Neutron scattering measurements of the magnetic excitations in single crystals of antiferromagnetic CaFe2As2 reveal steeply dispersive and well-defined spin waves up to an energy of approximately 100 meV. Magnetic excitations above 100 meV and up to the maximum energy of 200 meV are however broader in energy and momentum than the experimental resolution. While the low energy modes can be fit to a Heisenberg model, the total spectrum cannot be described as arising from excitations of a local moment system. Ab initio calculations of the dynamic magnetic susceptibility suggest that the high energy behavior is dominated by the damping of spin waves by particle-hole excitations.

Magnetic correlations in the paramagnetic phase of CaFe 2 As 2 ͑T N = 172 K͒ have been examined by means of inelastic neutron scattering from 180 K ͑ϳ1.05T N ͒ up to 300 K ͑1.8T N ͒. Despite the first-order nature of the magnetic ordering, strong but short-ranged antiferromagnetic ͑AFM͒ correlations are clearly observed. These correlations, which consist of quasielastic scattering centered at the wave vector Q AFM of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer ͑ប Ͼ 60 meV͒. The L dependence of the scattering implies rather weak interlayer coupling in the tetragonal c direction corresponding to nearly two-dimensional fluctuations in the ͑ab͒ plane. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. Similar to the cobalt-doped superconducting BaFe 2 As 2 compounds, these experimental features can be adequately reproduced by a scattering model that describes short-ranged and anisotropic spin correlations with overdamped dynamics.

Inelastic neutron scattering measurements on the low energy spin waves in CaFe 2 As 2 show that the magnetic exchange interactions in the Fe layers are exceptionally large and similar to the cuprates. However, the exchange between layers is ~10% of the coupling in the layers and the magnetism is more appropriately categorized as anisotropic threedimensional, in contrast to the two-dimensional cuprates. Band structure calculations of the spin dynamics and magnetic exchange interactions are in good agreement with the experimental data. PACS: 75.30.Ds, 78.70.Nx, 75.30.Et, However, the parent phases of the iron-arsenides are not insulators. Rather, they are metallic and, for the AFe 2 As 2 compounds, the AF ordering is strongly coupled to a structural transition from a high-temperature tetragonal structure to a low temperature orthorhombic structure.[12] One other notable difference between the cuprates and iron arsenides concerns the conditions necessary for SC. While doping charge carriers does indeed suppress AF and lead to superconductivity in both systems, it has recently been shown that pressure alone can destroy the AF state in CaFe 2 As 2 and lead to SC. [13,14] 3 Despite these differences, the energy scale and dimensionality (or anisotropy) of the magnetic interactions may actually be quite similar, possibly leading to a common origin for SC in these two families of compounds. In order to move beyond qualitative comparisons and address the relevance of magnetic interactions to SC in the ironarsenides, direct measurements of the energy scale and anisotropy of the magnetic interactions are necessary. Here we report results from inelastic neutron scattering from CaFe 2 As 2 , both below and above the AF ordering temperature, and demonstrate that the magnetic exchange interactions are exceptionally large, with a similar energy scale as the cuprates. Although the magnetic exchange between the Fe layers is relatively small (> ~10% of the in-plane exchange), it is substantially larger than that found for the cuprates (~0.001%). This anisotropic 3D magnetism is supported by theoretical calculations of the spin dynamics. Despite the first-order magnetostructural transition observed in CaFe 2 As 2 , spin correlations are observed to persist above the AF ordering temperature, attesting to the strength of the magnetism and supportive of a model of frustrated magnetism in the high-temperature tetragonal phase.CaFe 2 As 2 is a non-superconducting parent compound that becomes superconducting by either doping [15] or the application of pressure.[14] CaFe 2 As 2 orders into a columnartype AF structure (as shown in Fig 1a)) with a simultaneous structural transition from a tetragonal (I4/mmm) to an orthorhombic (Fmmm) crystal structure below T s = 172 K with a = 5.51 Å, b = 5.45 Å, and c = 11.66 Å.[12] For the inelastic neutron scattering study, single crystals of CaFe 2 As 2 were grown out of Sn flux using conventional high temperature solution growth techniques described previously. scattering plane (in orthorhombic...

Phytoglycogen is a naturally occurring polysaccharide nanoparticle made up of extensively branched glucose monomers. It has a number of unusual and advantageous properties, such as high water retention, low viscosity, and high stability in water, which make this biomaterial a promising candidate for a wide variety of applications. In this study, we have characterized the structure and hydration of aqueous dispersions of phytoglycogen nanoparticles using neutron scattering. Small angle neutron scattering results suggest that the phytoglycogen nanoparticles behave similar to hard sphere colloids and are hydrated by a large number of water molecules (each nanoparticle contains between 250% and 285% of its mass in water). This suggests that phytoglycogen is an ideal sample in which to study the dynamics of hydration water. To this end, we used quasielastic neutron scattering (QENS) to provide an independent and consistent measure of the hydration number, and to estimate the retardation factor (or degree of water slow-down) for hydration water translational motions. These data demonstrate a length-scale dependence in the measured retardation factors that clarifies the origin of discrepancies between retardation factor values reported for hydration water using different experimental techniques. The present approach can be generalized to other systems containing nanoconfined water.