1,741 research outputs found
Chaos vs. Linear Instability in the Vlasov Equation: A Fractal Analysis Characterization
In this work we discuss the most recent results concerning the Vlasov
dynamics inside the spinodal region. The chaotic behaviour which follows an
initial regular evolution is characterized through the calculation of the
fractal dimension of the distribution of the final modes excited. The ambiguous
role of the largest Lyapunov exponent for unstable systems is also critically
reviewed.Comment: 10 pages, RevTeX, 4 figures not included but available upon reques
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 ( and ) and to charged () 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.Comment: 21 pages, REVTEX, 8 ps-figures, to appear in Phys.Rev.C. e-mail:
[email protected]
Isovector Channel Role of Relativistic Mean Field Models in the Neutrino Mean Free Path
An improvement in the treatment of the isovector channel of relativistic mean
field (RMF) models based on effective field theory (E-RMF) is suggested, by
adding an isovector scalar (delta) meson and using a similar procedure to the
one used by Horowitz and Piekarewicz to adjust the isovector-vector channel in
order to achieve a softer density dependent symmetry energy of the nuclear
matter at high density. Their effects on the equation of state (EOS) at high
density and on the neutrino mean free path (NMFP) in neutron stars are
discussed.Comment: 20 pages, 8 figure
Isovector splitting of nucleon effective masses, ab-initio benchmarks and extended stability criteria for Skyrme energy functionals
We study the effect of the splitting of neutron and proton effective masses
with isospin asymmetry on the properties of the Skyrme energy density
functional. We discuss the ability of the latter to predict observable of
infinite matter and finite nuclei, paying particular attention to controlling
the agreement with ab-initio predictions of the spin-isospin content of the
nuclear equation of state, as well as diagnosing the onset of finite size
instabilities, which we find to be of critical importance. We show that these
various constraints cannot be simultaneously fulfilled by the standard Skyrme
force, calling at least for an extension of its P-wave part.Comment: 17 pages, 9 figures; Minor changes, references added; Accepted for
publication in Phys.Rev.
Neutron-antineutron Oscillations in the Trapping Box
We have reexamined the problem of oscillations for ultra-cold
neutrons (UCN) confined within a trap. We have shown that the growth of the
component with time is to a decent accuracy given by where is the mixing parameter,
sec in the neutron propagation time between subsequent collisions
with the trap walls. Possible corrections to this law and open questions are
discussed.Comment: 11 pages, LaTeX2
Screening Effects in Superfluid Nuclear and Neutron Matter within Brueckner Theory
Effects of medium polarization are studied for 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 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
Spontaneous breaking of rotational symmetry in superconductors
We show that homogeneous superconductors with broken spin/isospin symmetry
lower their energy via a transition to a novel superconducting state where the
Fermi-surfaces are deformed to a quasi-ellipsoidal form at zero total momentum
of Cooper pairs. In this state, the gain in the condensation energy of the
pairs dominates over the loss in the kinetic energy caused by the lowest order
(quadrupole) deformation of Fermi-surfaces from the spherically symmetric form.
There are two energy minima in general, corresponding to the deformations of
the Fermi-spheres into either prolate or oblate forms. The phase transition
from spherically symmetric state to the superconducting state with broken
rotational symmetry is of the first order.Comment: 5 pages, including 3 figures, published versio
Critical Enhancement of the In-medium Nucleon-Nucleon Cross Section at low Temperatures
The in-medium nucleon-nucleon cross section is calculated starting from the
thermodynamic T-matrix at finite temperatures. The corresponding
Bethe-Salpeter-equation is solved using a separable representation of the Paris
nucleon-nucleon-potential. The energy-dependent in-medium N-N cross section at
a given density shows a strong temperature dependence. Especially at low
temperatures and low total momenta, the in-medium cross section is strongly
modified by in-medium effects. In particular, with decreasing temperature an
enhancement near the Fermi energy is observed. This enhancement can be
discussed as a precursor of the superfluid phase transition in nuclear matter.Comment: 10 pages with 4 figures (available on request from the authors),
MPG-VT-UR 34/94 accepted for publication in Phys. Rev.
Neutrino Electromagnetic Form Factors Effect on the Neutrino Cross Section in Dense Matter
The sensitivity of the differential cross section of the interaction between
neutrino-electron with dense matter to the possibly nonzero neutrino
electromagnetic properties has been investigated. Here, the relativistic mean
field model inspired by effective field theory has been used to describe non
strange dense matter, both with and without the neutrino trapping. We have
found that the cross section becomes more sensitive to the constituent
distribution of the matter, once electromagnetic properties of the neutrino are
taken into account. The effects of electromagnetic properties of neutrino on
the cross section become more significant for the neutrino magnetic moment
mu_nu > 10^{-10} mu_B and for the neutrino charge radius R > 10^{-5} MeV^{-1}.Comment: 24 pages, 10 figures, submitted to Physical Review
Two-body correlation functions in nuclear matter with condensate
The density, spin and isospin correlation functions in nuclear matter with a
neutron-proton () condensate are calculated to study the possible
signatures of the BEC-BCS crossover in the low-density region. It is shown that
the criterion of the crossover (Phys. Rev. Lett. {\bf 95}, 090402 (2005)),
consisting in the change of the sign of the density correlation function at low
momentum transfer, fails to describe correctly the density-driven BEC-BCS
transition at finite isospin asymmetry or finite temperature. As an unambiguous
signature of the BEC-BCS transition, there can be used the presence (BCS
regime) or absence (BEC regime) of the singularity in the momentum distribution
of the quasiparticle density of states.Comment: Prepared with RevTeX4, 5p., 4 figure
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