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We have studied the physical properties of Nd2O3 with neutron diffraction, inelastic neutron scattering, heat capacity, and magnetic susceptibility measurements. Nd2O3 crystallizes in a trigonal structure, with Nd 3+ ions surrounded by cages of 7 oxygen anions. The crystal field spectrum consists of four excitations spanning the energy range 3-60 meV. The refined eigenfunctions indicate XY-spins in the ab plane. The Curie-Weiss temperature of θCW = −23.7(1) K was determined from magnetic susceptibility measurements. Heat capacity measurements show a sharp peak at 550 mK and a broader feature centered near 1.5 K. Neutron diffraction measurements show that the 550 mK transition corresponds to long-range anti-ferromagnetic order implying a frustration index of θCW /TN ≈ 43. These results indicate that Nd2O3 is a structurally and chemically simple model system for frustration caused by competing interactions with moments with predominate XY anisotropy.

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The application of a magnetic field transverse to the easy axis, Ising direction in the quasi-two dimensional Kagome staircase magnet, Co3V2O8, induces three quantum phase transitions at low temperatures, ultimately producing a novel high field polarized state, with two distinct sublattices. New time-of-flight neutron scattering techniques, accompanied by large angular access, high magnetic field infrastructure allow the mapping of a sequence of ferromagnetic and incommensurate phases and their accompanying spin excitations. At least one of the transitions to incommensurate phases at µ0Hc1 ∼ 6.25 T and µ0Hc2 ∼ 7 T is discontinuous, while the final quantum critical point at µ0Hc3 ∼ 13 T is continuous.

The magnetic excitation spectrum of the quantum magnet YbCl 3 is studied with inelastic neutron scattering. The spectrum exhibits an unusually sharp feature within a broad continuum, as well as conventional spin waves. By including both transverse and longitudinal channels of the neutron response, linear spin wave theory with a single Heisenberg interaction on the honeycomb lattice reproduces all of the key features in the spectrum. In particular, the broad continuum corresponds to a two-magnon contribution from the longitudinal channel, while the sharp feature within this continuum is identified as a Van Hove singularity in the joint density of states, which indicates the two-dimensional nature of the two-magnon continuum.We term these singularities magneto-caustic features in analogy with caustic features in ray optics where focused envelopes of light are generated when light passes through or reflects from curved or distorted surfaces. The experimental demonstration of a sharp Van Hove singularity in a two-magnon continuum is important because analogous features in potential two-spinon continua could distinguish quantum spin liquids from merely disordered systems. These results establish YbCl 3 as a nearly ideal two-dimensional honeycomb lattice material hosting strong quantum effects in the unfrustrated limit.

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