343 research outputs found
Non-collinear long-range magnetic ordering in HgCr2S4
The low-temperature magnetic structure of \HG has been studied by
high-resolution powder neutron diffraction. Long-range incommensurate magnetic
order sets in at T22K with propagation vector
\textbf{k}=(0,0,0.18). On cooling below T, the propagation vector
increases and saturates at the commensurate value \textbf{k}=(0,0,0.25). The
magnetic structure below T consists of ferromagnetic layers in the
\textit{ab}-plane stacked in a spiral arrangement along the \textit{c}-axis.
Symmetry analysis using corepresentations theory reveals a point group symmetry
in the ordered magnetic phase of 422 (D), which is incompatible with
macroscopic ferroelectricity. This finding indicates that the spontaneous
electric polarization observed experimentally cannot be coupled to the magnetic
order parameter
Prolate and temperature-responsive self-assemblies of amphiphilic random copolymers with perfluoroalkyl and polyoxyethylene side chains in solution
Two amphiphilic random copolymers, PEGMAx-co-FAy (x = 90 and 70 mol%), were synthesized by ATRP and their solutions were investigated as a function of solvent, concentration and temperature by DLS and SANS analyses. Both copolymers self-assembled in nanostructures by single-chain folding in water solutions over a wide range of temperatures. The values of the DLS hydrodynamic radius and the SANS radius of gyration were found to be ~4 nm and ~3.4–3.7 nm, respectively. Moreover, SANS showed the self-folded nanoassemblies to be prolated spheroids with ratio of polar/equatorial axes ~5:1 for PEGMA90-co-FA10 and ~2:1 for PEGMA70-co-FA30. On heating above a critical temperature Tc, multi-chain microassemblies were formed that reverted back to nanoassemblies on cooling below Tc. This temperature-responsive transition was fully and sharply reversible
Inelastic neutron scattering studies of the quantum frustrated magnet clinoatacamite, -Cu2(OD)3Cl, a proposed valence bond solid (VBS)
The frustrated magnet clinoatacamite, -Cu(OH)Cl, is
attracting a lot of interest after suggestions that at low temperature it forms
an exotic quantum state termed a Valence Bond Solid (VBS) made from dimerised
Cu () spins.\cite{Lee_clinoatacamite} Key to the arguments
surrounding this proposal were suggestions that the kagom\'e planes in the
magnetic pyrochlore lattice of clinoatacamite are only weakly coupled, causing
the system to behave as a quasi-2-dimensional magnet. This was reasoned from
the near 95 angles made at the bridging oxygens that mediate exchange
between the Cu ions that link the kagom\'e planes.
Recent work pointed out that this exchange model is inappropriate for
-Cu(OH)Cl, where the oxygen is present as a
-OH.\cite{Wills_JPC} Further, it used symmetry calculations and neutron
powder diffraction to show that the low temperature magnetic structure (
K) was canted and involved significant spin ordering on all the Cu
spins, which is incompatible with the interpretation of simultaneous VBS and
N\'eel ordering. Correspondingly, clinoatacamite is best considered a distorted
pyrochlore magnet. In this report we show detailed inelastic neutron scattering
spectra and revisit the responses of this frustrated quantum magnet.Comment: Proceedings of The International Conference on Highly Frustrated
Magnetism 2008 (HFM2008
Stochastic Modelling Approach to the Incubation Time of Prionic Diseases
Transmissible spongiform encephalopathies like the bovine spongiform
encephalopathy (BSE) and the Creutzfeldt-Jakob disease (CJD) in humans are
neurodegenerative diseases for which prions are the attributed pathogenic
agents. A widely accepted theory assumes that prion replication is due to a
direct interaction between the pathologic (PrPsc) form and the host encoded
(PrPc) conformation, in a kind of an autocatalytic process. Here we show that
the overall features of the incubation time of prion diseases are readily
obtained if the prion reaction is described by a simple mean-field model. An
analytical expression for the incubation time distribution then follows by
associating the rate constant to a stochastic variable log normally
distributed. The incubation time distribution is then also shown to be log
normal and fits the observed BSE data very well. The basic ideas of the
theoretical model are then incorporated in a cellular automata model. The
computer simulation results yield the correct BSE incubation time distribution
at low densities of the host encoded protein
On the weak confinement of kinks in the one-dimensional quantum ferromagnet CoNb2O6
In a recent paper Coldea et al (2010 Science {\bf 327} 177) report
observation of the weak confinement of kinks in the Ising spin chain
ferromagnet CoNb2O6 at low temperatures. To interpret the entire spectra of
magnetic excitations measured via neutron scattering, they introduce a
phenomenological model, which takes into account only the two-kink
configurations of the spin chain. We present the exact solution of this model.
