280 research outputs found
Probing Correlated Ground States with Microscopic Optical Model for Nucleon Scattering off Doubly-Closed-Shell Nuclei
The RPA long range correlations are known to play a significant role in
understanding the depletion of single particle-hole states observed in (e, e')
and (e, e'p) measurements. Here the Random Phase Approximation (RPA) theory,
implemented using the D1S force is considered for the specific purpose of
building correlated ground states and related one-body density matrix elements.
These may be implemented and tested in a fully microscopic optical model for NA
scattering off doubly-closed-shell nuclei. A method is presented to correct for
the correlations overcounting inherent to the RPA formalism. One-body density
matrix elements in the uncorrelated (i.e. Hartree-Fock) and correlated (i.e.
RPA) ground states are then challenged in proton scattering studies based on
the Melbourne microscopic optical model to highlight the role played by the RPA
correlations. Effects of such correlations which deplete the nuclear matter at
small radial distance (r 2 fm) and enhance its surface region, are getting
more and more sizeable as the incident energy increases. Illustrations are
given for proton scattering observables measured up to 201 MeV for the
O, Ca, Ca and Pb target nuclei. Handling the RPA
correlations systematically improves the agreement between scattering
predictions and data for energies higher than 150 MeV.Comment: 20 pages, 7 figure
Concepts of alpha-particle condensation
Certain aspects of the recently proposed antisymmetrised alpha particle
product state wave function, or THSR alpha cluster wave function, for the
description of the ground state in 8Be, the Hoyle state in 12C, and analogous
states in heavier nuclei, are elaborated in detail. For instance, the influence
of antisymmetrisation in the Hoyle state on the bosonic character of the alpha
particles is studied carefully. It is shown to be weak, so that bosonic aspects
are predominant. The de Broglie wave length of alpha particles in the Hoyle
state is shown to be much larger than the inter-alpha distance. It is pointed
out that the bosonic features of low density alpha gas states have measurable
consequences, one of which, that is enhanced multi-alpha decay properties,
likely already have been detected. Consistent with experiment, the width of the
proposed analogue to the Hoyle state in 16O at the excitation energy of
E_x=15.1 MeV is estimated to be very small (34 keV), lending credit to the
existence of heavier Hoyle-like states. The intrinsic single boson density
matrix of a self-bound Bose system can, under physically desirable boundary
conditions, be defined unambiguously. One eigenvalue then separates out, being
close to the number of alpha's in the system. Differences between Brink and
THSR alpha cluster wave functions are worked out. No cluster model of the Brink
type can describe the Hoyle state with a single configuration. On the contrary,
many superpositions of the Brink type are necessary, implying delocalisation
towards an alpha product state. It is shown that single alpha particle orbits
in condensates of different nuclei are almost the same. It is thus argued that
alpha particle antisymmetrised product states of the THSR type are a very
promising novel and useful concept in nuclear physics.Comment: 16 pages, 14 figures, to appear in PR
Superconductivity in Ultrasmall Metallic Grains
We develop a theory of superconductivity in ultrasmall (nm-scale) metallic
grains having a discrete electronic eigenspectrum with a mean level spacing of
order of the bulk gap. The theory is based on calculating the eigenspectrum
using a generalized BCS variational approach, whose applicability has been
extensively demonstrated in studies of pairing correlations in nuclear physics.
We discuss how conventional mean field theory breaks down with decreasing
sample size, how the so-called blocking effect weakens pairing correlations in
states with non-zero total spin, and how this affects the discrete
eigenspectrum's behavior in a magnetic field, which favors non-zero total spin.
In ultrasmall grains, spin magnetism dominates orbital magnetism, just as in
thin films in a parallel field; but whereas in the latter the magnetic-field
induced transition to a normal state is known to be first-order, we show that
in ultrasmall grains it is softened by finite size effects. Our calculations
qualitatively reproduce the magnetic-field dependent tunneling spectra for
individual aluminum grains measured recently by Ralph, Black and Tinkham. We
argue that previously-discussed parity effects for the odd-even ground state
energy difference are presently not observable for experimental reasons, and
propose an analogous parity effect for the pair-breaking energy that should be
observable provided that the grain size can be controlled sufficiently well.
