SPIN SUSCEPTIBILITY AND LOW-LYING EXCITATIONS IN THE HALDANE-GAP COMPOUND Y2BANIO5

(89)NMR frequency shifts K and spin-lattice relaxation rates 1/T-1 hav e been measured in the chain nickelate Y2BaNiO5. Between 40 and 300 K the anisotropic shift is proportional to the isotropic shift K-iso, wh ich 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: E(g) (K-iso)=80/-5 K, whereas E(g)(1/T-1) = 122+/-8 K. This is also unexpected, and i ndicates that the dynamic susceptibility chi(q,omega) is more strongly gapped for large q than for q = 0.