4,279 research outputs found
Effects of the Background Turbulence on the Relaxation of Ion Temperature Anisotropy in Space Plasmas
Turbulence in space plasmas usually exhibits two regimes separated by a
spectral break that divides the so called inertial and kinetic ranges. Large
scale magnetic fluctuations are dominated by non-linear MHD wave-wave
interactions following a -5/3 or -2 slope power-law spectrum. After the break,
at scales in which kinetic effects take place, the magnetic spectrum follows a
steeper power-law shape given by a spectral index .
Despite its ubiquitousness, the possible effects of a turbulent background
spectrum in the quasilinear relaxation of solar wind temperatures are usually
not considered. In this work, a quasilinear kinetic theory is used to study the
evolution of the proton temperatures in an initially turbulent collisionless
plasma composed by cold electrons and bi-Maxwellian protons, in which
electromagnetic waves propagate along a background magnetic field. Four wave
spectrum shapes are compared with different levels of wave intensity. We show
that a sufficient turbulent magnetic power can drive stable protons to
transverse heating, resulting in an increase in the temperature anisotropy and
the reduction of the parallel proton beta. Thus, stable proton velocity
distribution can evolve in such a way as to develop kinetic instabilities. This
may explain why the constituents of the solar wind can be observed far from
thermodynamic equilibrium and near the instability thresholds.Comment: 19 pages, 6 figures, accepted for publication in Frontiers in Physic
Sensitivity of the g-mode frequencies to pulsation codes and their parameters
From the recent work of the Evolution and Seismic Tools Activity (ESTA,
Lebreton et al. 2006; Monteiro et al. 2008), whose Task 2 is devoted to compare
pulsational frequencies computed using most of the pulsational codes available
in the asteroseismic community, the dependence of the theoretical frequencies
with non-physical choices is now quite well fixed. To ensure that the accuracy
of the computed frequencies is of the same order of magnitude or better than
the observational errors, some requirements in the equilibrium models and the
numerical resolutions of the pulsational equations must be followed. In
particular, we have verified the numerical accuracy obtained with the Saclay
seismic model, which is used to study the solar g-mode region (60 to
140Hz). We have compared the results coming from the Aarhus adiabatic
pulsation code (ADIPLS), with the frequencies computed with the Granada Code
(GraCo) taking into account several possible choices. We have concluded that
the present equilibrium models and the use of the Richardson extrapolation
ensure an accuracy of the order of in the determination of the
frequencies, which is quite enough for our purposes.Comment: 10 pages, 5 figures, accepted in Solar Physic
Recovering coherence from decoherence: a method of quantum state reconstruction
We present a feasible scheme for reconstructing the quantum state of a field
prepared inside a lossy cavity. Quantum coherences are normally destroyed by
dissipation, but we show that at zero temperature we are able to retrieve
enough information about the initial state, making possible to recover its
Wigner function as well as other quasiprobabilities. We provide a numerical
simulation of a Schroedinger cat state reconstruction.Comment: 8 pages, in RevTeX, 4 figures, accepted for publication in Phys. Rev.
A (november 1999
The Question of Low-Lying Intruder States in and Neighboring Nuclei
The presence of not yet detected intruder states in e.g. a
intruder at 9 excitation would affect the shape of the -delayed alpha spectra of and . In order to test the
plausibility of this assumption, shell model calculations with up to excitations in (and up to excitations in
) were performed. With the above restrictions on the model spaces, the
calculations did not yield any low-lying intruder state in . Another
approach -the simple deformed oscillator model with self-consistent frequencies
and volume conservation gives an intruder state in which is lower in
energy than the above shell model results, but its energy is still considerably
higher than 9 .Comment: 16 pages (RevTeX), 1 PS figure. To appear in Phys. Rev.
