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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 =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 . Since the absence of Néel order
was reported down to 1.2 K in the first paper ,
various measurements have been performed on this compound, most of which
looked for the Haldane gap.