1,677 research outputs found
Strong mobility degradation in ideal graphene nanoribbons due to phonon scattering
We investigate the low-field phonon-limited mobility in armchair graphene
nanoribbons (GNRs) using full-band electron and phonon dispersion relations. We
show that lateral confinement suppresses the intrinsic mobility of GNRs to
values typical of common bulk semiconductors, and very far from the impressive
experiments on 2D graphene. Suspended GNRs with a width of 1 nm exhibit a
mobility close to 500 cm^2/Vs at room temperature, whereas if the same GNRs are
deposited on HfO2 mobility is further reduced to about 60 cm^2/Vs due to
surface phonons. We also show the occurrence of polaron formation, leading to
band gap renormalization of ~118 meV for 1 nm-wide armchair GNRs.Comment: 11 pages, 4 figure
Atomistic quantum transport modeling of metal-graphene nanoribbon heterojunctions
We calculate quantum transport for metal-graphene nanoribbon heterojunctions
within the atomistic self-consistent Schr\"odinger/Poisson scheme. Attention is
paid on both the chemical aspects of the interface bonding as well the
one-dimensional electrostatics along the ribbon length. Band-bending and doping
effects strongly influence the transport properties, giving rise to conductance
asymmetries and a selective suppression of the subband formation. Junction
electrostatics and p-type characteristics drive the conduction mechanism in the
case of high work function Au, Pd and Pt electrodes, while contact resistance
becomes dominant in the case of Al.Comment: 4 pages, 5 figure
Simulation of hydrogenated graphene Field-Effect Transistors through a multiscale approach
In this work, we present a performance analysis of Field Effect Transistors
based on recently fabricated 100% hydrogenated graphene (the so-called
graphane) and theoretically predicted semi-hydrogenated graphene (i.e.
graphone). The approach is based on accurate calculations of the energy bands
by means of GW approximation, subsequently fitted with a three-nearest neighbor
(3NN) sp3 tight-binding Hamiltonian, and finally used to compute ballistic
transport in transistors based on functionalized graphene. Due to the large
energy gap, the proposed devices have many of the advantages provided by
one-dimensional graphene nanoribbon FETs, such as large Ion and Ion/Ioff
ratios, reduced band-to-band tunneling, without the corresponding disadvantages
in terms of prohibitive lithography and patterning requirements for circuit
integration
Subsidizing Religious Participation through Groups: A Model of the âMegachurchâ Strategy for Growth
Either despite or because of their non-traditional approach, megachurches have grown significantly in the United States since 1980. This paper models religious participation as an imperfect public good which, absent intervention, yields suboptimal participation by members from the churchâs perspective. Megachurches address this problem in part by employing secular-based group activities to subsidize religious participation that then translates into an increase in the attendeesâ religious investment. This strategy not only allows megachurches to attract and retain new members when many traditional churches are losing members but also results in higher levels of an individualâs religious capital. As a result, the megachurch may raise expectations of membersâ levels of commitment and faith practices. Data from the FACT2000 survey provide evidence that megachurches employ groups more extensively than other churches, and this approach is consistent with a strategy to use groups to help subsidize individualsâ religious investment. Religious capital rises among members of megachurches relative to members of non-megachurches as a result of this strategy
Effect of dephasing on the current statistics of mesoscopic devices
We investigate the effects of dephasing on the current statistics of
mesoscopic conductors with a recently developed statistical model, focusing in
particular on mesoscopic cavities and Aharonov-Bohm rings. For such devices, we
analyze the influence of an arbitrary degree of decoherence on the cumulants of
the current. We recover known results for the limiting cases of fully coherent
and totally incoherent transport and are able to obtain detailed information on
the intermediate regime of partial coherence for a varying number of open
channels. We show that dephasing affects the average current, shot noise, and
higher order cumulants in a quantitatively and qualitatively similar way, and
that consequently shot noise or higher order cumulants of the current do not
provide information on decoherence additional or complementary to what can be
already obtained from the average current.Comment: 4 pages, 4 figure
Electron dynamics in intentionally disordered semiconductor superlattices
We study the dynamical behavior of disordered quantum-well-based
semiconductor superlattices where the disorder is intentional and short-range
correlated. We show that, whereas the transmission time of a particle grows
exponentially with the number of wells in an usual disordered superlattice for
any value of the incident particle energy, for specific values of the incident
energy this time increases linearly when correlated disorder is included. As
expected, those values of the energy coincide with a narrow subband of extended
states predicted by the static calculations of Dom\'{\i}nguez-Adame {\em et
al.} [Phys. Rev. B {\bf 51}, 14 ,359 (1994)]; such states are seen in our
dynamical results to exhibit a ballistic regime, very close to the WKB
approximation of a perfect superlattice. Fourier transform of the output signal
for an incident Gaussian wave packet reveals a dramatic filtering of the
