5,453 research outputs found
How branching can change the conductance of ballistic semiconductor devices
We demonstrate that branching of the electron flow in semiconductor
nanostructures can strongly affect macroscopic transport quantities and can
significantly change their dependence on external parameters compared to the
ideal ballistic case even when the system size is much smaller than the mean
free path. In a corner-shaped ballistic device based on a GaAs/AlGaAs
two-dimensional electron gas we observe a splitting of the commensurability
peaks in the magnetoresistance curve. We show that a model which includes a
random disorder potential of the two-dimensional electron gas can account for
the random splitting of the peaks that result from the collimation of the
electron beam. The shape of the splitting depends on the particular realization
of the disorder potential. At the same time magnetic focusing peaks are largely
unaffected by the disorder potential.Comment: accepted for publication in Phys. Rev.
Direct strain and elastic energy evaluation in rolled-up semiconductor tubes by x-ray micro-diffraction
We depict the use of x-ray diffraction as a tool to directly probe the strain
status in rolled-up semiconductor tubes. By employing continuum elasticity
theory and a simple model we are able to simulate quantitatively the strain
relaxation in perfect crystalline III-V semiconductor bi- and multilayers as
well as in rolled-up layers with dislocations. The reduction in the local
elastic energy is evaluated for each case. Limitations of the technique and
theoretical model are discussed in detail.Comment: 32 pages (single column), 9 figures, 39 reference
Three-dimensional diffraction mapping by tuning the X-ray energy
Three-dimensional reciprocal-space maps of a single SiGe island around the Si(004) Bragg peak are recorded using an energy-tuning technique with a microfocused X-ray beam with compound refractive lenses as focusing optics
Integrating Beneficiation into Regolith Conveyance Systems
Regolith conveyance includes hauler/dumpers, hoppers, augers, pneumatic transport subsystems, and other elements. The features of the conveyance and the time the material stream spend in conveyance may be used synergistically to perform beneficiation, pre-processing (such as heating), and other tasks, thus reducing the mass and complexity of the overall ISRU system. Since the cost of spaceflight is largely driven by the cost of launching mass out of Earth's gravity well, the conveyance system should be leveraged in this way to the maximum extent
Influence of contact angle on slow evaporation in two-dimensional porous media
We study numerically the influence of contact angle on slow evaporation in
two-dimensional model porous media. For sufficiently low contact angles, the
drying pattern is fractal and can be predicted by a simple model combining the
invasion percolation model with the computation of the diffusive transport in
the gas phase. The overall drying time is minimum in this regime and is
independent of contact angle over a large range of contact angles up to the
beginning of a transition zone. As the contact angle increases in the
transition region, the cooperative smoothing mechanisms of the interface become
important and the width of the liquid gas interface fingers that form during
the evaporation process increases. The mean overall drying time increases in
the transition region up to an upper bound which is reached at a critical
contact angle \Theta_c. The increase in the drying time in the transition
region is explained in relation with the diffusional screening phenomenon
associated with the Laplace equation governing the vapor transport in the gas
phase. Above \Theta_c the drying pattern is character- ized by a flat traveling
front and the mean overall drying time becomes independent of the contact
angle. Drying time fluctuations are studied and are found to be important below
\Theta_c, i.e., when the pattern is fractal. The fluctuations are of the same
order of magnitude regardless of the value of contact angle in this range. The
fluctuations are found to die out abruptly at \Theta_c as the liquid gas
interface becomes a flat front
cAMP-dependent protein kinase A (PKA) regulates angiogenesis by modulating tip cell behavior in a Notch-independent manner
cAMP-dependent protein kinase A (PKA) is a ubiquitously expressed serine/threonine kinase that regulates a variety of cellular functions. Here, we demonstrate that endothelial PKA activity is essential for vascular development, specifically regulating the transition from sprouting to stabilization of nascent vessels. Inhibition of endothelial PKA by endothelial cell-specific expression of dominant-negative PKA in mice led to perturbed vascular development, hemorrhage and embryonic lethality at mid-gestation. During perinatal retinal angiogenesis, inhibition of PKA resulted in hypersprouting as a result of increased numbers of tip cells. In zebrafish, cell autonomous PKA inhibition also increased and sustained endothelial cell motility, driving cells to become tip cells. Although these effects of PKA inhibition were highly reminiscent of Notch inhibition effects, our data demonstrate that PKA and Notch independently regulate tip and stalk cell formation and behavior
Structural and magnetic properties of an InGaAs/FeSi superlattice in cylindrical geometry
The structure and the magnetic properties of an InGaAs/Fe3Si superlattice in
a cylindrical geometry are investigated by electron microscopy techniques,
x-ray diffraction and magnetometry. To form a radial superlattice, a
pseudomorphic InGaAs/Fe3As bilayer has been released from its substrate
self-forming into a rolled-up microtube. Oxide-free interfaces as well as areas
of crystalline bonding are observed and an overall lattice mismatch between
succeeding layers is determined. The cylindrical symmetry of the final radial
superlattice shows a significant effect on the magnetization behavior of the
rolled-up layers
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