846 research outputs found
Towards automated design of quantum cascade lasers
We present an advanced technique for the design and optimization of GaAs/AlGaAs quantum cascade laser structures. It is based on the implementation of the simulated annealing algorithm with the purpose of determining a set of design parameters that satisfy predefined conditions, leading to an enhancement of the device output characteristics. Two important design aspects have been addressed: improved thermal behavior, achieved by the use of higher conduction band offset materials, and a more efficient extraction mechanism, realized via a ladder of three lower laser states, with subsequent pairs separated by the optical phonon energy. A detailed analysis of performance of the obtained structures is carried out within a full self-consistent rate equations model of the carrier dynamics. The latter uses wave functions calculated by the transfer matrix method, and evaluates all relevant carrier–phonon and carrier–carrier scattering rates from each quantized state to all others within the same and neighboring periods of the cascade. These values are then used to form a set of rate equations for the carrier density in each state, enabling further calculation of the current density and gain as a function of the applied field and temperature. This paper addresses the application of the described procedure to the design of lambda~9 µm GaAs-based mid-infrared quantum cascade lasers and presents the output characteristics of some of the designed optimized structures. © 2005 American Institute of Physic
Genetic Programming For Cellular Automata Urban Inundation Modelling
Recent advances in Cellular Automata (CA) represent a new, computationally efficient method of simulating flooding in urban areas. A number of recent publications in this field have shown that CAs can be much more computationally efficient than methods than using standard shallow water equations (Saint Venant/Navier-Stokes equations). CAs operate using local state-transition rules that determine the progression of the flow from one cell in the grid to another cell, in many publications the Manning’s Formula is used as a simplified local state transition rule. Through the distributed interactions of the CA, computationally simplified urban flooding can be processed, although these methods are limited by the approximation represented by the Manning’s formula. Literature demonstrates that the viability of the Manning’s formula will break down with too large a time step, flow rates, too small a cell size, or too smooth roughness factor; Therefore further increases in computational efficiency could be gained with a better approximation, or rather one capable of producing the required simulation with enough accuracy at larger time steps, smaller cells sizes, smoother roughness factors. Genetic programming has the potential to be used to optimise state transition rules to maximise accuracy and minimise computation time. In this paper we present some preliminary findings on the use of genetic programming (GP) for deriving these rules automatically. The experimentation compares GP-derived rules with human created solutions based on the Mannings formula and findings indicate that the GP rules can improve on these approaches
The imprints of primordial non-gaussianities on large-scale structure: scale dependent bias and abundance of virialized objects
We study the effect of primordial nongaussianity on large-scale structure,
focusing upon the most massive virialized objects. Using analytic arguments and
N-body simulations, we calculate the mass function and clustering of dark
matter halos across a range of redshifts and levels of nongaussianity. We
propose a simple fitting function for the mass function valid across the entire
range of our simulations. We find pronounced effects of nongaussianity on the
clustering of dark matter halos, leading to strongly scale-dependent bias. This
suggests that the large-scale clustering of rare objects may provide a
sensitive probe of primordial nongaussianity. We very roughly estimate that
upcoming surveys can constrain nongaussianity at the level |fNL| <~ 10,
competitive with forecasted constraints from the microwave background.Comment: 16 pages, color figures, revtex4. v2: added references and an
equation. submitted to PRD. v3: simplified derivation, additional reference
Magnetotunnelling in resonant tunnelling structures with spin-orbit interaction
Magnetotunnelling spectroscopy of resonant tunnelling structures provides
information on the nature of the two-dimensional electron gas in the well. We
describe a model based on nonequilibrium Green's functions that allows for a
comprehensive study of the density of states, tunnelling currents and current
spin polarization. The investigated effects include the electron-phonon
interaction, interface roughness scattering, Zeeman effect and the Rashba
spin-orbit interaction. A qualitative agreement with experimental data is found
regarding the satellite peaks. The spin polarization is predicted to be larger
than ten percent for magnetic fields above 2 Tesla and having a structure even
at the satellite peaks. The Rashba effect is confirmed to be observable as a
beating pattern in the density of states but found to be too small to affect
the tunnelling current.Comment: 31 pages, 11 figure
Effects of Ellipticity and Shear on Gravitational Lens Statistics
We study the effects of ellipticity in lens galaxies and external tidal shear
from neighboring objects on the statistics of strong gravitational lenses. For
isothermal lens galaxies normalized so that the Einstein radius is independent
of ellipticity and shear, ellipticity {\it reduces} the lensing cross section
slightly, and shear leaves it unchanged. Ellipticity and shear can
significantly enhance the magnification bias, but only if the luminosity
function of background sources is steep. Realistic distributions of ellipticity
and shear {\it lower} the total optical depth by a few percent for most source
luminosity functions, and increase the optical depth only for steep luminosity
functions. The boost in the optical depth is noticeable (>5%) only for surveys
limited to the brightest quasars (L/L_* > 10). Ellipticity and shear broaden
the distribution of lens image separations but do not affect the mean.
