110 research outputs found
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Optimization of the Epitaxial Growth of Undoped GaN Waveguides in GaN-Based Laser Diodes Evaluated by Photoluminescence
Non-intentionally doped c-plane GaN layers are generally employed as p-side waveguide layers in violet/blue-emitting laser diodes. The recombination and diffusion of charge carriers in the p-side GaN waveguide influence the injection efficiency of holes into the InGaN quantum wells of these devices. In this study, the non-radiative recombination and the diffusivity in the [000-1] direction for charge carriers in such GaN layers are investigated by the photoluminescence of buried InGaN quantum wells, in addition to the GaN photoluminescence. The vertical charge carrier diffusion length and the diffusion constant in GaN were determined by evaluating the intensity from InGaN quantum wells in different depths below a top GaN layer. Additionally, the intensity from the buried InGaN quantum wells was found to be more sensitive to variations in the non-radiative recombination rate in the GaN layer than the intensity from the GaN itself. The study enables conclusions to be drawn on how the growth of a p-side GaN waveguide layer has to be optimized: (1) The charge carrier diffusivity in the [000-1] direction at device operation temperature is limited by phonon scattering and can be only slightly improved by material quality. (2) The use of TMGa (trimethylgallium) instead of TEGa (triethylgallium) as a precursor for the growth of GaN lowers the background silicon doping level and is advantageous for a large hole diffusion length. (3) Small growth rates below 0.5 ÎĽm/h when using TMGa or below 0.12 ÎĽm/h when using TEGa enhance non-radiative recombination. (4) A V/III gas ratio of 2200 or more is needed for low non-radiative recombination rates in GaN
Pion Interferometry for a Granular Source of Quark-Gluon Plasma Droplets
We examine the two-pion interferometry for a granular source of quark-gluon
plasma droplets. The evolution of the droplets is described by relativistic
hydrodynamics with an equation of state suggested by lattice gauge results.
Pions are assumed to be emitted thermally from the droplets at the freeze-out
configuration characterized by a freeze-out temperature . We find that the
HBT radius decreases if the initial size of the droplets decreases.
On the other hand, depends on the droplet spatial distribution and
is relatively independent of the droplet size. It increases with an increase in
the width of the spatial distribution and the collective-expansion velocity of
the droplets. As a result, the value of can lie close to
for a granular quark-gluon plasma source. The granular model of the emitting
source may provide an explanation to the RHIC HBT puzzle and may lead to a new
insight into the dynamics of the quark-gluon plasma phase transition.Comment: 5 pages, 4 figure
Correlated Gravitational Wave and Neutrino Signals from General-Relativistic Rapidly Rotating Iron Core Collapse
We present results from a new set of 3D general-relativistic hydrodynamic
simulations of rotating iron core collapse. We assume octant symmetry and focus
on axisymmetric collapse, bounce, the early postbounce evolution, and the
associated gravitational wave (GW) and neutrino signals. We employ a
finite-temperature nuclear equation of state, parameterized electron capture in
the collapse phase, and a multi-species neutrino leakage scheme after bounce.
