8,390 research outputs found
On the variational structure of breather solutions
In this paper we give a systematic and simple account that put in evidence
that many breather solutions of integrable equations satisfy suitable
variational elliptic equations, which also implies that the stability problem
reduces in some sense to the study of the spectrum of explicit linear
systems (\emph{spectral stability}), and the understanding of how bad
directions (if any) can be controlled using low regularity conservation laws.
We exemplify this idea in the case of the modified Korteweg-de Vries (mKdV),
Gardner, and sine-Gordon (SG) equations. Then we perform numerical simulations
that confirm, at the level of the spectral problem, our previous rigorous
results, where we showed that mKdV breathers are and stable,
respectively. In a second step, we also discuss the Gardner and the Sine-Gordon
cases, where the spectral study of a fourth-order linear matrix system is the
key element to show stability. Using numerical methods, we confirm that all
spectral assumptions leading to the stability of SG breathers
are numerically satisfied, even in the ultra-relativistic, singular regime. In
a second part, we study the periodic mKdV case, where a periodic breather is
known from the work of Kevrekidis et al. We rigorously show that these
breathers satisfy a suitable elliptic equation, and we also show numerical
spectral stability. However, we also identify the source of nonlinear
instability in the case described in Kevrekidis et al. Finally, we present a
new class of breather solution for mKdV, believed to exist from geometric
considerations, and which is periodic in time and space, but has nonzero mean,
unlike standard breathers.Comment: 55 pages; This paper is an improved version of our previous paper
1309.0625 and hence we replace i
Orbital eigenchannel analysis for ab-initio quantum transport calculations
We show how to extract the orbital contribution to the transport
eigenchannels from a first-principles quantum transport calculation in a
nanoscopic conductor. This is achieved by calculating and diagonalizing the
first-principles transmission matrix reduced to selected scattering
cross-sections. As an example, the orbital nature of the eigenchannels in the
case of Ni nanocontacts is explored, stressing the difficulties inherent to the
use of non-orthogonal basis sets and first-principles Hamiltonians.Comment: 5 pages, 5 figurs; replaced with final version, introduction revised;
to be published in PR
Modeling contact formation between atomic-sized gold tips via molecular dynamics
The formation and rupture of atomic-sized contacts is modelled by means of
molecular dynamics simulations. Such nano-contacts are realized in scanning
tunnelling microscope and mechanically controlled break junction experiments.
These instruments routinely measure the conductance across the nano-sized
electrodes as they are brought into contact and separated, permitting
conductance traces to be recorded that are plots of conductance versus the
distance between the electrodes. One interesting feature of the conductance
traces is that for some metals and geometric configurations a jump in the value
of the conductance is observed right before contact between the electrodes, a
phenomenon known as jump-to-contact. This paper considers, from a computational
point of view, the dynamics of contact between two gold nano-electrodes.
Repeated indentation of the two surfaces on each other is performed in two
crystallographic orientations of face-centred cubic gold, namely (001) and
(111). Ultimately, the intention is to identify the structures at the atomic
level at the moment of first contact between the surfaces, since the value of
the conductance is related to the minimum cross-section in the contact region.
Conductance values obtained in this way are determined using first principles
electronic transport calculations, with atomic configurations taken from the
molecular dynamics simulations serving as input structures.Comment: 6 pages, 4 figures, conference submissio
New determination of abundances and stellar parameters for a set of weak G-band stars
Weak G-band (wGb) stars are very peculiar red giants almost devoided of
carbon and often mildly enriched in lithium. Despite their very puzzling
abundance patterns, very few detailed spectroscopic studies existed up to a few
years ago, preventing any clear understanding of the wGb phenomenon. We
recently proposed the first consistent analysis of published data for 28 wGb
stars and identified them as descendants of early A-type to late B-type stars,
without being able to conclude on their evolutionary status or the origin of
their peculiar abundance pattern.
We used newly obtained high-resolution and high SNR spectra for 19 wGb stars
in the southern and northern hemisphere to homogeneously derive their
fundamental parameters, metallicities, as well as the spectroscopic abundances
for Li, C, N, O, Na, Sr, and Ba. We also computed dedicated stellar evolution
models that we used to determine the masses and to investigate the evolutionary
status and chemical history of the stars in our sample. We confirm that the wGb
stars are stars in the mass range 3.2 to 4.2 M. We suggest that a large
fraction could be mildly evolved stars on the SGB currently undergoing the 1st
DUP, while a smaller number of stars are more probably in the core He burning
phase at the clump. After analysing their abundance pattern, we confirm their
strong N enrichment anti-correlated with large C depletion, characteristic of
material fully processed through the CNO cycle to an extent not known in other
evolved intermediate-mass stars. However, we demonstrate here that such a
pattern is very unlikely due to self-enrichment. In the light of the current
observational constraints, no solid self-consistent pollution scenario can be
presented either, leaving the wGb puzzle largely unsolved.Comment: 19 pages , 14 figures, accepted for publication in Astronomy &
Astrophysic
Charging of highly resistive granular metal films
We have used the Scanning Kelvin probe microscopy technique to monitor the
charging process of highly resistive granular thin films. The sample is
connected to two leads and is separated by an insulator layer from a gate
electrode. When a gate voltage is applied, charges enter from the leads and
rearrange across the sample. We find very slow processes with characteristic
charging times exponentially distributed over a wide range of values, resulting
in a logarithmic relaxation to equilibrium. After the gate voltage has been
switched off, the system again relaxes logarithmically slowly to the new
equilibrium. The results cannot be explained with diffusion models, but most of
them can be understood with a hopping percolation model, in which the
localization length is shorter than the typical site separation. The technique
is very promising for the study of slow phenomena in highly resistive systems
and will be able to estimate the conductance of these systems when direct
macroscopic measurement techniques are not sensitive enough.Comment: 8 pages, 7 figure
Magnetic and orbital blocking in Ni nanocontacts
We address the fundamental question of whether magneto-resistance (MR) of
atomic-sized contacts of Nickel is very large because of the formation of a
domain wall (DW) at the neck. Using {\em ab initio} transport calculations we
find that, as in the case of non-magnetic electrodes, transport in Ni
nanocontacts depends very much on the orbital nature of the electrons. Our
results are in agreement with several experiments in the average value of the
conductance. On the other hand, contrary to existing claims, DW scattering does
{\em not} account for large MR in Ni nanocontacts.Comment: 5 pages, 3 Figure
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