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News Article | December 20, 2016
Site: phys.org

The material, a formulation of iron, sodium, copper and arsenic created by Rice graduate student Yu Song in the laboratory of physicist Pengcheng Dai, is described this week in the journal Nature Communications. Dai said Song's recipe—which involves mixing ingredients in a pure argon atmosphere, sealing them in niobium canisters and baking them at nearly 1,000 degrees Celsius—produces a layered alloy in which iron and copper separate into alternating stripes. This striping is critical for the material's usefulness in explaining the origins of high-temperature superconductivity, said RCQM Director Qimiao Si. "By forming this regular pattern, Yu Song has physically removed disorder from the system, and that is crucially important for being able to say something meaningful about what's going on electronically," said Si, a theoretical physicist who has worked to explain the origins of high-temperature superconductivity and similar phenomena for nearly two decades. High-temperature superconductivity was discovered in 1986. It occurs when electrons pair up and flow freely in layered alloys like Song's new creation. Dozens of high-temperature superconducting alloys have been created. Most are complex crystals that contain a transition metal—typically iron or copper—and other elements. High-temperature superconductors are typically terrible conductors at room temperature and only become superconductors when they are cooled to a critical temperature. "The central problem of high-temperature superconductivity is to understand the precise relationship between these two fundamental states of matter and the phase transition between them," said Dai, professor of physics and astronomy at Rice. "The macroscopic change is evident, but the microscopic origins of the behavior are open to interpretation, largely because there are many variables in play, and the relationship between them is both synergistic and nonlinear." Dai said two schools of thought "developed from the very beginning of this field. One was the itinerant camp, which argues that both states ultimately arise from itinerant electrons. After all, these materials are metals, even if they may be poor metals." The other camp is the localized camp, which argues that fundamentally new physics arise—due to electron-electron interactions—at the critical point at which the materials transition from one phase to the other. Dai said measurements on Song's new material support the localized theory. In particular, the new material is the first member of a class of iron-based superconductors called pnictides (pronounced NIK-tides) that can be tuned between two competing phases: the superconducting phase in which electrons flow with no resistance, and a "Mott insulating" phase in which electrons become locked in place and do not flow at all. "The discovery that Yu Song made is that this material is more correlated, which is evident because of the Mott insulating phase," Dai said. "This is the first time anyone has reported an iron-based superconductor that can be continuously tuned from the superconducting phase to the Mott insulating phase." Samples were made and some tests were performed at RCQM. Additional tests were performed at Chalk River Laboratories' Canadian Neutron Beam Center in Ontario, the National Institute for Standards and Technology's Center for Neutron Research in Maryland, Brookhaven National Laboratory in New York, Oak Ridge National Laboratory's High Flux Isotope Reactor in Tennessee and the Paul Scherrer Institute's Advanced Resonant Spectroscopies beamline in Switzerland. "In the paper, we showed that if the interaction was weak, then even replacing 50 percent of the iron with copper would still not be sufficient to produce the insulating state," Si said. "The fact that our experimentalists have managed to turn the system to be Mott insulating therefore provides direct evidence for strong electron-electron interactions in iron pnictides. That is an important step forward because it suggests that superconductivity should be tied up with these strong electron correlations." Explore further: Material turns 'schizophrenic' on way to superconductivity More information: Yu Song et al. A Mott insulator continuously connected to iron pnictide superconductors, Nature Communications (2016). DOI: 10.1038/ncomms13879


Drolle E.,University of Waterloo | Kucerka N.,Canadian Neutron Beam Center | Kucerka N.,Comenius University | Hoopes M.I.,University of Waterloo | And 5 more authors.
Biochimica et Biophysica Acta - Biomembranes | Year: 2013

