1,603 research outputs found
Stress relief as the driving force for self-assembled Bi nanolines
Stress resulting from mismatch between a substrate and an adsorbed material
has often been thought to be the driving force for the self-assembly of
nanoscale structures. Bi nanolines self-assemble on Si(001), and are remarkable
for their straightness and length -- they are often more than 400 nm long, and
a kink in a nanoline has never been observed. Through electronic structure
calculations, we have found an energetically favourable structure for these
nanolines that agrees with our scanning tunneling microscopy and photoemission
experiments; the structure has an extremely unusual subsurface structure,
comprising a double core of 7-membered rings of silicon. Our proposed structure
explains all the observed features of the nanolines, and shows that surface
stress resulting from the mismatch between the Bi and the Si substrate are
responsible for their self-assembly. This has wider implications for the
controlled growth of nanostructures on semiconductor surfaces.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
High power laser semiconductor interactions: A Monte Carlo study for silicon
In this article, we use Monte Carlo methods to study the interaction of high power laser pulses with electrons in the conduction band of semiconductors. The laser field is represented by a sinusoidal electric field which tends to cause an oscillatory motion in the electrons. The scattering of electrons from the lattice force the electrons to lose phase coherence with the field. The approach is applied to silicon. We use the approach to examine the carrier energy distribution and material breakdown due to the transfer of energy from the laser to the electrons followed by impact ionization. The impact ionization coefficient, α, and its dependence on the laser frequency and field strength is examined and compared to the values in a dc field. In general, the ac value is smaller than the dc value, but at low frequencies and high field strengths, the ac impact ionization coefficient approaches the dc value at the same rms field value. The importance of collisions in the energy transfer process is elucidated. © 1997 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70199/2/JAPIAU-81-4-1807-1.pd
The effect of strain on hot‐electron and hole longitudinal diffusion and noise in Si and Si0.9Ge0.1
Monte Carlo methods are used to model the electron and hole high‐field transport in both unstrained and compressively strained silicon and silicon‐germanium alloy. The data are analyzed to determine in what way the thermal noise properties of the carriers are affected by compressive, in‐plane strain. Results include the longitudinal diffusion coefficient, the longitudinal noise temperature, and the longitudinal noise spectral density, for electric fields in the range of 0–20 kV/cm. The results are qualitatively similar for silicon with 1% compressive biaxial strain and for Si0.9Ge0.1/Si(001). The effects of strain are found to be more pronounced for electrons than for holes and are primarily related to changes in the conductivity effective mass. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70547/2/JAPIAU-78-9-5454-1.pd
Calculation of electron and hole impact ionization coefficients in SiGe alloys
Silicon–germanium alloys offer a system where the ratio of the electron impact ionization coefficient (α) and hole impact ionization coefficient (β) varies from a value larger than unity (in high silicon content alloys), to a value smaller than unity (in high germanium content alloys). We report results for α and β for this alloy system. The electron results are based on a multivalley nonparabolic band structure. The hole results are based on a six‐band k⋅p model for low energies coupled to an eight‐band model for high energies. We find that for the alloy Si0.4Ge0.6, α∼β. Alloy scattering is found to play an important role in determining the impact ionization coefficient. For compositions around Si0.5Ge0.5, the strong alloy scattering is found to suppress the impact ionization coefficient. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70702/2/JAPIAU-80-12-6773-1.pd
Three-dimensional structures of the tracheal systems of Anopheles sinensis and Aedes togoi pupae
Mosquitoes act as a vector for the transmission of disease. The World Health Organization has recommended strict control of mosquito larvae because of their few, fixed, and findable features. The respiratory system of mosquito larvae and pupae in the water has a weak point. As aquatic organisms, mosquito larvae and pupae inhale atmosphere oxygen. However, the mosquito pupae have a non-feeding stage, unlike the larvae. Therefore, detailed study on the tracheal system of mosquito pupae is helpful for understanding their survival strategy. In this study, the three-dimensional (3D) structures of the tracheal systems of Anopheles sinensis and Aedes togoi pupae were comparatively investigated using synchrotron X-ray microscopic computed tomography. The respiratory frequencies of the dorsal trunks were also investigated. Interestingly, the pupae of the two mosquito species possess special tracheal systems of which the morphological and functional features are distinctively different. The respiratory frequency of Ae. togoi is higher than that of An. sinensis. These differences in the breathing phenomena and 3D structures of the respiratory systems of these two mosquito species provide an insight into the tracheal systems of mosquito pupae. ? 2017 The Author(s).111Ysciescopu
Mass inflation in f(R) gravity: A conjecture on the resolution of the mass inflation singularity
We study gravitational collapse of a charged black hole in f(R) gravity using
double-null formalism. We require cosmological stability to f(R) models; we
used the Starobinsky model and the R + (1/2)cR^2 model. Charged black holes in
f(R) gravity can have a new type of singularity due to higher curvature
corrections, the so-called f(R)-induced singularity, although it is highly
