1,183 research outputs found
\u27Dred Scott v. Sandford\u27 Analysis
The Scott v. Sandford decision will forever be known as a dark moment in America\u27s history. The Supreme Court chose to rule on a controversial issue, and they made the wrong decision. Scott v. Sandford is an example of what can happen when the Court chooses to side with personal opinion instead of what is right
Methods of regularization for computing orbits in celestial mechanics
Numerical and analytical methods for orbit computation in celestial mechanics during and beyond collision by introduction of regularized coordinate
Dynamical excitonic effects in metals and semiconductors
The dynamics of an electron--hole pair induced by the time--dependent
screened Coulomb interaction is discussed. In contrast to the case where the
static electron--hole interaction is considered we demonstrate the occurrence
of important dynamical excitonic effects in the solution of the Bethe--Salpeter
equation.This is illustrated in the calculated absorption spectra of noble
metals (copper and silver) and silicon. Dynamical corrections strongly affect
the spectra, partially canceling dynamical self--energy effects and leading to
good agreement with experiment.Comment: Accepted for publication on Phys. Rev. Let
Theory of stripe domains in magnetic shape memory alloys
The evolution of multivariant patterns in thin plates of magnetic shape
memory materials with an applied magnetic field was studied theoretically. A
geometrical domain-model is considered composed of straight stripe-like
martensite variants with constant internal magnetization (high anisotropy
limit) and magnetic domain wall orientation fixed by the twin boundaries.
Through integral transforms of the demagnetization energy, the micromagnetic
energy is cast into a form convenient for direct numerical evaluation and
analytical calculations. The equilibrium geometrical parameters of multivariant
patterns with straight and oblique twin boundaries have been derived as
functions of the applied field and the material parameters of a plate. It is
shown that the oblique multivariant states exist only in plates with
thicknesses L larger than a certain critical value L_0. In samples with L < L_0
a magnetic-field-driven transformation occurs directly between single variant
states.Comment: Materials for E-MRS Fall Meeting, Warsaw 2007. Corrected figure
Mechanical and SEM analysis of artificial comet nucleus samples
Since 1987 experiments dealing with comet nucleus phenomena have been carried out in the DFVLR space simulation chambers. The main objective of these experiments is a better understanding of thermal behavior, surface phenomena and especially the gas dust interaction. As a function of different sample compositions and exposure to solar irradiation (xenon-bulbs) crusts of different hardness and thickness were measured. The measuring device consists of a motor driven pressure foot (5 mm diameter), which is pressed into the sample. The applied compressive force is electronically monitored. The microstructure of the crust and dust residuals is investigated by scanning electron microscopy (SEM) techniques. Stress-depth profiles of an unirradiated and an irradiated model comet are given
Spin-sensitive Bleaching and Spin-Relaxation in QW's
Spin-sensitive saturation of absorption of infrared radiation has been
investigated in p-type GaAs QWs. It is shown that the absorption saturation of
circularly polarized radiation is mostly controlled by the spin relaxation time
of the holes. The saturation behavior has been investigated for different QW
widths and in dependence on the temperature with the result that the saturation
intensity substantially decreases with narrowing of QWs. Spin relaxation times
were experimentally obtained by making use of calculated (linear) absorption
coefficients for inter-subband transitions
Modifications of comet materials by the sublimation process: Results from simulation experiments
An active comet like comet Halley loses by sublimation a surface layer of the order of 1 m thickness per perihelion passage. In situ measurements show that water ice is the main constituent which contributes to the gas emission although even more volatile species (CO, NH3, CH4, CO2 etc.) have been identified. Dust particles which were embedded in the ices are carried by the sublimating gases. Measurements of the chemical composition of cometary grains indicate that they are composed of silicates of approximate chondritic composition and refractory carbonaceous material. Comet simulation experiments show that significant modifications of cometary materials occur due to sublimation process in near surface layers which have to be taken into account in order to derive the original state of the material
Spin relaxation times of 2D holes from spin sensitive bleaching of inter-subband absorption
We present spin relaxation times of 2D holes obtained by means of spin
sensitive bleaching of the absorption of infrared radiation in p-type
GaAs/AlGaAs quantum wells (QWs). It is shown that the saturation of
inter-subband absorption of circularly polarized radiation is mainly controlled
by the spin relaxation time of the holes. The saturation behavior has been
determined for different QW widths and in a wide temperature range with the
result that the saturation intensity substantially decreases with narrowing of
the QWs. Spin relaxation times are derived from the measured saturation
intensities by making use of calculated (linear) absorption coefficients for
direct inter-subband transitions. It is shown that spin relaxation is due to
the D'yakonov-Perel' mechanism governed by hole-hole scattering. The problem of
selection rules is addressed.Comment: 14 pages, 5 figure
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