361 research outputs found
Dynamic Kerr effect responses in the Terahertz-range
Dynamic Kerr effect measurements provide a simple realization of a nonlinear
experiment. We propose a field-off experiment where an electric field of one or
several sinusoidal cycles is applied to a sample in thermal equilibrium.
Afterwards, the evolution of the polarizability is measured. If such an
experiment is performed in the Terahertz-range it might provide valuable
information about the low-frequency dynamics in disordered systems. We treat
these dynamics in terms of a Brownian oscillator model and calculate the Kerr
effect response. It is shown that frequency-selective behaviour can be
expected. In the interesting case of underdamped vibrational motion we find
that the frequency-dependence of the phonon-damping can be determined from the
experiment. Also the behaviour of overdamped relaxational modes is discussed.
For typical glassy materials we estimate the magnitude of all relevant
quantities, which we believe to be helpful in experimental realizations.Comment: 26 pages incl. 5 figure
Collective and single-particle excitations in the photoyield spectrum of Al
Using angle- and energy-resolved photoyield spectroscopy, we investigate the properties of the multipole plasmon excitation. At higher energies, a systematic dependence of the photoyield on the photon angle of incidence is observed and explained on the basis of classical Fresnel theory, indicating the possibility of obtaining information about optical constants from such measurements. A feature above the multipole plasmon is assigned to the excitation of a bulk plasmon by the photon field
Photoinduced plasmon excitations in alkali-metal overlayers
Collective surface excitations in alkali-metal overlayers are observed using photoyield spectroscopy. Spectra for Na and K on Al(111) reveal a multipole surface plasmon and bulklike overlayer plasmon. In contrast, Li on Al exhibits only the multipole mode. In the submonolayer regime, all three alkali metals provide evidence for the threshold excitation. Time-dependent density-functional calculations for realistic alkali-metal overlayers agree well with these observations
MIMO Grid Impedance Identification of Three-Phase Power Systems: Parametric vs. Nonparametric Approaches
A fast and accurate grid impedance measurement of three-phase power systems
is crucial for online assessment of power system stability and adaptive control
of grid-connected converters. Existing grid impedance measurement approaches
typically rely on pointwise sinusoidal injections or sequential wideband
perturbations to identify a nonparametric grid impedance curve via fast Fourier
computations in the frequency domain. This is not only time-consuming, but also
inaccurate during time-varying grid conditions, while on top of that, the
identified nonparametric model cannot be immediately used for stability
analysis or control design. To tackle these problems, we propose to use
parametric system identification techniques (e.g., prediction error or subspace
methods) to obtain a parametric impedance model directly from time-domain
current and voltage data. Our approach relies on injecting wideband excitation
signals in the converter's controller and allows to accurately identify the
grid impedance in closed loop within one injection and measurement cycle. Even
though the underlying parametric system identification techniques are
well-studied in general, their utilization in a grid impedance identification
setup poses specific challenges, is vastly underexplored, and has not gained
adequate attention in urgent and timely power systems applications. To this
end, we demonstrate in numerical experiments how the proposed parametric
approach can accomplish a significant improvement compared to prevalent
nonparametric methods.Comment: 7 pages, 7 figure
Electronic structure of cubic gallium nitride films grown on GaAs
The composition, surface structure, and electronic structure of zinc blendeâGaN films grown on GaAs (100) and (110) by plasmaâassisted molecular beam epitaxy were investigated by means of core and valence level photoemission. Angleâresolved photoelectron spectra (photon energy 30â110 eV) exhibited emission from the Ga 3d and N 2s levels, as well as a clear peak structure in the valence band region. These peaks were found to shift with photon energy, indicative of direct transitions between occupied and unoccupied GaN bands. By using a free electron final band, we are able to derive the course of the bands along the ÎâX and ÎâKâX directions of the Brillouin zone and to determine the energy of critical points at the X point. The relative energies of the Ga 3d and nitrogen 2s bands were also studied, and a small amount of dispersion was detected in the latter. The resulting band structure is discussed in relation to existing band structure calculations
The well-preserved Late Neolithic dolmen burial of Oberbipp, Switzerland. Construction, use, and post-depositional processes
Excavation of the Late Neolithic dolmen of Oberbipp BE, Steingasse in the Swiss Central Plateau provided a
unique opportunity for a comprehensive study of the archaeological and anthropological evidence. In multidisciplinary
studies, we investigated the processes at work during construction, use, and abandonment of the
megalithic structure, as well as the dietary habits, subsistence strategy, and possible mobility of the Neolithic
population. Archaeological methods included micromorphology, archaeobiology, typology, use-wear analysis,
and geology. The anthropological investigation was complemented by an analysis of stable isotope ratios and
palaeogenetics. Local topography and the cover of alluvial sediments ensured an extraordinary conservation of
the monument. It allowed the preservation of the human remains of at least 42 individuals of both sexes and all
ages. The observation of the sedimentary and post-depositional processes, supplemented by an extensive series of
radiocarbon dates, allowed us to reconstruct the history of the dolmen in its environment and the definition of at
least two deposition phases. We found genetic evidence of lactase intolerance, a local population with a mixed
ancestry of early Anatolian farmers and Western hunter-gatherers, and a crop-based diet. Sparse remains of a
nearby Late Neolithic settlement sustain the interpretation that this is the burial site of a local farming community.
