41,199 research outputs found
Emergence of Classical Orbits in Few-Cycle Above-Threshold Ionization
The time-dependent Schr\"odinger equation for atomic hydrogen in few-cycle
laser pulses is solved numerically. Introducing a positive definite quantum
distribution function in energy-position space, a straightforward comparison of
the numerical ab initio results with classical orbit theory is facilitated.
Integration over position space yields directly the photoelectron spectra so
that the various pathways contributing to a certain energy in the photoelectron
spectra can be established in an unprecedented direct and transparent way.Comment: 4 pages, 4 figures REVTeX (manuscript with higher resolution figures
available at http://www.dieterbauer.de/publist.html
Determining the Nature of Late Gunn-Peterson Troughs with Galaxy Surveys
Recent observations have discovered long (up to ~110 Mpc/h), opaque
Gunn-Peterson troughs in the z ~ 5.5 Lyman-alpha forest, which are challenging
to explain with conventional models of the post-reionization intergalactic
medium. Here we demonstrate that observations of the galaxy populations in the
vicinity of the deepest troughs can distinguish two competing models for these
features: deep voids where the ionizing background is weak due to fluctuations
in the mean free path of ionizing photons would show a deficit of galaxies,
while residual temperature variations from extended, inhomogeneous reionization
would show an overdensity of galaxies. We use large (~550 Mpc/h) semi-numerical
simulations of these competing explanations to predict the galaxy populations
in the largest of the known troughs at z ~ 5.7. We quantify the strong
correlation of Lyman-alpha effective optical depth and galaxy surface density
in both models and estimate the degree to which realistic surveys can measure
such a correlation. While a spectroscopic galaxy survey is ideal, we also show
that a relatively inexpensive narrowband survey of Lyman-alpha-emitting
galaxies is ~90% likely to distinguish between the competing models.Comment: 12 pages, 16 figures. Submitted to Ap
Smectic Phases with Cubic Symmetry: The Splay Analog of the Blue Phase
We report on a construction for smectic blue phases, which have quasi-long
range smectic translational order as well as long range cubic or hexagonal
order. Our proposed structures fill space with a combination of minimal surface
patches and cylindrical tubes. We find that for the right range of material
parameters, the favorable saddle-splay energy of these structures can stabilize
them against uniform layered structures.Comment: 4 pages, 4 eps figures, RevTe
Tropical Pacific spatial trend patterns in observed sea level: internal variability and/or anthropogenic signature?
In this study we focus on the sea level trend pattern observed by satellite altimetry in the tropical Pacific over the 1993–2009 time span (i.e. 17 yr). Our objective is to investigate whether this 17-yr-long trend pattern was different before the altimetry era, what was its spatio-temporal variability and what have been its main drivers. We try to discriminate the respective roles of the internal variability of the climate system and of external forcing factors, in particular anthropogenic emissions (greenhouse gases and aerosols). On the basis of a 2-D past sea level reconstruction over 1950–2009 (based on a combination of observations and ocean modelling) and multi-century control runs (i.e. with constant, preindustrial external forcing) from eight coupled climate models, we have investigated how the observed 17-yr sea level trend pattern evolved during the last decades and centuries, and try to estimate the characteristic time scales of its variability. For that purpose, we have computed sea level trend patterns over successive 17-yr windows (i.e. the length of the altimetry record), both for the 60-yr long reconstructed sea level and the model runs. We find that the 2-D sea level reconstruction shows spatial trend patterns similar to the one observed during the altimetry era. The pattern appears to have fluctuated with time with a characteristic time scale of the order of 25–30 yr. The same behaviour is found in multi-centennial control runs of the coupled climate models. A similar analysis is performed with 20th century coupled climate model runs with complete external forcing (i.e. solar plus volcanic variability and changes in anthropogenic forcing). Results suggest that in the tropical Pacific, sea level trend fluctuations are dominated by the internal variability of the ocean–atmosphere coupled system. While our analysis cannot rule out any influence of anthropogenic forcing, it concludes that the latter effect in that particular region is stillhardly detectable
On Retardation Effects in Space Charge Calculations Of High Current Electron Beams
Laser-plasma accelerators are expected to deliver electron bunches with high
space charge fields. Several recent publications have addressed the impact of
space charge effects on such bunches after the extraction into vacuum.
Artifacts due to the approximation of retardation effects are addressed, which
are typically either neglected or approximated. We discuss a much more
appropriate calculation for the case of laser wakefield acceleration with
negligible retardation artifacts due to the calculation performed in the mean
rest frame. This presented calculation approach also aims at a validation of
other simulation approaches
Non-dipole recollision-gated double ionization and observable effects
Using a three-dimensional semiclassical model, we study double ionization for
strongly-driven He fully accounting for magnetic field effects. For linearly
and slightly elliptically polarized laser fields, we show that recollisions and
the magnetic field combined act as a gate. This gate favors more transverse -
with respect to the electric field - initial momenta of the tunneling electron
that are opposite to the propagation direction of the laser field. In the
absence of non-dipole effects, the transverse initial momentum is symmetric
with respect to zero. We find that this asymmetry in the transverse initial
momentum gives rise to an asymmetry in a double ionization observable. Finally,
we show that this asymmetry in the transverse initial momentum of the tunneling
electron accounts for a recently-reported unexpectedly large average sum of the
electron momenta parallel to the propagation direction of the laser field.Comment: Amended the focus of the paper and discussion. 9 pages, 7 figure
Mitigating optical losses in crystalline silicon thin film solar cells on glass
Liquid phase crystallized silicon thin film solar cells on glass provide efficiencies up to 14.2 . While open circuit voltage and fill factor are already comparable to wafer based devices, short circuit current density is reduced due to incomplete light absorption. This paper analyzes the losses of current device designs in experiment and one dimensional simulations, revealing the low absorber thickness of 15 20 amp; 956;m as well as the planar glass silicon interface as the main cause of non absorption. Interface textures, in particular a sinusoidal texture and a smooth anti reflective three dimensional SMART texture, are discussed concerning their potential to mitigate these losses, allowing to reduce losses at the glass silicon interface by at least 40 relative. Taking the electronic interface quality into account, the SMART texture is identified as the most promising texture for light management in liquid phase crystallized silicon thin film solar cells on glas
Nucleation dynamics in 2d cylindrical Ising models and chemotaxis
The aim of our work is to study the effect of geometry variation on
nucleation times and to address its role in the context of eukaryotic
chemotaxis (i.e. the process which allows cells to identify and follow a
gradient of chemical attractant). As a first step in this direction we study
the nucleation dynamics of the 2d Ising model defined on a cylindrical lattice
whose radius changes as a function of time. Geometry variation is obtained by
changing the relative value of the couplings between spins in the compactified
(vertical) direction with respect to the horizontal one. This allows us to keep
the lattice size unchanged and study in a single simulation the values of the
compactification radius which change in time. We show, both with theoretical
arguments and numerical simulations that squeezing the geometry allows the
system to speed up nucleation times even in presence of a very small energy gap
between the stable and the metastable states. We then address the implications
of our analysis for directional chemotaxis. The initial steps of chemotaxis can
be modelled as a nucleation process occurring on the cell membrane as a
consequence of the external chemical gradient (which plays the role of energy
gap between the stable and metastable phases). In nature most of the cells
modify their geometry by extending quasi-onedimensional protrusions (filopodia)
so as to enhance their sensitivity to chemoattractant. Our results show that
this geometry variation has indeed the effect of greatly decreasing the
timescale of the nucleation process even in presence of very small amounts of
chemoattractants.Comment: 27 pages, 6 figures and 2 table
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