The latter captures the important effects of deleptonization, neutrino cooling
and heating and enables approximate predictions for the neutrino luminosities
in the early evolution after core bounce. We consider 12-solar-mass and
40-solar-mass presupernova models and systematically study the effects of (i)
rotation, (ii) progenitor structure, and (iii) postbounce neutrino leakage on
dynamics, GW, and, neutrino signals. We demonstrate, that the GW signal of
rapidly rotating core collapse is practically independent of progenitor mass
and precollapse structure. Moreover, we show that the effects of neutrino
leakage on the GW signal are strong only in nonrotating or slowly rotating
models in which GW emission is not dominated by inner core dynamics. In rapidly
rotating cores, core bounce of the centrifugally-deformed inner core excites
the fundamental quadrupole pulsation mode of the nascent protoneutron star. The
ensuing global oscillations (f~700-800 Hz) lead to pronounced oscillations in
the GW signal and correlated strong variations in the rising luminosities of
antineutrino and heavy-lepton neutrinos. We find these features in cores that
collapse to protoneutron stars with spin periods <~ 2.5 ms and rotational
energies sufficient to drive hyper-energetic core-collapse supernova
explosions. Hence, joint GW + neutrino observations of a core collapse event
could deliver strong evidence for or against rapid core rotation. [abridged]Comment: 29 pages, 14 figures. Replaced with version matching published
versio
Equation of State in Relativistic Magnetohydrodynamics: variable versus constant adiabatic index
The role of the equation of state for a perfectly conducting, relativistic
magnetized fluid is the main subject of this work. The ideal constant
-law equation of state, commonly adopted in a wide range of
astrophysical applications, is compared with a more realistic equation of state
that better approximates the single-specie relativistic gas. The paper focus on
three different topics. First, the influence of a more realistic equation of
state on the propagation of fast magneto-sonic shocks is investigated. This
calls into question the validity of the constant -law equation of state
in problems where the temperature of the gas substantially changes across
hydromagnetic waves. Second, we present a new inversion scheme to recover
primitive variables (such as rest-mass density and pressure) from conservative
ones that allows for a general equation of state and avoids catastrophic
numerical cancellations in the non-relativistic and ultrarelativistic limits.
Finally, selected numerical tests of astrophysical relevance (including
magnetized accretion flows around Kerr black holes) are compared using
different equations of state. Our main conclusion is that the choice of a
realistic equation of state can considerably bear upon the solution when
transitions from cold to hot gas (or viceversa) are present. Under these
circumstances, a polytropic equation of state can significantly endanger the
solution.Comment: 14 pages, 14 figure
Application of 233Â nm far-UVC LEDs for eradication of MRSA and MSSA and risk assessment on skin models
A newly developed UVC LED source with an emission wavelength of 233 nm was proved on bactericidal efficacy and skin tolerability. The bactericidal efficacy was qualitatively analysed using blood agar test. Subsequently, quantitative analyses were performed on germ carrier tests using the MRSA strain DSM11822, the MSSA strain DSM799, S. epidermidis DSM1798 with various soil loads. Additionally, the compatibility of the germicidal radiation doses on excised human skin and reconstructed human epidermis was proved. Cell viability, DNA damage and production of radicals were assessed in comparison to typical UVC radiation from discharge lamps (222 nm, 254 nm) and UVB (280-380 nm) radiation for clinical assessment. At a dose of 40 mJ/cm(2), the 233 nm light source reduced the viable microorganisms by a log(10) reduction (LR) of 5 log(10) levels if no soil load was present. Mucin and protein containing soil loads diminished the effect to an LR of 1.5-3.3. A salt solution representing artificial sweat (pH 8.4) had only minor effects on the reduction. The viability of the skin models was not reduced and the DNA damage was far below the damage evoked by 0.1 UVB minimal erythema dose, which can be regarded as safe. Furthermore, the induced damage vanished after 24 h. Irradiation on four consecutive days also did not evoke DNA damage. The radical formation was far lower than 20 min outdoor visible light would cause, which is classified as low radical load and can be compensated by the antioxidant defence system
Scalar field induced oscillations of neutron stars and gravitational collapse
We study the interaction of massless scalar fields with self-gravitating
neutron stars by means of fully dynamic numerical simulations of the
Einstein-Klein-Gordon perfect fluid system. Our investigation is restricted to
spherical symmetry and the neutron stars are approximated by relativistic
polytropes. Studying the nonlinear dynamics of isolated neutron stars is very
effectively performed within the characteristic formulation of general
relativity, in which the spacetime is foliated by a family of outgoing light
cones. We are able to compactify the entire spacetime on a computational grid
and simultaneously impose natural radiative boundary conditions and extract
accurate radiative signals. We study the transfer of energy from the scalar
field to the fluid star. We find, in particular, that depending on the
compactness of the neutron star model, the scalar wave forces the neutron star
either to oscillate in its radial modes of pulsation or to undergo
gravitational collapse to a black hole on a dynamical timescale. The radiative
signal, read off at future null infinity, shows quasi-normal oscillations
before the setting of a late time power-law tail.Comment: 12 pages, 13 figures, submitted to Phys. Rev.
