INTEGER VS HALF-INTEGER-SPIN ANTIFERROMAGNETIC CHAINS, AND EXPERIMENTAL TEST OF THE HALDANE CONJECTURE - EXCITATIONS IN THE S=5 2 SYSTEM CSMNI3 VS EXCITATIONS IN THE S=1 CSNICL3/

By means of high-resolution inelastic neutron scattering on the quasi- one-dimensional Heisenberg antiferromagnet CsMnI3, striking difference s between the magnon spectrum of this S = 5/2 compound and that of the isomorphic Haldane system S = 1 CsNiCl3 are demonstrated. Since conve ntional spin wave theory provides a consistent description of the CsMn I3 properties but fails in case of CsNiCl3, this experiment gives stro ng support of Haldane's conjecture. It is shown that the deviations fr om standard spin wave theory as observed in CsNiCl3 are not due to fru stration effects but to the remainders of the Haldane ground state whi ch are still present even in the three-dimensionally ordered state of the S = 1 system CsNiCl3.