Evidence of higher mobility of females and kinship over three generations solely in the paternal line
suggests a virilocal community. Bone-altering pathologies support the assumption of a caring society
Exome sequencing reveals mutated SLC19A3 in patients with an early-infantile, lethal encephalopathy
To accomplish a diagnosis in patients with a rare unclassified disorder is difficult. In this study, we used magnetic resonance imaging pattern recognition analysis to identify patients with the same novel heritable disorder. Whole-exome sequencing was performed to discover the mutated gene. We identified seven patients sharing a previously undescribed magnetic resonance imaging pattern, characterized by initial swelling with T2 hyperintensity of the basal nuclei, thalami, cerebral white matter and cortex, pons and midbrain, followed by rarefaction or cystic degeneration of the white matter and, eventually, by progressive cerebral, cerebellar and brainstem atrophy. All patients developed a severe encephalopathy with rapid deterioration of neurological functions a few weeks after birth, followed by respiratory failure and death. Lactate was elevated in body fluids and on magnetic resonance spectroscopy in most patients. Whole-exome sequencing in a single patient revealed two predicted pathogenic, heterozygous missense mutations in the SLC19A3 gene, encoding the second thiamine transporter. Additional predicted pathogenic mutations and deletions were detected by Sanger sequencing in all six other patients. Pathology of brain tissue of two patients demonstrated severe cerebral atrophy and microscopic brain lesions similar to Leigh's syndrome. Although the localization of SLC19A3 expression in brain was similar in the two investigated patients compared to age-matched control subjects, the intensity of the immunoreactivity was increased. Previously published patients with SLC19A3 mutations have a milder clinical phenotype, no laboratory evidence of mitochondrial dysfunction and more limited lesions on magnetic resonance imaging. In some, cerebral atrophy has been reported. The identification of this new, severe, lethal phenotype characterized by subtotal brain degeneration broadens the phenotypic spectrum of SLC19A3 mutations. Recognition of the associated magnetic resonance imaging pattern allows a fast diagnosis in affected infant
oMEGACat I: MUSE spectroscopy of 300,000 stars within the half-light radius of Centauri
Omega Centauri ( Cen) is the most massive globular cluster of the
Milky Way and has been the focus of many studies that reveal the complexity of
its stellar populations and kinematics. However, most previous studies have
used photometric and spectroscopic datasets with limited spatial or magnitude
coverage, while we aim to investigate it having full spatial coverage out to
its half-light radius and stars ranging from the main sequence to the tip of
the red giant branch. This is the first paper in a new survey of Cen
that combines uniform imaging and spectroscopic data out to its half-light
radius to study its stellar populations, kinematics, and formation history. In
this paper, we present an unprecedented MUSE spectroscopic dataset combining 87
new MUSE pointings with previous observations collected from guaranteed time
observations. We extract spectra of more than 300,000 stars reaching more than
two magnitudes below the main sequence turn-off. We use these spectra to derive
metallicity and line-of-sight velocity measurements and determine robust
uncertainties on these quantities using repeat measurements. Applying quality
cuts we achieve signal-to-noise ratios of 16.47/73.51 and mean metallicity
errors of 0.174/0.031 dex for the main sequence stars (18 mag 22 mag) and red giant branch stars (16 mag 10
mag), respectively. We correct the metallicities for atomic diffusion and
identify foreground stars. This massive spectroscopic dataset will enable
future studies that will transform our understanding of Cen, allowing
us to investigate the stellar populations, ages, and kinematics in great
detail.Comment: 27 pages, 18 figures, 3 tables, accepted for publication in ApJ, the
catalog will be available in the online material of the published articl
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