9,744 research outputs found
A Theory of the Casimir Effect for Compact Regions
We develop a mathematically precise framework for the Casimir effect. Our
working hypothesis, verified in the case of parallel plates, is that only the
regularization-independent Ramanujan sum of a given asymptotic series
contributes to the Casimir pressure. As an illustration, we treat two cases:
parallel plates, identifying a previous cutoff free version (by G. Scharf and
W. W.) as a special case, and the sphere.We finally discuss the open problem of
the Casimir force for the cube. We propose an Ansatz for the exterior force and
argue why it may provide the exact solution, as well as an explanation of the
repulsive sign of the force.Comment: version published, 23 page
Minimal Equation Sets for Output Computation in Object-Oriented Models
Object-oriented models of complex physical systems
can have a very large number of equations and variables. For some applications, only a few output variables of the model are of actual interest. This paper presents an application of the well-known Tarjan’s algorithm, that allows to automatically select the minimal set of equations and variables required to compute
the time histories of selected outputs of a given model.
The application of the algorithm to a simple test case is illustrated in the paper
Second-order quantum nonlinear optical processes in single graphene nanostructures and arrays
Intense efforts have been made in recent years to realize nonlinear optical
interactions at the single-photon level. Much of this work has focused on
achieving strong third-order nonlinearities, such as by using single atoms or
other quantum emitters while the possibility of achieving strong second-order
nonlinearities remains unexplored. Here, we describe a novel technique to
realize such nonlinearities using graphene, exploiting the strong per-photon
fields associated with tightly confined graphene plasmons in combination with
spatially nonlocal nonlinear optical interactions. We show that in properly
designed graphene nanostructures, these conditions enable extremely strong
internal down-conversion between a single quantized plasmon and an entangled
plasmon pair, or the reverse process of second harmonic generation. A separate
issue is how such strong internal nonlinearities can be observed, given the
nominally weak coupling between these plasmon resonances and free-space
radiative fields. On one hand, by using the collective coupling to radiation of
nanostructure arrays, we show that the internal nonlinearities can manifest
themselves as efficient frequency conversion of radiative fields at extremely
low input powers. On the other hand, the development of techniques to
efficiently couple to single nanostructures would allow these nonlinear
processes to occur at the level of single input photons.Comment: 25 pages, 6 figure
Ultrafast Optical Control of the Electronic Properties of
We report on the temperature dependence of the electronic
properties, studied at equilibrium and out of equilibrium, by means of time and
angle resolved photoelectron spectroscopy. Our results unveil the dependence of
the electronic band structure across the Fermi energy on the sample
temperature. This finding is regarded as the dominant mechanism responsible for
the anomalous resistivity observed at T* 160 K along with the change of
the charge carrier character from holelike to electronlike. Having addressed
these long-lasting questions, we prove the possibility to control, at the
ultrashort time scale, both the binding energy and the quasiparticle lifetime
of the valence band. These experimental evidences pave the way for optically
controlling the thermoelectric and magnetoelectric transport properties of
Primordial Black Holes as Near Infrared Background sources
The near infrared background (NIRB) is the collective light from unresolved
sources observed in the band 1-10 m. The measured NIRB angular power
spectrum on angular scales arcmin exceeds by roughly two
order of magnitudes predictions from known galaxy populations. The nature of
the sources producing these fluctuations is still unknown. Here we test
primordial black holes (PBHs) as sources of the NIRB excess. Considering PBHs
as a cold dark matter (DM) component, we model the emission of gas accreting
onto PBHs in a cosmological framework. We account for both accretion in the
intergalactic medium (IGM) and in DM haloes. We self consistently derive the
IGM temperature evolution, considering ionization and heating due to X-ray
emission from PBHs. Besides CDM, we consider a model that accounts for
the modification of the linear matter power spectrum due to the presence of
PBHs; we also explore two PBH mass distributions, i.e. a -function and
a lognormal distribution. For each model, we compute the mean intensity and the
