85 research outputs found
The collisional evolution of undifferentiated asteroids and the formation of chondritic meteoroids
Most meteorites are fragments from recent collisions experienced in the
asteroid belt. In such a hyper-velocity collision, the smaller collision
partner is destroyed, whereas a crater on the asteroid is formed or it is
entirely disrupted, too. The present size distribution of the asteroid belt
suggests that an asteroid with 100 km radius is encountered times
during the lifetime of the Solar System by objects larger than 10 cm in radius;
the formed craters cover the surface of the asteroid about 100 times. We
present a Monte Carlo code that takes into account the statistical bombardment
of individual infinitesimally small surface elements, the subsequent compaction
of the underlying material, the formation of a crater and a regolith layer. For
the entire asteroid, 10,000 individual surface elements are calculated. We
compare the ejected material from the calculated craters with the shock stage
of meteorites with low petrologic type and find that these most likely stem
from smaller parent bodies that do not possess a significant regolith layer.
For larger objects, which accrete a regolith layer, a prediction of the
thickness depending on the largest visible crater can be made. Additionally, we
compare the crater distribution of an object initially 100 km in radius with
the shape model of the asteroid (21) Lutetia, assuming it to be initially
formed spherical with a radius that is equal to its longest present ellipsoid
length. Here, we find the shapes of both objects to show resemblance to each
other.Comment: Accepted by Ap
Real-time gauge theory simulations from stochastic quantization with optimized updating
We investigate simulations for gauge theories on a Minkowskian space-time
lattice. We employ stochastic quantization with optimized updating using
stochastic reweighting or gauge fixing, respectively. These procedures do not
affect the underlying theory but strongly improve the stability properties of
the stochastic dynamics, such that simulations on larger real-time lattices can
be performed.Comment: 29 pages, 9 figures, 2 tables (NPB version, minor changes
Near-field heat transfer in a scanning thermal microscope
We present measurements of the near-field heat transfer between the tip of a
thermal profiler and planar material surfaces under ultrahigh vacuum
conditions. For tip-sample distances below 10-8 m our results differ markedly
from the prediction of fluctuating electrodynamics. We argue that these
differences are due to the existence of a material-dependent small length scale
below which the macroscopic description of the dielectric properties fails, and
discuss a corresponding model which yields fair agreement with the available
data. These results are of importance for the quantitative interpretation of
signals obtained by scanning thermal microscopes capable of detecting local
temperature variations on surfaces
Schottky Solar Cells with CuInSâ‚‚ Nanocrystals as Absorber Material
Colloidal semiconductor nanocrystals with tunable optical properties
are promising materials for light harvesting in solar cells. So far, in particular
cadmiumand lead chalcogenide nanocrystals were intensively studied in this respect,
and the device performance has made rapid progress in recent years. In
contrast, less research efforts were undertaken to develop solar cells based on Cd- and
Pb-free nanoparticles as absorbermaterial. In the present work, we report on
Schottky solar cells with the absorber layermade of colloidal copper indiumdisulfide
nanocrystals. Absorber films with up to ∼ 500 nm thickness were realized
by a solution-based layer-by-layer deposition technique. The device performance
was systematically studied dependent on the absorber layer thickness. Decreasing
photocurrent densities with increasing thickness revealed charge transport to
be a limiting factor for the device performance
How to Reduce Charge Recombination in Organic Solar Cells: There Are Still Lessons to Learn from P3HT:PCBM
Suppressing charge recombination is key for organic solar cells to become
commercial reality. However, there is still no conclusive picture of how
recombination losses are influenced by the complex nanoscale morphology. Here,
new insight is provided by revisiting the P3HT:PCBM blend, which is still one
of the best performers regarding reduced recombination. By changing small
details in the annealing procedure, two model morphologies were prepared that
vary in phase separation, molecular order and phase purity, as revealed by
electron tomography and optical spectroscopy. Both systems behave very
similarly with respect to charge generation and transport, but differ
significantly in bimolecular recombination. Only the system containing P3HT
aggregates of high crystalline quality and purity is found to achieve
exceptionally low recombination rates. The high-quality aggregates support
charge delocalization, which assists the re-dissociation of interfacial
charge-transfer states formed upon the encounter of free carriers. For devices
with the optimized morphology, an exceptional long hole diffusion length is
found, which allows them to work as Shockley-type solar cells even in thick
junctions of 300 nm. In contrast, the encounter rate and the size of the
phase-separated domains appears to be less important.Comment: final version, journal reference and DOI adde
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