arXiv subjects

F. Matera

Publications and source records attributed to F. Matera.

27 records · Page 2Linked to original sources

arXiv2001

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.

nucl-th↗

arXiv2001

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.

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arXiv2001

Asymmetric nuclear matter in a Hartree-Fock approach to non-linear QHD

The Equation of State (EOS) for asymmetric nuclear matter is discussed starting from a phenomenological hadronic field theory of Serot-Walecka type including exchange terms. In a model with self interactions of the scalar sigma-meson we show that the Fock terms naturally lead to isospin effects in the nuclear EOS. These effects are quite large and dominate over the contribution due to isovector mesons. We obtain a potential symmetry term of "stiff" type, i.e. increasing with baryon density and an interesting behaviour of neutron/proton effective masses of relevance for transport properties of asymmetric dense matter.

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arXiv2000

Fock exchange terms in non-linear Quantum Hadrodynamics

We propose a method to introduce Fock term contributions in relativistic models of fermions coupled to mesons, including self-interactions for the mesonic fields. We show that effects on equilibrium properties and on the dynamical response of the fermionic system can be consistently accounted for. Some implications on equilibrium properties of asymmetric nuclear matter are discussed.

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arXiv2000

Density fluctuations and multifragmentation of nuclear matter

The density fluctuations of nuclear matter are studied within a mean-field model in wich fluctuations are generated by an external stochastic field. The constraints imposed on the random force by the fluctuation-dissipation theorem are analyzed. It is shown that in the proximity of the borders of the spinodal region the assumption of a white-noise stochastic field can be reliably used. The domain distribution of the liquid phase in the spinodal decomposition of nuclear matter is derived. The related distribution of fragment sizes compares favourably with the experimental fragment distribution observed in heavy ion collisions.

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arXiv1999

Kinetic-theory approach to low-energy collective modes in nuclei

Two different solutions of the linearized Vlasov equation for finite systems, characterized by fixed and moving-surface boundary conditions, are discussed in a unified perspective. A condition determining the eigenfrequencies of collective nuclear oscillations, that can be obtained from the moving-surface solution, is studied for isoscalar vibrations of lowest multipolarity. Analytic expressions for the friction and mass parameters related to the low-enegy surface excitations are derived and their value is compared to values given by other models. Both similarities and differences are found with respect to the other approaches, however the close agreement obtained in many cases with one of the other models suggests that, in spite of some important differences, the two approaches are substantially equivalent. The formalism based on the Vlasov equation is more transparent since it leads to analytical expressions that can be a basis for further improvement of the model.

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arXiv1999

Semiclassical theory of surface plasmons in spheroidal clusters

A microscopic theory of linear response based on the Vlasov equation is extended to systems having spheroidal equilibrium shape. The solution of the linearized Vlasov equation, which gives a semiclassical version of the random phase approximation, is studied for electrons moving in a deformed equilibrium mean field. The deformed field has been approximated by a cavity of spheroidal shape, both prolate and oblate. Contrary to spherical systems, there is now a coupling among excitations of different multipolarity induced by the interaction among constituents. Explicit calculations are performed for the dipole response of deformed clusters of different size. In all cases studied here the photoabsorption strength for prolate clusters always displays a typical double-peaked structure. For oblate clusters we find that the high--frequency component of the plasmon doublet can get fragmented in the medium size region ($N \sim 250$). This fragmentation is related to the presence of two kinds of three-dimensional electron orbits in oblate cavities. The possible scaling of our semiclassical equations with the valence electron number and density is investigated.

cond-mat.mes-hall↗

arXiv1998

Coulomb effects on growth of instabilities in asymmetric nuclear matter

We study the effects of the Coulomb interaction on the growth of unstable modes in asymmetric nuclear matter. In order to compare with previous calculations we use a semiclassical approach based on the linearized Vlasov equation. Moreover, a quantum calculation is performed within the R.P.A.. The Coulomb effects are a slowing down of the growth and the occurrence of a minimal wave vector for the onset of the instabilities. The quantum corrections cause a further decrease of the growth rates.

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arXiv1997

Relativistic Approach to Superfluidity in Nuclear Matter

Pairing correlations in symmetric nuclear matter are studied within a relativistic mean-field approximation based on a field theory of nucleons coupled to neutral ($σ$ and $ω$) and to charged ($\varrho$) mesons. The Hartree-Fock and the pairing fields are calculated in a self-consistent way. The energy gap is the result of a strong cancellation between the scalar and vector components of the pairing field. We find that the pair amplitude vanishes beyond a certain value of momentum of the paired nucleons. This fact determines an effective cutoff in the gap equation. The value of this cutoff gives an energy gap in agreement with the estimates of non relativistic calculations.

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