731 research outputs found
Self-consistency in the Projected Shell Model
The Projected Shell Model is a shell model theory built up over a deformed
BCS mean field. Ground state and excited bands in even-even nuclei are obtained
through diagonalization of a pairing plus quadrupole Hamiltonian in an angular
momentum projected 0-, 2-, and 4-quasiparticle basis. The residual
quadrupole-quadrupole interaction strength is fixed self-consistently with the
deformed mean field and the pairing constants are the same used in constructing
the quasiparticle basis. Taking as an example, we calculate
low-lying states and compare them with experimental data. We exhibit the effect
of changing the residual interaction strengths on the spectra. It is clearly
seen that there are many bandheads whose energies can
only be reproduced using the self-consistent strengths. It is thus concluded
that the Projected Shell Model is a model essentially with no free parameters.Comment: 13 pages, 10 figures, submitted to Nuclear Physics
The Hubbard model with smooth boundary conditions
We apply recently developed smooth boundary conditions to the quantum Monte
Carlo simulation of the two-dimensional Hubbard model. At half-filling, where
there is no sign problem, we show that the thermodynamic limit is reached more
rapidly with smooth rather than with periodic or open boundary conditions. Away
from half-filling, where ordinarily the simulation cannot be carried out at low
temperatures due to the existence of the sign problem, we show that smooth
boundary conditions allow us to reach significantly lower temperatures. We
examine pairing correlation functions away from half-filling in order to
determine the possible existence of a superconducting state. On a
lattice for , at a filling of and an inverse
temperature of , we did find enhancement of the -wave correlations
with respect to the non-interacting case, a possible sign of -wave
superconductivity.Comment: 16 pages RevTeX, 9 postscript figures included (Figure 1 will be
faxed on request
Pseudo SU(3) shell model: Normal parity bands in odd-mass nuclei
A pseudo shell SU(3) model description of normal parity bands in 159-Tb is
presented. The Hamiltonian includes spherical Nilsson single-particle energies,
the quadrupole-quadrupole and pairing interactions, as well as three rotor
terms. A systematic parametrization is introduced, accompained by a detailed
discussion of the effect each term in the Hamiltonian has on the energy
spectrum. Yrast and excited band wavefunctions are analyzed together with their
B(E2) values.Comment: 29 pages, 6 figure
Quasi-SU(3) truncation scheme for even-even sd-shell nuclei
The Quasi-SU(3) symmetry was uncovered in full pf and sdg shell-model
calculations for both even-even and odd-even nuclei. It manifests itself
through a dominance of single-particle and quadrupole-quadrupole terms in the
Hamiltonian used to describe well-deformed nuclei. A practical consequence of
the quasi-SU(3) symmetry is an efficient basis truncation scheme. In a recent
work was shown that when this type of Hamiltonian is diagonalized in an SU(3)
basis, only a few irreducible represntations (irreps) of SU(3) are needed to
describe the Yrast band, the leading S = 0 irrep augmented with the leading S =
1 irreps in the proton and neutron subspaces. In the present article the
quasi-SU(3) truncation scheme is used, in conjunction with a "realistic but
schematic" Hamiltonian that includes the most important multipole terms, to
describe the energy spectra and B(E2) transition strengths of 20-Ne, 22-Ne,
24-Mg and 28-Si. The effect of the size of the Hilbert space on both sets of
observables is discussed, as well as the structure of the Yrast band and the
importance of the various terms in the Hamiltonian.Comment: 30 pages, 8 figures. Submited to Nucl. Phys.
How good are the Garvey-Kelson predictions of nuclear masses?
The Garvey-Kelson relations are used in an iterative process to predict
nuclear masses in the neighborhood of nuclei with measured masses. Average
errors in the predicted masses for the first three iteration shells are smaller
than those obtained with the best nuclear mass models. Their quality is
comparable with the Audi-Wapstra extrapolations, offering a simple and
reproducible procedure for short range mass predictions. A systematic study of
the way the error grows as a function of the iteration and the distance to the
known masses region, shows that a correlation exists between the error and the
residual neutron-proton interaction, produced mainly by the implicit assumption
that varies smoothly along the nuclear landscape.Comment: 10 pages, 18 figure
A schematic model for QCD I: Low energy meson states
A simple model for QCD is presented, which is able to reproduce the meson
spectrum at low energy. The model is a Lipkin type model for quarks coupled to
gluons. The basic building blocks are pairs of quark-antiquarks coupled to a
definite flavor and spin. These pairs are coupled to pairs of gluons with spin
zero. The multiplicity problem, which dictates that a given experimental state
can be described in various manners, is removed when a particle-mixing
interaction is turned on. In this first paper of a series we concentrates on
the discussion of meson states at low energy, the so-called zero temperature
limit of the theory. The treatment of baryonic states is indicated, also.Comment: 29 pages, 6 figures. submitted to Phys. Rev.
Nuclear masses and the number of valence nucleons
An improved version of the liquid drop model is presented. The addition of two terms, linear and quadratic in the total number of valence nucleons (particles or holes), improves the description of atomic masses, which can be fitted with an r.m.s. error of 1.2 MeV. Predictions are analysed an compared with those of established models. (c) 2007 Elsevier B.V. All rights reserved
Backbending in Dy isotopes within the Projected Shell Model
A systematic study of the yrast band in 154-164 Dy isotopes using the
Projected Shell Model is presented. It is shown that, in the context of the
present model, enlarging the mean field deformation by about 20 % allows a very
good description of the spectrum of yrast band in these isotopes. The
dependence of the B(E2) values on angular momentum is also better described
when larger deformations are used. The observed oscillation of g-factors at low
spin states remains an open question for this model.Comment: 17 pages, 7 figures, submitted to Phys. Rev.
Self-Consistent Quasi-Particle RPA for the Description of Superfluid Fermi Systems
Self-Consistent Quasi-Particle RPA (SCQRPA) is for the first time applied to
a more level pairing case. Various filling situations and values for the
coupling constant are considered. Very encouraging results in comparison with
the exact solution of the model are obtained. The nature of the low lying mode
in SCQRPA is identified. The strong reduction of the number fluctuation in
SCQRPA vs BCS is pointed out. The transition from superfluidity to the normal
fluid case is carefully investigated.Comment: 23 pages, 18 figures and 1 table, submitted to Phys. Rev.
Linear response within the projection-based renormalization method: Many-body corrections beyond the random phase approximation
The explicit evaluation of linear response coefficients for interacting
many-particle systems still poses a considerable challenge to theoreticians. In
this work we use a novel many-particle renormalization technique, the so-called
projector-based renormalization method, to show how such coefficients can
systematically be evaluated. To demonstrate the prospects and power of our
approach we consider the dynamical wave-vector dependent spin susceptibility of
the two-dimensional Hubbard model and also determine the subsequent magnetic
phase diagram close to half-filling. We show that the superior treatment of
(Coulomb) correlation and fluctuation effects within the projector-based
renormalization method significantly improves the standard random phase
approximation results.Comment: 17 pages, 7 figures, revised versio
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