A New Open-Source Code for Spherically-Symmetric Stellar Collapse to Neutron Stars and Black Holes
We present the new open-source spherically-symmetric general-relativistic
(GR) hydrodynamics code GR1D. It is based on the Eulerian formulation of GR
hydrodynamics (GRHD) put forth by Romero-Ibanez-Gourgoulhon and employs
radial-gauge, polar-slicing coordinates in which the 3+1 equations simplify
substantially. We discretize the GRHD equations with a finite-volume scheme,
employing piecewise-parabolic reconstruction and an approximate Riemann solver.
GR1D is intended for the simulation of stellar collapse to neutron stars and
black holes and will also serve as a testbed for modeling technology to be
incorporated in multi-D GR codes. Its GRHD part is coupled to various
finite-temperature microphysical equations of state in tabulated form that we
make available with GR1D. An approximate deleptonization scheme for the
collapse phase and a neutrino-leakage/heating scheme for the postbounce epoch
are included and described. We also derive the equations for effective rotation
in 1D and implement them in GR1D. We present an array of standard test
calculations and also show how simple analytic equations of state in
combination with presupernova models from stellar evolutionary calculations can
be used to study qualitative aspects of black hole formation in failing
rotating core-collapse supernovae. In addition, we present a simulation with
microphysical EOS and neutrino leakage/heating of a failing core-collapse
supernova and black hole formation in a presupernova model of a 40 solar mass
zero-age main-sequence star. We find good agreement on the time of black hole
formation (within 20%) and last stable protoneutron star mass (within 10%) with
predictions from simulations with full Boltzmann neutrino radiation
hydrodynamics.Comment: 25 pages, 6 figures, 2 appendices. Accepted for publication to the
Classical and Quantum Gravity special issue for MICRA2009. Code may be
downloaded from http://www.stellarcollapse.org Update: corrected title, small
modifications suggested by the referees, added source term derivation in
appendix
EvoL: The new Padova T-SPH parallel code for cosmological simulations - I. Basic code: gravity and hydrodynamics
We present EvoL, the new release of the Padova N-body code for cosmological
simulations of galaxy formation and evolution. In this paper, the basic Tree +
SPH code is presented and analysed, together with an overview on the software
architectures. EvoL is a flexible parallel Fortran95 code, specifically
designed for simulations of cosmological structure formation on cluster,
galactic and sub-galactic scales. EvoL is a fully Lagrangian self-adaptive
code, based on the classical Oct-tree and on the Smoothed Particle
Hydrodynamics algorithm. It includes special features such as adaptive
softening lengths with correcting extra-terms, and modern formulations of SPH
and artificial viscosity. It is designed to be run in parallel on multiple CPUs
to optimize the performance and save computational time. We describe the code
in detail, and present the results of a number of standard hydrodynamical
tests.Comment: 33 pages, 49 figures, accepted on A&
Simulation techniques for cosmological simulations
Modern cosmological observations allow us to study in great detail the
evolution and history of the large scale structure hierarchy. The fundamental
problem of accurate constraints on the cosmological parameters, within a given
cosmological model, requires precise modelling of the observed structure. In
this paper we briefly review the current most effective techniques of large
scale structure simulations, emphasising both their advantages and
shortcomings. Starting with basics of the direct N-body simulations appropriate
to modelling cold dark matter evolution, we then discuss the direct-sum
technique GRAPE, particle-mesh (PM) and hybrid methods, combining the PM and
the tree algorithms. Simulations of baryonic matter in the Universe often use
hydrodynamic codes based on both particle methods that discretise mass, and
grid-based methods. We briefly describe Eulerian grid methods, and also some
variants of Lagrangian smoothed particle hydrodynamics (SPH) methods.Comment: 42 pages, 16 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 12; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
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