model-dependent. As the advanced time increases, the internal structure will
approach the Cauchy horizon, which may not be an inner apparent horizon. There
is mass inflation as one approaches the Cauchy horizon and hence the Cauchy
horizon may be a curvature singularity with nonzero area. However, the Ricci
scalar is finite for an out-going null observer. This can be integrated as
follows: Cosmologically stable higher curvature corrections of the Ricci scalar
made it bounded even in the presence of mass inflation. Finally, we conjecture
that if there is a general action including general higher curvature
corrections with cosmological stability, then the corrections can make all
curvature components finite even in the presence of mass inflation. This might
help us to resolve the problem of inner horizon instability of regular black
hole models.Comment: 31 pages, 15 figure
Nearly Massless Electrons in the Silicon Interface with a Metal Film
We demonstrate the realization of nearly massless electrons in the most
widely used device material, silicon, at the interface with a metal film. Using
angle-resolved photoemission, we found that the surface band of a monolayer
lead film drives a hole band of the Si inversion layer formed at the interface
with the film to have nearly linear dispersion with an effective mass about 20
times lighter than bulk Si and comparable to graphene. The reduction of mass
can be accounted for by repulsive interaction between neighboring bands of the
metal film and Si substrate. Our result suggests a promising way to take
advantage of massless carriers in silicon-based thin-film devices, which can
also be applied for various other semiconductor devices.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
Dynamical formation and evolution of (2+1)-dimensional charged black holes
In this paper, we investigate the dynamical formation and evolution of 2 +
1-dimensional charged black holes. We numerically study dynamical collapses of
charged matter fields in an anti de Sitter background and note the formation of
black holes using the double-null formalism. Moreover, we include re-normalized
energy-momentum tensors assuming the S-wave approximation to determine
thermodynamical back-reactions to the internal structures. If there is no
semi-classical effects, the amount of charge determines the causal structures.
If the charge is sufficiently small, the causal structure has a space-like
singularity. However, as the charge increases, an inner Cauchy horizon appears.
If we have sufficient charge, we see a space-like outer horizon and a time-like
inner horizon, and if we give excessive charge, black hole horizons disappear.
We have some circumstantial evidences that weak cosmic censorship is still
satisfied, even for such excessive charge cases. Also, we confirm that there is
mass inflation along the inner horizon, although the properties are quite
different from those of four-dimensional cases. Semi-classical back-reactions
will not affect the outer horizon, but they will affect the inner horizon. Near
the center, there is a place where negative energy is concentrated. Thus,
charged black holes in three dimensions have two types of curvature
singularities in general: via mass inflation and via a concentration of
negative energy. Finally, we classify possible causal structures.Comment: 40 pages, 15 figure
TCP throughput guarantee in the DiffServ Assured Forwarding service: what about the results?
Since the proposition of Quality of Service architectures by the IETF, the
interaction between TCP and the QoS services has been intensively studied. This
paper proposes to look forward to the results obtained in terms of TCP
throughput guarantee in the DiffServ Assured Forwarding (DiffServ/AF) service
and to present an overview of the different proposals to solve the problem. It
has been demonstrated that the standardized IETF DiffServ conditioners such as
the token bucket color marker and the time sliding window color maker were not
good TCP traffic descriptors. Starting with this point, several propositions
have been made and most of them presents new marking schemes in order to
replace or improve the traditional token bucket color marker. The main problem
is that TCP congestion control is not designed to work with the AF service.
Indeed, both mechanisms are antagonists. TCP has the property to share in a
fair manner the bottleneck bandwidth between flows while DiffServ network
provides a level of service controllable and predictable. In this paper, we
build a classification of all the propositions made during these last years and
compare them. As a result, we will see that these conditioning schemes can be
separated in three sets of action level and that the conditioning at the
network edge level is the most accepted one. We conclude that the problem is
still unsolved and that TCP, conditioned or not conditioned, remains
inappropriate to the DiffServ/AF service
Direct observation of the spin polarization in Au atomic wires on Si(553)
The spin-resolved electronic band structure of Au-induced metallic atomic wires on a vicinal silicon surface, Si(553), was investigated using spin-and angle-resolved photoelectron spectroscopy. We directly measured the spin polarization of three partially filled one-dimensional metallic bands, a one-third-filled band, and the doublet of nearly half-filled bands. For the half-filled doublet, the strong apparent spin polarization was observed near the Fermi energy with a minor out-of-plane spin component. This observation is consistent with the Rashba-type spin-orbit splitting and with a recent experiment on a similar doublet of Si(557)-Au. In contrast, the one-third-filled band does not show a substantial spin polarization within the experimental accuracy, indicating a much smaller spin splitting, if any. These results are discussed for the origin of the partially filled bands and for the intriguing broken-symmetry ground state observed at low temperature.X11116sciescopu
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