9,690 research outputs found
Towards electron transport measurements in chemically modified graphene: The effect of a solvent
Chemical functionalization of graphene modifies the local electron density of
the carbon atoms and hence electron transport. Measuring these changes allows
for a closer understanding of the chemical interaction and the influence of
functionalization on the graphene lattice. However, not only chemistry, in this
case diazonium chemistry, has an effect on the electron transport. Latter is
also influenced by defects and dopants resulting from different processing
steps. Here, we show that solvents used in the chemical reaction process change
the transport properties. In more detail, the investigated combination of
isopropanol and heating treatment reduces the doping concentration and
significantly increases the mobility of graphene. Furthermore, the isopropanol
treatment alone increases the concentration of dopants and introduces an
asymmetry between electron and hole transport which might be difficult to
distinguish from the effect of functionalization. The results shown in this
work demand a closer look on the influence of solvents used for chemical
modification in order to understand their influence
Dynamic Steerable Blocks in Deep Residual Networks
Filters in convolutional networks are typically parameterized in a pixel
basis, that does not take prior knowledge about the visual world into account.
We investigate the generalized notion of frames designed with image properties
in mind, as alternatives to this parametrization. We show that frame-based
ResNets and Densenets can improve performance on Cifar-10+ consistently, while
having additional pleasant properties like steerability. By exploiting these
transformation properties explicitly, we arrive at dynamic steerable blocks.
They are an extension of residual blocks, that are able to seamlessly transform
filters under pre-defined transformations, conditioned on the input at training
and inference time. Dynamic steerable blocks learn the degree of invariance
from data and locally adapt filters, allowing them to apply a different
geometrical variant of the same filter to each location of the feature map.
When evaluated on the Berkeley Segmentation contour detection dataset, our
approach outperforms all competing approaches that do not utilize pre-training.
Our results highlight the benefits of image-based regularization to deep
networks
Unraveling the acoustic electron-phonon interaction in graphene
Using a first-principles approach we calculate the acoustic electron-phonon
couplings in graphene for the transverse (TA) and longitudinal (LA) acoustic
phonons. Analytic forms of the coupling matrix elements valid in the
long-wavelength limit are found to give an almost quantitative description of
the first-principles based matrix elements even at shorter wavelengths. Using
the analytic forms of the coupling matrix elements, we study the acoustic
phonon-limited carrier mobility for temperatures 0-200 K and high carrier
densities of 10^{12}-10^{13} cm^{-2}. We find that the intrinsic effective
acoustic deformation potential of graphene is \Xi_eff = 6.8 eV and that the
temperature dependence of the mobility \mu ~ T^{-\alpha} increases beyond an
\alpha = 4 dependence even in the absence of screening when the full coupling
matrix elements are considered. The large disagreement between our calculated
deformation potential and those extracted from experimental measurements (18-29
eV) indicates that additional or modified acoustic phonon-scattering mechanisms
are at play in experimental situations.Comment: 7 pages, 3 figure
Simulations of energetic beam deposition: from picoseconds to seconds
We present a new method for simulating crystal growth by energetic beam
deposition. The method combines a Kinetic Monte-Carlo simulation for the
thermal surface diffusion with a small scale molecular dynamics simulation of
every single deposition event. We have implemented the method using the
effective medium theory as a model potential for the atomic interactions, and
present simulations for Ag/Ag(111) and Pt/Pt(111) for incoming energies up to
35 eV. The method is capable of following the growth of several monolayers at
realistic growth rates of 1 monolayer per second, correctly accounting for both
energy-induced atomic mobility and thermal surface diffusion. We find that the
energy influences island and step densities and can induce layer-by-layer
growth. We find an optimal energy for layer-by-layer growth (25 eV for Ag),
which correlates with where the net impact-induced downward interlayer
transport is at a maximum. A high step density is needed for energy induced
layer-by-layer growth, hence the effect dies away at increased temperatures,
where thermal surface diffusion reduces the step density. As part of the
