EFFECT OF SPIN-1 IMPURITIES IN DIMERIZED HEISENBERG-CHAINS

The effects of spin-1 impurities introduced in dimerized antiferromagn etic spin chains are studied using Lanczos diagonalization and quantum Monte Carlo simulations. First, the magnitude of the interaction betw een a Ni ion replacing a Cu ion is derived for the case study system C u-O-Cu. Then, it is shown that the introduction of S = 1 impurities le ads to the suppression of the original spin gap of the dimerized syste m and to the enhancement of the antiferromagnetic correlations close t o the impurity. These features are interpreted in terms of a valence b ond picture of the ground state of both the pure and the doped system. The similitudes and differences between the underlying mechanisms tha t produce these effects in the cases of nonmagnetic and S=1 impurities are analyzed. It is stressed that a singlet formed between the spin-1 impurity site and the two spin-1/2 neighbors spins behaves as an effe ctive nonmagnetic vacancy which may be partially responsible for the e nhancement of the antiferromagnetic correlations on the whole system. Implications of these results for experiment are also discussed.