Bond-versus-site doping models for off-chain-doped Haldane-gap system   Y$_2$ Ba Ni O$_5$
  Using the density matrix renormalization-group technique, we calculate the impurity energy levels for two different effective models of off-chain doping for quasi-one-dimensional Heisenberg chain compound Y$_2$ Ba Ni O$_5$: ferromagnetic bond doping and antiferromagnetic site spin-1/2 doping. Thresholds of the impurity strength for the appearance of localized states are found for both models. However, the ground-state and low-energy excitations for weak impurity strength are different for these two models and the difference can be detected by experiments. 
Jizhong Lou, Shaojin Qin, Zhaobin Su and Lu Yu