Surface properties of nuclear pairing with the Gogny force in a simplified model

Surface properties of neutron-neutron (T=1) pairing in semi-infinite nuclear matter in a hard wall potential are investigated in BCS approximation using the Gogny force. Surface enhancement of the gap function, pairing tensor and correlation energy density is put into evidence.Comment: 16 pages; 4 figures ; submitted to Phys. Lett.

Semi-Classical Description of the Average Pairing Properties in Nuclei

We present a new semi-classical theory for describing pairing in finite Fermi systems. It is based in taking the $\hbar \to 0$, i.e. Thomas-Fermi, limit of the gap equation written in the basis of the mean field (weak coupling). In addition to the position dependence of the Fermi momentum, the size dependence of the matrix elements of the pairing force is also taken into account in this theory. An example typical for the nuclear situation shows the improvement of this new approach over the standard Local Density Approximation. We also show that if in this approach some shell fluctuations are introduced in the level density, the arch structure displayed by the quantal gaps along isotopic chains is almost recovered. We also point out that in heavy drip line nuclei pairing is strongly reduced

Thomas-Fermi approximation to pairing in finite Fermi systems. The weak coupling regime

We present a new semiclassical theory for describing pairing in finite Fermi systems. It is based on taking the $\hbar \to 0$, i.e. Thomas-Fermi, limit of the gap equation written in the basis of the mean field (weak coupling). In addition to the position dependence of the Fermi momentum, the size dependence of the pairing force is also taken into account in this theory. Along isotopic chains the Thomas-Fermi gaps average the well known arch structure shown by the quantal gaps. This structure can be almost recovered in our formalism if some shell fluctuations are included in the level density. We point out that at the drip line nuclear pairing is strongly reduced. This fact is illustrated with the behavior of the gap in the inner crust of neutron stars.Comment: 9 pages, 4 figures, Many Body Conference 2011(Paris

Surface behaviour of the pairing gap in a slab of nuclear matter

The surface behaviour of the pairing gap previously studied for semi-infinite nuclear matter is analyzed in the slab geometry. The gap-shape function is calculated in two cases: (a) pairing with the Gogny force in a hard-wall potential and (b) pairing with the separable Paris interaction in a Saxon-Woods mean-field potential. It is shown that the surface features are preserved in the case of slab geometry, being almost independent of the width of the slab. It is also demonstrated that the surface enhancement is strengthened as the absolute value of chemical potential $|\mu|$ decreases which simulates the approach to the nucleon drip line.Comment: 12 pages, 2 figure

Determination of the stiffness of the nuclear symmetry energy from isospin diffusion

With an isospin- and momentum-dependent transport model, we find that the degree of isospin diffusion in heavy ion collisions at intermediate energies is affected by both the stiffness of the nuclear symmetry energy and the momentum dependence of the nucleon potential. Using a momentum dependence derived from the Gogny effective interaction, recent experimental data from NSCL/MSU on isospin diffusion are shown to be consistent with a nuclear symmetry energy given by $E_{\text{sym}}(\rho)\approx 31.6(\rho /\rho_{0})^{1.05}$ at subnormal densities. This leads to a significantly constrained value of about -550 MeV for the isospin-dependent part of the isobaric incompressibility of isospin asymmetric nuclear matter.Comment: 4 pages, 4 figures, 1 table, revised version, to appear in PR

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

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

Monopole giant resonances and nuclear compressibility in relativistic mean field theory

Isoscalar and isovector monopole oscillations that correspond to giant resonances in spherical nuclei are described in the framework of time-dependent relativistic mean-field (RMF) theory. Excitation energies and the structure of eigenmodes are determined from a Fourier analysis of dynamical monopole moments and densities. The generator coordinate method, with generating functions that are solutions of constrained RMF calculations, is also used to calculate excitation energies and transition densities of giant monopole states. Calculations are performed with effective interactions which differ in their prediction of the nuclear matter compression modulus K_nm. Both time-dependent and constrained RMF results indicate that empirical GMR energies are best reproduced by an effective force with K_nm \approx 270 MeV.Comment: 30 pages of LaTeX, 18 PS-figure