564 research outputs found
Universality aspects of the d=3 random-bond Blume-Capel model
The effects of bond randomness on the universality aspects of the simple
cubic lattice ferromagnetic Blume-Capel model are discussed. The system is
studied numerically in both its first- and second-order phase transition
regimes by a comprehensive finite-size scaling analysis. We find that our data
for the second-order phase transition, emerging under random bonds from the
second-order regime of the pure model, are compatible with the universality
class of the 3d random Ising model. Furthermore, we find evidence that, the
second-order transition emerging under bond randomness from the first-order
regime of the pure model, belongs to a new and distinctive universality class.
The first finding reinforces the scenario of a single universality class for
the 3d Ising model with the three well-known types of quenched uncorrelated
disorder (bond randomness, site- and bond-dilution). The second, amounts to a
strong violation of universality principle of critical phenomena. For this case
of the ex-first-order 3d Blume-Capel model, we find sharp differences from the
critical behaviors, emerging under randomness, in the cases of the
ex-first-order transitions of the corresponding weak and strong first-order
transitions in the 3d three-state and four-state Potts models.Comment: 12 pages, 12 figure
Uncovering the secrets of the 2d random-bond Blume-Capel model
The effects of bond randomness on the ground-state structure, phase diagram
and critical behavior of the square lattice ferromagnetic Blume-Capel (BC)
model are discussed. The calculation of ground states at strong disorder and
large values of the crystal field is carried out by mapping the system onto a
network and we search for a minimum cut by a maximum flow method. In finite
temperatures the system is studied by an efficient two-stage Wang-Landau (WL)
method for several values of the crystal field, including both the first- and
second-order phase transition regimes of the pure model. We attempt to explain
the enhancement of ferromagnetic order and we discuss the critical behavior of
the random-bond model. Our results provide evidence for a strong violation of
universality along the second-order phase transition line of the random-bond
version.Comment: 6 LATEX pages, 3 EPS figures, Presented by AM at the symposium
"Trajectories and Friends" in honor of Nihat Berker, MIT, October 200
Matrix Elements and Few-Body Calculations within the Unitary Correlation Operator Method
We employ the Unitary Correlation Operator Method (UCOM) to construct
correlated, low-momentum matrix elements of realistic nucleon-nucleon
interactions. The dominant short-range central and tensor correlations induced
by the interaction are included explicitly by an unitary transformation. Using
correlated momentum-space matrix elements of the Argonne V18 potential, we show
that the unitary transformation eliminates the strong off-diagonal
contributions caused by the short-range repulsion and the tensor interaction,
and leaves a correlated interaction dominated by low-momentum contributions. We
use correlated harmonic oscillator matrix elements as input for no-core shell
model calculations for few-nucleon systems. Compared to the bare interaction,
the convergence properties are dramatically improved. The bulk of the binding
energy can already be obtained in very small model spaces or even with a single
Slater determinant. Residual long-range correlations, not treated explicitly by
the unitary transformation, can easily be described in model spaces of moderate
size allowing for fast convergence. By varying the range of the tensor
correlator we are able to map out the Tjon line and can in turn constrain the
optimal correlator ranges.Comment: 16 pages, 9 figures, using REVTEX
The role of the continuum and the spurious 1- transitions in incoherent mu - e conversion rate calculations
By using the Continuum RPA (CRPA) method, the incoherent transition strength of the exotic mu - e conversion in the 208Pb and 40Ca nuclei is investigated. The question whether excited nuclear states lying high in the continuum give an important contribution to the incoherent rate is addressed. The admixture of spurious components in the rate coming from 1- excitations is investigated in detail by using the self-consistent CRPA with Skyrme interactions as well as a less consistent version and by employing two ways to remove the spurious strength: the use of effective operators or simply the exclusion of the spurious state appearing close to zero energy. In all cases, the correction achieved is quite large
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The Effect of the Short-Range Correlations on the Generalized Momentum Distribution in Finite Nuclei
The effect of dynamical short-range correlations on the generalized momentum
distribution in the case of , -closed shell
nuclei is investigated by introducing Jastrow-type correlations in the
harmonic-oscillator model. First, a low order approximation is considered and
applied to the nucleus He. Compact analytical expressions are derived and
numerical results are presented and the effect of center-of-mass corrections is
estimated. Next, an approximation is proposed for of
heavier nuclei, that uses the above correlated of He.
Results are presented for the nucleus O. It is found that the effect of
short-range correlations is significant for rather large values of the momenta
and/or and should be included, along with center of mass corrections
for light nuclei, in a reliable evaluation of in the whole
domain of and .Comment: 29 pages, 8 figures. Further results, figures and discussion for the
CM corrections are added. Accepted by Journal of Physics
The role of the continuum and the spurious 1⁻ transitions in incoherent μ⁻ - e⁻ conversion rate calculations
By using the Continuum RPA (CRPA) method, the incoherent transition strength of the exotic μ⁻ - e⁻ conversion in nuclei is investigated. The question whether excited nuclear states lying high in the continuum give an important contribution to the incoherent rate is addressed. Results for ⁴⁰Ca are compared with those obtained previously for ²⁰⁸Pb. For both nuclei we then investigate in detail the admixture of spurious components in the rate coming from 1⁻ excitations, within the self-consistent CRPA with Skyrme interactions as well as within a less consistent version. We employ and compare two methods for removing the spurious strength: the use of effective operators, as done in a previous work for ²⁰⁸Pb, or simply the exclusion of the spurious state appearing close to zero energy. In all cases, the correction achieved is quite large.Facultad de Ciencias Exacta
Hartree-Fock and Many-Body Perturbation Theory with Correlated Realistic NN-Interactions
We employ correlated realistic nucleon-nucleon interactions for the
description of nuclear ground states throughout the nuclear chart within the
Hartree-Fock approximation. The crucial short-range central and tensor
correlations, which are induced by the realistic interaction and cannot be
described by the Hartree-Fock many-body state itself, are included explicitly
by a state-independent unitary transformation in the framework of the unitary
correlation operator method (UCOM). Using the correlated realistic interaction
V_UCOM resulting from the Argonne V18 potential, bound nuclei are obtained
already on the Hartree-Fock level. However, the binding energies are smaller
than the experimental values because long-range correlations have not been
accounted for. Their inclusion by means of many-body perturbation theory leads
to a remarkable agreement with experimental binding energies over the whole
mass range from He-4 to Pb-208, even far off the valley of stability. The
observed perturbative character of the residual long-range correlations and the
apparently small net effect of three-body forces provides promising
perspectives for a unified nuclear structure description.Comment: 14 pages, 8 figures, 3 tables, using REVTEX
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