1,061 research outputs found
Multiple Time Scales in Diffraction Measurements of Diffusive Surface Relaxation
We grew SrTiO3 on SrTiO3 (001) by pulsed laser deposition, using x-ray
scattering to monitor the growth in real time. The time-resolved small angle
scattering exhibits a well-defined length scale associated with the spacing
between unit cell high surface features. This length scale imposes a discrete
spectrum of Fourier components and rate constants upon the diffusion equation
solution, evident in multiple exponential relaxation of the "anti-Bragg"
diffracted intensity. An Arrhenius analysis of measured rate constants confirms
that they originate from a single activation energy.Comment: 4 pages, 3 figure
Time-, Frequency-, and Wavevector-Resolved X-Ray Diffraction from Single Molecules
Using a quantum electrodynamic framework, we calculate the off-resonant
scattering of a broad-band X-ray pulse from a sample initially prepared in an
arbitrary superposition of electronic states. The signal consists of
single-particle (incoherent) and two-particle (coherent) contributions that
carry different particle form factors that involve different material
transitions. Single-molecule experiments involving incoherent scattering are
more influenced by inelastic processes compared to bulk measurements. The
conditions under which the technique directly measures charge densities (and
can be considered as diffraction) as opposed to correlation functions of the
charge-density are specified. The results are illustrated with time- and
wavevector-resolved signals from a single amino acid molecule (cysteine)
following an impulsive excitation by a stimulated X-ray Raman process resonant
with the sulfur K-edge. Our theory and simulations can guide future
experimental studies on the structures of nano-particles and proteins
Pair distribution function and structure factor of spherical particles
The availability of neutron spallation-source instruments that provide total
scattering powder diffraction has led to an increased application of real-space
structure analysis using the pair distribution function. Currently, the
analytical treatment of finite size effects within pair distribution refinement
procedures is limited. To that end, an envelope function is derived which
transforms the pair distribution function of an infinite solid into that of a
spherical particle with the same crystal structure. Distributions of particle
sizes are then considered, and the associated envelope function is used to
predict the particle size distribution of an experimental sample of gold
nanoparticles from its pair distribution function alone. Finally, complementing
the wealth of existing diffraction analysis, the peak broadening for the
structure factor of spherical particles, expressed as a convolution derived
from the envelope functions, is calculated exactly for all particle size
distributions considered, and peak maxima, offsets, and asymmetries are
discussed.Comment: 7 pages, 6 figure
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
A 4-unit-cell superstructure in optimally doped YBa2Cu3O6.92 superconductor
Using high-energy diffraction we show that a 4-unit-cell superstructure,
q0=(1/4,0,0), along the shorter Cu-Cu bonds coexists with superconductivity in
optimally doped YBCO. A complex set of anisotropic atomic displacements on
neighboring CuO chain planes, BaO planes, and CuO2 planes, respectively,
correlated over ~3-6 unit cells gives rise to diffuse superlattice peaks. Our
observations are consistent with the presence of Ortho-IV nanodomains
containing these displacements.Comment: Corrected typo in abstrac
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
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
Orbital Ordering Structures in (Nd,Pr)0.5Sr0.5MnO3 Manganite Thin Films on Perovskite (011) Substrates
Structural study of orbital-ordered manganite thin films has been conducted
using synchrotron radiation, and a ground state electronic phase diagram is
made. The lattice parameters of four manganite thin films, Nd0.5Sr0.5MnO3
(NSMO) or Pr0.5Sr0.5MnO3 (PSMO) on (011) surfaces of SrTiO3 (STO) or
[(LaAlO3){0.3}(SrAl0.5Ta0.5O3){0.7}] (LSAT), were measured as a function of
temperature. The result shows, as expected based on previous knowledge of bulk
materials, that the films' resistivity is closely related to their structures.
Observed superlattice reflections indicate that NSMO thin films have an
antiferro-orbital-ordered phase as their low-temperature phase while PSMO film
on LSAT has a ferro-orbital-ordered phase, and that on STO has no
orbital-ordered phase. A metallic ground state was observed only in films
having a narrow region of A-site ion radius, while larger ions favor
ferro-orbital-ordered structure and smaller ions stabilize
antiferro-orbital-ordered structure. The key to the orbital-ordering transition
in (011) film is found to be the in-plane displacement along [0-1 1] direction.Comment: 19pages, 11 figure
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