130 research outputs found
Reflectance and photoluminescence characterization of CdS and CdSe heteroepitaxial films deposited by laser ablation technique
Optical properties of CdSe and CdS films, deposited on sapphire substrate by means of pulsed laser ablation technique, have been investigated in order to study the effect of such a transparent substrate on the photoluminescence efficiency of the deposited epilayers. CdSe and CdS films present intrinsic (excitonic) emission at low temperature, differently from the same films deposited on quartz. The temperature dependence of the excitonic energy has been analyzed taking into account the contribution of both the thermal dilatation and electron-phonon interaction
Order-of-magnitude speedup for steady states and traveling waves via Stokes preconditioning in Channelflow and Openpipeflow
Steady states and traveling waves play a fundamental role in understanding
hydrodynamic problems. Even when unstable, these states provide the
bifurcation-theoretic explanation for the origin of the observed states. In
turbulent wall-bounded shear flows, these states have been hypothesized to be
saddle points organizing the trajectories within a chaotic attractor. These
states must be computed with Newton's method or one of its generalizations,
since time-integration cannot converge to unstable equilibria. The bottleneck
is the solution of linear systems involving the Jacobian of the Navier-Stokes
or Boussinesq equations. Originally such computations were carried out by
constructing and directly inverting the Jacobian, but this is unfeasible for
the matrices arising from three-dimensional hydrodynamic configurations in
large domains. A popular method is to seek states that are invariant under
numerical time integration. Surprisingly, equilibria may also be found by
seeking flows that are invariant under a single very large Backwards-Euler
Forwards-Euler timestep. We show that this method, called Stokes
preconditioning, is 10 to 50 times faster at computing steady states in plane
Couette flow and traveling waves in pipe flow. Moreover, it can be carried out
using Channelflow (by Gibson) and Openpipeflow (by Willis) without any changes
to these popular spectral codes. We explain the convergence rate as a function
of the integration period and Reynolds number by computing the full spectra of
the operators corresponding to the Jacobians of both methods.Comment: in Computational Modelling of Bifurcations and Instabilities in Fluid
Dynamics, ed. Alexander Gelfgat (Springer, 2018
Nucleosynthesis Calculations for the Ejecta of Neutron Star Coalescences
We present the results of fully dynamical r-process network calculations for
the ejecta of neutron star mergers (NSMs). The late stages of the inspiral and
the final violent coalescence of a neutron star binary have been calculated in
detail using a 3D hydrodynamics code (Newtonian gravity plus backreaction
forces emerging from the emission of gravitational waves) and a realistic
nuclear equation of state. The found trajectories for the ejecta serve as input
for dynamical r-process calculations where all relevant nuclear reactions
(including beta-decays depositing nuclear energy in the expanding material) are
followed. We find that all the ejected material undergoes r-process. For an
initial Ye close to 0.1 the abundance distributions reproduce very accurately
the solar r-process pattern for nuclei with A above 130. For lighter nuclei
strongly underabundant (as compared to solar) distributions are encountered. We
show that this behaviour is consistent with the latest observations of very
old, metal-poor stars, despite simplistic arguments that have recently been
raised against the possibility of NSM as possible sources of Galactic r-process
material.Comment: 5 pages, 2 figures, proceedings of Nuclei in the Cosmos 2000, to be
published in Nucl. Phys. A; minor correctio
Real-Time Online Monitoring of the Ion Range by Means of Prompt Secondary Radiations
International audiencePrompt secondary radiations such as gamma rays and protons can be used for ion-range monitoring during ion therapy either on an energy-slice basis or on a pencil-beam basis. We present a review of the ongoing activities in terms of detector developments, imaging, experimental and theoretical physics issues concerning the correlation between the physical dose and hadronic processe
Managing design and designers for strategic renewal
In this article, we propose a framework for understanding how design and designers may contribute to strategic renewal in producers of traditional and hi-tech consumer durables. Building on a study of outstanding innovators in product design, we describe design-driven renewal as a four-phase process stimulated and supported by design, combining continuous product innovation with the periodic revision of the strategic course of the company. For each phase, we discuss the specific role of managers and the most common pitfalls that arise from poor management of the process
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