356,625 research outputs found
Dirac-Brueckner-Hartree-Fock calculations for isospin asymmetric nuclear matter based on improved approximation schemes
We present Dirac-Brueckner-Hartree-Fock calculations for isospin asymmetric
nuclear matter which are based on improved approximations schemes. The
potential matrix elements have been adapted for isospin asymmetric nuclear
matter in order to account for the proton-neutron mass splitting in a more
consistent way. The proton properties are particularly sensitive to this
adaption and its consequences, whereas the neutron properties remains almost
unaffected in neutron rich matter. Although at present full Brueckner
calculations are still too complex to apply to finite nuclei, these
relativistic Brueckner results can be used as a guidance to construct a density
dependent relativistic mean field theory, which can be applied to finite
nuclei. It is found that an accurate reproduction of the
Dirac-Brueckner-Hartree-Fock equation of state requires a renormalization of
these coupling functions.Comment: 34 pages, 9 figures, submitted to Eur. Phys. J.
Momentum, Density, and Isospin dependence of the Symmetric and Asymmetric Nuclear Matter Properties
Properties of symmetric and asymmetric nuclear matter have been investigated
in the relativistic Dirac-Brueckner-Hartree-Fock approach based on projection
techniques using the Bonn A potential. The momentum, density, and isospin
dependence of the optical potentials and nucleon effective masses are studied.
It turns out that the isovector optical potential depends sensitively on
density and momentum, but is almost insensitive to the isospin asymmetry.
Furthermore, the Dirac mass and the nonrelativistic mass
which parametrizes the energy dependence of the single particle spectrum, are
both determined from relativistic Dirac-Brueckner-Hartree-Fock calculations.
The nonrelativistic mass shows a characteristic peak structure at momenta
slightly above the Fermi momentum \kf. The relativistic Dirac mass shows a
proton-neutron mass splitting of in isospin asymmetric
nuclear matter. However, the nonrelativistic mass has a reversed mass splitting
which is in agreement with the results from
nonrelativistic calculations.Comment: 25 pages, 12 figures, to appear in Physical Review
Coupling constants and transition potentials for hadronic decay modes of a meson
Within the independent-harmonic-oscillator model for quarks inside a hadron,
a rigorous method is presented for the calculation of coupling constants and
transition potentials for hadronic decay, as needed in a multi-channel
description of mesons.Comment: 19 pages, 4 figure
Evaluation of an Extended Day Program in the Netherlands: A Randomized Field Experiment
Policies that aim at improving student achievement frequently increase instructional time, for example by means of an extended day program. There is, however, hardly any evidence that these programs are effective, and the few studies that allow causal inference indicate that we should expect neutral to small effects of such programs. This study conducts a randomized field experiment to estimate the effect of an extended day program in seven Dutch elementary schools on math and reading achievement. The empirical results show that this three-month program had a modest but non-significant effect on math, and no significant effect on reading achievement.Extended Day; Increased Instructional Time; Random Assignment; Field Experiment
Web assisted teaching: an undergraduate experience
The emergence of the Internet has created a number of claims as to the future of education and the possibility of dramatically changing the way in which education is delivered. Much of the attention has focussed on the adoption of teaching methods that are solely web-based. We set out to incorporate web-based teaching as support for more traditional teaching methods to improve the learning outcomes for students. This first step into web-based teaching was developed to harness the benefits of web-based teaching tools without supplanting traditional teaching methods.
The aim of this paper is to report our experience with web-assisted teaching in two undergraduate courses, Accounting Information Systems and Management Accounting Services, during 2000. The paper evaluates the approach taken and proposes a tentative framework for developing future web-assisted teaching applications.
We believe that web-assisted and web-based teaching are inevitable outcomes of the telecommunications and computer revolution and that academics cannot afford to become isolated from the on-line world. A considered approach is needed to ensure the integration of web-based features into the overall structure of a course. The components of the course material and the learning experiences students are exposed to need to be structured and delivered in a way that ensures they support student learning rather than replacing one form of learning with another. Therefore a careful consideration of the structure, content, level of detail and time of delivery needs to be integrated to create a course structure that provides a range of student learning experiences that are complimentary rather than competing.
The feedback was positive from both extramural (distance) and internal students, demonstrating to us that web sites can be used as an effective teaching tool in support of more traditional teaching methods as well as a tool for distance education. The ability to harness the positives of the web in conjunction with more traditional teaching modes is one that should not be overlooked in the move to adopt web based instruction methods. Web-based teaching need not be seen as an all or nothing divide but can be used as a useful way of improving the range and type of learning experiences open to students.
The Web challenges traditional methods and thinking but it also provides tools to develop innovative solutions to both distance and on campus learning. Further research is needed to determine how we can best meet the needs of our students while maintaining high quality learning outcomes
Large anomalous magnetic moment in three-dimensional Dirac and Weyl semimetals
We investigate the effect of Coulomb interactions on the electromagnetic
response of three-dimensional Dirac and Weyl semimetals. In a calculation
reminiscent of Schwinger's seminal work on quantum electrodynamics, we find
three physically distinct effects for the anomalous magnetic moment of the
relativisticlike quasiparticles in the semimetal. In the case of nonzero
doping, the anomalous magnetic moment is finite at long wavelengths and
typically orders of magnitude larger than Schwinger's result. We also find
interesting effects of one of the three new Hamiltonian terms on the
topological surface states at the interface between vacuum and a Weyl
semimetal. We conclude that observation of these effects should be within
experimental reach.Comment: 6 pages, 3 figures; second version contains discussion of Rashba spin
orbit coupling due to inversion symmetry breaking at the surfac
On the Variational Principle for Generalized Gibbs Measures
We present a novel approach to establishing the variational principle for
Gibbs and generalized (weak and almost) Gibbs states. Limitations of a
thermodynamical formalism for generalized Gibbs states will be discussed. A new
class of intuitively weak Gibbs measures is introduced, and a typical example
is studied. Finally, we present a new example of a non-Gibbsian measure arising
from an industrial application.Comment: To appear in Markov Processes and Related Fields, Proceedings
workshop Gibbs-nonGibb
Observation of Stueckelberg oscillations in dipole-dipole interactions
We have observed Stueckelberg oscillations in the dipole-dipole interaction
between Rydberg atoms with an externally applied radio-frequency field. The
oscillating RF field brings the interaction between cold Rydberg atoms in two
separated volumes into resonance. We observe multi-photon transitions when
varying the amplitude of the RF-field and the static electric field offset. The
angular momentum states we use show a quadratic Stark shift, which leads to a
fundamentally different behavior than linearly shifting states. Both cases are
studied theoretically using the Floquet approach and are compared. The
amplitude of the sidebands, related to the interaction strength, is given by
the Bessel function in the linearly shifting case and by the generalized Bessel
function in the quadratically shifting case. The oscillatory behavior of both
functions corresponds to Stueckelberg oscillations, an interference effect
described by the semi-classical Landau-Zener-Stueckelberg model. The
measurements prove coherent dipole-dipole interaction during at least 0.6
micro-seconds
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