Ellipticity and shear naturally increase the abundance of quadruple lenses
relative to double lenses, especially for steep source luminosity functions,
but the effect is not enough (by itself) to explain the observed
quadruple-to-double ratio. With such small changes to the optical depth and
image separation distribution, ellipticity and shear have a small effect on
cosmological constraints from lens statistics: neglecting the two leads to
biases of just Delta Omega_M = 0.00 \pm 0.01 and Delta Omega_Lambda = -0.02 \pm
0.01 (where the errorbars represent statistical uncertainties in our
calculations).Comment: Optical depth normalization discussed. Matches the published versio
Circulating calcitonin and carcinoembryonic antigen m-RNA detected by RT-PCR as tumour markers in medullary thyroid carcinoma
Detection of local relapse or metastasis in medullary thyroid carcinoma (MTC) continue to pose a major diagnostic challenge. Besides established diagnostic studies such as serum calcitonin (CT) and carcinoembryonic antigen (CEA), molecular detection of circulating tumour cells may be an additional diagnostic tool for the early detection of disease recurrence. We performed reverse transcription-polymerase chain reaction (RT-PCR) on blood samples from patients diagnosed with MTC disease using primers specific for CT and CEA, respectively. CT mRNA was not detectable in peripheral blood of all patients with MTC (n = 11) and all controls (n = 32). CEA mRNA was significantly more often detected patients with MTC (72.7%) than in controls (34.4%; p = 0.038; Fisher exact test). With an example of a patient with MTC and massive tumour mass in the neck we demonstrate the failure of detection of CT mRNA over a period of 6 months, whereas CEA mRNA could be detected in peripheral blood of this patient. As a consequence, CT mRNA detected by RT-PCR in the peripheral blood can not be recommended as a tumour marker in MTC. However, the use of carcinoembryonic mRNA may provide a significant improvement in diagnosis of recurrent disease in MTC. © 2001 Cancer Research Campaign http://www.bjcancer.co
The Uncorrelated Universe: Statistical Anisotropy and the Vanishing Angular Correlation Function in WMAP Years 1-3
The large-angle (low-ell) correlations of the Cosmic Microwave Background
(CMB) as reported by the Wilkinson Microwave Anisotropy Probe (WMAP) after
their first year of observations exhibited statistically significant anomalies
compared to the predictions of the standard inflationary big-bang model. We
suggested then that these implied the presence of a solar system foreground, a
systematic correlated with solar system geometry, or both. We re-examine these
anomalies for the data from the first three years of WMAP's operation. We show
that, despite the identification by the WMAP team of a systematic correlated
with the equinoxes and the ecliptic, the anomalies in the first-year Internal
Linear Combination (ILC) map persist in the three-year ILC map, in all-but-one
case at similar statistical significance. The three-year ILC quadrupole and
octopole therefore remain inconsistent with statistical isotropy -- they are
correlated with each other (99.6% C.L.), and there are statistically
significant correlations with local geometry, especially that of the solar
system. The angular two-point correlation function at scales >60 degrees in the
regions outside the (kp0) galactic cut, where it is most reliably determined,
is approximately zero in all wavebands and is even more discrepant with the
best fit LambdaCDM inflationary model than in the first-year data - 99.97% C.L.
for the new ILC map. The full-sky ILC map, on the other hand, has a
non-vanishing angular two-point correlation function, apparently driven by the
region inside the cut, but which does not agree better with LambdaCDM. The role
of the newly identified low-ell systematics is more puzzling than reassuring.Comment: 18 pages, 7 figures, minor additions to refs and conclusions in v2.
High resolution figures, multipole vector code and other information can be
found at http://www.phys.cwru.edu/projects/mpvectors
Assessment of the effectiveness of a risk-reduction measure on pluvial flooding and economic loss in Eindhoven, the Netherlands
Open Access journalCopyright © 2013 The Authors. Published by Elsevier Ltd.12th International Conference on Computing and Control for the Water Industry, CCWI2013Cities are increasingly assessing and reducing pluvial flood risk. Quantitative assessment of the effectiveness of risk-reduction measures is required. We use hydraulic simulation with GIS-based financial analysis to assess the pluvial flood risk for Eindhoven, The Netherlands. Analysis is carried out for four scenarios: two rainfall events, with and without separation of the combined sewer-stormwater network. Flooding statistics show how the risk-reduction measure impacts local flooding. Financial analysis demonstrates the saving resulting from the risk-reduction measure. Expected annual damage is reduced by c.€130,500. City authorities are better equipped in making cost-benefit decisions regarding implementation of pluvial flood risk-reduction measures.EC FP7 project PREPARED: Enabling Chang
Assessing Financial Loss due to Pluvial Flooding and the Efficacy of Risk-Reduction Measures in the Residential Property Sector
The final publication is available at Springer via http://dx.doi.org/10.1007/s11269-014-0833-6A novel quantitative risk assessment for residential properties at risk of pluvial flooding in Eindhoven, The Netherlands, is presented. A hydraulic model belonging to Eindhoven was forced with low return period rainfall events (2, 5 and 10-year design rainfalls). Three scenarios were analysed for each event: a baseline and two risk-reduction scenarios. GIS analysis identified areas where risk-reduction measures had the greatest impact. Financial loss calculations were carried out using fixed-threshold and probabilistic approaches. Under fixed-threshold assessment, per-event Expected Annual Damage (EAD) reached €38.2 m, with reductions of up to €454,000 resulting from risk-reduction measures. Present costs of flooding reach €1.43bn when calculated over a 50-year period. All net-present value figures for the risk-reduction measures are negative. Probabilistic assessment yielded EAD values up to more than double those of the fixed-threshold analysis which suggested positive net-present value. To the best of our knowledge, the probabilistic method based on the distribution of doorstep heights has never before been introduced for pluvial flood risk assessment. Although this work suggests poor net-present value of risk-reduction measures, indirect impacts of flooding, damage to infrastructure and the potential impacts of climate change were omitted. This work represents a useful first step in helping Eindhoven prepare for future pluvial flooding. The analysis is based on software and tools already available at the municipality, eliminating the need for software upgrading or training. The approach is generally applicable to similar cities.European Commission Seventh Framework Program (EC FP7
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