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[en] Piperazinium hexachlorodicuprate is shown to be a frustrated quasi-two-dimensional quantum Heisenberg antiferromagnet with a gapped spectrum. Zero-field inelastic neutron scattering and susceptibility and specific- heat measurements as a function of applied magnetic field are presented. At T=1.5 K, the magnetic excitation spectrum is dominated by a single propagating mode with a gap, Δ=1 meV, and bandwidth of ∼1.8 meV in the (h0l) plane. The mode has no dispersion along the b^* direction indicating that neighboring a-c planes of the triclinic structure are magnetically decoupled. The heat capacity shows a reduction of the gap as a function of applied magnetic field in agreement with a singlet-triplet excitation spectrum. A field-induced ordered phase is observed in heat capacity and magnetic susceptibility measurements for magnetic fields greater than H_c1∼7.5 T. Analysis of the neutron-scattering data reveals the important exchange interactions and indicates that some of these are highly frustrated

[en] Inelastic neutron scattering on deuterated single-crystal samples is used to study Haldane-gap excitations in the new S=1 one-dimensional quantum antiferromagnet Ni(C₅D₁₄N₂)₂N₃(PF₆), that was recently recognized as an ideal model system for high-field studies. The Haldane gap energies Δ_x=0.42(3) meV, Δ_y=0.52(6) meV, and Δ_z=1.9(1) meV, for excitations polarized along the a, b, and c crystallographic axes, respectively, are measured. The dispersion relation is studied for momentum transfers both along and perpendicular to the chains' direction. The in-chain exchange constant J=2.8 meV is found to be much larger than interchain coupling, J_y=1.8(4)x10^-3 meV and J_x=4(3)x10^-4 meV, along the b and a axes, respectively. The results are discussed in the context of future experiments in high magnetic fields

[en] The report describes a new multi-detector powder neutron diffractometer installed at the DR3 reactor at Risoe. The report gives details of the design criteria and operating instructions in order to ease operation of the instrument by actual and potential users. The individual components (collimators; monochromater, detectors) are described in detail. An example of a study of the oxygen uptake of YBa₂Cu₃O_7x is given. 13 ills. (author)

[en] Neutron scattering experiments on UPt₃ and U/sub 1-x/Th/sub x/Pt₃ are reviewed. At relatively high energies (/approximately/5 MeV), the magnetic fluctuation spectrum is modulated by the structure factor derived from short-range antiferromagnetic correlations where the two U ions in each unit cell are oppositely polarized. In contrast, at low energies (≤ 1 MeV), the diffuse inelastic scattering is associated with antiferromagnetic correlations where the unit cell is doubled. Nominally pure UPt₃ exhibits magnetic order with the wavevector corresponding to this doubling and a static moment of 0.02 +- 0.01 μ/sub B/. (U/sub 1-x/Th/sub x/)Pt₃ with x ≅ 0.05 exhibits ordering of the same type, but with a much larger static moment, 0.65 +- 0.1 μ/sub B/. Neutron scattering measurements on UPt₃ with implications for superconductivity are surveyed. 23 refs., 7 figs

[en] Inelastic neutron scattering is used to characterize spin fluctuations in the d -electron heavy fermion spinel LiV₂O ₄ . The spin-relaxation rate, Γ_Q , for Q=0.6 {angstrom}^-1 is 1.4(2)meV at low temperatures and increases linearly with temperature at a rate of 0.46(8)k_B . There is antiferromagnetic short-range order at low temperatures with a characteristic wave vector Q_c=0.64(2) {angstrom}^-1 and a correlation length of 6(1) {angstrom} . While warming shifts intensity towards lower Q , the staggered susceptibility peaks at a finite wave vector for T<80 K . The data are compared with conventional heavy fermion systems, geometrically frustrated insulating magnets, and recent theories for LiV₂O ₄

[en] Field-induced commensurate transverse magnetic ordering is observed in the Haldane-gap compound Ni(C₅D₁₄ N₂)₂N₃ (PF₆ ) by means of neutron diffraction. Depending on the direction of applied field, the high-field phase is shown to be either a three-dimensional ordered Neel state or a short-range ordered state with dominant two-dimensional spin correlations. The structure of the high-field phase is determined, and properties of the observed quantum phase transition are discussed

[en] We summarize neutron-scattering studies of the antiferromagnetic correlations in UPt₃. The differences between correlations persisting on short and long time scales are reviewed. The short-lived correlations are shown to be responsible for the bulk susceptibility peak near 15 K. We describe the diffraction evidence for a coupling between antiferromagnetic and superconducting order parameters. (orig.)

[en] Cold neutron spectroscopy is a unique tool in condensed matter physics that provides direct access to the many body response functions at low energies. In this paper, we describe modern instrumentation for cold neutron triple-axis spectroscopy and present an example of research performed using the technique

No abstract available