1,711 research outputs found
Weak gravitational lensing of finite beams
The standard theory of weak gravitational lensing relies on the infinitesimal
light beam approximation. In this context, images are distorted by convergence
and shear, the respective sources of which unphysically depend on the
resolution of the distribution of matter---the so-called Ricci-Weyl problem. In
this letter, we propose a strong-lensing-inspired formalism to describe the
lensing of finite beams. We address the Ricci-Weyl problem by showing
explicitly that convergence is caused by the matter enclosed by the beam,
regardless of its distribution. Furthermore, shear turns out to be
systematically enhanced by the finiteness of the beam. This implies, in
particular, that the Kaiser-Squires relation between shear and convergence is
violated, which could have profound consequences on the interpretation of weak
lensing surveys.Comment: 6 pages, 2 figures, v2: matches published version, some typos
correcte
Gate-modulated thermopower in disordered nanowires: I. Low temperature coherent regime
Using a one-dimensional tight-binding Anderson model, we study a disordered
nanowire in the presence of an external gate which can be used for depleting
its carrier density (field effect transistor device configuration). In this
first paper, we consider the low temperature coherent regime where the electron
transmission through the nanowire remains elastic. In the limit where the
nanowire length exceeds the electron localization length, we derive three
analytical expressions for the typical value of the thermopower as a function
of the gate potential, in the cases where the electron transport takes place
(i) inside the impurity band of the nanowire, (ii) around its band edges and
eventually (iii) outside its band. We obtain a very large enhancement of the
typical thermopower at the band edges, while the sample to sample fluctuations
around the typical value exhibit a sharp crossover from a Lorentzian
distribution inside the impurity band towards a Gaussian distribution as the
band edges are approached.Comment: 13 pages, 8 figures, final version as publishe
The theory of stochastic cosmological lensing
On the scale of the light beams subtended by small sources, e.g. supernovae,
matter cannot be accurately described as a fluid, which questions the
applicability of standard cosmic lensing to those cases. In this article, we
propose a new formalism to deal with small-scale lensing as a diffusion
process: the Sachs and Jacobi equations governing the propagation of narrow
light beams are treated as Langevin equations. We derive the associated
Fokker-Planck-Kolmogorov equations, and use them to deduce general analytical
results on the mean and dispersion of the angular distance. This formalism is
applied to random Einstein-Straus Swiss-cheese models, allowing us to: (1) show
an explicit example of the involved calculations; (2) check the validity of the
method against both ray-tracing simulations and direct numerical integrations
of the Langevin equation. As a byproduct, we obtain a
post-Kantowski-Dyer-Roeder approximation, accounting for the effect of tidal
distortions on the angular distance, in excellent agreement with numerical
results. Besides, the dispersion of the angular distance is correctly
reproduced in some regimes.Comment: 37+13 pages, 8 figures. A few typos corrected. Matches published
versio
Interpretation of the Hubble diagram in a nonhomogeneous universe
In the standard cosmological framework, the Hubble diagram is interpreted by
assuming that the light emitted by standard candles propagates in a spatially
homogeneous and isotropic spacetime. However, the light from "point
sources"--such as supernovae--probes the Universe on scales where the
homogeneity principle is no longer valid. Inhomogeneities are expected to
induce a bias and a dispersion of the Hubble diagram. This is investigated by
considering a Swiss-cheese cosmological model, which (1) is an exact solution
of the Einstein field equations, (2) is strongly inhomogeneous on small scales,
but (3) has the same expansion history as a strictly homogeneous and isotropic
universe. By simulating Hubble diagrams in such models, we quantify the
influence of inhomogeneities on the measurement of the cosmological parameters.
Though significant in general, the effects reduce drastically for a universe
dominated by the cosmological constant.Comment: 25 pages, 25 figures. Typos and Eqs. (2.15), (2.17), (4.16), (4.20),
and (4.43) corrected. Fig. 25 updated. Matches published versio
Fast Membranes Hemifusion via Dewetting between Lipid Bilayers
The behavior of lipid bilayer is important to understand the functionality of
cells like the trafficking of ions between cells. Standard procedures to
explore the properties of lipid bilayer and hemifused states typically use
either supported membranes or vesicles. Both techniques have several
shortcoming in terms of bio relevance or accessibility for measurements. In
this article the formation of individual free standing hemifused states between
model cell membranes is studied using an optimized microfluidic scheme which
allows for simultaneous optical and electrophysiological measurements. In a
first step, two model membranes are formed at a desired location within a
microfluidic device using a variation of the droplet interface bilayer (DiB)
technique. In a second step, the two model membranes are brought into contact
forming a single hemifused state. For all tested lipids, the hemifused state
between free standing membranes form within hundreds of milliseconds, i.e.
several orders of magnitude faster than reported in literature. The formation
of a hemifused state is observed as a two stage process, whereas the second
stage can be explained as a dewetting process in no-slip boundary condition.
