333 research outputs found
Proton pygmy resonances: predictions for N=20 isotones
We study theoretically the low-energy electric-dipole response of N=20
isotones. We present results from a quasiparticle random-phase approximation
(QRPA) and a continuum random-phase approximation (CRPA), and we compare them
with results for the mirror Z=20 nuclei. According to our analysis, enhanced E1
strength is expected energetically well below the giant dipole resonance in the
proton-rich isotones. Large amounts of E1 strength in the asymmetric N=20
isotones are predicted, unlike their equally asymmetric Z=20 mirror nuclei,
pointing unambiguously to the role of structural effects such as loose binding.
A proton-skin oscillation could develop especially in 46Fe. The proper
description of non localized threshold transitions and the nucleon effective
mass in mean-field treatments may affect theoretical predictions. We call for
systematic theoretical investigations to quantify the role bulk-matter
properties, in anticipation of measurements of E1 transitions in proton-rich
nuclei.Comment: 10 pages, incl. 9 figures and 2 tables; v2: some rephrasing and
clarifications, corrected Fig.
Learning morphological phenomena of Modern Greek an exploratory approach
This paper presents a computational model for the description of concatenative morphological phenomena of modern Greek (such as inflection, derivation and compounding) to allow learners, trainers and developers to explore linguistic processes through their own constructions in an interactive openâended multimedia environment. The proposed model introduces a new language metaphor, the âpuzzleâmetaphorâ (similar to the existing âturtleâmetaphorâ for concepts from mathematics and physics), based on a visualized unificationâlike mechanism for pattern matching. The computational implementation of the model can be used for creating environments for learning through design and learning by teaching
Fine structure of the isoscalar giant quadrupole resonance in 40Ca due to Landau damping?
The fragmentation of the Isoscalar Giant Quadrupole Resonance (ISGQR) in 40Ca
has been investigated in high energy-resolution experiments using proton
inelastic scattering at E_p = 200 MeV. Fine structure is observed in the region
of the ISGQR and its characteristic energy scales are extracted from the
experimental data by means of a wavelet analysis. The experimental scales are
well described by Random Phase Approximation (RPA) and second-RPA calculations
with an effective interaction derived from a realistic nucleon-nucleon
interaction by the Unitary Correlation Operator Method (UCOM). In these results
characteristic scales are already present at the mean-field level pointing to
their origination in Landau damping, in contrast to the findings in heavier
nuclei and also to SRPA calculations for 40Ca based on phenomenological
effective interactions, where fine structure is explained by the coupling to
two-particle two-hole (2p-2h) states.Comment: Phys. Lett. B, in pres
Isoscalar dipole coherence at low energies and forbidden E1 strength
In 16O and 40Ca an isoscalar, low-energy dipole transition (IS-LED)
exhausting approximately 4% of the isoscalar dipole (ISD) energy-weighted sum
rule is experimentally known, but conspicuously absent from recent theoretical
investigations of ISD strength. The IS-LED mode coincides with the so-called
isospin-forbidden E1 transition. We report that for N=Z nuclei up to 100Sn the
fully self-consistent Random-Phase-Approximation with finite-range forces,
phenomenological and realistic, yields a collective IS-LED mode, typically
overestimating its excitation energy, but correctly describing its IS strength
and electroexcitation form factor. The presence of E1 strength is solely due to
the Coulomb interaction between the protons and the resulting isospin-symmetry
breaking. The smallness of its value is related to the form of the transition
density, due to translational invariance. The calculated values of E1 and ISD
strength carried by the IS-LED depend on the effective interaction used.
