937 research outputs found
A universal Hamiltonian for the motion and the merging of Dirac cones in a two-dimensional crystal
We propose a simple Hamiltonian to describe the motion and the merging of
Dirac points in the electronic spectrum of two-dimensional electrons. This
merging is a topological transition which separates a semi-metallic phase with
two Dirac cones from an insulating phase with a gap. We calculate the density
of states and the specific heat. The spectrum in a magnetic field B is related
to the resolution of a Schrodinger equation in a double well potential. They
obey the general scaling law e_n \propto B^{2/3} f_n(Delta /B^{2/3}. They
evolve continuously from a sqrt{n B} to a linear (n+1/2)B dependence, with a
[(n+1/2)B]^{2/3} dependence at the transition. The spectrum in the vicinity of
the topological transition is very well described by a semiclassical
quantization rule. This model describes continuously the coupling between
valleys associated with the two Dirac points, when approaching the transition.
It is applied to the tight-binding model of graphene and its generalization
when one hopping parameter is varied. It remarkably reproduces the low field
part of the Rammal-Hofstadter spectrum for the honeycomb lattice.Comment: 18 pages, 15 figure
Hadron Production in Heavy Ion Collisions
We review hadron production in heavy ion collisions with emphasis on pion and
kaon production at energies below 2 AGeV and on partonic collectivity at RHIC
energies.Comment: 31 pages, 26 figures, accepted for publication in Landolt-Boernstein
Volume 1-23
Designing for interaction
At present, the design of computer-supported group-based learning (CS)GBL) is often based on subjective decisions regarding tasks, pedagogy and technology, or concepts such as ‘cooperative learning’ and ‘collaborative learning’. Critical review reveals these concepts as insufficiently substantial to serve as a basis for (CS)GBL design. Furthermore, the relationship between outcome and group interaction is rarely specified a priori. Thus, there is a need for a more systematic approach to designing (CS)GBL that focuses on the elicitation of expected interaction processes. A framework for such a process-oriented methodology is proposed. Critical elements that affect interaction are identified: learning objectives, task-type, level of pre-structuring, group size and computer support. The proposed process-oriented method aims to stimulate designers to adopt a more systematic approach to (CS)GBL design according to the interaction expected, while paying attention to critical elements that affect interaction. This approach may bridge the gap between observed quality of interaction and learning outcomes and foster (CS)GBL design that focuses on the heart of the matter: interaction
Stability of circular orbits of spinning particles in Schwarzschild-like space-times
Circular orbits of spinning test particles and their stability in
Schwarzschild-like backgrounds are investigated. For these space-times the
equations of motion admit solutions representing circular orbits with particles
spins being constant and normal to the plane of orbits. For the de Sitter
background the orbits are always stable with particle velocity and momentum
being co-linear along them. The world-line deviation equations for particles of
the same spin-to-mass ratios are solved and the resulting deviation vectors are
used to study the stability of orbits. It is shown that the orbits are stable
against radial perturbations. The general criterion for stability against
normal perturbations is obtained. Explicit calculations are performed in the
case of the Schwarzschild space-time leading to the conclusion that the orbits
are stable.Comment: eps figures, submitted to General Relativity and Gravitatio
Systematics of collective correlation energies from self-consistent mean-field calculations
The collective ground-state correlations stemming from low-lying quadrupole
excitations are computed microscopically. To that end, the self-consistent
mean-field model is employed on the basis of the Skyrme-Hartre-Fock (SHF)
functional augmented by BCS pairing. The microscopic-macroscopic mapping is
achieved by quadrupole-constrained mean-field calculations which are processed
further in the generator-coordinate method (GCM) at the level of the Gaussian
overlap approximation (GOA).
We study the correlation effects on energy, charge radii, and surface
thickness for a great variety of semi-magic nuclei. A key issue is to work out
the influence of variations of the SHF functional. We find that collective
ground-state correlations (GSC) are robust under change of nuclear bulk
properties (e.g., effective mass, symmetry energy) or of spin-orbit coupling.
Some dependence on the pairing strength is observed. This, however, does not
change the general conclusion that collective GSC obey a general pattern and
that their magnitudes are rather independent of the actual SHF parameters.Comment: 13 pages, 13 figure
Construction of Field Algebras with Quantum Symmetry from Local Observables
It has been discussed earlier that ( weak quasi-) quantum groups allow for
conventional interpretation as internal symmetries in local quantum theory.
From general arguments and explicit examples their consistency with (braid-)
statistics and locality was established. This work addresses to the
reconstruction of quantum symmetries and algebras of field operators. For every
algebra \A of observables satisfying certain standard assumptions, an
appropriate quantum symmetry is found. Field operators are obtained which act
on a positive definite Hilbert space of states and transform covariantly under
the quantum symmetry. As a substitute for Bose/Fermi (anti-) commutation
relations, these fields are demonstrated to obey local braid relation.Comment: 50 pages, HUTMP 93-B33
Density dependent hadron field theory for neutron stars with antikaon condensates
We investigate and condensation in -equilibrated
hyperonic matter within a density dependent hadron field theoretical model. In
this model, baryon-baryon and (anti)kaon-baryon interactions are mediated by
the exchange of mesons. Density dependent meson-baryon coupling constants are
obtained from microscopic Dirac Brueckner calculations using Groningen and Bonn
A nucleon-nucleon potential. It is found that the threshold of antikaon
condensation is not only sensitive to the equation of state but also to
antikaon optical potential depth. Only for large values of antikaon optical
potential depth, condensation sets in even in the presence of negatively
charged hyperons. The threshold of condensation is always reached
after condensation. Antikaon condensation makes the equation of state
softer thus resulting in smaller maximum mass stars compared with the case
without any condensate.Comment: 20 pages, 7 figures; final version to appear in Physical Review
Learning difficulties : a portuguese perspective of a universal issue
In this article we present findings of a study that was conducted with the purpose of deepening the knowledge about the field of learning difficulties in Portugal. Therefore, within these findings we will discuss across several cultural boundaries, themes related with the existence of learning difficulties as a construct, the terminology, the political, social and scientific influences on the field, and the models of identification and of ongoing school support for students. While addressing the above-mentioned themes we will draw attention to the different, yet converging, international understandings of learning difficulties
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