91 research outputs found
Resonant X-ray diffraction studies on the charge ordering in magnetite
Here we show that the low temperature phase of magnetite is associated with
an effective, although fractional, ordering of the charge. Evidence and a
quantitative evaluation of the atomic charges are achieved by using resonant
x-ray diffraction (RXD) experiments whose results are further analyzed with the
help of ab initio calculations of the scattering factors involved. By
confirming the results obtained from X-ray crystallography we have shown that
RXD is able to probe quantitatively the electronic structure in very complex
oxides, whose importance covers a wide domain of applications.Comment: 4 pages 4 figures, accepted for publication in PR
Revised structural phase diagram of (Ba0.7Ca0.3TiO3)-(BaZr0.2Ti0.8O3)
The temperature-composition phase diagram of barium calcium titanate zirconate (x(Ba0.7Ca0.3TiO3)(1-x)(BaZr0.2Ti0.8O3); BCTZ) has been reinvestigated using high-resolution synchrotron x-ray powder diffraction. Contrary to previous reports of an unusual rhombohedral-tetragonal phase transition in this system, we have observed an intermediate orthorhombic phase, isostructural to that present in the parent phase, BaTiO3, and we identify the previously assigned T-R transition as a T-O transition. We also observe the O-R transition coalescing with the previously observed triple point, forming a phase convergence region. The implication of the orthorhombic phase in reconciling the exceptional piezoelectric properties with the surrounding phase diagram is discussed
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
Step by step capping and strain state of GaN/AlN quantum dots studied by grazing incidence diffraction anomalous fine structure
The investigation of small size embedded nanostructures, by a combination of
complementary anomalous diffraction techniques, is reported. GaN Quantum Dots
(QDs), grown by molecular beam epitaxy in a modified Stranski-Krastanow mode,
are studied in terms of strain and local environment, as a function of the AlN
cap layer thickness, by means of grazing incidence anomalous diffraction. That
is, the X-ray photons energy is tuned across the Ga absorption K-edge which
makes diffraction chemically selective. Measurement of \textit{hkl}-scans,
close to the AlN (30-30) Bragg reflection, at several energies across the Ga
K-edge, allows the extraction of the Ga partial structure factor, from which
the in-plane strain of GaN QDs is deduced. From the fixed-Q energy-dependent
diffracted intensity spectra, measured for diffraction-selected iso-strain
regions corresponding to the average in-plane strain state of the QDs,
quantitative information regarding composition and the out-of-plane strain has
been obtained. We recover the in-plane and out-of-plane strains in the dots.
The comparison to the biaxial elastic strain in a pseudomorphic layer indicates
a tendency to an over-strained regime.Comment: submitted to PR
Detailed Structure of a CDW in a Quenched Random Field
Using high resolution x-ray scattering, we have measured the structure of the
Q_1 CDW in Ta-doped NbSe_3. Detailed line shape analysis of the data
demonstrates that two length scales are required to describe the phase-phase
correlation function. Phase fluctuations with wavelengths less than a new
length scale are suppressed and this is identified with the amplitude
coherence length. We find that xi_a* = 34.4 \pm 10.3 angstroms. Implications
for the physical mechanisms responsible for pinning are discussed.Comment: revtex 3.0, 3 postscript uuencoded figure
X-ray anomalous scattering investigations on the charge order in -NaVO
Anomalous x-ray diffraction studies show that the charge ordering in
-NaVO is of zig-zag type in all vanadium ladders. We
have found that there are two models of the stacking of layers along
\emph{c-}direction, each of them consisting of 2 degenerated patterns, and that
the experimental data is well reproduced if the 2 patterns appears
simultaneously. We believe that the low temperature structure contains stacking
faults separating regions corresponding to the four possible patterns.Comment: Submitted to Phys. Rev. Lett., 4 pages, 4 eps figures inserted in the
tex
Interplay of inequivalent atomic positions in resonant x-ray diffraction of Fe3BO6
'Forbidden' Bragg reflections of iron orthoborate Fe3BO6 were studied
theoretically and experimentally in the vicinity of the iron K edge. Their
energy spectra are explained as resulting from the interference of x-rays
scattered from two inequivalent crystallographic sites occupied by iron ions.
This particular structure property gives rise to complex azimuthal dependences
of the reflection intensities in the pre-edge region as they result from the
interplay of site specific dipole-quadrupole and quadrupole-quadrupole resonant
scattering. Also evidenced is an isotropic character of the absorption
spectrum. Self-absorption correction to the diffraction data, as well as
possible contributions of thermal vibrations and magnetic order, are discussed.
Particular care is given to extracting clean spectra from the data, and it is
demonstrated that excellent results can be obtained even from measurements that
appear corrupted by several effects such as poor crystal quality and multiple
scattering.Comment: 12 page
Synthesis, Characterization, and Finite Size Effects on Electrical Transport of Nanoribbons of the Charge-Density Wave Conductor NbSe3
NbSe3 exhibits remarkable anisotropy in most of its physical properties and
has been a model system for studies of quasi-one-dimensional
charge-density-wave (CDW) phenomena. Herein, we report the synthesis,
characterization, and electrical transport of single-crystalline NbSe3
nanoribbons by a facile one-step vapour transport process involving the
transport of selenium powder onto a niobium foil substrate. Our investigations
aid the understanding of the CDW nature of NbSe3 and the growth process of the
material. They also indicate that NbSe3 nanoribbons have enhanced CDW
properties compared to those of the bulk phase due to size confinement effects,
thus expanding the search for new mesoscopic phenomena at the nanoscale level.
Single nanoribbon measurements on the electrical resistance as a function of
temperature show charge-density wave transitions at 59 K and 141 K. We also
demonstrate significant enhancement in the depinning effect and sliding regimes
mainly attributed to finite size effects.Comment: Version accepted for publicatio
X-Ray Scattering Measurements of the Transient Structure of a Driven Charge-Density-Wave
We report time-resolved x-ray scattering measurements of the transient
structural response of the sliding {\bf Q} charge-density-wave (CDW) in
NbSe to a reversal of the driving electric field. The observed time scale
characterizing this response at 70K varies from 15 msec for driving
fields near threshold to 2 msec for fields well above threshold. The
position and time-dependent strain of the CDW is analyzed in terms of a
phenomenological equation of motion for the phase of the CDW order parameter.
The value of the damping constant, eV
seconds \AA, is in excellent agreement with the value
determined from transport measurements. As the driving field approaches
threshold from above, the line shape becomes bimodal, suggesting that the CDW
does not depin throughout the entire sample at one well-defined voltage.Comment: revtex 3.0, 7 figure
- …