952 research outputs found
A Formalism for Scattering of Complex Composite Structures. 2 Distributed Reference Points
Recently we developed a formalism for the scattering from linear and acyclic
branched structures build of mutually non-interacting sub-units.{[}C. Svaneborg
and J. S. Pedersen, J. Chem. Phys. 136, 104105 (2012){]} We assumed each
sub-unit has reference points associated with it. These are well defined
positions where sub-units can be linked together. In the present paper, we
generalize the formalism to the case where each reference point can represent a
distribution of potential link positions. We also present a generalized
diagrammatic representation of the formalism. Scattering expressions required
to model rods, polymers, loops, flat circular disks, rigid spheres and
cylinders are derived. and we use them to illustrate the formalism by deriving
the generic scattering expression for micelles and bottle brush structures and
show how the scattering is affected by different choices of potential link
positions.Comment: Paper no. 2 of a serie
Structural and magnetic properties of CoPt mixed clusters
In this present work, we report a structural and magnetic study of mixed
Co58Pt42 clusters. MgO, Nb and Si matrix can be used to embed clusters,
avoiding any magnetic interactions between particles. Transmission Electron
Microscopy (TEM) observations show that Co58Pt42 supported isolated clusters
are about 2nm in diameter and crystallized in the A1 fcc chemically disordered
phase. Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing
Incidence Wide Angle X-ray Scattering (GIWAXS) reveal that buried clusters
conserve these properties, interaction with matrix atoms being limited to their
first atomic layers. Considering that 60% of particle atoms are located at
surface, this interactions leads to a drastic change in magnetic properties
which were investigated with conventional magnetometry and X-Ray Magnetic
Circular Dichro\"{i}sm (XMCD). Magnetization and blocking temperature are
weaker for clusters embedded in Nb than in MgO, and totally vanish in silicon
as silicides are formed. Magnetic volume of clusters embedded in MgO is close
to the crystallized volume determined by GIWAXS experiments. Cluster can be
seen as a pure ferromagnetic CoPt crystallized core surrounded by a
cluster-matrix mixed shell. The outer shell plays a predominant role in
magnetic properties, especially for clusters embedded in niobium which have a
blocking temperature 3 times smaller than clusters embedded in MgO
Nanocrystallization and Amorphization Induced by Reactive Nitrogen Sputtering in Iron and Permalloy
Thin films of iron and permalloy Ni80Fe20 were prepared using an Ar+N2
mixture with magnetron sputtering technique at ambient temperature. The
nitrogen partial pressure, during sputtering process was varied in the range of
0 to 100%, keeping the total gas flow at constant. At lower nitrogen pressures
RN2<33% both Fe and NiFe, first form a nanocrystalline structure and an
increase in nitrogen partail pressure results in formation of an amorphous
structure. At intermediate nitrogen partial pressures, nitrides of Fe and NiFe
were obtained while at even higher nitrogen partial pressures, nitrides
themselves became nanocrystalline or amorphous. The surface, structural and
magnetic properties of the deposited films were studied using x-ray reflection
and diffraction, transmission electron microscopy, polarized neutron
reflectivity and using a DC extraction magnetometer. The growth behavior for
amorphous film was found different as compared with poly or nanocrystalline
films. The soft-magnetic properties of FeN were improved on nanocrystallization
while those of NiFeN were degraded. A mechanism inducing nanocrystallization
and amorphization in Fe and NiFe due to reactive nitrogen sputtering is
discussed in the present article.Comment: 13 Pages, 15 Figure
Characterization of the glass transition in vitreous silica by temperature scanning small-angle X-ray scattering
The temperature dependence of the x-ray scattering in the region below the
first sharp diffraction peak was measured for silica glasses with low and high
OH content (GE-124 and Corning 7980). Data were obtained upon scanning the
temperature at 10, 40 and 80 K/min between 400 K and 1820 K. The measurements
resolve, for the first time, the hysteresis between heating and cooling through
the glass transition for silica glass, and the data have a better signal to
noise ratio than previous light scattering and differential thermal analysis
data. For the glass with the higher hydroxyl concentration the glass transition
is broader and at a lower temperature. Fits of the data to the
Adam-Gibbs-Fulcher equation provide updated kinetic parameters for this very
strong glass. The temperature derivative of the observed X-ray scattering
matches that of light scattering to within 14%.Comment: EurophysicsLetters, in pres
Plasmonic Airy Beam on Metal Surface
Optical Airy beam, as a novel non-diffracting and self-accelerating wave
packet, has generated great enthusiasm since its first realization in 2007,
owing to its unique physics and exciting applications. Here, we report a new
