2,880 research outputs found

### Screening Effects in Superfluid Nuclear and Neutron Matter within Brueckner Theory

Effects of medium polarization are studied for $^1S_0$ pairing in neutron and
nuclear matter. The screening potential is calculated in the RPA limit,
suitably renormalized to cure the low density mechanical instability of nuclear
matter. The selfenergy corrections are consistently included resulting in a
strong depletion of the Fermi surface. All medium effects are calculated based
on the Brueckner theory. The $^1S_0$ gap is determined from the generalized gap
equation. The selfenergy corrections always lead to a quenching of the gap,
which is enhanced by the screening effect of the pairing potential in neutron
matter, whereas it is almost completely compensated by the antiscreening effect
in nuclear matter.Comment: 8 pages, 6 Postscript figure

### Spin-polarized states of nuclear matter

The equations of state of spin-polarized nuclear matter and pure neutron
matter are studied in the framework of the Brueckner-Hartree-Fock theory
including a three-body force. The energy per nucleon $E_A(\delta)$ calculated
in the full range of spin polarization ${\delta} =
\frac{\rho_{\uparrow}-\rho_{\downarrow}}{\rho}$ for symmetric nuclear matter
and pure neutron matter fulfills a parabolic law. In both cases the
spin-symmetry energy is calculated as a function of the baryonic density along
with the related quantities such as the magnetic susceptibility and the Landau
parameter $G_0$. The main effect of the three-body force is to strongly reduce
the degenerate Fermi gas magnetic susceptibility even more than the value with
only two body force. The EOS is monotonically increasing with the density for
all spin-aligned configurations studied here so that no any signature is found
for a spontaneous transition to a ferromagnetic state.Comment: Contribution to GISELDA Meeting, 14-18 January, 2002 (Frascati), to
appear in World Scientific (Singapore

### Transport parameters in neutron stars from in-medium NN cross sections

We present a numerical study of shear viscosity and thermal conductivity of
symmetric nuclear matter, pure neutron matter and $\beta$-stable nuclear
matter, in the framework of the Brueckner theory. The calculation of in-medium
cross sections and nucleon effective masses is performed with a consistent two
and three body interaction. The investigation covers a wide baryon density
range as requested in the applications to neutron stars. The results for the
transport coefficients in $\beta$-stable nuclear matter are used to make
preliminary predictions on the damping time scales of non radial modes in
neutron stars

### Isospin singlet (pn) pairing and quartetting contribution to the binding energy of nuclei

Isospin singlet (pn) pairing as well as quartetting in nuclei is expected to
arise near the symmetry line $N=Z$. Empirical values can be deduced from the
nuclear binding energies applying special filters. Within the local density
approximation, theoretical estimates for finite nuclei are obtained from
results for the condensation energy of asymmetric nuclear matter. It is shown
that the isospin singlet condensation energy drops down abruptly for |N-Z|~4
for medium nuclei in the region A=40. Furthermore, alpha-like quartetting and
the influence of excitations are discussed.Comment: 19 pages, 19 figures, submitted to PR

### Microscopic three-body force for asymmetric nuclear matter

Brueckner calculations including a microscopic three-body force have been
extended to isospin asymmetric nuclear matter. The effects of the three-body
force on the equation of state and on the single-particle properties of nuclear
matter are discussed with a view to possible applications in nuclear physics
and astrophysics. It is shown that, even in the presence of the three-body
force, the empirical parabolic law of the energy per nucleon vs isospin
asymmetry $\beta=(N-Z)/A$ is fulfilled in the whole asymmetry range
$0\le\beta\le 1$ up to high densities. The three-body force provides a strong
enhancement of symmetry energy increasing with the density in good agreement
with relativistic approaches. The Lane's assumption that proton and neutron
mean fields linearly vary vs the isospin parameter is violated at high density
in the presence of the three-body force. Instead the momentum dependence of the
mean fields is rather insensitive to three body force which brings about a
linear isospin deviation of the neutron and proton effective masses. The
isospin effects on multifragmentation events and collective flows in heavy-ion
collisions are briefly discussed along with the conditions for direct URCA
processes to occur in the neutron-star cooling.Comment: 11 pages, 7 figure

### Screening of nuclear pairing in nuclear and neutron matter

The screening potential in the $^1S_0$ and $^3S_1$ pairing channels in
neutron and nuclear matter in different approximations is discussed. It is
found that the vertex corrections to the potential are much stronger in nuclear
matter than in neutron matter.Comment: 11 pages, 8 figures, revtex4 styl

### Medium polarization in asymmetric nuclear matter

The influence of the core polarization on the effective nuclear interaction
of asymmetric nuclear matter is calculated in the framework of the induced
interaction theory. The strong isospin dependence of the density and spin
density fluctuations is studied along with the interplay between the neutron
and proton core polarizations. Moving from symmetric nuclear matter to pure
neutron matter the crossover of the induced interaction from attractive to
repulsive in the spin singlet state is determined as a function of the isospin
imbalance.The density range in which it occurs is also determined. For the spin
triplet state the induced interaction turns out to be always repulsive. The
implications of the results for the neutron star superfluid phases are shortly
discussed.Comment: 6 pages, 4 figure

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