Finally, experimental evidence is pointed out that the dominant role played by
time-reversed pairs of states, well-established in bulk and in dirty
superconductors, persists also in ultrasmall grains.Comment: 21 pages RevTeX, 12 EPS figures included, uses epsf.st
Time-Dependent Gutzwiller Theory for Multiband Hubbard Models
Based on the variational Gutzwiller theory, we present a method for the
computation of response functions for multiband Hubbard models with general
local Coulomb interactions. The improvement over the conventional random-phase
approximation is exemplified for an infinite-dimensional two-band Hubbard model
where the incorporation of the local multiplet-structure leads to a much larger
sensitivity of ferromagnetism on the Hund coupling. Our method can be
implemented into LDA+Gutzwiller schemes and will therefore be an important tool
for the computation of response functions for strongly correlated materials.Comment: 4 pages, 3 figure
A quantum Monte-Carlo method for fermions, free of discretization errors
In this work we present a novel quantum Monte-Carlo method for fermions,
based on an exact decomposition of the Boltzmann operator . It
can be seen as a synthesis of several related methods. It has the advantage
that it is free of discretization errors, and applicable to general
interactions, both for ground-state and finite-temperature calculations. The
decomposition is based on low-rank matrices, which allows faster calculations.
As an illustration, the method is applied to an analytically solvable model
(pairing in a degenerate shell) and to the Hubbard model.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Let
Shell Effects in Nuclei with Vector Self-Coupling of Omega Meson in Relativistic Hartree-Bogoliubov Theory
Shell effects in nuclei about the stability line are investigated within the
framework of the Relativistic Hartree-Bogoliubov (RHB) theory with
self-consistent finite-range pairing. Using 2-neutron separation energies of Ni
and Sn isotopes, the role of - and -meson couplings on the
shell effects in nuclei is examined. It is observed that the existing
successful nuclear forces (Lagrangian parameter sets) based upon the nonlinear
scalar coupling of -meson exhibit shell effects which are stronger than
suggested by the experimental data. We have introduced nonlinear vector
self-coupling of -meson in the RHB theory. It is shown that the
inclusion of the vector self-coupling of -meson in addition to the
nonlinear scalar coupling of -meson provides a good agreement with the
experimental data on shell effects in nuclei about the stability line. A
comparison of the shell effects in the RHB theory is made with the Hartree-Fock
Bogoliubov approach using the Skyrme force SkP. It is shown that the
oft-discussed shell quenching with SkP is not consistent with the available
experimental data.Comment: 34 pages latex, 18 ps figures, replaced with minor corrections in
some figures, accepted for publication in Phys. Rev.
The Density Matrix Renormalization Group for finite Fermi systems
The Density Matrix Renormalization Group (DMRG) was introduced by Steven
White in 1992 as a method for accurately describing the properties of
one-dimensional quantum lattices. The method, as originally introduced, was
based on the iterative inclusion of sites on a real-space lattice. Based on its
enormous success in that domain, it was subsequently proposed that the DMRG
could be modified for use on finite Fermi systems, through the replacement of
real-space lattice sites by an appropriately ordered set of single-particle
levels. Since then, there has been an enormous amount of work on the subject,
ranging from efforts to clarify the optimal means of implementing the algorithm
to extensive applications in a variety of fields. In this article, we review
these recent developments. Following a description of the real-space DMRG
method, we discuss the key steps that were undertaken to modify it for use on
finite Fermi systems and then describe its applications to Quantum Chemistry,
ultrasmall superconducting grains, finite nuclei and two-dimensional electron
systems. We also describe a recent development which permits symmetries to be
taken into account consistently throughout the DMRG algorithm. We close with an
outlook for future applications of the method.Comment: 48 pages, 17 figures Corrections made to equation 19 and table
Energetics and Structural Properties of Trapped Two-Component Fermi Gases
Using two different numerical methods, we study the behavior of two-component
Fermi gases interacting through short-range s-wave interactions in a harmonic
trap. A correlated Gaussian basis-set expansion technique is used to determine
the energies and structural properties, i.e., the radial one-body densities and
pair distribution functions, for small systems with either even or odd , as
functions of the s-wave scattering length and the mass ratio of the
two species. Particular emphasis is put on a discussion of the angular momentum
of the system in the BEC-BCS crossover regime. At unitarity, the excitation
spectrum of the four-particle system with total angular momentum L=0 is
calculated as a function of the mass ratio . The results are analyzed
from a hyperspherical perspective, which offers new insights into the problem.
Additionally, fixed-node diffusion Monte Carlo calculations are performed for
equal-mass Fermi gases with up to N=30 atoms. We focus on the odd-even
oscillations of the ground state energy of the equal-mass unitary system having
up to N=30 particles, which are related to the excitation gap of the system.
Furthermore, we present a detailed analysis of the structural properties of
these systems.Comment: 22 pages, 21 figure
- …