413 research outputs found
Generic Finite Size Enhancement of Pairing in Mesoscopic Fermi Systems
The finite size dependent enhancement of pairing in mesoscopic Fermi systems
is studied under the assumption that the BCS approach is valid and that the two
body force is size independent. Different systems are investigated such as
superconducting metallic grains and films as well atomic nuclei. It is shown
that the finite size enhancement of pairing in these systems is in part due to
the presence of a surface which accounts quite well for the data of nuclei and
explains a good fraction of the enhancement in Al grains.Comment: Updated version 17/02/0
Isospin non-equilibrium in heavy-ion collisions at intermediate energies
We study the equilibration of isospin degree of freedom in intermediate
energy heavy-ion collisions using an isospin-dependent BUU model. It is found
that there exists a transition from the isospin equilibration at low energies
to non-equilibration at high energies as the beam energy varies across the
Fermi energy in central, asymmetric heavy-ion collisions. At beam energies
around 55 MeV/nucleon, the composite system in thermal equilibrium but isospin
non-equilibrium breaks up into two primary hot residues with N/Z ratios closely
related to those of the target and projectile respectively. The decay of these
forward-backward moving residues results in the strong isospin asymmetry in
space and the dependence of the isotopic composition of fragments on the N/Z
ratios of the target and projectile. These features are in good agreement with
those found recently in experiments at NSCL/MSU and TAMU, implications of these
findings are discussed.Comment: 9 pages, latex, + 3 figures available upon reques
Collective decision making and social interaction rules in mixed-species flocks of songbirds
Associations in mixed-species foraging groups are common in animals, yet have rarely been explored in the context of collective behaviour. Despite many investigations into the social and ecological conditions under which individuals should form groups, we still know little about the specific behavioural rules that individuals adopt in these contexts, or whether these can be generalized to heterospecifics. Here, we studied collective behaviour in flocks in a community of five species of woodland passerine birds. We adopted an automated data collection protocol, involving visits by RFID-tagged birds to feeding stations equipped with antennae, over two winters, recording 91 576 feeding events by 1904 individuals. We demonstrated highly synchronized feeding behaviour within patches, with birds moving towards areas of the patch with the largest proportion of the flock. Using a model of collective decision making, we then explored the underlying decision rule birds may be using when foraging in mixed-species flocks. The model tested whether birds used a different decision rule for conspecifics and heterospecifics, and whether the rules used by individuals of different species varied. We found that species differed in their response to the distribution of conspecifics and heterospecifics across foraging patches. However, simulating decisions using the different rules, which reproduced our data well, suggested that the outcome of using different decision rules by each species resulted in qualitatively similar overall patterns of movement. It is possible that the decision rules each species uses may be adjusted to variation in mean species abundance in order for individuals to maintain the same overall flock-level response. This is likely to be important for maintaining coordinated behaviour across species, and to result in quick and adaptive flock responses to food resources that are patchily distributed in space and time
Optical detection of a BCS transition of Lithium-6 in harmonic traps
We study the detection of a BCS transition within a sample of Lithium--6
atoms confined in a harmonic trap. Using the local density approximation we
calculate the pair correlation function in the normal and superfluid state at
zero temperature. We show that the softening of the Fermi hole associated with
a BCS transition leads to an observable increase in the intensity of
off--resonant light scattered from the atomic cloud at small angles.Comment: 7 pages, 3 figures, submitted to Europhysics Letter
Glandular areas associated with the male genitalia in Triatoma rubrofasciata (Triatominae, Reduviidae, Hemiptera) and other Reduviidae
Moment of inertia of a trapped superfluid gas of Fermions
The moment of inertia Q of a trapped superfluid gas of atomic Fermions (6Li)
is calculated as a function of the temperature. At zero temperature the moment
of inertia takes on the irrotational flow value. Only for T very close to Tc
rigid rotation is attained. It is proposed that future measurements of the
rotational energy will unambiguously reveal whether the system is in a
superfluid state or not.Comment: 43 pages, 5 figures ; accepted in Phys. Rev.
Systematics of Fission Barriers in Superheavy Elements
We investigate the systematics of fission barriers in superheavy elements in
the range Z = 108-120 and N = 166-182. Results from two self-consistent models
for nuclear structure, the relativistic mean-field (RMF) model as well as the
non-relativistic Skyrme-Hartree-Fock approach are compared and discussed. We
restrict ourselves to axially symmetric shapes, which provides an upper bound
on static fission barriers. We benchmark the predictive power of the models
examining the barriers and fission isomers of selected heavy actinide nuclei
for which data are available. For both actinides and superheavy nuclei, the RMF
model systematically predicts lower barriers than most Skyrme interactions. In
particular the fission isomers are predicted too low by the RMF, which casts
some doubt on recent predictions about superdeformed ground states of some
superheavy nuclei. For the superheavy nuclei under investigation, fission
barriers drop to small values around Z = 110, N = 180 and increase again for
heavier systems. For most of the forces, there is no fission isomer for
superheavy nuclei, as superdeformed states are in most cases found to be
unstable with respect to octupole distortions.Comment: 17 pages REVTEX, 12 embedded eps figures. corrected abstrac
Equation of state of asymmetric nuclear matter and collisions of neutron-rich nuclei
The ratio of pre-equilibrium neutrons to protons from collisions of
neutron-rich nuclei is studied as a function of their kinetic energies. This
ratio is found to be sensitive to the density dependence of the nuclear
symmetry energy, but is independent of the compressibility of symmetric nuclear
matter and the in-medium nucleon-nucleon cross sections. The experimental
measurement of this ratio thus provides a novel means for determining the
nuclear equation of state of asymmetric nuclear matter.Comment: 11 pages + 3 postscript figures, Phys. Rev. Lett. (1997) in pres
Mean field theory for global binding systematics
We review some possible improvements of mean field theory for application to
nuclear binding systematics. Up to now, microscopic theory has been less
successful than models starting from the liquid drop in describing accurately
the global binding systematics. We believe that there are good prospects to
develop a better global theory, using modern forms of energy density
functionals and treating correlation energies systematically by the RPA.Comment: RevTex, 17 pages, 5 eps figures. To be published in Yadernaya Fizika,
special edition for the 90th birthday of Professor A.B. Migda
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