The formed hemifusion states are long living and a single fusion event can be
observed when triggered by an applied electric field as demonstrated for
monoolein
Scanning Gate Microscopy of Quantum Contacts Under Parallel Magnetic Field: Beating Patterns Between Spin-Split Transmission Peaks or Channel Openings
We study the conductance of an electron interferometer created in a two
dimensional electron gas between a nanostructured contact and the depletion
region induced by the charged tip of a scanning gate microscope. Using
non-interacting models, we study the beating pattern of interference fringes
exhibited by the images giving as a function of the tip position when a
parallel magnetic field is applied. The analytical solution of a simplified
model allows us to distinguish between two cases: (i) If the field is applied
everywhere, the beating of Fabry-P\'erot oscillations of opposite spins gives
rise to interference rings which can be observed at low temperatures when the
contact is open between spin-split transmission resonances. (ii) If the field
acts only upon the contact, the interference rings cannot be observed at low
temperatures, but only at temperatures of the order of the Zeeman energy. For a
contact made of two sites in series, a model often used for describing an
inversion-symmetric double-dot setup, a pseudo-spin degeneracy is broken by the
inter-dot coupling and a similar beating effect can be observed without
magnetic field at temperatures of the order of the interdot coupling.
Eventually, numerical studies of a quantum point contact with quantized
conductance plateaus confirm that a parallel magnetic field applied everywhere
or only upon the contact gives rises to similar beating effects between
spin-split channel openings.Comment: 11 pages, 17 figure
Le dépôt électronique pour les cours au Canada (une idée qui arrive à point nommé) : une réponse à un document de travail publié par la Cour suprême du Canada recommandant des stratégies pour choisir un FSDE
"Ce texte se veut une réflexion sur les points à considérer avant l'adoption d'un système de dépôt électronique accessible à toutes les cours du Canada. En prenant pour exemple l'expérience torontoise, l'auteur souligne une série d'éléments à considérer avant la mise en œuvre d'un tel processus, à savoir : La nécessité de tenir compte des coûts associés à la formation des avocats, des juges, ainsi que du personnel juridique; L'attachement au document "" papier "" et le manque d'unanimité quant à la valeur du document électronique; et L'impact négatif que peut avoir l'informatisation des données sur la vie privée des contribuables. L'auteur conclu en ventant l'aspect centralisé du projet de dépôt électronique proposé par la Cour suprême en soulignant toutefois qu'un tel projet devra nécessairement évaluer les besoins des utilisateurs du système afin d'en tenir compte lors de sa conception.""This paper consists of a number of considerations which should be examined before the adoption of a nation-wide e-filling system. While using the Toronto experiment as his pattern, the author underlines a series of elements to be evaluated before the creation of such a project, such as: The need to take into consideration the costs associated with training layers, judges, and legal personnel; The importance attributed to paper and the lack of common ground between jurisdictions when it comes to the value of electronic documents; and The negative impact that online accessibility can have towards citizens' privacy. The author concludes with a note commending the fact that the Supreme Court's proposed system would be centralized, but warns that for such a project to be successful, it must evaluate and take into consideration the needs of its users.
Absorbing/Emitting Phonons with one dimensional MOSFETs
We consider nanowires in the field effect transistor device configuration.
Modeling each nanowire as a one dimensional lattice with random site
potentials, we study the heat exchanges between the nanowire electrons and the
substrate phonons, when electron transport is due to phonon-assisted hops
between localized states. Shifting the nanowire conduction band with a metallic
gate induces different behaviors. When the Fermi potential is located near the
band center, a bias voltage gives rise to small local heat exchanges which
fluctuate randomly along the nanowire. When it is located near one of the band
edges, the bias voltage yields heat currents which flow mainly from the
substrate towards the nanowire near one boundary of the nanowire, and in the
opposite direction near the other boundary. This opens interesting perspectives
for heat management at submicron scales: Arrays of parallel gated nanowires
could be used for a field control of phonon emission/absorption.Comment: 9 pages, 11 figure
Collective waves in dense and confined microfluidic droplet arrays
Excitation mechanisms for collective waves in confined dense one-dimensional
microfluidic droplet arrays are investigated by experiments and computer
simulations. We demonstrate that distinct modes can be excited by creating
specific `defect' patterns in flowing droplet trains. Excited longitudinal
modes exhibit a short-lived cascade of pairs of laterally displacing droplets.
Transversely excited modes obey the dispersion relation of microfluidic phonons
and induce a coupling between longitudinal and transverse modes, whose origin
is the hydrodynamic interaction of the droplets with the confining walls.
Moreover, we investigate the long-time behaviour of the oscillations and
discuss possible mechanisms for the onset of instabilities. Our findings
demonstrate that the collective dynamics of microfluidic droplet ensembles can
be studied particularly well in dense and confined systems. Experimentally, the
ability to control microfluidic droplets may allow to modulate the refractive
index of optofluidic crystals which is a promising approach for the production
of dynamically programmable metamaterials.Comment: 13 pages, 17 figure
On the stability and causality of scalar-vector theories
Various extensions of standard inflationary models have been proposed
recently by adding vector fields. Because they are generally motivated by
large-scale anomalies, and the possibility of statistical anisotropy of
primordial fluctuations, such models require to introduce non-standard
couplings between vector fields on the one hand, and either gravity or scalar
fields on the other hand. In this article, we study models involving a vector
field coupled to a scalar field. We derive restrictive necessary conditions for
these models to be both stable (Hamiltonian bounded by below) and causal
(hyperbolic equations of motion).Comment: 20 pages, references added, v2 matches published version in JCA
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