original signal, which makes us confident that devices based on this property
may be designed and used for nanotechnological applications. This is more so in
view of the possibility of controllingthe outp ut band using a dc electric
field, which we also discuss. In the conclusion we summarize our results and
present an outlook for future developments arising from this work.Comment: 10 pagex, RevTex, 13 Postscript figures. Physical Review B (in press
ďťżUnderwater acoustic channel properties ďťżin the Gulf of Naples and their effects ďťżon digital data transmission
ďťżIn this paper we studied the physical properties of the Gulf of Naples (Southern Italy) for its use as a commu-
nication channel for the acoustic transmission of digital data acquired by seismic instruments on the seafloor to
a moored buoy. The acoustic link will be assured by high frequency acoustic modems operating with a central
frequency of 100 kHz and a band pass of 10 kHz. The main operational requirements of data transmission con-
cern the near horizontal acoustic link, the maximum depth of the sea being about 300 m and the planned hori-
zontal distance between seismic instruments and buoy 2 km. This study constructs the signal-to-noise ratio maps
to understand the limits beyond which the clarity of the transmission is no longer considered reliable. Using ray-
theory, we compute the amplitudes of a transmitted signal at a grid of 21Ă12 receivers to calculate the transmis-
sion loss at each receiver. The signal-to-noise ratio is finally computed for each receiver knowing also the trans-
mitter source level and the acoustic noise level in the Gulf of Naples. The results show that the multipath effects
predominate over the effects produced by the sound velocity gradient in the sea in the summer period. In the case
of omnidirectional transmitters with a Source Level (SL) of 165 dB and a baud rate of 2.4 kbit/s, the results al-
so show that distances of 1400-1600 m can be reached throughout the year for transmitter-receiver connections
below 50 m depth in the underwater acoustic channel
Three Dimensional Visualization and Fractal Analysis of Mosaic Patches in Rat Chimeras: Cell Assortment in Liver, Adrenal Cortex and Cornea
The production of organ parenchyma in a rapid and reproducible manner is critical to normal development. In chimeras produced by the combination of genetically distinguishable tissues, mosaic patterns of cells derived from the combined genotypes can be visualized. These patterns comprise patches of contiguously similar genotypes and are different in different organs but similar in a given organ from individual to individual. Thus, the processes that produce the patterns are regulated and conserved. We have previously established that mosaic patches in multiple tissues are fractal, consistent with an iterative, recursive growth model with simple stereotypical division rules. Fractal dimensions of various tissues are consistent with algorithmic models in which changing a single variable (e.g. daughter cell placement after division) switches the mosaic pattern from islands to stripes of cells. Here we show that the spiral pattern previously observed in mouse cornea can also be visualized in rat chimeras. While it is generally held that the pattern is induced by stem cell division dynamics, there is an unexplained discrepancy in the speed of cellular migration and the emergence of the pattern. We demonstrate in chimeric rat corneas both island and striped patterns exist depending on the age of the animal. The patches that comprise the pattern are fractal, and the fractal dimension changes with the age of the animal and indicates the constraint in patch complexity as the spiral pattern emerges. The spiral patterns are consistent with a loxodrome. Such data are likely to be relevant to growth and cell division in organ systems and will help in understanding how organ parenchyma are generated and maintained from multipotent stem cell populations located in specific topographical locations within the organ. Ultimately, understanding algorithmic growth is likely to be essential in achieving organ regeneration in vivo or in vitro from stem cell populations
Inferences on the source mechanisms of the 1930 Irpinia (Southern Italy) earthquake from simulations of the kinematic rupture process
We examine here a number of parameters that define the source of the earthquake that occurred on 23rd July 1930
in Southern Italy (in the Irpinia region). Starting from the source models proposed in different studies, we have simulated
the acceleration field for each hypothesized model, and compared it with the macroseismic data. We then
used the hybrid stochastic-deterministic technique proposed by Zollo et al. (1997) for the simulation of the ground
motion associated with the rupture of an extended fault. The accelerations simulated for several sites were associated
with the intensities using the empirical relationship proposed by Trifunac and Brady (1975), before being compared
with the available data from the macroseismic catalogue. A good reproduction of the macroseismic field is
provided by a normal fault striking in Apenninic direction (approximately NW-SE) and dipping 55° toward the SW
Mechanical Systems: Symmetry and Reduction
Reduction theory is concerned with mechanical systems with symmetries. It constructs a
lower dimensional reduced space in which associated conservation laws are taken out and
symmetries are \factored out" and studies the relation between the dynamics of the given
system with the dynamics on the reduced space. This subject is important in many areas,
such as stability of relative equilibria, geometric phases and integrable systems
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