5.1.4 Experimental evidence for Haldane's conjecture

 The first material investigated to test Haldane's conjecture was a hexagonal ABX₃-type material CsNiCl₃. With inelastic neutron scattering measurements [72,73,74], an energy gap was observed above the Néel temperature $T_{\rm N}$=4.9 K. The authors identified the observed energy gap as the Haldane gap, but the existence of 3D Néel order made a clear conclusion difficult.

Another model material Ni(C₂H₈N₈)₂NO₂(ClO₄) (NENP) was reported by J. P. Renard et al. in 1987 [75]. Since the absence of Néel order was reported down to 1.2 K in the first paper [75], various measurements have been performed on this compound, most of which looked for the Haldane gap.