596 research outputs found
Two-particle spatial correlations in superfluid nuclei
We discuss the effect of pairing on two-neutron space correlations in
deformed nuclei. The spatial correlations are described by the pairing tensor
in coordinate space calculated in the HFB approach. The calculations are done
using the D1S Gogny force. We show that the pairing tensor has a rather small
extension in the relative coordinate, a feature observed earlier in spherical
nuclei. It is pointed out that in deformed nuclei the coherence length
corresponding to the pairing tensor has a pattern similar to what we have found
previously in spherical nuclei, i.e., it is maximal in the interior of the
nucleus and then it is decreasing rather fast in the surface region where it
reaches a minimal value of about 2 fm. This minimal value of the coherence
length in the surface is essentially determined by the finite size properties
of single-particle states in the vicinity of the chemical potential and has
little to do with enhanced pairing correlations in the nuclear surface. It is
shown that in nuclei the coherence length is not a good indicator of the
intensity of pairing correlations. This feature is contrasted with the
situation in infinite matter.Comment: 14 pages, 17 figures, submitted to PR
Convergence of Particle-Hole Expansions for the Description of Nuclear Correlations
The convergence properties of a multiparticle-multihole (mp-mh) configuration
mixing approach whose purpose is to describe ground state correlations in
nuclei without particle number and Pauli violations is investigated in the case
of an exactly solvable pairing hamiltonian. Two different truncation schemes
are tested by looking at quantities as correlation energies and single-particle
occupation probabilities. Results show that pairing correlations present in
usual superfluid nuclei can be accurately described using up to 6 particle-6
hole excitations, a convergence fast enought for envisaging extensions to fully
microscopic calculations.Comment: 8 pages, 4 figure
Description of nuclear systems with a self-consistent configuration-mixing approach. I: Theory, algorithm, and application to the C test nucleus
Although self-consistent multi-configuration methods have been used for
decades to address the description of atomic and molecular many-body systems,
only a few trials have been made in the context of nuclear structure. This work
aims at the development of such an approach to describe in a unified way
various types of correlations in nuclei, in a self-consistent manner where the
mean-field is improved as correlations are introduced. The goal is to reconcile
the usually set apart Shell-Model and Self-Consistent Mean-Field methods. This
approach is referred as "variational multiparticle-multihole configuration
mixing method". It is based on a double variational principle which yields a
set of two coupled equations that determine at the same time the expansion
coefficients of the many-body wave function and the single particle states. The
formalism is derived and discussed in a general context, starting from a
three-body Hamiltonian. Links to existing many-body techniques such as the
formalism of Green's functions are established. First applications are done
using the two-body D1S Gogny effective force. The numerical procedure is tested
on the C nucleus in order to study the convergence features of the
algorithm in different contexts. Ground state properties as well as
single-particle quantities are analyzed, and the description of the first
state is examined. This study allows to validate our numerical algorithm and
leads to encouraging results. In order to test the method further, we will
realize in the second article of this series, a systematic description of more
nuclei and observables obtained by applying the newly-developed numerical
procedure with the same Gogny force. As raised in the present work,
applications of the variational multiparticle-multihole configuration mixing
method will however ultimately require the use of an extended and more
constrained Gogny force.Comment: 22 pages, 18 figures, accepted for publication in Phys. Rev. C. v2:
minor corrections and references adde
Generic strong coupling behavior of Cooper pairs in the surface of superfluid nuclei
With realistic HFB calculations, using the D1S Gogny force, we reveal a
generic behavior of concentration of small sized Cooper pairs (2-3 fm) in the
surface of superfluid nuclei. This study confirms and extends previous results
given in the literature that use more schematic approaches.Comment: 5 pages, 5 figure
Atom-molecule collisions in an optically trapped gas
Cold inelastic collisions between confined cesium (Cs) atoms and Cs
molecules are investigated inside a CO laser dipole trap. Inelastic
atom-molecule collisions can be observed and measured with a rate coefficient
of cm s, mainly independent of the
molecular ro-vibrational state populated. Lifetimes of purely atomic and
molecular samples are essentially limited by rest gas collisions. The pure
molecular trap lifetime ranges 0,3-1 s, four times smaller than the atomic one,
as is also observed in a pure magnetic trap. We give an estimation of the
inelastic molecule-molecule collision rate to be cm
s
Particle-particle random phase approximation applied to Beryllium isotopes
This work is dedicated to the study of even-even 8-14 Be isotopes using the
particle-particle Random Phase Approximation that accounts for two-body
correlations in the core nucleus. A better description of energies and
two-particle amplitudes is obtained in comparison with models assuming a
neutron closed-shell (or subshell) core. A Wood-Saxon potential corrected by a
phenomenological particle-vibration coupling term has been used for the
neutron-core interaction and the D1S Gogny force for the neutron-neutron
interaction. Calculated ground state properties as well as excited state ones
are discussed and compared to experimental data. In particular, results suggest
the same 2s_1/2-1p_1/2 shell inversion in 13Be as in 11Be.Comment: to appear in Phys. Rev.
