Chalk River Laboratories

About: Chalk River Laboratories is a based out in . It is known for research contribution in the topics: Neutron diffraction & Neutron scattering. The organization has 2297 authors who have published 2700 publications receiving 73287 citations.

The structure of ammonium chloride by neutron diffraction

Abstract: From the intensities of powder diffraction lines the positions of the deuterons in N${\mathrm{D}}_{4}$Cl have been determined at liquid nitrogen and room temperatures. The experimental results confirm at low temperature the space group $T_{d}^{}{}_{}{}^{1}$ and the tetrahedral structure of the ammonium ion with an $N\ensuremath{-}D$ separation 1.03A. At room temperature the $T_{d}^{}{}_{}{}^{1}$ space group persists. Neither free rotation nor disordered orientations of the ammonium ions have been observed. Details concerning deuteron vibrations are discussed.

Spin dynamics near the critical doping in weakly superconducting underdoped Y Ba 2 Cu 3 O 6.35 ( T c = 18 K )

Abstract: From neutron inelastic and elastic scattering we have determined the magnetic structure and fluctuations in the $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.35}$ (YBCO) superconductor $({T}_{c}=18\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ near the boundary of the superconducting phase. The long-range ordered collinear spins of the insulating antiferromagnet are replaced by a commensurate central mode arising from slow, isotropically polarized, short-range spin correlations extending over four planar unit cells. The inelastic spectrum up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ is also broad in wave vector and commensurate. In contrast to the resonance peak of higher-${T}_{c}$ superconductors, the spins exhibit a single overdamped spectrum whose rate of relaxation $\ensuremath{\Gamma}$ decreases on cooling and saturates at $2\ensuremath{\Gamma}=5\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ below $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. As the relaxation rate saturates, the quasistatic spin correlations grow and become resolution limited in energy. The spin susceptibility above $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ follows the same $\ensuremath{\omega}∕T$ scaling relation found for the monolayer ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ system, indicating that the dominant energy scale is set by the temperature. Below $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the scale length is geometric and not linked by velocity to dynamic widths. Despite the large differences from an antiferromagnet, we show that integrated intensity conserves the total moment sum rule, and that on cooling the spectral weight transfers from the inelastic spin relaxation to the elastic central peak. There is no observable suppression of the spin fluctuations or central mode upon the onset of superconductivity. The spins respond not to coherent charge pairs but to hole doping, allowing coexistence of glassy short-range spin order with superconductivity. Since the physics of the weakly superconducting system ${\mathrm{YBCO}}_{6.35}$ must connect continuously with that in more strongly superconducting ${\mathrm{YBCO}}_{6.5}$, we find that neither incommensurate stripe-like spin modulations nor a well-defined neutron spin resonance are essential for the onset with doping of pairing in a high-temperature cuprate superconductor.

Asymmetric distribution of cholesterol in unilamellar vesicles of monounsaturated phospholipids.

Abstract: We have studied the effect of cholesterol on curved bilayers using 600 A unilamellar vesicles made of monounsaturated lipids. From small-angle X-ray scattering experiments we were able to detect an asymmetric distribution of lipid densities across certain bilayers. We discovered that, with the exception of diC24:1PC bilayers, monounsaturated diacylphosphatidylcholine lipids (diCn:1PC, n = 14, 16, 18, 20, and 22) form symmetric bilayers. However, the addition of 44 mol % cholesterol resulted in some of these bilayers (i.e., n = 14, 16, and 18) to become asymmetric, where cholesterol was found to distribute unequally between the bilayer’s two leaflets. This finding is potentially of relevance to biological membranes made up of different types of lipids and whose local curvature may be dictated by lipid composition.

Hierarchical, Self-Similar Structure in Native Squid Pen

Abstract: The structure of native squid pen (gladius) was investigated in two different species on different length scales. By combining microscopy, atomic force microscopy (AFM), and X-ray diffraction, the experiments probed length scales from millimetres down to nanometres. The gladii showed a hierarchical, self-similar structure in the optical experiments with fibres of different size oriented along the long axis of the gladius. The fibre-like structure was reproduced at the nanoscale in AFM measurements and fibres with diameters of 500 μm, 100 μm, 10 μm, 2 μm and 0.2 μm were observed. Their molecular structure was determined using X-ray diffraction. In the squid gladius, the chitin molecules are known to form nano-crystallites of monoclinic lattice symmetry wrapped in a protein layer, resulting in β-chitin nano-fibrils. Signals corresponding to the α-coil protein phase and β-chitin crystallites were observed in the X-ray experiments and their orientation with respect to the fibre-axis was determined. The size of a nano-fibril was estimated from the X-ray experiments to be about 150 × 300 A. About 100 of these nano-fibrils are needed to form a 0.2 μm thick micro-fibre. We found that the molecular structure is highly anisotropic with ∼90% of the α-coils and β-chitin crystallites oriented along the fibre-axis, indicating a strong correlation between the macroscale structure and molecular orientation.

Hydrogen bonding in ikaite, CaCO3.6H2O

Abstract: Ikaite is a metastable hexahydrate of calcium carbonate, forming in aqueous conditions near freezing conditions. Neutron- powder diffraction data of synthetic deuterated ikaite, collected at seven temperatures from 4 to 270 K, were refined. The linear thermal expansion coefficients are quite anisotropic, being smaller in the direction of the C-O bond. A review of the hydrogen-bonding scheme around the (CaCO 0 3 ) ion pair is given and an additional weak potential hydrogen bond is suggested. Temperature-dependent growth of the atomic displacement factors of the carbonate O1 atom agrees with the previous suggestion of a possible low-frequency, hindered librational mode of the carbonate group.