angular power spectrum of the NIRB produced by PBHs with mass
1-. In the limiting case in which the entirety of DM
is made of PBHs, the PBH emission contributes per cent to the observed
NIRB fluctuations. This value decreases to per cent if current
constraints on the abundance of PBHs are taken into account. We conclude that
PBHs are ruled out as substantial contributors to the NIRB.Comment: Accepted for publication in MNRA
A Damage Detection Approach for Axially Loaded Beam-like Structures Based on Gaussian Mixture Model
Axially loaded beam-like structures represent a challenging case study for unsupervised learning vibration-based damage detection. Under real environmental and operational conditions, changes in axial load cause changes in the characteristics of the dynamic response that are significantly greater than those due to damage at an early stage. In previous works, the authors proposed the adoption of a multivariate damage feature composed of eigenfrequencies of multiple vibration modes. Successful results were obtained by framing the problem of damage detection as that of unsupervised outlier detection, adopting the well-known Mahalanobis squared distance (MSD) to define an effective damage index. Starting from these promising results, a novel approach based on unsupervised learning data clustering is proposed in this work, which increases the sensitivity to damage and significantly reduces the uncertainty associated with the results, allowing for earlier damage detection. The novel approach, which is based on Gaussian mixture model, is compared with the benchmark one based on the MSD, under the effects of an uncontrolled environment and, most importantly, in the presence of real damage due to corrosion
Reduced Basis Method for Parametrized Elliptic Optimal Control Problems
We propose a suitable model reduction paradigm-the certified reduced basis method (RB)-for the rapid and reliable solution of parametrized optimal control problems governed by partial differential equations. In particular, we develop the methodology for parametrized quadratic optimization problems with elliptic equations as a constraint and infinite-dimensional control variable. First, we recast the optimal control problem in the framework of saddle-point problems in order to take advantage of the already developed RB theory for Stokes-type problems. Then, the usual ingredients of the RB methodology are called into play: a Galerkin projection onto a low-dimensional space of basis functions properly selected by an adaptive procedure; an affine parametric dependence enabling one to perform competitive offline-online splitting in the computational procedure; and an efficient and rigorous a posteriori error estimate on the state, control, and adjoint variables as well as on the cost functional. Finally, we address some numerical tests that confirm our theoretical results and show the efficiency of the proposed technique. Copyright \ua9 by SIAM. Unauthorized reproduction of this article is prohibited
Casimir force on a piston
We consider a massless scalar field obeying Dirichlet boundary conditions on
the walls of a two-dimensional L x b rectangular box, divided by a movable
partition (piston) into two compartments of dimensions a x b and (L-a) x b. We
compute the Casimir force on the piston in the limit L -> infinity. Regardless
of the value of a/b, the piston is attracted to the nearest end of the box.
Asymptotic expressions for the Casimir force on the piston are derived for a <<
b and a >> b.Comment: 10 pages, 1 figure. Final version, accepted for publication in Phys.
Rev.
Voluntary nicotine consumption triggers in vivo potentiation of cortical excitatory drives to midbrain dopaminergic neurons
International audienceActive response to either natural or pharmacological reward causes synaptic modifications to excitatory synapses on dopamine (DA) neurons of the ventral tegmental area (VTA). Here, we examine these modifications using nicotine, the main addictive component of tobacco, which is a potent regulator of VTA DA neurons. Using an in vivo electrophysiological technique, we investigated the role of key components of the limbic circuit, the infralimbic cortex (ILCx) and the bed nucleus of the stria terminalis (BNST), in operant behaviors related to nicotine reward. Our results indicated that nicotine self-administration in rats, but not passive delivery, triggers hyperactivity of VTA DA neurons. The data suggest that potentiation of the ILCx-BNST excitatory pathway is involved in these modifications in VTA DA neurons. Thus, recruitment of these specific excitatory inputs to VTA DA neurons may be a neural correlate for the learned association between active responding and the reward experience
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