Integer vs. Half-Integer-Spin Antiferromagnetic Chains, an Experimental Test of the Haldane Conjecture: Excitations in the S = 5/2 System CsMnI 3 vs. Excitations in the S = 1 CsNiCl 3

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Abstract
By means of high-resolution inelastic neutron scattering on the quasi-one-dimensional Heisenberg antiferromagnet CsMnI3, striking differences between the magnon spectrum of this S = 5/2 compound and that of the isomorphic Haldane system S = 1 CsNiCl3 are demonstrated. Since conventional spin wave theory provides a consistent description of the CsMnI3 properties but fails in case of CsNiCl3, this experiment gives strong support of Haldane's conjecture. It is shown that the deviations from standard spin wave theory as observed in CsNiCl3 are not due to frustration effects but to the remainders of the Haldane ground state which are still present even in the three-dimensionally ordered state of the S = 1 system CsNiCl3.