The explicit expressions for the two-kink bound-state energy spectra and for
the relative intensities of neutron scattering on these magnetic modes are
obtained in terms of the Bessel function.Comment: 18 pages, 9 figures; v2: figures 1,3,4 replaced, few misprints
correcte
Microbial nitrogen cycling on the Greenland Ice Sheet
Nitrogen inputs and microbial nitrogen cycling were investigated along a 79 km transect into the Greenland Ice Sheet (GrIS) during the main ablation season in summer 2010. The depletion of dissolved nitrate and production of ammonium (relative to icemelt) in cryoconite holes on Leverett Glacier, within 7.5 km of the ice sheet margin, suggested microbial uptake and ammonification respectively. Positive in situ acetylene assays indicated nitrogen fixation both in a debris-rich 100 m marginal zone and up to 5.7 km upslope on Leverett Glacier (with rates up to 16.3 μmoles C<sub>2</sub>H<sub>4</sub> m<sup>−2</sup> day<sup>−1</sup>). No positive acetylene assays were detected > 5.7 km into the ablation zone of the ice sheet. Potential nitrogen fixation only occurred when concentrations of dissolved and sediment-bound inorganic nitrogen were undetectable. Estimates of nitrogen fluxes onto the transect suggest that nitrogen fixation is likely of minor importance to the overall nitrogen budget of Leverett Glacier and of negligible importance to the nitrogen budget on the main ice sheet itself. Nitrogen fixation is however potentially important as a source of nitrogen to microbial communities in the debris-rich marginal zone close to the terminus of the glacier, where nitrogen fixation may aid the colonization of subglacial and moraine-derived debris
Spin dynamics in bulk MnNiGa and Mn1.4Pt0.9Pd0.1Sn investigated by muon spin relaxation
Martin Gleghorn
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X-Ray Analysis of Oxygen-induced Perpendicular Magnetic Anisotropy in Pt/Co/AlOx trilayer
X-ray spectroscopy measurements have been performed on a series of Pt/Co/AlOx
trilayers to investigate the role of Co oxidation in the perpendicular magnetic
anisotropy of the Co/AlOx interface. It is observed that high temperature
annealing modifies the magnetic properties of the Co layer, inducing an
enhancement of the perpendicular magnetic anisotropy. The microscopic
structural properties are analyzed via X-ray Absorption Spectroscopy, X-ray
Magnetic Circular Dichroism and X-ray Photoelectron Spectroscopy measurements.
It is shown that annealing enhances the amount of interfacial oxide, which may
be at the origin of a strong perpendicular magnetic anisotropy
Spring Thaw Ionic Pulses Boost Nutrient Availability and Microbial Growth in Entombed Antarctic Dry Valley Cryoconite Holes
The seasonal melting of ice entombed cryoconite holes on McMurdo Dry Valley glaciers provides oases for life in the harsh environmental conditions of the polar desert where surface air temperatures only occasionally exceed 0°C during the Austral summer. Here we follow temporal changes in cryoconite hole biogeochemistry on Canada Glacier from fully frozen conditions through the initial stages of spring thaw toward fully melted holes. The cryoconite holes had a mean isolation age from the glacial drainage system of 3.4 years, with an increasing mass of aqueous nutrients (dissolved organic carbon, total nitrogen, total phosphorus) with longer isolation age. During the initial melt there was a mean nine times enrichment in dissolved chloride relative to mean concentrations of the initial frozen holes indicative of an ionic pulse, with similar mean nine times enrichments in nitrite, ammonium, and dissolved organic matter. Nitrate was enriched twelve times and dissolved organic nitrogen six times, suggesting net nitrification, while lower enrichments for dissolved organic phosphorus and phosphate were consistent with net microbial phosphorus uptake. Rates of bacterial production were significantly elevated during the ionic pulse, likely due to the increased nutrient availability. There was no concomitant increase in photosynthesis rates, with a net depletion of dissolved inorganic carbon suggesting inorganic carbon limitation. Potential nitrogen fixation was detected in fully melted holes where it could be an important source of nitrogen to support microbial growth, but not during the ionic pulse where nitrogen availability was higher. This study demonstrates that ionic pulses significantly alter the timing and magnitude of microbial activity within entombed cryoconite holes, and adds credence to hypotheses that ionic enrichments during freeze-thaw can elevate rates of microbial growth and activity in other icy habitats, such as ice veins and subglacial regelation zones
Bundling up carbon nanotubes through Wigner defects
We show, using ab initio total energy density functional theory, that the
so-called Wigner defects, an interstitial carbon atom right besides a vacancy,
which are present in irradiated graphite can also exist in bundles of carbon
nanotubes. Due to the geometrical structure of a nanotube, however, this defect
has a rather low formation energy, lower than the vacancy itself, suggesting
that it may be one of the most important defects that are created after
electron or ion irradiation. Moreover, they form a strong link between the
nanotubes in bundles, increasing their shear modulus by a sizeable amount,
clearly indicating its importance for the mechanical properties of nanotube
bundles.Comment: 5 pages and 4 figure
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