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 few sites, using White's density matrix renormalization group (DMRG) method. For the S = 1 chain there exists only one length scale in the system which determines the behavior of the one-and two-point correlation functions both around the local perturbation and near the free boundary. For the critical, half-odd-integer spin cases the exponent of the spin-spin correlation function was found to be η = 1, and the exponent of the decay of the site mag-netization around the perturbed site is x m = η/2. Close to a free boundary, however, the behavior is completely different for S = 1/2 and S > 1/2.