62 research outputs found
Persistence of characteristics of an ordered flux line lattice above the second peak in
We report Small Angle Neutron Scattering measurements of the flux lines
lattice (FLL) in . As previously reported, the
scattered intensity decreases strongly when the magnetic field is increased,
but it remains measurable far above the second peak. The direct observation of
Bragg peaks proves that the characteristics of a lattice are still present. No
structural features related to a symmetry breaking, such as a liquid like or an
amorphous state, can be observed. However, the associated scattered intensity
is very low and is difficult to explain. We discuss the coexistence between two
FLL states as a possible interpretation.Comment: accepted for publication in Phys Rev
Small angle neutron scattering study of the step-like magnetic transformation in Pr0.70Ca0.30MnO3
Small angle neutron scattering (SANS) magnetic and electrical transport
measurements were performed to study a single crystal of Pr0.7Ca0.3MnO3, a
colossal magnetoresistive (CMR) material. While the magnetic field induced
transformation of this phase separated compound consisting of an
antiferromagnetic insulating phase (AFI) and a ferromagnetic insulating phase
(FI), is continuous at high temperature (above 5K), at lower temperature a step
like transformation is observed (around 5T at 2K). Macroscopic magnetization
measurements and SANS indicate that this transformation occurs by the formation
of mesoscopic ferromagnetic metallic (FM) domains in the AFI phase, and,
eventually, in the FI phase. Although above 5K this transformation is
continuous, below 5K a magnetization step marks the abrupt transition from a
large scale FI/AFI phase separation to a large scale phase separation between
AFI, FI and FM phases. Our results suggest that relaxation of elastic strains
inherent to the coexistence of these different phases plays a crucial role in
the mechanism of these transformations. The occurrence of magnetization steps
could result from an intrinsic behavior of the AFI phase at low temperature
Microphase separation in Pr0.67Ca0.33MnO3 by small angle neutron scattering
We have evidenced by small angle neutron scattering at low temperature the
coexistence of ferromagnetism (F) and antiferromagnetism (AF) in
Pr0.67Ca0.33MnO3. The results are compared to those obtained in
Pr0.80Ca0.20MnO3 and Pr0.63Ca0.37MnO3, which are F and AF respectively.
Quantitative analysis shows that the small angle scattering is not due to a
mesoscopic mixing but to a nanoscopic electronic and magnetic ''red cabbage''
structure, in which the ferromagnetic phase exists in form of thin layers in
the AF matrix (stripes or 2D ''sheets'').Comment: 4 figure
Structure of the flux lines lattice in NbSe2: Equilibrium state and influence of the magnetic history
We have performed small-angle neutron scattering (SANS) of the flux line
lattice (FLL) in a Fe doped NbSe_2 sample which presents a large peak effect in
the critical current. The scattered intensity and the width of the Bragg peaks
of the equilibrium FLL indicate an ordered structure in the peak effect zone.
The history dependence in the FLL structure has been studied using field cooled
and zero field cooled procedures, and each state shows the same intensity of
Bragg scattering and good orientational order. These results strongly suggest
that the peak effect is unrelated to a bulk disordering transition, and confirm
the role of a heterogeneous distribution of screening current.Comment: accepted in Phys. Rev.
