| Summary: | Magnetic model systems Li2VOSi04 , Pb2VO(P04)2 and SrZnVO(P04)2 were studied using polarised neutron scattering. The vanadium 3 = 1/2 ions form . two-dimensional square lattices, where the cross-bond exchange, J2 , along the diagonal can compete with the nearest-neighbour (NN) exchange, J1, along the sides of the square. The magnetic ground state for all these compounds is a collinear antiferromagnet (CAF), i.e. ferromagnetic (F:M) lines arranged in an antiferromagnetic (AF) way. This is in contrast to the more usual Neel antiferromagnet (NAF), characteristic of spin systems dominated by AF NN exchange. The ordered moment is reduced in all cases, especially for Pb2VO(P04)2 and SrZnVO(P04)2' and at the same time a considerable diffuse background appears, due to quantum zero-point fluctuations. This quantum disordered state was also studied in the paramagnetic phase using polarised diffuse scattering techniques, as a way of extracting the magnetic exchanges in the absence of large single crystals. The 3(Q) measurements of powdered Pb2VO(P04 )2 and SrZnVO(P04)2 samples revealed a completely new area of the so-called J1 - J2 phase diagram, being the first materials with FM J1 and AF J2 interactions. Single crystals of the transition metal oxides GdV03 and LuV03 were studied with neutrons and resonant x-rays. In this case, the vanadium lattice has an almost cubic symmetry, resulting in degenerate many-body orbital states that compete. A combination of the above techniques on GdV03 yielded a spin structure consisting of FM chains with AF order between them (C-type), while the orbitals were found to order with orientations staggered in all three directions (G-type). The orbital ordering x-ray energy spectra have been modeled with ab initio calculations. Spinwave dispersions were investigated in inelastic neutron studies of LuV03 , which has both C- and G-spin (and orbital) phases at different temperatures. The exotic gapped C-spin phase is explained in terms of a recently developed theory with orbital fluctuations.and an orbital Peierls effect.
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