The cell membrane plays an important role in the molecular mechanism of amyloid toxicity associated with Alzheimer's disease. The membrane's chemical composition and the incorporation of small molecules, such as melatonin and cholesterol, can alter its structure and physical properties, thereby affecting its interaction with amyloid peptides. Both melatonin and cholesterol have been recently linked to amyloid toxicity. Melatonin has been shown to have a protective role against amyloid toxicity. However, the underlying molecular mechanism of this protection is still not well understood, and cholesterol's role remains controversial. We used small-angle neutron diffraction (SAND) from oriented lipid multi-layers, small-angle neutron scattering (SANS) from unilamellar vesicles experiments and Molecular Dynamics (MD) simulations to elucidate non-specific interactions of melatonin and cholesterol with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dipalmitoyl-sn-glycero- 3-phosphocholine (DPPC) model membranes. We conclude that melatonin decreases the thickness of both model membranes by disordering the lipid hydrocarbon chains, thus increasing membrane fluidity. This result is in stark contrast to the much accepted ordering effect induced by cholesterol, which causes membranes to thicken. © 2013 Elsevier B.V.


Alsop R.J.,McMaster University | Toppozini L.,McMaster University | Marquardt D.,Brock University | Kucerka N.,Canadian Neutron Beam Center | And 5 more authors.
Biochimica et Biophysica Acta - Biomembranes | Year: 2015

Aspirin and other non-steroidal anti-inflammatory drugs have a high affinity for phospholipid membranes, altering their structure and biophysical properties. Aspirin has been shown to partition into the lipid head groups, thereby increasing membrane fluidity. Cholesterol is another well known mediator of membrane fluidity, in turn increasing membrane stiffness. As well, cholesterol is believed to distribute unevenly within lipid membranes leading to the formation of lipid rafts or plaques. In many studies, aspirin has increased positive outcomes for patients with high cholesterol. We are interested if these effects may be, at least partially, the result of a non-specific interaction between aspirin and cholesterol in lipid membranes. We have studied the effect of aspirin on the organization of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) membranes containing cholesterol. Through Langmuir-Blodgett experiments we show that aspirin increases the area per lipid and decreases compressibility at 32.5 mol% cholesterol, leading to a significant increase of fluidity of the membranes. Differential scanning calorimetry provides evidence for the formation of meta-stable structures in the presence of aspirin. The molecular organization of lipids, cholesterol and aspirin was studied using neutron diffraction. While the formation of rafts has been reported in binary DPPC/cholesterol membranes, aspirin was found to locally disrupt membrane organization and lead to the frustration of raft formation. Our results suggest that aspirin is able to directly oppose the formation of cholesterol structures through non-specific interactions with lipid membranes. © 2014 Elsevier B.V. All rights reserved.


Sediako D.G.,Canadian Neutron Beam Center | Kasprzak W.,Natural Resources Canada
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2015

Understanding of the kinetics of solid-phase evolution in solidification of hypereutectic aluminum alloys is a key to control their as-cast microstructure and resultant mechanical properties, and in turn, to enhance the service characteristics of actual components. This study was performed to evaluate the solidification kinetics for three P-modified hypereutectic Al-19 pct Si alloys: namely, Al-Si binary alloy and with the subsequent addition of 2.8 pct Cu and 2.8 pct Cu + 0.7 pct Mg. Metallurgical evaluation included thermodynamic calculations of the solidification process using the FactSage™ 6.2 software package, as well as experimental thermal analysis, and in situ neutron diffraction. The study revealed kinetics of solid α-Al, solid Si, Al2Cu, and Mg2Si evolution, as well as the individual effects of Cu and Mg alloying additions on the solidification path of the Al-Si system. Various techniques applied in this study resulted in some discrepancies in the results. For example, the FactSage computations, in general, resulted in 281 K to 286 K (8 °C to 13 °C) higher Al-Si eutectic temperatures than the ones recorded in the thermal analysis, which are also ~278 K (~5 °C) higher than those observed in the in situ neutron diffraction. None of the techniques can provide a definite value for the solidus temperature, as this is affected by the chosen calculation path [283 K to 303 K (10 °C to 30 °C) higher for equilibrium solidification vs non-equilibrium] for the FactSage analysis; and further complicated by evolution of secondary Al-Cu and Mg-Si phases that commenced at the end of solidification. An explanation of the discrepancies observed and complications associated with every technique applied is offered in the paper. © 2015, Published with permission of Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources.