Field dependence of the electronic phase separation in Pr0.67Ca0.33MnO3 by small angle magnetic neutron scattering
We have studied by small angle neutron scattering the evolution induced by
the application of magnetic field of the coexistence of ferromagnetism (F) and
antiferromagnetism (AF) in a crystal of PrCaMnO. The
results are compared to magnetic measurements which provide the evolution of
the ferromagnetic fraction. These results show that the growth of the
ferromagnetic phase corresponds to an increase of the thickness of the
ferromagnetic ''cabbage'' sheets
Radial collapse of carbon nanotubes for conductivity optimized polymer composites
The optimization of the electronic conduction of carbon nanotube polymer composites is studied by tuning the radial geometry of the carbon nanotubes in a compression cycle. We have investigated the structural evolution of multi-walled carbon nanotubes in a polyamide matrix as a function of applied high pressure. Combining high resolution electron microscopy and small angle neutron scattering experiments, we conclude that the nanotube radial cross-section is irreversibly deformed following applied pressures up to 5 GPa. Studying highly percolated composites we observe that the sample resistivity drastically decreases with pressure up to about 2 GPa with no further change up to the maximum 5 GPa applied pressure. An important hysteresis is observed upon decompression which leads to an enhanced electrical conductivity of the composite in all the studied compression cycles with maximum pressures ranging from 1 to 5 GPa. Modelling the radial collapse of single-walled carbon nanotubes shows that the modified radial geometry can considerably improve the electronic transport properties in contacted carbon nanotube junctions. Our results open opportunities for engineering nanotube composites by controlling the radial collapse
Effect of Nanoparticle Size on the Morphology of Adsorbed Surfactant Layers
The surface aggregates structure of dimethyldodecylamine-N-oxide (C12DAO) in
three silica dispersions of different particle sizes (16 - 42 nm) was studied
by small-angle neutron scattering (SANS) in a H2O/D2O solvent mixture matching
the silica. At the experimental conditions (pH 9) the surfactant exists in its
nonionic form and the structure of the adsorbed layer is not affected by added
electrolyte. It is found that C12DAO forms spherical surface micelles of 2 nm
diameter on the 16 nm silica particles, but oblate ellipsoidal surface micelles
are formed on the 27 and 42 nm particles. The dimensions of these oblate
surface aggregates (minor and major semi-axes Rn and Rlat) are similar to those
of C12DAO micelles in the aqueous solutions. It is concluded that the
morphological transition from spherical to ellipsoidal surface aggregates is
induced by the surface curvature of the silica particles. A comparison of the
shape and dimensions of the surface aggregates formed by C12DAO and C12E5 on
the 16 nm silica particles demonstrates that the nature of the surfactant head
group does not determine the morphology of the surface aggregates, but has a
strong influence on the number of surface aggregates per particle, due to the
different interactions of the head groups with the silica surface
Distribution of Transport Current in a Type II Superconductor Studied by Small Angle Neutron Scattering
We report Small-Angle Neutron Scattering (SANS) measurements on the vortex
lattice in a PbIn polycrystal in the presence of an applied current. Using the
rocking curves as a probe of the distribution of current in the sample, we
observe that vortex pinning is due to the surface roughness. This leads to a
surface current that persists in the flux flow region. We show the influence of
surface treatments on the distribution of this current.Comment: 5 pages 5 figures. accepted for publication in Phys Rev Let
In vivo Bioluminescence Imaging of Ca(2+) Signalling in the Brain of Drosophila
Many different cells' signalling pathways are universally regulated by Ca(2+) concentration [Ca(2+)] rises that have highly variable amplitudes and kinetic properties. Optical imaging can provide the means to characterise both the temporal and spatial aspects of Ca(2+) signals involved in neurophysiological functions. New methods for in vivo imaging of Ca(2+) signalling in the brain of Drosophila are required for probing the different dynamic aspects of this system. In studies here, whole brain Ca(2+) imaging was performed on transgenic flies with targeted expression of the bioluminescent Ca(2+) reporter GFP-aequorin (GA) in different neural structures. A photon counting based technique was used to undertake continuous recordings of cytosolic [Ca(2+)] over hours. Time integrals for reconstructing images and analysis of the data were selected offline according to the signal intensity. This approach allowed a unique Ca(2+) response associated with cholinergic transmission to be identified by whole brain imaging of specific neural structures. Notably, [Ca(2+)] transients in the Mushroom Bodies (MBs) following nicotine stimulation were accompanied by a delayed secondary [Ca(2+)] rise (up to 15 min. later) in the MB lobes. The delayed response was sensitive to thapsigargin, suggesting a role for intra-cellular Ca(2+) stores. Moreover, it was reduced in dunce mutant flies, which are impaired in learning and memory. Bioluminescence imaging is therefore useful for studying Ca(2+) signalling pathways and for functional mapping of neurophysiological processes in the fly brain
Non-Invasive In Vivo Imaging of Calcium Signaling in Mice
Rapid and transient elevations of Ca2+ within cellular microdomains play a critical role in the regulation of many signal transduction pathways. Described here is a genetic approach for non-invasive detection of localized Ca2+ concentration ([Ca2+]) rises in live animals using bioluminescence imaging (BLI). Transgenic mice conditionally expressing the Ca2+-sensitive bioluminescent reporter GFP-aequorin targeted to the mitochondrial matrix were studied in several experimental paradigms. Rapid [Ca2+] rises inside the mitochondrial matrix could be readily detected during single-twitch muscle contractions. Whole body patterns of [Ca2+] were monitored in freely moving mice and during epileptic seizures. Furthermore, variations in mitochondrial [Ca2+] correlated to behavioral components of the sleep/wake cycle were observed during prolonged whole body recordings of newborn mice. This non-invasive imaging technique opens new avenues for the analysis of Ca2+ signaling whenever whole body information in freely moving animals is desired, in particular during behavioral and developmental studies
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