62 research outputs found
Numerical solution of the Boltzmann equation for the collective modes of trapped Fermi gases
We numerically solve the Boltzmann equation for trapped fermions in the
normal phase using the test-particle method. After discussing a couple of tests
in order to estimate the reliability of the method, we apply it to the
description of collective modes in a spherical harmonic trap. The numerical
results are compared with those obtained previously by taking moments of the
Boltzmann equation. We find that the general shape of the response function is
very similar in both methods, but the relaxation time obtained from the
simulation is significantly longer than that predicted by the method of
moments. It is shown that the result of the method of moments can be corrected
by including fourth-order moments in addition to the usual second-order ones
and that this method agrees very well with our numerical simulations.Comment: 13 pages, 8 figures, accepted for publication in Phys. Rev.
Nuclear energy density optimization: Shell structure
Nuclear density functional theory is the only microscopical theory that can
be applied throughout the entire nuclear landscape. Its key ingredient is the
energy density functional. In this work, we propose a new parameterization
UNEDF2 of the Skyrme energy density functional. The functional optimization is
carried out using the POUNDerS optimization algorithm within the framework of
the Skyrme Hartree-Fock-Bogoliubov theory. Compared to the previous
parameterization UNEDF1, restrictions on the tensor term of the energy density
have been lifted, yielding a very general form of the energy density functional
up to second order in derivatives of the one-body density matrix. In order to
impose constraints on all the parameters of the functional, selected data on
single-particle splittings in spherical doubly-magic nuclei have been included
into the experimental dataset. The agreement with both bulk and spectroscopic
nuclear properties achieved by the resulting UNEDF2 parameterization is
comparable with UNEDF1. While there is a small improvement on single-particle
spectra and binding energies of closed shell nuclei, the reproduction of
fission barriers and fission isomer excitation energies has degraded. As
compared to previous UNEDF parameterizations, the parameter confidence interval
for UNEDF2 is narrower. In particular, our results overlap well with those
obtained in previous systematic studies of the spin-orbit and tensor terms.
UNEDF2 can be viewed as an all-around Skyrme EDF that performs reasonably well
for both global nuclear properties and shell structure. However, after adding
new data aiming to better constrain the nuclear functional, its quality has
improved only marginally. These results suggest that the standard Skyrme energy
density has reached its limits and significant changes to the form of the
functional are needed.Comment: 18 pages, 13 figures, 12 tables; resubmitted for publication to Phys.
Rev. C after second review by refere
Trap anharmonicity and sloshing mode of a Fermi gas
For a gas trapped in a harmonic potential, the sloshing (or Kohn) mode is
undamped and its frequency coincides with the trap frequency, independently of
the statistics, interaction and temperature of the gas. However, experimental
trap potentials have usually Gaussian shape and anharmonicity effects appear as
the temperature and, in the case of Fermions, the filling of the trap are
increased. We study the sloshing mode of a degenerate Fermi gas in an
anharmonic trap within the Boltzmann equation, including in-medium effects in
both the transport and collision terms. The calculated frequency shifts and
damping rates of the sloshing mode due to the trap anharmonicity are in
satisfactory agreement with the available experimental data. We also discuss
higher-order dipole, octupole, and bending modes and show that the damping of
the sloshing mode is caused by its coupling to these modes.Comment: 10 pages; v2: comparison with damping data added, minor extensions
and correction
Control of defect-mediated tunneling barrier heights in ultrathin MgO films
The impact of oxygen vacancies on local tunneling properties across
rf-sputtered MgO thin films was investigated by optical absorption spectroscopy
and conducting atomic force microscopy. Adding O to the Ar plasma during
MgO growth alters the oxygen defect populations, leading to improved local
tunneling characteristics such as a lower density of current hotspots and a
lower tunnel current amplitude. We discuss a defect-based potential landscape
across ultrathin MgO barriers.Comment: 4 pages, 4 figure
The pi -> pi pi process in nuclei and the restoration of chiral symmetry
The results of an extensive campaign of measurements of the pi -> pi pi
process in the nucleon and nuclei at intermediate energies are presented. The
measurements were motivated by the study of strong pi pi correlations in
nuclei. The analysis relies on the composite ratio C_{pi pi}^A, which accounts
for the clear effect of the nuclear medium on the (pi pi) system. The
comparison of the C_{pi pi}^A distributions for the (pi pi)_{I=J=0} and (pi
pi)_{I=0,J=2} systems to the model predictions indicates that the C_{pi pi}^A
behavior in proximity of the 2m_pi threshold is explainable through the partial
restoration of chiral symmetry in nuclei.Comment: accepted for publication in Nucl. Phys.
