745 research outputs found
Two Component Heat Diffusion Observed in CMR Manganites
We investigate the low-temperature electron, lattice, and spin dynamics of
LaMnO_3 (LMO) and La_0.7Ca_0.3MnO_3 (LCMO) by resonant pump-probe reflectance
spectroscopy. Probing the high-spin d-d transition as a function of time delay
and probe energy, we compare the responses of the Mott insulator and the
double-exchange metal to the photoexcitation. Attempts have previously been
made to describe the sub-picosecond dynamics of CMR manganites in terms of a
phenomenological three temperature model describing the energy transfer between
the electron, lattice and spin subsystems followed by a comparatively slow
exponential decay back to the ground state. However, conflicting results have
been reported. Here we first show clear evidence of an additional component in
the long term relaxation due to film-to-substrate heat diffusion and then
develop a modified three temperature model that gives a consistent account for
this feature. We confirm our interpretation by using it to deduce the bandgap
in LMO. In addition we also model the non-thermal sub-picosecond dynamics,
giving a full account of all observed transient features both in the insulating
LMO and the metallic LCMO.Comment: 6 pages, 5 figures http://link.aps.org/doi/10.1103/PhysRevB.81.064434
v2: Abstract correcte
Anharmonic softening of Raman active phonons in Iron-Pnictides; estimating the Fe isotope effect due to anharmonic expansion
We present Raman measurements on the iron-pnictide superconductors
CeFeAsO_{1-x}F_{x} and NdFeAsO{1-x}F_{x}. Modeling the Fe-As plane in terms of
harmonic and a cubic anharmonic Fe-As interaction we calculate the temperature
dependence of the energy and lifetime of the Raman active Fe B_{1g} mode and
fit to the observed energy shift. The shifts and lifetimes are in good
agreement with those measured also in other Raman studies which demonstrate
that the phonon spectrum is well represented by phonon-phonon interactions
without any significant electronic contribution. We also estimate the
anharmonic expansion from Fe (56->54) isotope substitution to \Delta a=5.1
10^{-4}\AA and \Delta d_{Fe-As}= 2.510^{-4}\AA and the shift of harmonic zero
point fluctuations of bond lengths <=3 10^{-5}\AA^2, giving a total
relative average decrease of electronic hopping integrals of |\delta t|/t<= 2.0
10^{-4}. The results poses a serious challenge for any theory of
superconductivity in the pnictides that does not include electron-phonon
interactions to produce a sizable Fe-isotope effect.Comment: 7 pages, 6 figure
Effects of hydrogen bonding on supercooled liquid dynamics and the implications for supercooled water
The supercooled state of bulk water is largely hidden by unavoidable
crystallization, which creates an experimentally inaccessible temperature
regime - a 'no man's land'. We address this and circumvent the crystallization
problem by systematically studying the supercooled dynamics of hydrogen bonded
oligomeric liquids (glycols), where water corresponds to the chain-ends alone.
This novel approach permits a 'dilution of water' by altering the hydrogen bond
concentration via variations in chain length. We observe a dynamic crossover in
the temperature dependence of the structural relaxation time for all glycols,
consistent with the common behavior of most supercooled liquids. We find that
the crossover becomes more pronounced for increasing hydrogen bond
concentrations, which leads to the prediction of a marked dynamic transition
for water within 'no man's land' at T~220 K. Interestingly, the predicted
transition thus takes place at a temperature where a so called 'strong-fragile'
transition has previously been suggested. Our results, however, imply that the
dynamic transition of supercooled water is analogous to that commonly observed
in supercooled liquids. Moreover, we find support also for the existence of a
secondary relaxation of water with behavior analogous to that of the secondary
relaxation observed for the glycols.Comment: 20 pages, 5 figures; corrected typos, title changed, small clarifying
text changes, two labels removed from Fig. 2
Use of near infrared reflectance spectroscopy to predict nitrogen uptake by winter wheat within fields with high variability in organic matter
In this study, the ability to predict N-uptake in winter wheat crops using NIR-spectroscopy on soil samples was evaluated. Soil samples were taken in unfertilized plots in one winter wheat field during three years (1997-1999) and in another winter wheat field nearby in one year (2000). Soil samples were analyzed for organic C content and their NIR-spectra. N-uptake was measured as total N-content in aboveground plant materials at harvest. Models calibrated to predict N-uptake were internally cross validated and validated across years and across fields. Cross-validated calibrations predicted N-uptake with an average error of 12.1 to 15.4 kg N ha-1. The standard deviation divided by this error (RPD) ranged between 1.9 and 2.5. In comparison, the corresponding calibrations based on organic C alone had an error from 11.7 to 28.2 kg N ha-1 and RPDs from 1.3 to 2.5. In three of four annual calibrations within a field, the NIR-based calibrations worked better than the organic C based calibrations. The prediction of N-uptake across years, but within a field, worked slightly better with an organic C based calibration than with a NIR based one, RPD = 1.9 and 1.7 respectively. Across fields, the corresponding difference was large in favour of the NIR-calibration, RPD = 2.5 for the NIR-calibration and 1.5 for the organic C calibration. It was concluded that NIR-spectroscopy integrates information about organic C with other relevant soil components and therefore has a good potential to predict complex functions of soils such as N-mineralization. A relatively good agreement of spectral relationships to parameters related to the N-mineralization of datasets across the world suggests that more general models can be calibrated
Propylene Carbonate Reexamined: Mode-Coupling Scaling without Factorisation ?
