353 research outputs found
Quasiparticle light elements and quantum condensates in nuclear matter
Nuclei in dense matter are influenced by the medium. In the cluster mean
field approximation, an effective Schr\"odinger equation for the -particle
cluster is obtained accounting for the effects of the surrounding medium, such
as self-energy and Pauli blocking. Similar to the single-baryon states (free
neutrons and protons), the light elements (, internal quantum
state ) are treated as quasiparticles with energies that depend on the center of mass momentum , the temperature
, and the total densities of neutrons and protons, respectively.
We consider the composition and thermodynamic properties of nuclear matter at
low densities. At low temperatures, quartetting is expected to occur.
Consequences for different physical properties of nuclear matter and finite
nuclei are discussed.Comment: 5 pages, 1 figure, 2 table
Light nuclei quasiparticle energy shift in hot and dense nuclear matter
Nuclei in dense matter are influenced by the medium. In the cluster mean
field approximation, an effective Schr\"odinger equation for the -particle
cluster is obtained accounting for the effects of the correlated medium such as
self-energy, Pauli blocking and Bose enhancement. Similar to the single-baryon
states (free neutrons and protons), the light elements (,
internal quantum state ) are treated as quasiparticles with energies
. These energies depend on the center of mass
momentum , as well as temperature and the total densities
of neutrons and protons, respectively. No equilibrium is considered so
that (or the corresponding chemical potentials ) are
fixed independently.
For the single nucleon quasiparticle energy shift, different approximate
expressions such as Skyrme or relativistic mean field approaches are well
known. Treating the -particle problem in appropriate approximations, results
for the cluster quasiparticle shifts are given. Properties of dense nuclear
matter at moderate temperatures in the subsaturation density region considered
here are influenced by the composition. This in turn is determined by the
cluster quasiparticle energies, in particular the formation of clusters at low
densities when the temperature decreases, and their dissolution due to Pauli
blocking as the density increases. Our finite-temperature Green function
approach covers different limiting cases: The low-density region where the
model of nuclear statistical equilibrium and virial expansions can be applied,
and the saturation density region where a mean field approach is possible
Uncertainties and robustness of the ignition process in type Ia supernovae
It is widely accepted that the onset of the explosive carbon burning in the
core of a CO WD triggers the ignition of a SN Ia. The features of the ignition
are among the few free parameters of the SN Ia explosion theory. We explore the
role for the ignition process of two different issues: firstly, the ignition is
studied in WD models coming from different accretion histories. Secondly, we
estimate how a different reaction rate for C-burning can affect the ignition.
Two-dimensional hydrodynamical simulations of temperature perturbations in the
WD core ("bubbles") are performed with the FLASH code. In order to evaluate the
impact of the C-burning reaction rate on the WD model, the evolution code
FLASH_THE_TORTOISE from Lesaffre et al. (2006) is used. In different WD models
a key role is played by the different gravitational acceleration in the
progenitor's core. As a consequence, the ignition is disfavored at a large
distance from the WD center in models with a larger central density, resulting
from the evolution of initially more massive progenitors. Changes in the C
reaction rate at T < 5e8 K slightly influence the ignition density in the WD
core, while the ignition temperature is almost unaffected. Recent measurements
of new resonances in the C-burning reaction rate (Spillane et al. 2007) do not
affect the core conditions of the WD significantly. This simple analysis,
performed on the features of the temperature perturbations in the WD core,
should be extended in the framework of the state-of-the-art numerical tools for
studying the turbulent convection and ignition in the WD core. Future
measurements of the C-burning reactions cross section at low energy, though
certainly useful, are not expected to affect dramatically our current
understanding of the ignition process.Comment: 7 pages, 5 figures, A&A accepte
Critical Temperature for -Particle Condensation within a Momentum Projected Mean Field Approach
Alpha-particle (quartet) condensation in homogeneous spin-isospin symmetric
nuclear matter is investigated. The usual Thouless criterion for the critical
temperature is extended to the quartet case. The in-medium four-body problem is
strongly simplified by the use of a momentum projected mean field ansatz for
the quartet. The self-consistent single particle wave functions are shown and
discussed for various values of the density at the critical temperature
Deuteron life-time in hot and dense nuclear matter near equilibrium
We consider deuteron formation in hot and dense nuclear matter close to
equilibrium and evaluate the life-time of the deuteron fluctuations within the
linear response theory. To this end we derive a generalized linear Boltzmann
equation where the collision integral is related to equilibrium correlation
functions. In this framework we then utilize finite temperature Green functions
to evaluate the collision integrals. The elementary reaction cross section is
evaluated within the Faddeev approach that is suitably modified to reflect the
properties of the surrounding hot and dense matter.Comment: 15 pages, 5 figure
Four-particle condensate in strongly coupled fermion systems
Four-particle correlations in fermion systems at finite temperatures are
investigated with special attention to the formation of a condensate. Instead
of the instability of the normal state with respect to the onset of pairing
described by the Gorkov equation, a new equation is obtained which describes
the onset of quartetting. Within a model calculation for symmetric nuclear
matter, we find that below a critical density, the four-particle condensation
(alpha-like quartetting) is favored over deuteron condensation (triplet
pairing). This pairing-quartetting competition is expected to be a general
feature of interacting fermion systems, such as the excition-biexciton system
in excited semiconductors. Possible experimental consequences are pointed out.Comment: LaTeX, 11 pages, 2 figures, uses psfig.sty (included), to be
published in Phys. Rev. Lett., tentatively scheduled for 13 April 1998
(Volume 80, Number 15
The Nonlinear Debye-Onsager Relaxation Effect in Weakly Ionized Plasmas
A weakly ionized plasma under the influence of a strong electric field is
considered. Supposing a local Maxwellian distribution for the electron momenta
the plasma is described by hydrodynamic equations for the pair distribution
functions. These equations are solved and the relaxation field is calculated
for an arbitrary field strength. It is found that the relaxation effect becomes
lower with increasing strength of the electrical field.Comment: 4 pages, 1 figur
Decoherence in an exactly solvable qubit model with initial qubit-environment correlations
We study a model of dephasing (decoherence) in a two-state quantum system
(qubit) coupled to a bath of harmonic oscillators. An exact analytic solution
for the reduced dynamics of a two-state system in this model has been obtained
previously for factorizing initial states of the combined system. We show that
the model admits exact solutions for a large class of correlated initial states
which are typical in the theory of quantum measurements. We derive exact
expressions for the off-diagonal elements of the qubit density matrix, which
hold for an arbitrary strength of coupling between the qubit and the bath. The
influence of initial correlations on decoherence is considered for different
bath spectral densities. Time behavior of the qubit entropy in the decoherence
process is discussed.Comment: 10 pages, 5 figure
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