2,259 research outputs found
Corrector estimates in homogenization of a nonlinear transmission problem for diffusion equations in connected domains
This paper is devoted to the homogenization of a nonlinear transmission problem stated in a two-phase domain. We consider a system of linear diffusion equations defined in a periodic domain consisting of two disjoint phases that are both connected sets separated by a thin interface. Depending on the field variables, at the interface, nonlinear conditions are imposed to describe interface reactions. In the variational setting of the problem, we prove the homogenization theorem and a bidomain averaged model. The periodic unfolding technique is used to obtain the residual error estimate with a first-order corrector. © 2019 The Authors. Mathematical Methods in the Applied Sciences published by John Wiley & Sons Ltd
Elliptical dichroism: operating principle of planar chiral metamaterials
We employ a homogenization technique based on the Lorentz electronic theory
to show that planar chiral structures (PCSs) can be described by an effective
dielectric tensor similar to that of biaxial elliptically dichroic crystals.
Such a crystal is shown to behave like a PCS insofar as it exhibits its
characteristic optical properties, namely, co-rotating elliptical polarization
eigenstates and asymmetric, direction-dependent transmission for
left/right-handed incident wave polarization.Comment: 3 pages, version as accepted in Optics Letters but before final
shortening
Orbital-assisted metal-insulator transition in VO
We found direct experimental evidence for an orbital switching in the V 3d
states across the metal-insulator transition in VO. We have used
soft-x-ray absorption spectroscopy at the V edges as a sensitive
local probe, and have determined quantitatively the orbital polarizations.
These results strongly suggest that, in going from the metallic to the
insulating state, the orbital occupation changes in a manner that charge
fluctuations and effective band widths are reduced, that the system becomes
more 1-dimensional and more susceptible to a Peierls-like transition, and that
the required massive orbital switching can only be made if the system is close
to a Mott insulating regime
Rapid and selective absorption of plant defense compounds from the gut of a sequestering insect
Many herbivorous insects exploit defense compounds produced by their host plants for protection against predators. Ingested plant defense compounds are absorbed via the gut epithelium and stored in the body, a physiological process that is currently not well understood. Here, we investigated the absorption of plant defense compounds from the gut in the horseradish flea beetle, Phyllotreta armoraciae, a specialist herbivore known to selectively sequester glucosinolates from its brassicaceous host plants. Feeding experiments using a mixture of glucosinolates and other glucosides not found in the host plants showed a rapid and selective uptake of glucosinolates in adult beetles. In addition, we provide evidence that this uptake mainly takes place in the foregut, whereas the endodermal midgut is the normal region of absorption. Absorption via the foregut epithelium is surprising as the apical membrane is covered by a chitinous intima. However, we could show that this cuticular layer differs in its structure and overall thickness between P. armoraciae and a non-sequestering leaf beetle. In P. armoraciae, we observed a thinner cuticle with a less dense chitinous matrix, which might facilitate glucosinolate absorption. Our results show that a selective and rapid uptake of glucosinolates from the anterior region of the gut contributes to the selective sequestration of glucosinolates in P. armoraciae
Enhancing joint reconstruction and segmentation with non-convex Bregman iteration
All imaging modalities such as computed tomography (CT), emission tomography
and magnetic resonance imaging (MRI) require a reconstruction approach to
produce an image. A common image processing task for applications that utilise
those modalities is image segmentation, typically performed posterior to the
reconstruction. We explore a new approach that combines reconstruction and
segmentation in a unified framework. We derive a variational model that
consists of a total variation regularised reconstruction from undersampled
measurements and a Chan-Vese based segmentation. We extend the variational
regularisation scheme to a Bregman iteration framework to improve the
reconstruction and therefore the segmentation. We develop a novel alternating
minimisation scheme that solves the non-convex optimisation problem with
provable convergence guarantees. Our results for synthetic and real data show
that both reconstruction and segmentation are improved compared to the
classical sequential approach
Accurate photon echo timing by optical freezing of exciton dephasing and rephasing in quantum dots
Semiconductor quantum dots are excellent candidates for ultrafast coherent manipulation of qubits by laser pulses on picosecond timescales or even faster. In inhomogeneous ensembles a macroscopic optical polarization decays rapidly due to dephasing, which, however, is reversible in photon echoes carrying complete information about the coherent ensemble dynamics. Control of the echo emission time is mandatory for applications. Here, we propose a concept to reach this goal. In a two-pulse photon echo sequence, we apply an additional resonant control pulse with multiple of 2π area. Depending on its arrival time, the control slows down dephasing or rephasing of the exciton ensemble during its action. We demonstrate for self-assembled (In,Ga)As quantum dots that the photon echo emission time can be retarded or advanced by up to 5 ps relative to its nominal appearance time without control. This versatile protocol may be used to obtain significantly longer temporal shifts for suitably tailored control pulses
