Brookhaven National Laboratory

Title: Haldane spin chains in uniform and staggered field.

Abstract: Integer-spin 1D antiferromagnets (AKA Haldane-gap systems) are by now very well understood both experimentally and theoretically. So well in fact, that neutron scatterers are getting rather desperate at  trying to find "something new and exciting" to do in this area. One obvious "exercise" would be to study the behavior of Haldane antiferromagnets in strong magnetic fields. The issue has been previously addressed theoretically in some detail, and many aspects of the problem were probed with neutron scattering on real quasi-1D materials. Unfortunately, the much-anticipated field-induced quantum phase transition to a state with long-range order could never be actually observed for various "technical" reasons. The recently discovered NDMAP (  Ni(C5H14N2)2N3(PF6) ) is the first compound where such a transition appears to be within reach of experimentalists. The first results of neutron scattering  experiments on NDMAP will be presented. A totally different, but non the less interesting problem is that of a Haldane spin chain in  an external STAGGERED field. A beautiful realization of this model  is found in 2-1-1-5 rare earth nickelates. Neutron scattering studies of these materials not only provide a comprehensive picture of the magnetism in this class of oxides, but also reveal some very general and fundamental properties of gapped quantum spin chains.