Impurities in s=1 Heisenberg antiferromagnets

The s=1 Heisenberg antiferromagnet is studied in the presence of two kinds of local impurities. First, a perturbed antiferromagnetic bond with J’≠J at the center of an even-length open chain is considered. Using the density-matrix renormalization-group method we find that, for sufficiently strong or weak J’, a bound state is localized at the impurity site, giving rise to an energy level in the Haldane gap. The energy of the bound state is in …