Orbital M1 versus E2 strength in deformed nuclei: A new energy weighted sum rule
Within the unified model of Bohr and Mottelson we derive the following linear
energy weighted sum rule for low energy orbital 1 excitations in even-even
deformed nuclei S_{\rm LE}^{\rm lew} (M_1^{\rm orb}) \cong (6/5) \epsilon
(B(E2; 0^+_1 \rightarrow 2_1^+ K=0)/Z e^2^2) \mu^2_N with B(E2) the E2
strength for the transition from the ground state to the first excited state in
the ground state rotational band, the charge r.m.s. radius squared and
the binding energy per nucleon in the nuclear ground state. It is
shown that this energy weighted sum rule is in good agreement with available
experimental data. The sum rule is derived using a simple ansatz for the
intrinsic ground state wave function that predicts also high energy 1
strength at 2 carrying 50\% of the total moment of the
orbital M1 operator.Comment: REVTEX (3.0), 9 pages, RU924
Giant and reversible extrinsic magnetocaloric effects in La0.7Ca0.3MnO3 films due to strain
Large thermal changes driven by a magnetic field have been proposed for
environmentally friendly energy efficient refrigeration, but only a few
materials which suffer hysteresis show these giant magnetocaloric effects. Here
we create giant and reversible extrinsic magnetocaloric effects in epitaxial
films of the ferromagnetic manganite La0.7Ca0.3MnO3 using strain mediated
feedback from BaTiO3 substrates near a first-order structural phase transition.
Our findings should inspire the discovery of giant magnetocaloric effects in a
wide range of magnetic materials, and the parallel development of
nanostructured bulk samples for practical applications.Comment: 32 pages, 1 Table, 5 figures, supplementary informatio
Non-Scissors-Mode Behaviour of Isovector Magnetic Dipole Orbital Transitions Involving Isospin Transfer
We study the response of isovector orbital magnetic dipole (IOMD) transitions
to the quadrupole-quadrupole () interaction, to the
isospin-conserving pairing interaction (ICP) and to combinations of both. We
find qualitatively different behaviours for transitions in which the final
isospin differs from the initial isospin versus cases where the two isospins
are the same. For even-even nuclei with ground states
such as and , the summed IOMD from the ground
state to all the states in the space does not
vanish when the interaction is turned off. The pairing interaction
(ICP) alone leads to a finite transition rate. For nuclei with
ground states such as and , the summed IOMD
vanish when the interaction is turned off, as is expected in
a good scissors-mode behaviour. However this is not the case for the
corresponding sum of the IOMD transitions. In (but not
in ) the sum of the IOMD transitions is remarkably
insensitive to the strengths of both the and the ICP interactions.
In an energy weighted-sum is similarly insensitive. All our
calculations were carried out in the space.Comment: 19 pages (including 5 figures). submitted to Nucl. Phys.
Charge and matter distributions and form factors of light, medium and heavy neutron-rich nuclei
Results of charge form factors calculations for several unstable neutron-rich
isotopes of light, medium and heavy nuclei (He, Li, Ni, Kr, Sn) are presented
and compared to those of stable isotopes in the same isotopic chain. For the
lighter isotopes (He and Li) the proton and neutron densities are obtained
within a microscopic large-scale shell-model, while for heavier ones Ni, Kr and
Sn the densities are calculated in deformed self-consistent mean-field Skyrme
HF+BCS method. We also compare proton densities to matter densities together
with their rms radii and diffuseness parameter values. Whenever possible
comparison of form factors, densities and rms radii with available experimental
data is also performed. Calculations of form factors are carried out both in
plane wave Born approximation (PWBA) and in distorted wave Born approximation
(DWBA). These form factors are suggested as predictions for the future
experiments on the electron-radioactive beam colliders where the effect of the
neutron halo or skin on the proton distributions in exotic nuclei is planned to
be studied and thereby the various theoretical models of exotic nuclei will be
tested.Comment: 26 pages, 11 figures, 3 tables, accepted for publication in Phys.
Rev.
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