Stability of the Haldane phase in anisotropic magnetic ladders

We have considered the properties of anisotropic two-leg ladder models with S=1/2 or S=1 spins on the rungs, using White's density matrix renormalization group method. We have generalized the method by taking into account the symmetries of the model in order to reduce the dimensions of the matrix to be diagonalized, thereby making possible to consider more states. The boundaries in the parameter space of the extended region, where the Haldane phase exists, are estimated.Comment: 19 pages, 5 figure