3D Reconstruction of VZV Infected Cell Nuclei and PML Nuclear Cages by Serial Section Array Scanning Electron Microscopy and Electron Tomography
Varicella-zoster virus (VZV) is a human alphaherpesvirus that causes varicella (chickenpox) and herpes zoster (shingles). Like all herpesviruses, the VZV DNA genome is replicated in the nucleus and packaged into nucleocapsids that must egress across the nuclear membrane for incorporation into virus particles in the cytoplasm. Our recent work showed that VZV nucleocapsids are sequestered in nuclear cages formed from promyelocytic leukemia protein (PML) in vitro and in human dorsal root ganglia and skin xenografts in vivo. We sought a method to determine the three-dimensional (3D) distribution of nucleocapsids in the nuclei of herpesvirus-infected cells as well as the 3D shape, volume and ultrastructure of these unique PML subnuclear domains. Here we report the development of a novel 3D imaging and reconstruction strategy that we term Serial Section Array-Scanning Electron Microscopy (SSA-SEM) and its application to the analysis of VZV-infected cells and these nuclear PML cages. We show that SSA-SEM permits large volume imaging and 3D reconstruction at a resolution sufficient to localize, count and distinguish different types of VZV nucleocapsids and to visualize complete PML cages. This method allowed a quantitative determination of how many nucleocapsids can be sequestered within individual PML cages (sequestration capacity), what proportion of nucleocapsids are entrapped in single nuclei (sequestration efficiency) and revealed the ultrastructural detail of the PML cages. More than 98% of all nucleocapsids in reconstructed nuclear volumes were contained in PML cages and single PML cages sequestered up to 2,780 nucleocapsids, which were shown by electron tomography to be embedded and cross-linked by an filamentous electron-dense meshwork within these unique subnuclear domains. This SSA-SEM analysis extends our recent characterization of PML cages and provides a proof of concept for this new strategy to investigate events during virion assembly at the single cell level
Multiple Rabi rotations of trions in InGaAs quantum dots observed by photon echo spectroscopy with spatially shaped laser pulses
We study Rabi rotations arising in intensity-dependent photon echoes from an
ensemble of self-assembled InGaAs quantum dots. To achieve a uniform
distribution of intensities within the excited ensemble, we introduce flattop
intensity profiles of picosecond laser pulses. This allows us to overcome the
damping of Rabi rotations imposed by the spatial inhomogeneity of Rabi
frequencies by a Gaussian laser profile. Using photon echo polarimetry, we
distinguish between the coherent optical responses from exciton and trion
ensembles. Here, we demonstrate that a photo-induced charging of the quantum
dots leads to a significant reduction of the number of neutral quantum dots
under resonant excitation with intensive optical pulses with areas exceeding
. The trion ensemble shows robust Rabi rotations when the area
of the refocussing pulse is increased up to 5.5. We analyze the remaining
attenuation of Rabi rotations by theoretical modeling of excitation induced
dephasing, inhomogeneity of dipole moments, and coupling to acoustic phonons.
The latter is identified as the dominating mechanism resulting in a loss of
optical coherence during the action of the involved optical pulses
Challenges in QCD matter physics - The Compressed Baryonic Matter experiment at FAIR
Substantial experimental and theoretical efforts worldwide are devoted to
explore the phase diagram of strongly interacting matter. At LHC and top RHIC
energies, QCD matter is studied at very high temperatures and nearly vanishing
net-baryon densities. There is evidence that a Quark-Gluon-Plasma (QGP) was
created at experiments at RHIC and LHC. The transition from the QGP back to the
hadron gas is found to be a smooth cross over. For larger net-baryon densities
and lower temperatures, it is expected that the QCD phase diagram exhibits a
rich structure, such as a first-order phase transition between hadronic and
partonic matter which terminates in a critical point, or exotic phases like
quarkyonic matter. The discovery of these landmarks would be a breakthrough in
our understanding of the strong interaction and is therefore in the focus of
various high-energy heavy-ion research programs. The Compressed Baryonic Matter
(CBM) experiment at FAIR will play a unique role in the exploration of the QCD
phase diagram in the region of high net-baryon densities, because it is
designed to run at unprecedented interaction rates. High-rate operation is the
key prerequisite for high-precision measurements of multi-differential
observables and of rare diagnostic probes which are sensitive to the dense
phase of the nuclear fireball. The goal of the CBM experiment at SIS100
(sqrt(s_NN) = 2.7 - 4.9 GeV) is to discover fundamental properties of QCD
matter: the phase structure at large baryon-chemical potentials (mu_B > 500
MeV), effects of chiral symmetry, and the equation-of-state at high density as
it is expected to occur in the core of neutron stars. In this article, we
review the motivation for and the physics programme of CBM, including
activities before the start of data taking in 2022, in the context of the
worldwide efforts to explore high-density QCD matter.Comment: 15 pages, 11 figures. Published in European Physical Journal
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