The dynamic susceptibility of propylene carbonate in the moderately viscous
regime above is reinvestigated by incoherent neutron and
depolarised light scattering, and compared to dielectric loss and solvation
response. Depending on the strength of relaxation, a more or less
extended scaling regime is found. Mode-coupling fits yield consistently
and K, although different positions of the
susceptibility minimum indicate that not all observables have reached the
universal asymptotics
Grey Literature and Professional Knowledge Making
International audienceWhat does grey literature mean? What role does it play in the production and dissemination of practitioner knowledge? How do reports, presentations and communications, working papers and other un-published material contribute to professional, extra-academic knowledge making? The following paper tries to provide some elements for a better understanding of grey literature, with examples from different collections and disciplines. Moreover, it puts the focus on critical issues like standards, identifiers and quality, and it discusses the impact of open science, i.e. the movement to make scientific research, data and dissemination accessible to all levels of an inquiring society, amateur or professional
Theoretical study of the thermal behavior of free and alumina-supported Fe-C nanoparticles
The thermal behavior of free and alumina-supported iron-carbon nanoparticles
is investigated via molecular dynamics simulations, in which the effect of the
substrate is treated with a simple Morse potential fitted to ab initio data. We
observe that the presence of the substrate raises the melting temperature of
medium and large nanoparticles ( = 0-0.16, = 80-1000, non-
magic numbers) by 40-60 K; it also plays an important role in defining the
ground state of smaller Fe nanoparticles ( = 50-80). The main focus of our
study is the investigation of Fe-C phase diagrams as a function of the
nanoparticle size. We find that as the cluster size decreases in the
1.1-1.6-nm-diameter range the eutectic point shifts significantly not only
toward lower temperatures, as expected from the Gibbs-Thomson law, but also
toward lower concentrations of C. The strong dependence of the maximum C
solubility on the Fe-C cluster size may have important implications for the
catalytic growth of carbon nanotubes by chemical vapor deposition.Comment: 13 pages, 11 figures, higher quality figures can be seen in article 9
at http://alpha.mems.duke.edu/wahyu
The role of oxygen vacancies in SrTiO3 at the LaAlO3/SrTiO3 interface
Strontium titanate, SrTiO3, a widely used substrate material for electronic
oxide thin film devices, has provided many interesting features. In a
combination with a similar oxide material, LaAlO3, it has recently received
great interest. It was suggested that two-dimensional electron gas is formed at
the interface between SrTiO3 and LaAlO3, resulting in high electrical
conductivity and mobility. In this report we demonstrate that the transport
properties in those heterostructures are very sensitive to the deposition
parameters during thin film growth. Using cathode- and photoluminescence
studies in conjunction with measurements of electrical transport properties and
microstructure we show that the electronic properties observed at a
LaAlO3/SrTiO3 interface can be explained by oxygen reduced SrTiO3. In addition,
we demonstrate that oxygen can be pushed in and out of the sample, but that
re-oxygenation of an initially oxygen depleted LaAlO3/SrTiO3 heterostructure is
partly prevented by the presence of the film.Comment: 19 pages, 5 figure
Universal and non-universal features of glassy relaxation in propylene carbonate
It is demonstrated that the susceptibility spectra of supercooled propylene
carbonate as measured by depolarized-light-scattering, dielectric-loss, and
incoherent quasi-elastic neutron-scattering spectroscopy within the GHz window
are simultaneously described by the solutions of a two-component schematic
model of the mode-coupling theory (MCT) for the evolution of glassy dynamics.
It is shown that the universal beta-relaxation-scaling laws, dealing with the
asymptotic behavior of the MCT solutions, describe the qualitative features of
the calculated spectra. But the non-universal corrections to the scaling laws
render it impossible to achieve a complete quantitative description using only
the leading-order-asymptotic results.Comment: 37 pages, 16 figures, to be published in Phys. Rev.
High-intensity activity is more strongly associated with metabolic health in children compared to sedentary time: a cross-sectional study of the I.Family cohort
Background: Physical activity (PA) during childhood is important for preventing future metabolic syndrome (MetS). To examine the relationship between PA and MetS in more detail, accurate measures of PA are needed. Previous studies have only utilized a small part of the information available from accelerometer measured PA. This study investigated the association between measured PA and MetS in children with a new method for data processing and analyses that enable more detailed interpretation of PA intensity level. Methods: The association between PA pattern and risk factors related to MetS was investigated in a cross- sectional sample of children (n = 2592, mean age 10.9 years, 49.4% male) participating in the European multicenter I. Family study. The risk factors examined include body mass index, blood pressure, high-density lipoprotein cholesterol, insulin resistance and a combined risk factor score (MetS score). PA was measured by triaxial accelerometers and raw data was processed using the 10 Hz frequency extended method (FEM). The PA output was divided into an intensity spectrum and the association with MetS risk factors was analyzed by partial least squares regression. Results: PA patterns differed between the European countries investigated, with Swedish children being most active and Italian children least active. Moderate intensity physical activity was associated with lower insulin resistance (R2 = 2.8%), while vigorous intensity physical activity was associated with lower body mass index (R2 = 3.6%), MetS score (R2 = 3.1%) and higher high-density lipoprotein cholesterol (R2 = 2.3%). PA of all intensities was associated with lower systolic- and diastolic blood pressure, although the associations were weaker than for the other risk factors (R2 = 1.5% and R2 = 1.4%). However, the multivariate analysis implies that the entire PA pattern must be considered. The main difference in PA was observed between normal weight and overweight children. Conclusions: The present study suggests a greater importance of more PA corresponding to an intensity of at least brisk walking with inclusion of high-intense exercise, rather than a limited time spent sedentary, in the association to metabolic health in children. The methods of data processing and statistical analysis enabled accurate analysis and interpretation of the health benefits of high intensity PA that have not been shown previously. © 2021, The Author(s)
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