Spin susceptibility and low-lying excitations in the Haldane-gap compoundY2BaNiO5

Abstract

${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR frequency shifts K and spin-lattice relaxation rates 1/${\mathit{T}}_1$ have been measured in the chain nickelate ${\mathrm{Y}}_2$ ${\mathrm{BaNiO}}_5$. Between 40 and 300 K the anisotropic shift is proportional to the isotropic shift ${\mathit{K}}_{\mathrm{iso}}$, which is incompatible with existing theory of the spin susceptibility in Haldane-gap antiferromagnets. In addition, different energy gaps are obtained from shift and spin-lattice relaxation data: ${\mathit{E}}_{\mathit{g}}$(${\mathit{K}}_{\mathrm{iso}}$)=80±5 K, whereas ${\mathit{E}}_{\mathit{g}}$(1/${\mathit{T}}_1$)=122±8 K. This is also unexpected, and indicates that the dynamic susceptibility χ(q,ω) is more strongly gapped for large q than for q=0.