Bond-versus-site doping models for the off-chain-doped Haldane-gap system Y2BaNiO5

Using the density matrix renormalization-group technique, we calculate the impurity energy levels for two different effective models of off-chain dopi ng for quasi-one-dimensional Heisenberg chain compound Y2BaNiO5: ferromagne tic bond doping and antiferromagnetic site spin-1/2 doping. Thresholds of t he impurity strength for the appearance of localized slates are found for b oth models. However, the ground-state and low-energy excitations for weak i mpurity strength are different for these two models and the difference can be detected by experiments. [S0163-1829(98)07640-1].