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.

Pressure-induced crystallization of a spin liquid

Abstract: Liquids are expected to crystallize at low temperature. The only exception is helium, which can remain liquid at 0 K, owing to quantum fluctuations1, 2. Similarly, the atomic magnetic moments (spins) in a magnet are expected to order at a temperature scale set by the Curie–Weiss temperature thetaCW (ref. 3). Geometrically frustrated magnets represent an exception. In these systems, the pairwise spin interactions cannot be simultaneously minimized because of the lattice symmetry4. This can stabilize a liquid-like state of short-range-ordered fluctuating moments well below thetaCW (refs 5–7). Here we use neutron scattering to observe the spin liquid state in a geometrically frustrated system, Tb2Ti2O7, under conditions of high pressure (approx9 GPa) and low temperature (approx1 K). This compound is a three-dimensional magnet with thetaCW = -19 K, where the negative value indicates antiferromagnetic interactions. At ambient pressure Tb2Ti2O7 remains in a spin liquid state down to at least 70 mK (ref. 8). But we find that, under high pressure, the spins start to order or 'crystallize' below 2.1 K, with antiferromagnetic order coexisting with liquid-like fluctuations. These results indicate that a spin liquid/solid mixture can be induced by pressure in geometrically frustrated systems.

Method for obtaining structure and interactions from oriented lipid bilayers

Abstract: Precise calculations are made of the scattering intensity I(q) from an oriented stack of lipid bilayers using a realistic model of fluctuations. The quantities of interest include the bilayer bending modulus Kc , the interbilayer interaction modulus B, and bilayer structure through the form factor F(qz). It is shown how Kc and B may be obtained from data at large qz where fluctuations dominate. Good estimates of F(qz) can be made over wide ranges of qz by using I(q )i nq regions away from the peaks and for qrfi0 where details of the scattering domains play little role. Rough estimates of domain sizes can also be made from smaller qz data. Results are presented for data taken on fully hydrated, oriented DOPC bilayers in the La phase. These results illustrate the advantages of oriented samples compared to powder samples.

Location of Cholesterol in DMPC Membranes. A Comparative Study by Neutron Diffraction and Molecular Mechanics Simulation

Abstract: The vertical location of 30 mol % cholesterol in a hydrated dimyristoylphosphatidylcholine (DMPC) membrane was determined by neutron diffraction on annealed samples containing deuterated or protonated cholesterol at 10, 20, 25, 30, and 50 °C. The sterol was deuterium-labeled in positions 2, 2, 3, 4, 4, and 6, and proton−deuterium contrast techniques were used to locate the position of the labeled part of the steroid in the membrane. Cholesterol is found well embedded in the membrane, with ring A at 16.3 ± 0.5 A from the bilayer center at 10 °C. This location linearly decreases to 15.1 ± 0.5 A at 50 °C, demonstrating that the sterol is not expelled from the membrane on crossing the former gel-to-fluid phase transition of pure DMPC (24 °C). Molecular dynamics were also performed on well-hydrated membranes in the presence and absence of cholesterol. Neutron scattering 1D profiles were then calculated for comparison with experimental neutron scattering data. The profile obtained from pure fluid-phase lipids i...