15,938 research outputs found
Dirac Cosmology and the Acceleration of the Contemporary Universe
A model is suggested to unify the Einstein GR and Dirac Cosmology. There is
one adjusted parameter in our model. After adjusting the parameter
in the model by using the supernova data, we have calculated the gravitational
constant and the physical quantities of , and by using the present day quantities as the initial conditions and
found that the equation of state parameter equals to -0.83, the
ratio of the density of the addition creation and the
ratio of the density of the matter including multiplication creation, radiation
and normal matter at present. The results are self-consistent
and in good agreement with present knowledge in cosmology. These results
suggest that the addition creation and multiplication creation in Dirac
cosmology play the role of the dark energy and dark matter.Comment: 13 pages, 8 figure
Limits on Phase Separation for Two-Dimensional Strongly Correlated Electrons
From calculations of the high temperature series for the free energy of the
two-dimensional t-J model we construct series for ratios of the free energy per
hole. The ratios can be extrapolated very accurately to low temperatures and
used to investigate phase separation. Our results confirm that phase separation
occurs only for J/t greater than 1.2. Also, the phase transition into the phase
separated state has Tc of approximately 0.25J for large J/t.Comment: 4 pages, 6 figure
Numerical simulation of strongly nonlinear and dispersive waves using a Green-Naghdi model
We investigate here the ability of a Green-Naghdi model to reproduce strongly
nonlinear and dispersive wave propagation. We test in particular the behavior
of the new hybrid finite-volume and finite-difference splitting approach
recently developed by the authors and collaborators on the challenging
benchmark of waves propagating over a submerged bar. Such a configuration
requires a model with very good dispersive properties, because of the
high-order harmonics generated by topography-induced nonlinear interactions. We
thus depart from the aforementioned work and choose to use a new Green-Naghdi
system with improved frequency dispersion characteristics. The absence of dry
areas also allows us to improve the treatment of the hyperbolic part of the
equations. This leads to very satisfying results for the demanding benchmarks
under consideration
Bond-versus-site doping models for off-chain-doped Haldane-gap system Y Ba Ni O
Using the density matrix renormalization-group technique, we calculate the
impurity energy levels for two different effective models of off-chain doping
for quasi-one-dimensional Heisenberg chain compound Y Ba Ni O:
ferromagnetic bond doping and antiferromagnetic site spin-1/2 doping.
Thresholds of the impurity strength for the appearance of localized states are
found for both models. However, the ground-state and low-energy excitations for
weak impurity strength are different for these two models and the difference
can be detected by experiments.Comment: 5 pages, 5 eps figures included, to be published in Phys. Rev.
On the absence of bound-state stabilization through short ultra-intense fields
We address the question of whether atomic bound states begin to stabilize in
the short ultra-intense field limit. We provide a general theory of ionization
probability and investigate its gauge invariance. For a wide range of
potentials we find an upper and lower bound by non-perturbative methods, which
clearly exclude the possibility that the ultra intense field might have a
stabilizing effect on the atom. For short pulses we find almost complete
ionization as the field strength increases.Comment: 34 pages Late
On the Influence of Pulse Shapes on Ionization Probability
We investigate analytical expressions for the upper and lower bounds for the
ionization probability through ultra-intense shortly pulsed laser radiation. We
take several different pulse shapes into account, including in particular those
with a smooth adiabatic turn-on and turn-off. For all situations for which our
bounds are applicable we do not find any evidence for bound-state
stabilization.Comment: 21 pages LateX, 10 figure
Competition of ferromagnetic and antiferromagnetic spin ordering in nuclear matter
In the framework of a Fermi liquid theory it is considered the possibility of
ferromagnetic and antiferromagnetic phase transitions in symmetric nuclear
matter with Skyrme effective interaction. The zero temperature dependence of
ferromagnetic and antiferromagnetic spin polarization parameters as functions
of density is found for SkM, SGII effective forces. It is shown that in the
density domain, where both type of solutions of self--consistent equations
exist, ferromagnetic spin state is more preferable than antiferromagnetic one.Comment: 9p., 3 figure
Perturbation theory for the one-dimensional optical polaron
The one-dimensional optical polaron is treated on the basis of the
perturbation theory in the weak coupling limit. A special matrix diagrammatic
technique is developed. It is shown how to evaluate all terms of the
perturbation theory for the ground-state energy of a polaron to any order by
means of this technique. The ground-state energy is calculated up to the eighth
order of the perturbation theory. The effective mass of an electron is obtained
up to the sixth order of the perturbation theory. The radius of convergence of
the obtained series is estimated. The obtained results are compared with the
results from the Feynman polaron theory.Comment: 9 pages, 2 figures, RevTeX, to be published in Phys. Rev. B (2001)
Ap
Ionization Probabilities through ultra-intense Fields in the extreme Limit
We continue our investigation concerning the question of whether atomic bound
states begin to stabilize in the ultra-intense field limit. The pulses
considered are essentially arbitrary, but we distinguish between three
situations. First the total classical momentum transfer is non-vanishing,
second not both the total classical momentum transfer and the total classical
displacement are vanishing together with the requirement that the potential has
a finite number of bound states and third both the total classical momentum
transfer and the total classical displacement are vanishing. For the first two
cases we rigorously prove, that the ionization probability tends to one when
the amplitude of the pulse tends to infinity and the pulse shape remains fixed.
In the third case the limit is strictly smaller than one. This case is also
related to the high frequency limit considered by Gavrila et al.Comment: 16 pages LateX, 2 figure
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