In-medium pi-pi Correlation Induced by Partial Restoration of Chiral Symmetry
We show that both the linear and the non-linear chiral models give an
enhancement of the pi-pi cross section near the 2pi threshold in the
scalar-iso-scalar (I=J=0) channel in nuclear matter. The reduction of the
chiral condensate, i.e., the partial chiral restoration in nuclear matter, is
responsible for the enhancement in both cases. We extract an effective
4pi-nucleon vertex which is responsible for the enhancement but has not been
considered in the non-liear models for in-medium pi-pi interaction. Relation of
this vertex and a next-to-leading order terms in the heavy-baryon chiral
lagrangian, L_piN^(2), is also discussed.Comment: 5 pages, 5 eps figure, REVTe
Medium Modification of The Pion-Pion Interaction at Finite Density
We discuss medium modifications of the unitarized pion-pion interaction in
the nuclear medium. We incorporate both the effects of chiral symmetry
restoration and the influence of collective nuclear pionic modes originating
from the p-wave coupling of the pion to delta-hole configurations. We show in
particular that the dropping of the sigma meson mass significantly enhances the
low energy structure created by the in-medium collective pionic modes.Comment: 26 pages, 7 figures included, Latex fil
The meson in a nuclear medium through two pion photoproduction
We present theoretical results for production on
nucleons and nuclei in the kinematical region where the scalar isoscalar amplitude is influenced by the pole. The final state interaction
of the pions modified by the nuclear medium produces a spectacular shift of
strength of the two pion invariant mass distribution induced by the moving of
the pole to lower masses and widths as the nuclear density increases.Comment: 14 pages, 7 figure
General properties of the pion production reaction in nuclear matter
The pion production reaction on was
studied at incident pion energies of = 240, 260, 280, 300, and
320 MeV. The experiment was performed using the pion-channel at TRIUMF,
and multiparticle events, () and
(), were detected with the CHAOS spectrometer. Results
are reported in the form of both differential and total cross sections, and are
compared to theoretical predictions and the reaction phase space. The present
investigation of the T-dependence of the
reaction complements earlier examinations of the A-dependence of the reaction,
which was measured using , , , , , and
targets at 280 MeV. Some general properties of the
pion-induced pion production reaction in nuclear matter will be presented,
based on the combined results of the two studies.Comment: 23 pages, Latex, accepted for publication in Nucl. Phys.
The interaction in nuclear matter from a study of the reactions
The pion-production reactions were studied on
, , , and nuclei at an incident pion energy
of =283 MeV. Pions were detected in coincidence using the CHAOS
spectrometer. The experimental results are reduced to differential cross
sections and compared to both theoretical predictions and the reaction phase
space. The composite ratio between the
invariant masses on nuclei and on the nucleon is also presented. Near the
threshold pion pairs couple to when produced in
the reaction channel. There is a marked near-threshold
enhancement of which is consistent with theoretical
predictions addressing the partial restoration of chiral symmetry in nuclear
matter. Furthermore, the behaviour of is well
described when the restoration of chiral symmetry is combined with standard
P-wave renormalization of pions in nuclear matter. On the other hand, nuclear
matter only weakly influences , which displays a flat
behaviour throughout the energy range regardless of .Comment: 30 pages, 16 figures, PS format, accepted for publication in Nucl.
Phys
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