Attention is drawn to the possibility that in N-not-equal-Z nuclei this
distinct mode of IS surface vibration can develop as such or mix strongly with
skin modes and thus influence the pygmy dipole strength as well as the ISD
strength function. In general, theoretical models currently in use may be unfit
to predict its precise position and strength, if at all its existence.Comment: 9 pages, 6 figures, EPJA submitte
Universality of the Ising and the S=1 model on Archimedean lattices: A Monte Carlo determination
The Ising model S=1/2 and the S=1 model are studied by efficient Monte Carlo
schemes on the (3,4,6,4) and the (3,3,3,3,6) Archimedean lattices. The
algorithms used, a hybrid Metropolis-Wolff algorithm and a parallel tempering
protocol, are briefly described and compared with the simple Metropolis
algorithm. Accurate Monte Carlo data are produced at the exact critical
temperatures of the Ising model for these lattices. Their finite-size analysis
provide, with high accuracy, all critical exponents which, as expected, are the
same with the well known 2d Ising model exact values. A detailed finite-size
scaling analysis of our Monte Carlo data for the S=1 model on the same lattices
provides very clear evidence that this model obeys, also very well, the 2d
Ising model critical exponents. As a result, we find that recent Monte Carlo
simulations and attempts to define effective dimensionality for the S=1 model
on these lattices are misleading. Accurate estimates are obtained for the
critical amplitudes of the logarithmic expansions of the specific heat for both
models on the two Archimedean lattices.Comment: 9 pages, 11 figure
Missed prediction of the neutron halo in Mg
Halo phenomena have long been an important frontier in both experimental and
theoretical nuclear physics. Mg was identified as a halo nucleus in 2014
and remains the heaviest nuclear halo system to date. While the halo phenomenon
in Mg was not predicted before the discovery, its description has been
still challenging afterwards. In this Letter, we report a microscopic,
self-consistent, and density-functional independent description of the neutron
halo in Mg by the deformed relativistic Hartree-Bogoliubov theory in
continuum (DRHBc) that was developed in 2010. The experimental neutron
separation energies and empirical matter radii of neutron-rich magnesium
isotopes as well as the deformed -wave halo characteristics of Mg are
well reproduced without any free parameters. The DRHBc theory investigated only
even-even magnesium isotopes in previous works and for that reason missed
predicting Mg as a halo nucleus before 2014. Although the core and the
halo of Mg are both prolate, higher-order shape decoupling on the
hexadecapole and hexacontatetrapole levels is predicted.Comment: 8 pages, 4 figures, 1 tabl
Critical aspects of three-dimensional anisotropic spin-glass models
We study the three-dimensional Ising model with a longitudinal
anisotropic bond randomness on the simple cubic lattice. The random exchange
interaction is applied only in the direction, whereas in the other two
directions, - planes, we consider ferromagnetic exchange. By implementing
an effective parallel tempering scheme, we outline the phase diagram of the
model and compare it to the corresponding isotropic one, as well as to a
previously studied anisotropic (transverse) case. We present a detailed
finite-size scaling analysis of the ferromagnetic - paramagnetic and spin glass
- paramagnetic transition lines, and we also discuss the ferromagnetic - spin
glass transition regime. We conclude that the present model shares the same
universality classes with the isotropic model, but at the symmetric point has a
considerably higher transition temperature from the spin-glass state to the
paramagnetic phase. Our data for the ferromagnetic - spin glass transition line
are supporting a forward behavior in contrast to the reentrant behavior of the
isotropic model.Comment: 10 pages, 9 eps figures, 1 table, corrected symbolis
Collective excitations in the Unitary Correlation Operator Method and relativistic QRPA studies of exotic nuclei
The collective excitation phenomena in atomic nuclei are studied in two
different formulations of the Random Phase Approximation (RPA): (i) RPA based
on correlated realistic nucleon-nucleon interactions constructed within the
Unitary Correlation Operator Method (UCOM), and (ii) relativistic RPA (RRPA)
derived from effective Lagrangians with density-dependent meson-exchange
interactions. The former includes the dominant interaction-induced short-range
central and tensor correlations by means of an unitary transformation. It is
shown that UCOM-RPA correlations induced by collective nuclear vibrations
recover a part of the residual long-range correlations that are not explicitly
included in the UCOM Hartree-Fock ground state. Both RPA models are employed in
studies of the isoscalar monopole resonance (ISGMR) in closed-shell nuclei
across the nuclide chart, with an emphasis on the sensitivity of its properties
on the constraints for the range of the UCOM correlation functions. Within the
Relativistic Quasiparticle RPA (RQRPA) based on Relativistic Hartree-Bogoliubov
model, the occurrence of pronounced low-lying dipole excitations is predicted
in nuclei towards the proton drip-line. From the analysis of the transition
densities and the structure of the RQRPA amplitudes, it is shown that these
states correspond to the proton pygmy dipole resonance.Comment: 15 pages, 4 figures, submitted to Physics of Atomic Nuclei,
conference proceedings, "Frontiers in the Physics of Nucleus", St.
Petersburg, 28. June-1. July, 200
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