.

As listed above, most of the experimental results on NENP supported (1) existence of the Haldane gap, and (2) a singlet ground state described by the Valence Bond Solid picture.

One interesting question regarding the Haldane ground state is what happens to the gap and the ground state, when charge is doped to the Haldane system. Since there have been discussions that high-$T_{\rm c}$ superconductivity may be realized when a spin-gap system is doped with charge, charge doping to the Haldane system is an interesting process from this point of view. Unfortunately, the Haldane material NENP does not allow charge doping. But recently, a charge-dopable Haldane compound (Y₂BaNiO₅) has been discovered, which is introduced in the next section.