A note on the Landauer principle in quantum statistical mechanics
The Landauer principle asserts that the energy cost of erasure of one bit of
information by the action of a thermal reservoir in equilibrium at temperature
T is never less than . We discuss Landauer's principle for quantum
statistical models describing a finite level quantum system S coupled to an
infinitely extended thermal reservoir R. Using Araki's perturbation theory of
KMS states and the Avron-Elgart adiabatic theorem we prove, under a natural
ergodicity assumption on the joint system S+R, that Landauer's bound saturates
for adiabatically switched interactions. The recent work of Reeb and Wolf on
the subject is discussed and compared
Cooper pair sizes in superfluid nuclei in a simplified model
Cooper pair sizes are evaluated in a simple harmonic oscillator model
reproducing the values of sophisticated HFB calculations. Underlying reasons
for the very small sizes of 2.0-2.5 fm of Cooper pairs in the surface of nuclei
are analysed. It is shown that the confining properties of the nuclear volume
is the dominating effect. It is argued that for Cooper pair sizes LDA is
particularly inadapted.Comment: 8 pages, 6 figure
Full counting statistics and phase diagram of a dissipative Rydberg gas
Ultra-cold gases excited to strongly interacting Rydberg states are a
promising system for quantum simulations of many-body systems. For off-resonant
excitation of such systems in the dissipative regime, highly correlated
many-body states exhibiting, among other characteristics, intermittency and
multi-modal counting distributions are expected to be created. So far,
experiments with Rydberg atoms have been carried out in the resonant,
non-dissipative regime. Here we realize a dissipative gas of rubidium Rydberg
atoms and measure its full counting statistics for both resonant and
off-resonant excitation. We find strongly bimodal counting distributions in the
off-resonant regime that are compatible with intermittency due to the
coexistence of dynamical phases. Moreover, we measure the phase diagram of the
system and find good agreement with recent theoretical predictions. Our results
pave the way towards detailed studies of many-body effects in Rydberg gases.Comment: 12 pages, 5 figure
Properties of sunspots in cycle 23: I. Dependence of brightness on sunspot size and cycle phase
In this paper we investigate the dependence of umbral core brightness, as
well as the mean umbral and penumbral brightness on the phase of the solar
cycle and on the size of the sunspot. Albregtsen & Maltby (1978) reported an
increase in umbral core brightness from the early to the late phase of solar
cycle from the analysis of 13 sunspots which cover solar cycles 20 and 21. Here
we revisit this topic by analysing continuum images of more than 160 sunspots
observed by the MDI instrument on board the SOHO spacecraft for the period
between 1998 March to 2004 March, i.e. a sizable part of solar cycle 23. The
advantage of this data set is its homogeneity, with no seeing fluctuations. A
careful stray light correction, which is validated using the Mercury transit of
7th May, 2003, is carried out before the umbral and penumbral intensities are
determined. The influence of the Zeeman splitting of the nearby NiI spectral
line on the measured 'continuum' intensity is also taken into account. We did
not observe any significant variation in umbral core, mean umbral and mean
penumbral intensities with solar cycle, which is in contrast to earlier
findings for the umbral core intensity. We do find a strong and clear
dependence of the umbral brightness on sunspot size, however. The penumbral
brightness also displays a weak dependence. The brightness-radius relationship
has numerous implications, some of which, such as those for the energy
transport in umbrae, are pointed out.Comment: 16 pages, 21 postscript figures, accepted for publication in A&
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