517 research outputs found
Nuclear forward scattering in particulate matter: dependence of lineshape on particle size distribution
In synchrotron Moessbauer spectroscopy, the nuclear exciton polariton
manifests itself in the lineshape of the spectra of nuclear forward scattering
(NFS) Fourier-transformed from time domain to frequency domain. This lineshape
is generally described by the convolution of two intensity factors. One of them
is Lorentzian related to free decay. We derived the expressions for the second
factor related to Frenkel exciton polariton effects at propagation of
synchrotron radiation in Moessbauer media. Parameters of this Frenkelian shape
depend on the spatial configuration of Moessbauer media. In a layer of uniform
thickness, this factor is found to be a simple hypergeometric function. Next,
we consider the particles spread over a 2D surface or diluted in non-Moessbauer
media to exclude an overlap of ray shadows by different particles. Deconvolving
the purely polaritonic component of linewidths is suggested as a simple
procedure sharpening the experimental NFS spectra in frequency domain. The
lineshapes in these sharpened spectra are theoretically expressed via the
parameters of the particle size distributions (PSD). Then, these parameters are
determined through least-squares fitting of the line shapes.Comment: 13 pages, 12 figure
Anisotropic phonon DOS: the application of Rietveld and Mossbauer texture analysis in aligned powders
While synthesizing the single crystals of novel materials is not always
feasible, orienting the layered polycrystals becomes an attractive method in
the studies of angular dependencies of inelastic scattering of x-rays or
neutrons. Putting in use the Rietveld analysis of layered structures in novel
manganites and cuprates we develop the studies of their anisotropic properties
with oriented powders instead of single crystals. Densities of phonon states
(DOS) and atomic thermal displacememts (ATD) are anisotropic in the A-site
ordered manganites LnBaMn2Oy of both y=5 and y=6 series (Ln=Y, La, Sm, Gd). We
establish the angular dependence of DOS on textures of arbitrary strengths,
link the textures observed by x-ray and gamma-ray techniques, and solve the
problem of disentanglement of Goldanskii-Karyagin effect (GKE) and texture in
Moessbauer spectra.Comment: 6 pages; 5 figures, revise
Dispersion of the odd magnetic resonant mode in near-optimally doped Bi2Sr2CaCu2O8+d
We report a neutron scattering study of the spin excitation spectrum in the
superconducting state of slightly overdoped Bi2Sr2CaCu2O8+d system (Tc=87 K).
We focus on the dispersion of the resonance peak in the superconducting state
that is due to a S=1 collective mode. The measured spin excitation spectrum
bears a strong similarity to the spectrum of the YBa2Cu3O6+x system for a
similar doping level i.e. x= 0.95-1), which consists of intersecting upward-
and downward-dispersing branches. A close comparison of the threshold of the
electron-hole spin flip continuum, deduced from angle resolved photo-emission
measurements in the same system, indicates that the magnetic response in the
superconducting state is confined, in both energy and momentum, below the
gapped Stoner continuum. In contrast to YBa2Cu3O6+x, the spin excitation
spectrum is broader than the experimental resolution. In the framework of an
itinerant-electron model, we quantitatively relate this intrinsic energy width
to the superconducting gap distribution observed in scanning tunnelling
microscopy experiments. Our study further suggests a significant in-plane
anisotropy of the magnetic response.Comment: 10 figure
Anomalously large oxygen-ordering contribution to the thermal expansion of untwinned YBa2Cu3O6.95 single crystals: a glass-like transition near room temperature
We present high-resolution capacitance dilatometry studies from 5 - 500 K of
untwinned YBa2Cu3Ox (Y123) single crystals for x ~ 6.95 and x = 7.0. Large
contributions to the thermal expansivities due to O-ordering are found for x ~
6.95, which disappear below a kinetic glass-like transition near room
temperature. The kinetics at this glass transition is governed by an energy
barrier of 0.98 +- 0.07 eV, in very good agreement with other O-ordering
studies. Using thermodynamic arguments, we show that O-ordering in the Y123
system is particularly sensitive to uniaxial pressure (stress) along the chain
axis and that the lack of well-ordered chains in Nd123 and La123 is most likely
a consequence of a chemical-pressure effect.Comment: 4 pages, 3 figures, submitted to PR
Condensation of a tetrahedra rigid-body libration mode in HoBaCo4O7 : the origin of phase transition at 355 K
