359 research outputs found
Entanglement and four wave mixing effects in the dissipation free nonlinear interaction of two photons at a single atom
We investigate the nonlinear interaction between two photons in a single
input pulse at an atomic two level nonlinearity. A one dimensional model for
the propagation of light to and from the atom is used to describe the precise
spatiotemporal coherence of the two photon state. It is shown that the
interaction generates spatiotemporal entanglement in the output state similar
to the entanglement observed in parametric downconversion. A method of
generating photon pairs from coherent pump light using this quantum mechanical
four wave mixing process is proposed.Comment: 10 pages, including 3 figures, correction in eq.(7), updated
references, final version for publication in PR
Teacher interventions in small group work in secondary mathematics and science lessons
Collaborative problem solving, when students work in pairs or small groups on a curriculum-related task, has become an increasingly common feature of classroom education. This paper reports a study of a topic which has received relatively little attention: how teachers can most usefully intervene when students are working in a group, but have encountered some sort of problem. The data used comes from a large scale interventional study of mathematics and science teaching in secondary schools in south east England, in which interactions between teachers and students were recorded in their usual classrooms. We identify the typical problem situations which lead to teachers’ interventions, and describe the different ways teachers were observed to intervene. We examine the different types of intervention, and consider how effective they are in helping group work proceed in a productive manner. Finally, we offer some conclusions about the practical implications of these findings.This is the final version of the article. It was first available from Taylor & Francis via http://dx.doi.org/10.1080/09500782.2015.112536
Quest for a Nuclear Georeactor
Knowledge about the interior of our planet is mainly based on the
interpretation of seismic data from earthquakes and nuclear explosions, and of
composition of meteorites. Additional observations have led to a wide range of
hypotheses on the heat flow from the interior to the crust, the abundance of
certain noble gases in gasses vented from volcanoes and the possibility of a
nuclear georeactor at the centre of the Earth. This paper focuses on a proposal
for an underground laboratory to further develop antineutrinos as a tool to map
the distribution of radiogenic heat sources, such as the natural radionuclides
and the hypothetical nuclear georeactor.Comment: Invited talk presented at the International Symposium on Radiation
Physics, Cape Town, 2003. Manuscript is submitted to Radiation Physics and
Chemistr
Collective modes of asymmetric nuclear matter in Quantum HadroDynamics
We discuss a fully relativistic Landau Fermi liquid theory based on the
Quantum Hadro-Dynamics () effective field picture of Nuclear Matter
({\it NM}).
From the linearized kinetic equations we get the dispersion relations of the
propagating collective modes. We focus our attention on the dynamical effects
of the interplay between scalar and vector channel contributions. A beautiful
``mirror'' structure in the form of the dynamical response in the
isoscalar/isovector degree of freedom is revealed, with a complete parallelism
in the role respectively played by the compressibility and the symmetry energy.
All that strongly supports the introduction of an explicit coupling to the
scalar-isovector channel of the nucleon-nucleon interaction. In particular we
study the influence of this coupling (to a -meson-like effective field)
on the collective response of asymmetric nuclear matter (). Interesting
contributions are found on the propagation of isovector-like modes at normal
density and on an expected smooth transition to isoscalar-like oscillations at
high baryon density. Important ``chemical'' effects on the neutron-proton
structure of the mode are shown. For dilute we have the isospin
distillation mechanism of the unstable isoscalar-like oscillations, while at
high baryon density we predict an almost pure neutron wave structure of the
propagating sounds.Comment: 18 pages (LATEX), 8 Postscript figures, uses "epsfig
A microscopic estimate of the nuclear matter compressibility and symmetry energy in relativistic mean-field models
The relativistic mean-field plus RPA calculations, based on effective
Lagrangians with density-dependent meson-nucleon vertex functions, are employed
in a microscopic analysis of the nuclear matter compressibility and symmetry
energy. We compute the isoscalar monopole and the isovector dipole response of
Pb, as well as the differences between the neutron and proton radii for
Pb and several Sn isotopes. The comparison of the calculated excitation
energies with the experimental data on the giant monopole resonance in
Pb, restricts the nuclear matter compression modulus of structure
models based on the relativistic mean-field approximation to MeV. The isovector giant dipole resonance in Pb, and the
available data on differences between neutron and proton radii, limit the range
of the nuclear matter symmetry energy at saturation (volume asymmetry) to 32
MeV 36 MeV.Comment: 16 pages, 6 figure
Relativistic Random-Phase Approximation with density-dependent meson-nucleon couplings
The matrix equations of the relativistic random-phase approximation (RRPA)
are derived for an effective Lagrangian characterized by density-dependent
meson-nucleon vertex functions. The explicit density dependence of the
meson-nucleon couplings introduces rearrangement terms in the residual two-body
interaction, that are essential for a quantitative description of excited
states. Illustrative calculations of the isoscalar monopole, isovector dipole
and isoscalar quadrupole response of Pb, are performed in the fully
self-consistent RRPA framework, based on effective interactions with a
phenomenological density dependence adjusted to nuclear matter and ground-state
properties of spherical nuclei. The comparison of the RRPA results on multipole
giant resonances with experimental data constrains the parameters that
characterize the isoscalar and isovector channel of the density-dependent
effective interactions.Comment: RevTeX, 8 eps figures, submitted to Phys. Rev.
Density Functional Theory for a Confined Fermi System with Short-Range Interaction
Effective field theory (EFT) methods are applied to density functional theory
(DFT) as part of a program to systematically go beyond mean-field approaches to
medium and heavy nuclei. A system of fermions with short-range, natural
interactions and an external confining potential (e.g., fermionic atoms in an
optical trap) serves as a laboratory for studying DFT/EFT. An effective action
formalism leads to a Kohn-Sham DFT by applying an inversion method
order-by-order in the EFT expansion parameter. Representative results showing
the convergence of Kohn-Sham calculations at zero temperature in the local
density approximation (LDA) are compared to Thomas-Fermi calculations and to
power-counting estimates.Comment: 36 pages, 20 figures, RevTeX
Relativistic Mean Field Model with Generalized Derivative Nucleon-Meson Couplings
The quantum hadrodynamics (QHD) model with minimal nucleon-meson couplings is
generalized by introducing couplings of mesons to derivatives of the nucleon
field in the Lagrangian density. This approach allows an effective description
of a state-dependent in-medium interaction in the mean-field approximation.
Various parametrizations for the generalized couplings are developed and
applied to infinite nuclear matter. In this approach, scalar and vector
self-energies depend on both density and momentum similarly as in the
Dirac-Brueckner theory. The Schr\"{o}diger-equivalent optical potential is much
less repulsive at high nucleon energies as compared to standard relativistic
mean field models and thus agrees better with experimental findings. The
derivative couplings in the extended model have significant effects on
properties of symmetric nuclear matter and neutron matter.Comment: 35 pages, 1 table, 10 figure
- …