16,067 research outputs found
Statistical multifragmentation model with discretized energy and the generalized Fermi breakup. I. Formulation of the model
The Generalized Fermi Breakup recently demonstrated to be formally equivalent
to the Statistical Multifragmentation Model, if the contribution of excited
states are included in the state densities of the former, is implemented. Since
this treatment requires the application of the Statistical Multifragmentation
Model repeatedly on the hot fragments until they have decayed to their ground
states, it becomes extremely computational demanding, making its application to
the systems of interest extremely difficult. Based on exact recursion formulae
previously developed by Chase and Mekjian to calculate the statistical weights
very efficiently, we present an implementation which is efficient enough to
allow it to be applied to large systems at high excitation energies. Comparison
with the GEMINI++ sequential decay code shows that the predictions obtained
with our treatment are fairly similar to those obtained with this more
traditional model.Comment: 8 pages, 6 figure
Dipole-induced vortex ratchets in superconducting films with arrays of micromagnets
We investigate the transport properties of superconducting films with
periodic arrays of in-plane magnetized micromagnets. Two different magnetic
textures are studied: a square array of magnetic bars and a close-packed array
of triangular microrings. As confirmed by MFM imaging, the magnetic state of
both systems can be adjusted to produce arrays of almost point-like magnetic
dipoles. By carrying out transport measurements with ac drive, we observed
experimentally a recently predicted ratchet effect induced by the interaction
between superconducting vortices and the magnetic dipoles. Moreover, we find
that these magnetic textures produce vortex-antivortex patterns, which have a
crucial role on the transport properties of this hybrid system.Comment: 4 pages, 4 figure
Orbital magnetism in axially deformed sodium clusters: From scissors mode to dia-para magnetic anisotropy
Low-energy orbital magnetic dipole excitations, known as scissors mode (SM),
are studied in alkali metal clusters. Subsequent dynamic and static effects are
explored. The treatment is based on a self-consistent microscopic approach
using the jellium approximation for the ionic background and the Kohn-Sham mean
field for the electrons. The microscopic origin of SM and its main features
(structure of the mode in light and medium clusters, separation into low- and
high-energy plasmons, coupling high-energy M1 scissors and E2 quadrupole
plasmons, contributions of shape isomers, etc) are discussed. The scissors M1
strength acquires large values with increasing cluster size. The mode is
responsible for the van Vleck paramagnetism of spin-saturated clusters. Quantum
shell effects induce a fragile interplay between Langevin diamagnetism and van
Vleck paramagnetism and lead to a remarkable dia-para anisotropy in magnetic
susceptibility of particular light clusters. Finally, several routes for
observing the SM experimentally are discussed.Comment: 21 pages, 7 figure
Orbital Magnetic Dipole Mode in Deformed Clusters: A Fully Microscopic Analysis
The orbital M1 collective mode predicted for deformed clusters in a schematic
model is studied in a self-consistent random-phase-approximation approach which
fully exploits the shell structure of the clusters. The microscopic mechanism
of the excitation is clarified and the close correlation with E2 mode
established. The study shows that the M1 strength of the mode is fragmented
over a large energy interval. In spite of that, the fraction remaining at low
energy, well below the overwhelming dipole plasmon resonance, is comparable to
the strength predicted in the schematic model. The importance of this result in
view of future experiments is stressed.Comment: 10 pages, 3 Postscript figures, uses revte
Averaging out magnetic forces with fast rf-sweeps in an optical trap for metastable chromium atoms
We introduce a novel type of time-averaged trap, in which the internal state
of the atoms is rapidly modulated to modify magnetic trapping potentials. In
our experiment, fast radiofrequency (rf) linear sweeps flip the spin of atoms
at a fast rate, which averages out magnetic forces. We use this procedure to
optimize the accumulation of metastable chomium atoms into an optical dipole
trap from a magneto-optical trap. The potential experienced by the metastable
atoms is identical to the bare optical dipole potential, so that this procedure
allows for trapping all magnetic sublevels, hence increasing by up to 80
percent the final number of accumulated atoms.Comment: 4 pages, 4 figure
The Statistical Multifragmentation Model with Skyrme Effective Interactions
The Statistical Multifragmentation Model is modified to incorporate the
Helmholtz free energies calculated in the finite temperature Thomas-Fermi
approximation using Skyrme effective interactions. In this formulation, the
density of the fragments at the freeze-out configuration corresponds to the
equilibrium value obtained in the Thomas-Fermi approximation at the given
temperature. The behavior of the nuclear caloric curve at constant volume is
investigated in the micro-canonical ensemble and a plateau is observed for
excitation energies between 8 and 10 MeV per nucleon. A kink in the caloric
curve is found at the onset of this gas transition, indicating the existence of
a small excitation energy region with negative heat capacity. In contrast to
previous statistical calculations, this situation takes place even in this case
in which the system is constrained to fixed volume. The observed phase
transition takes place at approximately constant entropy. The charge
distribution and other observables also turn out to be sensitive to the
treatment employed in the calculation of the free energies and the fragments'
volumes at finite temperature, specially at high excitation energies. The
isotopic distribution is also affected by this treatment, which suggests that
this prescription may help to obtain information on the nuclear equation of
state
Decoherence and thermalization dynamics of a quantum oscillator
We introduce the quantitative measures characterizing the rates of
decoherence and thermalization of quantum systems. We study the time evolution
of these measures in the case of a quantum harmonic oscillator whose relaxation
is described in the framework of the standard master equation, for various
initial states (coherent, `cat', squeezed and number). We establish the
conditions under which the true decoherence measure can be approximated by the
linear entropy . We show that at low temperatures and for
highly excited initial states the decoherence process consists of three
distinct stages with quite different time scales. In particular, the `cat'
states preserve 50% of the initial coherence for a long time interval which
increases logarithmically with increase of the initial energy.Comment: 24 pages, LaTex, 8 ps figures, accepted for publication in J. Opt.
Isotopic Scaling in Nuclear Reactions
A three parameter scaling relationship between isotopic distributions for
elements with Z has been observed that allows a simple description of
the dependence of such distributions on the overall isospin of the system. This
scaling law (termed iso-scaling) applies for a variety of reaction mechanisms
that are dominated by phase space, including evaporation, multifragmentation
and deeply inelastic scattering. The origins of this scaling behavior for the
various reaction mechanisms are explained. For multifragmentation processes,
the systematics is influenced by the density dependence of the asymmetry term
of the equation of state.Comment: 10 Pages, 2 Figure
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