8,650 research outputs found
Mean-Field Description of Fusion Barriers with Skyrme's Interaction
Fusion barriers are determined in the framework of the Skyrme energy-density
functional together with the semi-classical approach known as the Extended
Thomas-Fermi method. The barriers obtained in this way with the Skyrme
interaction SkM* turn out to be close to those generated by phenomenological
models like those using the proximity potentials. It is also shown that the
location and the structure of the fusion barrier in the vicinity of its maximum
and beyond can be quite accurately described by a simple analytical form
depending only on the masses and the relative isospin of target and projectile
nucleus.Comment: 7 pages, latex, 5 figure
Requisite variety and intercultural teams: to what extent is Ashby's law useful ?
Requisite variety and intercultural teams: To what extent is Ashby's law useful? The “Law of Requisite Variety” (LRV) is frequently evoked to explain the design, functioning and performance of intercultural teams. But to what extent does the law really enhance understanding in this particular field? The authors consider that LRV has rarely been questioned in-depth in management studies. The paper briefly details LRV in the cybernetics context before “translating” it to social systems, organizations and intercultural teams. Using a qualitative case-study method, the case of an intercultural team is analysed and questioned from the perspective of LRV. The results show that LRV superficially fits the composition of this team, but is unable to explain the human and social dynamics that evolve during the work process.law of requisite variety; requisite variety; intercultural teams ; complexity ; diversity; systems; case study
1.6 GHz VLBI Observations of SN 1979C: almost-free expansion
We report on 1.6 GHz Very-Long-Baseline-Interferometry (VLBI) observations of
supernova SN 1979C made on 18 November 2002. We derive a model-dependent
supernova size. We also present a reanalysis of VLBI observations made by us on
June 1999 and by other authors on February 2005. We conclude that, contrary to
our earlier claim of strong deceleration in the expansion, SN 1979C has been
undergoing almost-free expansion (; ) for over
25 years.Comment: 4 pages, 4 figures; submitted to A&A on 14 May 2009. Accepted on 7
Jul 200
Semiclassical approaches to nuclear dynamics
The extended Gutzwiller trajectory approach is presented for the
semiclassical description of nuclear collective dynamics, in line with the main
topics of the fruitful activity of V.G. Solovjov. Within the Fermi-liquid
droplet model, the leptodermous effective surface approximation was applied to
calculations of energies, sum rules and transition densities for the
neutron-proton asymmetry of the isovector giant-dipole resonance and found to
be in good agreement with the experimental data. By using the Strutinsky shell
correction method, the semiclassical collective transport coefficients such as
nuclear inertia, friction, stiffness, and moments of inertia can be derived
beyond the quantum perturbation approximation of the response function theory
and the cranking model.The averaged particle-number dependence of the low-lying
collective vibrational states are described in good agreement with basic
experimental data, mainly due to an enhancement of the collective inertia as
compared to its irrotational flow value. Shell components of the moment of
inertia are derived in terms of the periodic-orbit free-energy shell
corrections. A good agreement between the semiclassical extended Thomas-Fermi
moments of inertia with shell corrections and the quantum results is obtained
for different nuclear deformations and particle numbers. Shell effects are
shown to be exponentially dampted out with increasing temperature in all the
transport coefficients.Comment: 83 pages, 39 figures, 4 tables, corrected typos and improved Englis
Fission-Fragment Mass Distribution and Particle Evaporation at low Energies
Fusion-fission dynamics is investigated with a special emphasis on fusion
reactions at low energy for which shell effects and pairing correlations can
play a crucial role leading in particular to multi-modal fission. To follow the
dynamical evolution of an excited and rotating nucleus we solve a 2-dimensional
Langevin equation taking explicitly light-particle evaporation into account.
The confrontation theory-experiment is demonstrated to give interesting
information on the model presented, its qualities as well as its shortcomings.Comment: 19 pages, latex, 24 eps-figure
A critical examination of compound stability predictions from machine-learned formation energies
Machine learning has emerged as a novel tool for the efficient prediction of material properties, and claims have been made that machine-learned models for the formation energy of compounds can approach the accuracy of Density Functional Theory (DFT). The models tested in this work include five recently published compositional models, a baseline model using stoichiometry alone, and a structural model. By testing seven machine learning models for formation energy on stability predictions using the Materials Project database of DFT calculations for 85,014 unique chemical compositions, we show that while formation energies can indeed be predicted well, all compositional models perform poorly on predicting the stability of compounds, making them considerably less useful than DFT for the discovery and design of new solids. Most critically, in sparse chemical spaces where few stoichiometries have stable compounds, only the structural model is capable of efficiently detecting which materials are stable. The nonincremental improvement of structural models compared with compositional models is noteworthy and encourages the use of structural models for materials discovery, with the constraint that for any new composition, the ground-state structure is not known a priori. This work demonstrates that accurate predictions of formation energy do not imply accurate predictions of stability, emphasizing the importance of assessing model performance on stability predictions, for which we provide a set of publicly available tests
Radio emission of SN1993J: the complete picture. I. Re-analysis of all the available VLBI data
We have performed a complete re-calibration and re-analysis of all the
available VLBI observations of supernova SN1993J, following an homogeneous and
well-defined methodology. Observations of SN1993J at 69 epochs, spanning 13
years, were performed by two teams, which used different strategies and
analysis tools. The results obtained by each group are similar, but their
conclusions on the supernova expansion and the shape and evolution of the
emitting region differ significantly. From our analysis of the combined set of
observations, we have obtained an expansion curve with unprecedented time
resolution and coverage. We find that the data from both teams are compatible
when analyzed with the same methodology. One expansion index () is enough to model the expansion observed at 1.7\,GHz, while two
expansion indices ( and ), separated
by a break time, days, are needed to model the data, at
frequencies higher than 1.7\,GHz, up to day 4000 after explosion. We thus
confirm the wavelength dependence of the size of the emitting region reported
by one of the groups. We also find that all sizes measured at epochs later than
day 4000 after explosion are systematically smaller than our model predictions.
We estimate the fractional shell width (, average of all epochs
and frequencies) and the level of opacity to the radio emission by the ejecta.
We find evidence of a spectral-index radial gradient in the supernova shell,
which is indicative of a frequency-dependent ejecta opacity. Finally, we study
the distribution and evolution of the azimuthal anisotropies (hot spots) found
around the radio shell during the expansion. These anisotropies have
intensities of % of the mean flux density of the shell, and appear to
systematically evolve during the expansion.Comment: 13 pages, 9 figures, accepted for publication in A&
The First VLBI Image of the Young, Oxygen-Rich Supernova Remnant in NGC 4449
We report on sensitive 1.4-GHz VLBI radio observations of the unusually
luminous supernova remnant SNR 4449-1 in the galaxy NGC 4449, which gave us the
first well-resolved image of this object. The remnant's radio morphology
consists of two approximately parallel bright ridges, suggesting similarities
to the barrel shape seen for many older Galactic supernova remnants or possibly
to SN 1987A. The angular extent of the remnant is 65 x 40 mas, corresponding to
(3.7 x 2.3) x 10^{18} (D/3.8 Mpc) cm. We also present a new, high
signal-to-noise optical spectrum. By comparing the remnant's linear size to the
maximum velocities measured from optical lines, as well as using constraints
from historical images, we conclude that the supernova explosion occurred
between ~1905 and 1961, likely around 1940. The age of the remnant is therefore
likely ~70 yr. We find that SNR 4449-1's shock wave is likely still interacting
with the circumstellar rather than interstellar medium.Comment: 7 pages, Accepted for publication in MNRA
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