Experimental evidence for the Haldane gap in a spin-1 nearly isotropic, antiferromagnetic chain

Neutron scattering has shown that the lowest spin excitations in the spin-1 antiferromagnet ${\mathrm{CsNiCl}}_3$ in its one-dimensional phase occur at finite frequency. After allowance for the known weak interchain coupling the gap is found to be in good agreement with recent numerical calculations. The results support the Haldane conjecture that in integral-spin chains with isotropic interactions a gap separates the ground state from the excitations.