form of this intriguing wave packet - plasmonic Airy beam, which is
experimentally realized on a silver surface for the first time. By particular
diffraction processes in a carefully designed nano-array structure, a novel
planar Airy beam of surface plasmon polariton (SPP) is directly generated and a
structural dependent phase tuning method is proposed to modulate the beam
properties. This SPP Airy beam is regarded as a two-dimensional (2D)
subwavelength counterpart of the 3D optical Airy beam in free space, allowing
for on-chip photonic manipulations. Moreover, it possibly suggests a
breakthrough in recognition of this unique wave packet in a polariton regime
after its previous evolution from free particle to pure optical wave
Not just fractal surfaces, but surface fractal aggregates: Derivation of the expression for the structure factor and its applications
Densely packed surface fractal aggregates form in systems with high local volume fractions of particles with very short diffusion lengths, which effectively means that particles have little space to move. However, there are no prior mathematical models, which would describe scattering from such surface fractal aggregates and which would allow the subdivision between inter- and intraparticle interferences of such aggregates. Here, we show that by including a form factor function of the primary particles building the aggregate, a finite size of the surface fractal interfacial sub-surfaces can be derived from a structure factor term. This formalism allows us to define both a finite specific surface area for fractal aggregates and the fraction of particle interfacial sub-surfaces at the perimeter of an aggregate. The derived surface fractal model is validated by comparing it with an ab initio approach that involves the generation of a "brick-in-a-wall" von Koch type contour fractals. Moreover, we show that this approach explains observed scattering intensities from in situ experiments that followed gypsum (CaSO4 · 2H2O) precipitation from highly supersaturated solutions. Our model of densely packed "brick-in-a-wall" surface fractal aggregates may well be the key precursor step in the formation of several types of mosaic- and meso-crystals
Small Angle Scattering by Fractal Aggregates: A Numerical Investigation of the Crossover Between the Fractal Regime and the Porod Regime
Fractal aggregates are built on a computer using off-lattice cluster-cluster
aggregation models. The aggregates are made of spherical particles of different
sizes distributed according to a Gaussian-like distribution characterised by a
mean and a standard deviation . The wave vector dependent
scattered intensity is computed in order to study the influence of the
particle polydispersity on the crossover between the fractal regime and the
Porod regime. It is shown that, given , the location of the
crossover decreases as increases. The dependence of on
can be understood from the evolution of the shape of the center-to-center
interparticle-distance distribution function.Comment: RevTex, 4 pages + 6 postscript figures, compressed using "uufiles",
published in Phys. Rev. B 50, 1305 (1994
Exact limiting relation between the structure factors in neutron and x-ray scattering
The ratio of the static matter structure factor measured in experiments on
coherent X-ray scattering to the static structure factor measured in
experiments on neutron scattering is considered. It is shown theoretically that
this ratio in the long-wavelength limit is equal to the nucleus charge at
arbitrary thermodynamic parameters of a pure substance (the system of nuclei
and electrons, where interaction between particles is pure Coulomb) in a
disordered equilibrium state. This result is the exact relation of the quantum
statistical mechanics. The experimental verification of this relation can be
done in the long wavelength X-ray and neutron experiments.Comment: 7 pages, no figure
Molecular dynamics simulations of lead clusters
Molecular dynamics simulations of nanometer-sized lead clusters have been
performed using the Lim, Ong and Ercolessi glue potential (Surf. Sci. {\bf
269/270}, 1109 (1992)). The binding energies of clusters forming crystalline
(fcc), decahedron and icosahedron structures are compared, showing that fcc
cuboctahedra are the most energetically favoured of these polyhedral model
structures. However, simulations of the freezing of liquid droplets produced a
characteristic form of ``shaved'' icosahedron, in which atoms are absent at the
edges and apexes of the polyhedron. This arrangement is energetically favoured
for 600-4000 atom clusters. Larger clusters favour crystalline structures.
Indeed, simulated freezing of a 6525-atom liquid droplet produced an imperfect
fcc Wulff particle, containing a number of parallel stacking faults. The
effects of temperature on the preferred structure of crystalline clusters below
the melting point have been considered. The implications of these results for
the interpretation of experimental data is discussed.Comment: 11 pages, 18 figues, new section added and one figure added, other
minor changes for publicatio
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