1,539 research outputs found
Low-energy modes of spin-imbalanced Fermi gases in BCS phase
The low-energy modes of a spin-imbalanced superfluid Fermi gas in the
Bardeen-Cooper-Schrieffer (BCS) side are studied. The gas is assumed to be
sufficiently dilute so that the pairing of atoms can be considered effective
only in s-wave between fermions of different internal state. The order
parameter at equilibrium is determined by the mean-field approximation, while
the properties of the collective modes are calculated within a Gaussian
approximation for the fluctuations of the order parameter. In particular we
investigate the effects of asymmetry between the populations of the two
different components and of temperature on the frequency and damping of
collective modes. It is found that the temperature does not much affect the
frequency and the damping of the modes, whereas an increase of the imbalance
shifts the frequency toward lower values and enhances the damping sensitively.
Besides the Bogoliubov-Anderson phonons, we observe modes at zero frequency for
finite values of the wave-number. These modes indicate that an instability
develops driving the system toward two separate phases, normal and superfluid.Comment: 7 pages, 4 figures, submitted to European Physical Journal D for
publicatio
Kinetic-theory description of isoscalar dipole modes
A semiclassical model, based on a solution of the Vlasov equation for finite
systems with moving-surface, is employed to study the isoscalar dipole modes in
nuclei. It is shown that, by taking into account the surface degree of freedom,
it is possible to obtain an exact treatment of the centre of mass motion. It is
also shown that a method often used to subtract the spurious strength in RPA
calculations does not always give the correct result. An alternative analytical
formula for the intrinsic strength function is derived in a simple
confined-Fermi-gas model. In this model the intrinsic isoscalar dipole strength
displays essentially a two-resonance structure, hence there are two relevant
modes. The interaction between nucleons couples these two modes and changes the
compressibility of the system. The evolution of the strength profile is then
studied as a function of the compressibility of the nuclear fluid. Comparison
with available data favours values of the incompressibility parameter of
nuclear matter smaller than those suggested by the analysis of the monopole
``breathing'' mode.Comment: 17 pages, 4 figures, revised version to be published in Nucl. Phys.
Unified semiclassical approach to isoscalar collective modes in heavy nuclei
A semiclassical model based on the solution of the Vlasov equation for finite
systems with a sharp moving surface has been used to study the isoscalar
quadrupole and octupole collective modes in heavy spherical nuclei. Within this
model, a unified description of both low-energy surface modes and higher-energy
giant resonances has been achieved by introducing a coupling between surface
vibrations and the motion of single nucleons. Analytical expressions for the
collective response functions of different multipolarity can be derived by
using a separable approximation for the residual interaction between nucleons.
The response functions obtained in this way give a good qualitative description
of the quadrupole and octupole response in heavy nuclei. Although shell effects
are not explicitly included in the theory, our semiclassical response functions
are very similar to the quantum ones. This happens because of the well known
close relation between classical trajectories and shell structure. The role
played by particular nucleon trajectories and their connection with various
features of the nuclear response is displayed most clearly in the present
approach, we discuss in some detail the damping of low-energy octupole
vibrations and give an explicit expression showing that only nucleons moving on
triangular orbits can contribute to this damping.Comment: 9 pages, 2 figures, Talk presented at the 8th International Spring
Seminar on Nuclear Physics on Key Topics in Nuclear Structure, Paestum,
Italy, May 23-27, 200
Influence of vector interactions on the hadron-quark/gluon phase transition
The hadron-quark/gluon phase transition is studied in the two-phase model. As
a further study of our previous work, both the isoscalar and isovector vector
interactions are included in the Polyakov loop modified Nambu--Jona-Lasinio
model (PNJL) for the quark phase. The relevance of the exchange (Fock) terms is
stressed and suitably accounted for. The calculation shows that the isovector
vector interaction delays the phase transition to higher densities and the
range of the mixed phase correspondingly shrinks. Meanwhile the asymmetry
parameter of quark matter in the mixed phase decreases with the strengthening
of this interaction channel. This leads to some possible observation signals
being weakened, although still present. We show that these can be rather
general effects of a repulsion in the quark phase due to the symmetry energy.
This is also confirmed by a simpler calculation with the MIT--Bag model.
However, the asymmetry parameter of quark matter is slightly enhanced with the
inclusion of the isoscalar vector interaction, but the phase transition will be
moved to higher densities. The largest uncertainty on the phase transition lies
in the undetermined coupling constants of the vector interactions. In this
respect new data on the mixed phase obtained from Heavy Ion Collisions at
Intermediate Energies appear very important.Comment: submitted to Phys. Rev.
Instabilities in a Mean-field dynamics of Asymmetric Nuclear Matter
We discuss the features of instabilities in asymmetric nuclear matter, in
particular the relation between the nature of fluctuations, the types of
instabilities and the properties of the interaction. We show a chemical
instability appears as an instability against isoscalar-like fluctuations. Then
starting from phenomenological hadronic field theory (QHD), including exchange
terms, we discuss the symmetry energy and the relation to the dynamical
response inside the spinodal region.Comment: 8 pages, 5 Postscript figures, talk at Cortona 2000 Conference, Oct.
17 - Oct. 20, Italy, World Scientific (in press
- …