EXCITATIONS OF CSMNI3 (S=5 2) IN MAGNETIC-FIELD, IN COMPARISON TO CSNICL3 (S=1) - A TEST OF THE HALDANE CONJECTURE/

In the past unusual features in the excitation spectrum of the S = I c ompound CsNiCl3 have been attributed to remaining Haldane correlations in the 3D ordered state. One of these features is the degenerate trip let excitation at the 1D AF zone center, which is split by the magneti c field. Recently a higher-order spin-wave expansion has been proposed , in order to explain the experiment without assuming the Haldane corr elations which are typical for integer spins. We studied CsNiCl3, and CsMnI3, the S = a counterparts of CsNiCl3,and demonstrate striking dif ferences. The energies and polarizations of CsMnI3 are well explained by linear spin-wave theory. The experimental findings on CsNiCl3, howe ver, disagree with the higher-order spin-wave theory, while a theory o f Affleck which takes into account Haldane correlations correctly mode ls the qualitative behaviour. Hence we have shown that the degeneracy of three modes at the 1D AF zone center of CsNiCl3, and their strong h eld dependence, are caused by Haldane correlations which are only pres ent in integer-spin systems.