130 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
Surface acoustic wave propagation in graphene film
Surface acoustic wave (SAW) propagation in a graphene film on the surface of piezoelectric crystals was studied at the BESSY II synchrotron radiation source. Talbot effect enabled the visualization of the SAW propagation on the crystal surface with the graphene film in a real time mode, and high-resolution x-ray diffraction permitted the determination of the SAW amplitude in the graphene/piezoelectric crystal system. The influence of the SAW on the electrical properties of the graphene film was examined. It was shown that the changing of the SAW amplitude enables controlling the magnitude and direction of current in graphene film on the surface of piezoelectric crystals
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
Breit-Wheeler Process in Intense Short Laser Pulses
Energy-angular distributions of electron-positron pair creation in collisions
of a laser beam and a nonlaser photon are calculated using the -matrix
formalism. The laser field is modeled as a finite pulse, similar to the
formulation introduced in our recent paper in the context of Compton scattering
[Phys. Rev. A {\bf 85}, 062102 (2012)]. The nonperturbative regime of pair
creation is considered here. The energy spectra of created particles are
compared with the corresponding spectra obtained using the modulated plane wave
approximation for the driving laser field. A very good agreement in these two
cases is observed, provided that the laser pulse is sufficiently long. For
short pulse durations, this agreement breaks down. The sensitivity of pair
production to the polarization of a driving pulse is also investigated. We show
that in the nonperturbative regime, the pair creation yields depend on the
polarization of the pulse, reaching their maximal values for the linear
polarization. Therefore, we focus on this case. Specifically, we analyze the
dependence of pair creation on the relative configuration of linear
polarizations of the laser pulse and the nonlaser photon. Lastly, we
investigate the carrier-envelope phase effect on angular distributions of
created particles, suggesting the possibility of phase control in relation to
the pair creation processes.Comment: 13 pages, 8 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
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
Observation of sagittal X ray diffraction by surface acoustic waves in Bragg geometry
X-ray Bragg diffraction in sagittal geometry on a Y-cut langasite crystal (La(3)Ga(5)SiO(14)) modulated by Λ = 3 µm Rayleigh surface acoustic waves was studied at the BESSY II synchrotron radiation facility. Owing to the crystal lattice modulation by the surface acoustic wave diffraction, satellites appear. Their intensity and angular separation depend on the amplitude and wavelength of the ultrasonic superlattice. Experimental results are compared with the corresponding theoretical model that exploits the kinematical diffraction theory. This experiment shows that the propagation of the surface acoustic waves creates a dynamical diffraction grating on the crystal surface, and this can be used for space–time modulation of an X-ray beam
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
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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