184 research outputs found
Correlated two-photon emission by transitions of Dirac-Volkov states in intense laser fields: QED predictions
In an intense laser field, an electron may decay by emitting a pair of
photons. The two photons emitted during the process, which can be interpreted
as a laser-dressed double Compton scattering, remain entangled in a
quantifiable way: namely, the so-called concurrence of the photon polarizations
gives a gauge-invariant measure of the correlation of the hard gamma rays. We
calculate the differential rate and concurrence for a backscattering setup of
the electron and photon beam, employing Volkov states and propagators for the
electron lines, thus accounting nonperturbatively for the electron-laser
interaction. The nonperturbative results are shown to differ significantly
compared to those obtained from the usual double Compton scattering.Comment: 32 pages, 12 figure
Friedmann Universes and Exact Solutions in String Cosmology
We show that the classical null strings generate the Hilbert-Einstein gravity
corresponding to D-dimensional Friedmann universes.Comment: 8 pages, LATE
Muon pair creation from positronium in a circularly polarized laser field
We study elementary particle reactions that result from the interaction of an
atomic system with a very intense laser wave of circular polarization. As a
specific example, we calculate the rate for the laser-driven reaction , where the electron and positron originate from a positronium
atom or, alternatively, from a nonrelativistic plasma. We distinguish
accordingly between the coherent and incoherent channels of the process. Apart
from numerical calculations, we derive by analytical means compact formulas for
the corresponding reaction rates. The rate for the coherent channel in a laser
field of circular polarization is shown to be damped because of the destructive
interference of the partial waves that constitute the positronium ground-state
wave packet. Conditions for the observation of the process via the dominant
incoherent channel in a circularly polarized field are pointed out
Scanning Electron Microscopy Observation of the Interaction Between the Surface Acoustic Waves and Regular Domain Structures in the LiNbO3 Crystals
This paper reports a scanning electron microscope study of the interaction between the surface acoustic waves and regular domain structures in LiNbO3 crystals. The regular domain structures in LiNbO3 crystals were formed by the method of the thermo-electric treatment after growth. We investigated two modes of interaction: the surface-acoustic-waves propagate along and across the regular domain structures. It is shown that the regular domain structures in the first case can be used as an acoustical wave-guide, because the power-flow vector of the surface acoustic waves has the direction along the domain structure. Also we observed that the surface acoustic wave inverts the voltage contrast of the image in the scanning electron microscope by π during the process of the propagation across the domain walls
Image Registration and Predictive Modeling: Learning the Metric on the Space of Diffeomorphisms
We present a method for metric optimization in the Large Deformation Diffeomorphic Metric Mapping (LDDMM) framework, by treating the induced Riemannian metric on the space of diffeomorphisms as a kernel in a machine learning context. For simplicity, we choose the kernel Fischer Linear Discriminant Analysis (KLDA) as the framework. Optimizing the kernel parameters in an Expectation-Maximization framework, we define model fidelity via the hinge loss of the decision function. The resulting algorithm optimizes the parameters of the LDDMM norm-inducing differential operator as a solution to a group-wise registration and classification problem. In practice, this may lead to a biology-aware registration, focusing its attention on the predictive task at hand such as identifying the effects of disease. We first tested our algorithm on a synthetic dataset, showing that our parameter selection improves registration quality and classification accuracy. We then tested the algorithm on 3D subcortical shapes from the Schizophrenia cohort Schizconnect. Our Schizophrenia-Control predictive model showed significant improvement in ROC AUC compared to baseline parameters
Caustic structures in the spectrum of x-ray Compton scattering off electrons driven by a short intense laser pulse
We study the Compton scattering of x-rays off electrons that are driven by a
relativistically intense short optical laser pulse. The frequency spectrum of
the laser-assisted Compton radiation shows a broad plateau in the vicinity of
the laser-free Compton line due to a nonlinear mixing between x-ray and laser
photons. Special emphasis is placed on how the shape of the short assisting
laser pulse affects the spectrum of the scattered x-rays. In particular, we
observe sharp peak structures in the plateau region, whose number and locations
are highly sensitive to the laser pulse shape. These structures are interpreted
as spectral caustics by using a semiclassical analysis of the laser-assisted
QED matrix element
String Propagation in Bianchi Type I models: Dynamical anisotropy Damping and Consequences
A generic ansatz is introduced which provides families of exact solutions to
the equations of motion and constraints for null-strings in Bianchi type I
cosmological models. This is achieved irrespective of the form of the metric.
Within classes of dilaton cosmologies a backreaction mapping relation is
established where the null string leads to more or less anisotropic members of
the family. The equations of motion and constraints for the generic model are
casted in their first order form and integrated both analytically and
numerically.Comment: 37 pages, 11 figure
Tailoring surface properties, biocompatibility and corrosion behavior of stainless steel by laser induced periodic surface treatment towards developing biomimetic stents
Laser-Induced Periodic Surface Structures (LIPSS) holds great potential for regenerative biomedicine. Creating highly precise LIPSS enables to generate biomimetic implant surfaces with improved properties. The present study focuses on the fabrication and investigation of laser-treated stainless steel samples with applied linear LIPSS patterns with grooves made by means of a picosecond laser system using wavelengths of 1064 nm and 532 nm. To investigate properties of the laser-treated surfaces and to understand the basics of cell-surface interactions between the LIPSS and human Umbilical Cord Mesenchymal Stem Cells (UCMSC), flat stainless steel samples with various applied nanopatterns were used. Such LIPSSs demonstrated higher surface roughness, good biocompatibility, lower wettability and higher corrosion resistance compared to the untreated (polished) spec-imens. The surface roughness of laser-treated samples was in microscale that enabled adhesion and migration of endothelial cells, thus increasing the likelihood for endothelialisation. This thereby could reduce the chances for the development of Late Stent Thrombosis (LST) and In-Stent Restenosis (ISR). Furthermore, laser textured surfaces demonstrated an environment supportive for cell attachment, proliferation and alignment with the nanogroves. Therefore, application of the biomimetic nanopatterns could help to overcome frequent post-surgery complications after the stent implantation
Association between prenatal alcohol exposure and children's facial shape:a prospective population-based cohort study
Tensionless String in the Notoph Background
We study the interaction between a tensionless (null) string and an
antisymmetric background field B_{ab} using a 2-component spinor formalism. A
geometric condition for the absence of such an interaction is formulated. We
show that only one gauge-invariant degree of freedom of the field B_{ab} does
not satisfy this condition. Identification of this degree of freedom with the
notoph field \phi of Ogievetskii-Polubarinov-Kalb-Ramond is suggested.
Application of a two-component spinor formalism allows us a reduction of the
complete system of non-linear partial differential equations and constraints
governing the interacting null string dynamics to a system of linear
differential equations for the basis spinors of the spin-frame. We find that
total effect of the interaction is contained in a single derivation coefficient
which is identified with the notoph field.Comment: 15 pages, no figures, RevTeX 3.
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