development of the method, we present molecular dynamics simulations of single
atom-surface collisions on flat parts of the surface and near straight steps,
we identify microscopic mechanisms by which the energy influences the growth,
and we discuss the nature of the energy-induced atomic mobility
On the universality of compact polymers
Fully packed loop models on the square and the honeycomb lattice constitute
new classes of critical behaviour, distinct from those of the low-temperature
O(n) model. A simple symmetry argument suggests that such compact phases are
only possible when the underlying lattice is bipartite. Motivated by the hope
of identifying further compact universality classes we therefore study the
fully packed loop model on the square-octagon lattice. Surprisingly, this model
is only critical for loop weights n < 1.88, and its scaling limit coincides
with the dense phase of the O(n) model. For n=2 it is exactly equivalent to the
selfdual 9-state Potts model. These analytical predictions are confirmed by
numerical transfer matrix results. Our conclusions extend to a large class of
bipartite decorated lattices.Comment: 13 pages including 4 figure
Graphene on metals: a Van der Waals density functional study
We use density functional theory (DFT) with a recently developed van der
Waals density functional (vdW-DF) to study the adsorption of graphene on Al,
Cu, Ag, Au, Pt, Pd, Co and Ni(111) surfaces. In constrast to the local density
approximation (LDA) which predicts relatively strong binding for Ni,Co and Pd,
the vdW-DF predicts weak binding for all metals and metal-graphene distances in
the range 3.40-3.72 \AA. At these distances the graphene bandstructure as
calculated with DFT and the many-body GW method is basically unaffected
by the substrate, in particular there is no opening of a band gap at the
-point.Comment: 4 pages, 3 figure
Phase diagram and critical exponents of a Potts gauge glass
The two-dimensional q-state Potts model is subjected to a Z_q symmetric
disorder that allows for the existence of a Nishimori line. At q=2, this model
coincides with the +/- J random-bond Ising model. For q>2, apart from the usual
pure and zero-temperature fixed points, the ferro/paramagnetic phase boundary
is controlled by two critical fixed points: a weak disorder point, whose
universality class is that of the ferromagnetic bond-disordered Potts model,
and a strong disorder point which generalizes the usual Nishimori point. We
numerically study the case q=3, tracing out the phase diagram and precisely
determining the critical exponents. The universality class of the Nishimori
point is inconsistent with percolation on Potts clusters.Comment: Latex, 7 pages, 3 figures, v2: 1 reference adde
The packing of two species of polygons on the square lattice
We decorate the square lattice with two species of polygons under the
constraint that every lattice edge is covered by only one polygon and every
vertex is visited by both types of polygons. We end up with a 24 vertex model
which is known in the literature as the fully packed double loop model. In the
particular case in which the fugacities of the polygons are the same, the model
admits an exact solution. The solution is obtained using coordinate Bethe
ansatz and provides a closed expression for the free energy. In particular we
find the free energy of the four colorings model and the double Hamiltonian
walk and recover the known entropy of the Ice model. When both fugacities are
set equal to two the model undergoes an infinite order phase transition.Comment: 21 pages, 4 figure
An integrated Rotorcraft Avionics/Controls Architecture to support advanced controls and low-altitude guidance flight research
Salient design features of a new NASA/Army research rotorcraft--the Rotorcraft-Aircrew Systems Concepts Airborne Laboratory (RASCAL) are described. Using a UH-60A Black Hawk helicopter as a baseline vehicle, the RASCAL will be a flying laboratory capable of supporting the research requirements of major NASA and Army guidance, control, and display research programs. The paper describes the research facility requirements of these programs together with other critical constraints on the design of the research system. Research program schedules demand a phased development approach, wherein specific research capability milestones are met and flight research projects are flown throughout the complete development cycle of the RASCAL. This development approach is summarized, and selected features of the research system are described. The research system includes a real-time obstacle detection and avoidance system which will generate low-altitude guidance commands to the pilot on a wide field-of-view, color helmet-mounted display and a full-authority, programmable, fault-tolerant/fail-safe, fly-by-wire flight control system
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