizatio n around the perturbed site is x(m) = eta/2 Close to a free boundary, howev er, the behavior is completely different for S= 1/2 and S>1/2.