Ramezanipour F.,McMaster University | Greedan J.E.,McMaster University | Siewenie J.,Los Alamos National Laboratory | Proffen T.,Los Alamos National Laboratory | And 3 more authors.
Inorganic Chemistry | Year: 2011

Sr2FeMnO5+y was synthesized under two different conditions, in air and in argon, both of which resulted in a cubic, Pm3̄m, structure with no long-range ordering of oxygen vacancies. The unit cell constants were found to be a0 = 3.89328(1) Å for argon (y = 0.0) and a0 = 3.83075(3) Å for air (y = 0.5). In contrast, Ca2FeMnO5 retains long-range brownmillerite oxygen vacancy ordering for either air or argon synthesis. Remarkably, Sr 2FeMnO5.0 oxidizes spontaneously in air at room temperature. A neutron pair distribution function (NPDF) study of Sr 2FeMnO5.0(Ar) showed evidence for local, brownmillerite-like ordering of oxygen vacancies for short distances up to 5 Å. Mössbauer spectroscopy results indicate more than one Fe site for Sr2FeMnO5+y(Ar and air), consistent with the noncubic local structure found by NPDF analysis. The isomer shifts and quadrupole splittings in both air- and argon-synthesized materials are consistent with the 3+ oxidation state for Fe in sites with coordination number four or five. This is confirmed by an L-edge XANES study. Mn is almost entirely in the 3+ state for Sr2FeMnO5.0(Ar), whereas Mn4+ is predominantly present for Sr2FeMnO5.5(air). Magnetic susceptibility data show zero-field-cooled/field-cooled (ZFC/FC) divergences near 50 K for the Ar sample and 25 K for the air sample, whereas Ca2FeMnO5 is long-range G-type antiferromagnetically ordered at 407(2) K. Hyperfine magnetic splitting, observed in temperature-dependent Mössbauer measurements, indicates short-range magnetic correlations that persist up to 150 K for Sr2FeMnO5.0(Ar) and 100 K for Sr 2FeMnO5.5(air), well above the ZFC/FC divergence temperatures. Neutron diffraction data confirm the absence of long-range magnetic ordering at room temperature and 4 K for Sr2FeMnO 5.0(Ar) but indicate the presence of domains with short-range G-type order at 4 K with an average dimension of ∼50 Å (y = 0); thus, this material is actually a superparamagnet rather than a true spin glass. In sharp contrast, corresponding data for Sr2FeMnO5.5(air) show mainly a very weak magnetic Bragg peak, indicating that ∼4% of the sample has G-type antiferromagnetic ordering at 4 K. © 2011 American Chemical Society.


Toppozini L.,McMaster University | Meinhardt S.,Johannes Gutenberg University Mainz | Armstrong C.L.,McMaster University | Yamani Z.,Canadian Neutron Beam Center | And 5 more authors.
Physical Review Letters | Year: 2014

Rafts, or functional domains, are transient nano-or mesoscopic structures in the plasma membrane and are thought to be essential for many cellular processes such as signal transduction, adhesion, trafficking, and lipid or protein sorting. Observations of these membrane heterogeneities have proven challenging, as they are thought to be both small and short lived. With a combination of coarse-grained molecular dynamics simulations and neutron diffraction using deuterium labeled cholesterol molecules, we observe raftlike structures and determine the ordering of the cholesterol molecules in binary cholesterol-containing lipid membranes. From coarse-grained computer simulations, heterogenous membranes structures were observed and characterized as small, ordered domains. Neutron diffraction was used to study the lateral structure of the cholesterol molecules. We find pairs of strongly bound cholesterol molecules in the liquid-disordered phase, in accordance with the umbrella model. Bragg peaks corresponding to ordering of the cholesterol molecules in the raftlike structures were observed and indexed by two different structures: a monoclinic structure of ordered cholesterol pairs of alternating direction in equilibrium with cholesterol plaques, i.e., triclinic cholesterol bilayers. © 2014 American Physical Society.


