CaFe_{2}O_{4} is a S=5/2 anisotropic antiferromagnet based upon zig-zag chains having two competing magnetic structures, denoted as the A (↑↑↓↓) and B (↑↓↑↓) phases, which differ by the c-axis stacking of ferromagnetic stripes. We apply neutron scattering to demonstrate that the competing A and B phase order parameters result in magnetic antiphase boundaries along c which freeze on the time scale of ∼1  ns at the onset of magnetic order at 200 K. Using high resolution neutron spectroscopy, we find quantized spin wave levels and measure 9 such excitations localized in regions ∼1-2 c-axis lattice constants in size. We discuss these in the context of solitary magnons predicted to exist in anisotropic systems. The magnetic anisotropy affords both competing A+B orders as well as localization of spin excitations in a classical magnet.

The recently presented LOPLS-AA all-atom force field for long hydrocarbon chains, based on the OPLS-AA force field, was extended to alcohols, esters, and glyceryl monooleate (GMO) lipids as a model lipid. Dihedral angles were fitted against high level ab initio calculations, and ester charges were increased to improve their hydration properties. Additionally, the ester Lennard-Jones parameters were readjusted to reproduce experimental liquid bulk properties, densities, and heats of vaporization. This extension enabled the setup of LOPLS-AA parameters for GMO molecules. The properties of the lipid force field were tested for the liquid-crystalline phase of a GMO bilayer. The obtained area per lipid for GMO is in good agreement with experiment. Additionally, the lipid dynamics on the subpicosecond to the nanosecond time scale is in excellent agreement with results from time-of-flight (TOF) quasielastic neutron scattering (QENS) experiments on a multilamellar monoolein system, enabling here for the first time the critical evaluation of the short-time dynamics obtained from a molecular dynamics simulation of a membrane system.

CaFe_{2}O_{4} is an anisotropic S=5/2 antiferromagnet with two competing A (↑↑↓↓) and B (↑↓↑↓) magnetic order parameters separated by static antiphase boundaries at low temperatures. Neutron diffraction and bulk susceptibility measurements, show that the spins near these boundaries are weakly correlated and a carry an uncompensated ferromagnetic moment that can be tuned with a magnetic field. Spectroscopic measurements find these spins are bound with excitation energies less than the bulk magnetic spin waves and resemble the spectra from isolated spin clusters. Localized bound orphaned spins separate the two competing magnetic order parameters in CaFe_{2}O_{4}.

The spin dynamics of mixed-valence YbB12 has been studied by inelastic neutron scattering on a high-quality single crystal. In the Kondo-insulating regime realized at low temperature, the spectra exhibit a spin-gap structure with two sharp, dispersive, in-gap excitations athω ≈ 14.5 and ≈ 20 meV. The lower mode is shown to be associated with short-range correlations near the antiferromagnetic wave vector q0 = ( ). Its properties are in overall agreement with those expected for a "spin exciton" branch in an indirect hybridization gap semiconductor.Heavy-fermion compounds exhibit a whole spectrum of unconventional low-temperature behaviors, basically reflecting the existence of a very small energy scale, of the order of a few tens of kelvin, in the electron subsystem [1]. This energy scale is a hallmark of strong electron correlations and, in the metallic case, is associated with the Kondo temperature below which the heavy-quasiparticle Fermi liquid forms.In insulating compounds, on the other hand -so-called "Kondo insulators" (KI) or "mixed-valence semiconductors" (MVSC), such as CeNiSn, Ce 3 Bi 4 Pt 3 , SmB 6 , YbB 12 , or UPtSn -it corresponds to the opening of a very narrow, temperaturedependent, energy gap in the electron density of states [2]. The physical origin of this insulating state is still incompletely understood. It has been argued [3] that a number of aspects can be explained in terms of a oneelectron band picture, with a "hybridization gap" forming at low temperature in the electronic density of states at the Fermi energy [4]. However, there is growing evidence that strong electron-electron correlations are central to the emergence of the gap behavior, and that their effects cannot be reduced to a mere renormalization of quasiparticle states. The spin dynamics of these systems is also peculiar: in most examples studied to date, inelastic neutron scattering (INS) spectra typically exhibit a spin-gap response (∆ s ∼ 1 − 10 meV) at low temperature, which seems directly related to the KI state and disappears rapidly when a single-site fluctuation regime is recovered by heating [2]. Information on Q dependences has remained rather scarce, and to a large extent inconclusive, either because of complex anisotropy effects as in CeNiSn, or because measurements were carried out only on polycrystal samples. YbB 12 is a promising candidate for further investigations: it is an archetype KI compound [5] with a simple NaCl-type crystal structure (interpenetrating f cc sublattices of Yb ions and B 12 cuboctahedra), and previous inelastic neutron scattering (INS) experiments on powder [6,7,8] have indicated the presence of two narrow magnetic excitations near the spingap edge. Early single-crystal measurements [9] were interpreted in terms of a single dispersive low-energy mode with high intensity along [111], but the form of the Q dependence was not clearly established. In this Letter, we report a detailed investigation of the low-energy spin dynamics in YbB 12 showing that there indeed exist two distinct excitat...

Inelastic neutron scattering experiments have been performed on the archetype compound YbB(12), using neutron polarization analysis to separate the magnetic signal from the phonon background. With decreasing temperature, components characteristic for a single-site spin-fluctuation dynamics are suppressed, giving place to specific, strongly Q-dependent, low-energy excitations near the spin-gap edge. This crossover is discussed in terms of a simple crystal-field description of the incoherent high-temperature state and a predominantly local mechanism for the formation of the low-temperature singlet ground state.