3,553 research outputs found
Domain structure of epitaxial Co films with perpendicular anisotropy
Epitaxial hcp Cobalt films with pronounced c-axis texture have been prepared
by pulsed lased deposition (PLD) either directly onto Al2O3 (0001) single
crystal substrates or with an intermediate Ruthenium buffer layer. The crystal
structure and epitaxial growth relation was studied by XRD, pole figure
measurements and reciprocal space mapping. Detailed VSM analysis shows that the
perpendicular anisotropy of these highly textured Co films reaches the
magnetocrystalline anisotropy of hcp-Co single crystal material. Films were
prepared with thickness t of 20 nm < t < 100 nm to study the crossover from
in-plane magnetization to out-of-plane magnetization in detail. The analysis of
the periodic domain pattern observed by magnetic force microscopy allows to
determine the critical minimum thickness below which the domains adopt a pure
in-plane orientation. Above the critical thickness the width of the stripe
domains is evaluated as a function of the film thickness and compared with
domain theory. Especially the discrepancies at smallest film thicknesses show
that the system is in an intermediate state between in-plane and out-of-plane
domains, which is not described by existing analytical domain models
Magnetic helicity and cosmological magnetic field
The magnetic helicity has paramount significance in nonlinear saturation of
galactic dynamo. We argue that the magnetic helicity conservation is violated
at the lepton stage in the evolution of early Universe. As a result, a
cosmological magnetic field which can be a seed for the galactic dynamo obtains
from the beginning a substantial magnetic helicity which has to be taken into
account in the magnetic helicity balance at the later stage of galactic dynamo.Comment: 11 pages, no figures; v3: new references and new paragraphs added,
discussion extended, some mistypings correcte
Astrophysical significance of the anisotropic kinetic alpha effect
The generation of large scale flows by the anisotropic kinetic alpha (AKA)
effect is investigated in simulations with a suitable time-dependent space- and
time-periodic anisotropic forcing lacking parity invariance. The forcing
pattern moves relative to the fluid, which leads to a breaking of the Galilean
invariance as required for the AKA effect to exist. The AKA effect is found to
produce a clear large scale flow pattern when the Reynolds number, R, is small
as only a few modes are excited in linear theory. In this case the
non-vanishing components of the AKA tensor are dynamically independent of the
Reynolds number. For larger values of R, many more modes are excited and the
components of the AKA tensor are found to decrease rapidly with increasing
value of R. However, once there is a magnetic field (imposed and of sufficient
strength, or dynamo-generated and saturated) the field begins to suppress the
AKA effect, regardless of the value of R. It is argued that the AKA effect is
unlikely to be astrophysically significant unless the magnetic field is weak
and R is small.Comment: 8 pages, 10 figures, submitted to A&
Preliminary Results on gamma gamma -> Ks K pi from CLEO
We analyzed 13.8 fb^{-1} of the integrated e+e- luminosity collected at 10.6
GeV center-of-mass energy with the CLEO II and II.V detectors to study
exclusive two-photon production of single hadronic resonances. We searched for
hadrons decaying into Ks K pi when both leptons remain undetected. In this
analysis we studied the detection efficiency and evaluated systematic errors
using independent data samples. We estimated 90% CL upper limits on the
products of the two-photon partial widths of (pseudo)scalar hadrons with masses
below 1.7 GeV/c2 and their branching fractions into Ks K pi. Our preliminary
results are marginally consistent with the first observation of eta(1440) in
two-photon collisions by the L3 experiment.Comment: 4 pages, 1 figure, proceedings contribution for PANIC'0
The equivalent circuit approach for the electrical diagnostics of dielectric barrier discharges: The classical theory and recent developments
Measurements of current and voltage are the basic diagnostics for electrical discharges. However, in the case of dielectric barrier discharges (DBDs), the measured current and voltage waveforms are influenced by the discharge reactor geometry, and thus, interpretation of measured quantities is required to determine the discharge properties. This contribution presents the main stages of the development of electrical diagnostics of DBDs, which are based on lumped electrical elements. The compilation and revision of the contributions to the equivalent circuit approach are targeted to indicate: (1) the interconnection between the stage of development, (2) its applicability, and (3) the current state-of-the-art of this approach. © 2019 by the authors
Turbulent transport in hydromagnetic flows
The predictive power of mean-field theory is emphasized by comparing theory
with simulations under controlled conditions. The recently developed test-field
method is used to extract turbulent transport coefficients both in kinematic as
