40,881 research outputs found
Communications for Next Generation single chip computers
It is the thesis of this report that much of what is presently thought to require specialized VLSI functions might instead be achieved by combinations of fast general purpose single chip computers with upgraded communication facilities. To this end, the characteristics of applications
of this nature are first surveyed briefly and some working principles established. In the light of these, three different chip philosophies are explored in some detail. This study shows that some upgrading of typical
single chip I/O will definitely be necessary, but that this upgrading does not have to be complex and that true multiprocessor-multibus operation could be achieved without excessive cost
Reaction paths of alane dissociation on the Si(001) surface
Building on our earlier study, we examine the kinetic barriers to
decomposition of alane, AlH, on the Si(001) surface, using the nudged
elastic band (NEB) approach within DFT. We find that the initial decomposition
to AlH with two H atoms on the surface proceeds without a significant barrier.
There are several pathways available to lose the final hydrogen, though these
present barriers of up to 1 eV. Incorporation is more challenging, with the
initial structures less stable in several cases than the starting structures,
just as was found for phosphorus. We identify a stable route for Al
incorporation following selective surface hydrogen desorption (e.g. by STM
tip). The overall process parallels PH, and indicates that atomically
precise acceptor doping should be possible.Comment: 19 pages, 8 figures, submitted to J. Physics.: Condens. Matte
Low-loss directional cloaks without superluminal velocity or magnetic response
The possibility of making an optically large (many wavelengths in diameter)
object appear invisible has been a subject of many recent studies. Exact
invisibility scenarios for large (relative to the wavelength) objects involve
(meta)materials with superluminal phase velocity (refractive index less than
unity) and/or magnetic response. We introduce a new approximation applicable to
certain device geometries in the eikonal limit: piecewise-uniform scaling of
the refractive index. This transformation preserves the ray trajectories, but
leads to a uniform phase delay. We show how to take advantage of phase delays
to achieve a limited (directional and wavelength-dependent) form of
invisibility that does not require loss-ridden (meta)materials with
superluminal phase velocities.Comment: 3 pages, 2 figure
Alane adsorption and dissociation on the Si(001) surface
We used DFT to study the energetics of the decomposition of alane, AlH3, on
the Si(001) surface, as the acceptor complement to PH3. Alane forms a dative
bond with the raised atoms of silicon surface dimers, via the Si atom lone
pair. We calculated the energies of various structures along the pathway of
successive dehydrogenation events following adsorption: AlH2, AlH and Al,
finding a gradual, significant decrease in energy. For each stage, we analyse
the structure and bonding, and present simulated STM images of the lowest
energy structures. Finally, we find that the energy of Al atoms incorporated
into the surface, ejecting a Si atom, is comparable to Al adatoms. These
findings show that Al incorporation is likely to be as precisely controlled as
P incorporation, if slightly less easy to achieve.Comment: Submitted to J. Phys.: Condens. Matte
Flow stabilization with active hydrodynamic cloaks
We demonstrate that fluid flow cloaking solutions based on active
hydrodynamic metamaterials exist for two-dimensional flows past a cylinder in a
wide range of Reynolds numbers, up to approximately 200. Within the framework
of the classical Brinkman equation for homogenized porous flow, we demonstrate
using two different methods that such cloaked flows can be dynamically stable
for in the range 5-119. The first, highly efficient, method is based on a
linearization of the Brinkman-Navier-Stokes equation and finding the
eigenfrequencies of the least stable eigen-perturbations; the second method is
a direct, numerical integration in the time domain. We show that, by
suppressing the Karman vortex street in the weekly turbulent wake, porous flow
cloaks can raise the critical Reynolds number up to about 120, or five times
greater than for a bare, uncloaked cylinder.Comment: 5 pages, 3 figure
Fluid flow control with transformation media
We introduce a new concept for the manipulation of fluid flow around
three-dimensional bodies. Inspired by transformation optics, the concept is
based on a mathematical idea of coordinate transformations and physically
implemented with anisotropic porous media permeable to the flow of fluids. In
two situations - for an impermeable object placed either in a free-flowing
fluid or in a fluid-filled porous medium - we show that the object can be
coated with an inhomogeneous, anisotropic permeable medium, such as to preserve
the flow that would have existed in the absence of the object. The proposed
fluid flow cloak eliminates downstream wake and compensates viscous drag,
hinting us at the possibility of novel propulsion techniques.Comment: 4 pages, 7 figure
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