Rietveld profiles, Moessbauer spectra and x-ray absorption fine structure
(XAFS) were analyzed through the structural phase transition at Ts = 355 K in
HoBaCo4O7. Excess of the oxygen content over O7 was avoided via annealing the
samples in argon flow at 600 degree C. Space groups (S.G.) Pbn21c and P63mc
were used to refine the structure parameters in the low- and high-temperature
phases, respectively. Additionally, the Cmc21 symmetry was considered as a
concurrent model of structure of the low-temperature phase. In the
high-temperature phase, severe anisotropy of thermal motion of the major part
of the oxygen atoms was observed. This anisotropic motion turns to be quenched
as the sample is cooled below Ts. The variation of quadrupole splitting near Ts
is not similar to a steplike anomaly frequently seen at the charge-ordering
transition. We observe instead a dip-like anomaly of the average quadrupole
splitting near Ts. Narrow distribution of the electric field gradient (EFG)
over different cobalt sites is observed and explained on the basis of
point-charge model. XAFS spectra show no evidence of significant difference
between YBaCo4O7 (T > Ts) and HoBaCo4O7 (T < Ts). The origin of the transition
at Ts is ascribed to the condensation of the libration phonon mode associated
with the rigid-body rotational movements of the starlike tetrahedral units, the
building blocks of kagome network. It is shown that the condensation of the
libration mode is not compatible with translation symmetry for the hexagonal
S.G., but compatible for the orthorhombic S.G. The orthorhombic lattice
parameters and EFG components (Vxx, Vyy, Vzz) vary smoothly with temperature at
approaching Ts and closely follow each other.Comment: 13 figure
The design of a randomised controlled trial to evaluate the (cost-) effectiveness of the posterolateral versus the direct anterior approach for THA (POLADA - trial)
Background: Total hip arthroplasty (THA) is one of the most successful orthopaedic procedures. Because of the increasing number of THAs, a growing demand for faster recovery and a greater emphasis on cost-effectiveness, minimally invasive THAs have been introduced in the last decades. The direct anterior approach is a minimally invasive, tissue-sparing approach in which intermuscular planes are used. Theoretically, this approach should result in a faster recovery of physical functioning and higher health-related quality of life. Methods/design: A randomised controlled trial will be performed. Patients will be randomly allocated to undergo THA by means of the anterior or posterolateral approach. Both the intervention and control group will consist of two subgroups: 1) patients with a good bone stock who will receive an uncemented femoral stem, and 2) patients with a poor bone stock who will receive a cemented femoral stem. Patients between 18 and 90 years with primary or secondary osteoarthritis will be included. Physical functioning and health-related quality of life will be assessed by means of questionnaires. Additionally, performance based tests will be performed to objectively assess the physical functioning. Cost-effectiveness will be assessed by obtaining data on medical costs in and outside the hospital and other nonmedical costs. Measurements will take place preoperatively, two and six weeks, three months and one year postoperatively. Discussion: There is some evidence that the anterior approach results in reduced tissue damage and faster recovery in the direct postoperative period, compared to the posterolateral approach. However, there is still a lack of well-designed studies that have confirmed the better outcomes and cost-effectiveness of the anterior approach. Therefore, the purpose of this study is to assess the physical functioning, health related quality of life and the cost-effectiveness of the anterior approach, compared to the conventional posterolateral approach
Flux-Line Lattice Structures in Untwinned YBa2Cu3O
A small angle neutron scattering study of the flux-line lattice in a large
single crystal of untwinned YBa2Cu3O is presented. In fields parallel to the
c-axis, diffraction spots are observed corresponding to four orientations of a
hexagonal lattice, distorted by the a-b anisotropy. A value for the anisotropy,
the penetration depth ratio, of 1.18(2) was obtained. The high quality of the
data is such that second order diffraction is observed, indicating a well
ordered FLL. With the field at 33 degrees to c a field dependent re-orientation
of the lattice is observed around 3T.Comment: 4 pages, 4 figure
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