well as nonlinear and quasi-kinematic cases. A striking example of the
quasi-kinematic method is provided by magnetic buoyancy-driven flows that
produce an alpha effect and turbulent diffusion.Comment: 17 pages, 6 figures, topical issue of Physica Scripta on turbulent
mixing and beyon
Optimization of a charge-state analyzer for ECRIS beams
A detailed experimental and simulation study of the extraction of a 24 keV
He-ion beam from an ECR ion source and the subsequent beam transport through an
analyzing magnet is presented. We find that such a slow ion beam is very
sensitive to space-charge forces, but also that the neutralization of the
beam's space charge by secondary electrons is virtually complete for beam
currents up to at least 0.5 mA. The beam emittance directly behind the
extraction system is 65 pi mm mrad and is determined by the fact that the ion
beam is extracted in the strong magnetic fringe field of the ion source. The
relatively large emittance of the beam and its non-paraxiality lead, in
combination with a relatively small magnet gap, to significant beam losses and
a five-fold increase of the effective beam emittance during its transport
through the analyzing magnet. The calculated beam profile and phase-space
distributions in the image plane of the analyzing magnet agree well with
measurements. The kinematic and magnet aberrations have been studied using the
calculated second-order transfer map of the analyzing magnet, with which we can
reproduce the phase-space distributions of the ion beam behind the analyzing
magnet. Using the transfer map and trajectory calculations we have worked out
an aberration compensation scheme based on the addition of compensating
hexapole components to the main dipole field by modifying the shape of the
poles. The simulations predict that by compensating the kinematic and geometric
aberrations in this way and enlarging the pole gap the overall beam transport
efficiency can be increased from 16 to 45%
Dissociation in a polymerization model of homochirality
A fully self-contained model of homochirality is presented that contains the
effects of both polymerization and dissociation. The dissociation fragments are
assumed to replenish the substrate from which new monomers can grow and undergo
new polymerization. The mean length of isotactic polymers is found to grow
slowly with the normalized total number of corresponding building blocks.
Alternatively, if one assumes that the dissociation fragments themselves can
polymerize further, then this corresponds to a strong source of short polymers,
and an unrealistically short average length of only 3. By contrast, without
dissociation, isotactic polymers becomes infinitely long.Comment: 16 pages, 6 figures, submitted to Orig. Life Evol. Biosp
On the precession of the isolated pulsar PSR B1828-11
Analysis of both pulse timing and pulse shape variations of the isolated
pulsar PSR B1828-11 shows highly correlated and strong Fourier power at periods
\~ 1000, 500, and 250 d (Stairs et al. 2000). The only description based on a
free precession of star's rigid crust coupled to the magnetic dipole torque,
explains the 500-component, as the fundamental Fourier frequency, with its
harmonic 250-component (Link & Epstein 2001). In this paper, we show that if
the dipole moment vector varies with time with a period nearly equal to the
longest (probably fundamental) observed period (~ 1000 d), the dipole torque
may produce the all other harmonics. We also find the second and fourth
harmonics at periods ~ 500 and 250 d are dominant for small wobble angle ~ 3^o
and large field's inclination angle 89^o.Comment: 11 pages, discussion is change
Monte-Carlo simulation of events with Drell-Yan lepton pairs from antiproton-proton collisions
The complete knowledge of the nucleon spin structure at leading twist
requires also addressing the transverse spin distribution of quarks, or
transversity, which is yet unexplored because of its chiral-odd nature.
Transversity can be best extracted from single-spin asymmetries in fully
polarized Drell-Yan processes with antiprotons, where valence contributions are
involved anyway. Alternatively, in single-polarized Drell-Yan the transversity
happens convoluted with another chiral-odd function, which is likely to be
responsible for the well known (and yet unexplained) violation of the Lam-Tung
sum rule in the corresponding unpolarized cross section. We present Monte-Carlo
simulations for the unpolarized and single-polarized Drell-Yan at different center-of-mass energies in both
configurations where the antiproton beam hits a fixed proton target or it
collides on another proton beam. The goal is to estimate the minimum number of
events needed to extract the above chiral-odd distributions from future
measurements at the HESR ring at GSI. It is important to study the feasibility
of such experiments at HESR in order to demonstrate that interesting spin
physics can be explored already using unpolarized antiprotons.Comment: Deeply revised text with improved discussion of kinematics and
results; added one table; 12 figures. Accepted for publication in Phys. Rev.
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