333 research outputs found
Explicit Integration of Extremely-Stiff Reaction Networks: Quasi-Steady-State Methods
A preceding paper demonstrated that explicit asymptotic methods generally
work much better for extremely stiff reaction networks than has previously been
shown in the literature. There we showed that for systems well removed from
equilibrium explicit asymptotic methods can rival standard implicit codes in
speed and accuracy for solving extremely stiff differential equations. In this
paper we continue the investigation of systems well removed from equilibrium by
examining quasi-steady-state (QSS) methods as an alternative to asymptotic
methods. We show that for systems well removed from equilibrium, QSS methods
also can compete with, or even exceed, standard implicit methods in speed, even
for extremely stiff networks, and in many cases give somewhat better
integration speed than for asymptotic methods. As for asymptotic methods, we
will find that QSS methods give correct results, but with non-competitive
integration speed as equilibrium is approached. Thus, we shall find that both
asymptotic and QSS methods must be supplemented with partial equilibrium
methods as equilibrium is approached to remain competitive with implicit
methods.Comment: Updated reference
Solution of the Nuclear Shell Model by Symmetry-Dictated Truncation
The dynamical symmetries of the Fermion Dynamical Symmetry Model are used as
a principle of truncation for the spherical shell model. Utilizing the usual
principle of energy-dictated truncation to select a valence space, and
symmetry-dictated truncation to select a collective subspace of that valence
space, we are able to reduce the full shell model space to one of manageable
dimensions with modern supercomputers, even for the heaviest nuclei. The
resulting shell model then consists of diagonalizing an effective Hamiltonian
within the restricted subspace. This theory is not confined to any symmetry
limits, and represents a full solution of the original shell model if the
appropriate effective interaction of the truncated space can be determined. As
a first step in constructing that interaction, we present an empirical
determination of its matrix elements for the collective subspace with no broken
pairs in a representative set of nuclei with . We demonstrate
that this effective interaction can be parameterized in terms of a few
quantities varying slowly with particle number, and is capable of describing a
broad range of low-energy observables for these nuclei. Finally we give a brief
discussion of extending these methods to include a single broken collective
pair.Comment: invited paper for J. Phys. G, 57 pages, Latex, 18 figures a macro are
available under request at [email protected]
Temperature-dependent gap equations and their solutions in the SU(4) model of high-temperature superconductivity
Temperature-dependent gap equations in the SU(4) model of high-Tc
superconductivity are derived and analytical solutions are obtained. Based on
these solutions, a generic gap diagram describing the features of energy gaps
as functions of doping P is presented and a phase diagram illustrating the
phase structure as a function of temperature T and doping P is sketched. A
special doping point P_q occurs naturally in the solutions that separates two
phases at temperature T = 0: a pure superconducting phase on one side (P > P_q)
and a phase with superconductivity strongly suppressed by antiferromagnetism on
the other (P < P_q). We interpret P_q as a quantum phase transition point.
Moreover, the pairing gap is found to have two solutions for P < P_q: a small
gap that is associated with competition between superconductivity and
antiferromagnetism and is responsible for the ground state superconductivity,
and a large gap without antiferromagnetic suppression that corresponds to a
collective excited state. A pseudogap appears in the solutions that terminates
at P_q and originates from the competition between d-wave superconductivity and
antiferromagnetism. Nevertheless, this conclusion does not contradict the
preformed pair picture conceptually if the preformed pairs are generally
defined as any pairs formed before pairing condensation.Comment: 23 pages, 5 color figure
Neutrino-driven supernovae: Boltzmann neutrino transport and the explosion mechanism
Core-collapse supernovae are, despite their spectacular visual display,
neutrino events. Virtually all of the 10^53 ergs of gravitational binding
energy released in the formation of the nascent neutron star is carried away in
the form of neutrinos and antineutrinos of all three flavors, and these
neutrinos are primarily responsible for powering the explosion. This mechanism
depends sensitively on the neutrino transport between the neutrinospheres and
the shock. In light of this, we have performed a comparison of multigroup
Boltzmann neutrino transport (MGBT) and multigroup flux-limited diffusion
(MGFLD) in post-core bounce environments. Differences in the mean inverse flux
factors, luminosities, and RMS energies translate to heating rates that are up
to 2 times larger for Boltzmann transport, with net cooling rates below the
gain radius that are typically 0.8 times the MGFLD rates. These differences are
greatest at earlier postbounce times for a given progenitor mass, and for a
given postbounce time, greater for greater progenitor mass. The increased
differences with increased progenitor mass suggest that the net heating
enhancement from MGBT is potentially robust and self-regulated.Comment: 7 pages, 2 figures, 1 table; LaTex using iopconf.sty; To appear in:
Proceedings of The Second Oak Ridge Symposium on Atomic & Nuclear
Astrophysic
Recommended from our members
Theoretical nuclear structure and astrophysics. Progress report for 1996
This research effort is directed toward theoretical support and guidance for the fields of radioactive ion beam physics, gamma ray spectroscopy, computational and nuclear astrophysics, and the interface between these disciplines. The authors report substantial progress in all those areas. One measure of progress is publications and invited material. The research described here has led to more than 43 papers that are published, accepted, or submitted to refereed journals, and to 15 invited presentations at conferences and workshops
Recommended from our members
Theoretical nuclear structure and astrophysics. Progress report for 1993--1995
This research effort is directed toward theoretical support and guidance for the developing fields of radioactive ion beam (RIB) physics, computational and nuclear astrophysics, and the interface between these disciplines. The authors are concerned both with the application of existing technologies and concepts to guide the initial RIB program, and the development of new ideas and new technologies to influence the longer-term future of nuclear structure physics and astrophysics. The authors report substantial progress in both areas. One measure of progress is publications and invited material. The research described here has led to more than 70 papers that are published, accepted, or submitted to refereed journals, and to 46 invited presentations at conferences and workshops
Universality of Symmetry and Mixed-symmetry Collective Nuclear States
The global correlation in the observed variation with mass number of the
and summed transition strengths is examined for rare earth nuclei. It is
shown that a theory of correlated and fermion pairs with a simple
pairing plus quadrupole interaction leads naturally to this universality. Thus
a unified and quantitative description emerges for low-lying quadrupole and
dipole strengths.Comment: In press, Phys. Rev. Lett. 199
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