Tomlinson P.,University of British Columbia | Azizi-Alizamini H.,University of British Columbia | Poole W.J.,University of British Columbia | Sinclair C.W.,University of British Columbia | Gharghouri M.A.,Canadian Neutron Beam Center
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2013

The multiaxial deformation of magnesium alloys is important for developing reliable, robust models for both the forming of components and also analysis of in-service performance of structures, for example, in the case of crash worthiness. The current study presents a combination of unique biaxial experimental tests and biaxial crystal plasticity simulations using a visco-plastic self-consistent (VPSC) formulation conducted on a relatively weak AZ80 cast texture. The experiments were conducted on tubular samples which are loaded in axial tension or compression along the tube and with internal pressure to generate hoop stresses orthogonal to the axial direction. The results were analyzed in stress and strain space and also in terms of the evolution of crystallographic texture. In general, it was found that the VPSC simulations matched well with the experiments. However, some differences were observed for cases where basal 〈a〉 slip and {101̄2} extension twinning were in close competition such as in the biaxial tension quadrant of the plastic potential. The evolution of texture measured experimentally and predicted from the VPSC simulations was qualitatively in good agreement. Finally, experiments and VPSC simulations were conducted on a second AZ80 material which had a stronger initial texture and a higher level of mechanical anisotropy. In the previous case, the agreement between experiments and simulations was good, but a larger difference was observed in the biaxial tension quadrant of the plastic potential. © The Minerals, Metals & Materials Society and ASM International 2013.


Yang F.-C.,McMaster University | Peters R.D.,McMaster University | Dies H.,McMaster University | Rheinstadter M.C.,McMaster University | Rheinstadter M.C.,Canadian Neutron Beam Center
Soft Matter | Year: 2014

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 Å. 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. This journal is © the Partner Organisations 2014.


Zhu Z.,Ecole Polytechnique de Montréal | Gharghouri M.A.,Canadian Neutron Beam Center | Pelton A.D.,Ecole Polytechnique de Montréal
Journal of Chemical Thermodynamics | Year: 2016

All available phase diagram data for the (Nd + Mg + Zn) system were critically assessed. In-situ neutron diffraction (ND) experiments were performed on selected samples to identify phases and transition temperatures. A critical thermodynamic evaluation and optimization of the (Nd + Mg + Zn) system was carried out and model parameters for the thermodynamic properties of all phases were obtained. The phase transformation behaviour of selected samples was well resolved from the ND experiments and experimental values were used to refine the thermodynamic model parameters. © 2015 Elsevier Inc. All rights reserved.


Elsayed A.,Ryerson University | Sediako D.,Canadian Neutron Beam Center | Ravindran C.,Ryerson University
Canadian Metallurgical Quarterly | Year: 2015

In situ neutron diffraction was used to examine the solidification behaviour of Mg-6 wt-%Al and Mg-9 wt-%Al alloys. Samples of each Mg-Al alloy were heated above their liquidus temperatures and stepwise cooled to 420°C while simultaneously collecting neutron scattering intensities. The solidified alloys were examined using scanning electron microscopy. Mainly blocky Mg17Al12 was found in Mg-6 wt-%Al alloy while branched Mg17Al12 adjacent to a large network of fine lamellar Mg17Al12 was found in the Mg-9 wt-%Al alloy. The neutron diffraction data accurately described the fraction solid growth as represented by the (10-11) crystallographic plane over the entire solidification regime. The fraction solid of the Mg-6 wt-%Al alloy rose quickly at temperatures just below the liquidus point and rapidly approached 100% until solidification was complete while the Mg-9 wt-%Al alloy showed a more linear transition from liquid to solid. Neutron diffraction was also capable of detecting the formation of eutectic Mg17Al12 in the Mg-9 wt-%Al alloy. This research demonstrates unique possibilities in using neutron diffraction for further understanding of nucleation, eutectic formation and solid phase evolution of Mg alloys. © 2015 Canadian Institute of Mining, Metallurgy and Petroleum Published by Maney on behalf of the Institute.

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