[Show abstract][Hide abstract] ABSTRACT: We suggest a small set of fission observables to be used as test cases for
validation of theoretical calculations. The purpose is to provide common data
to facilitate the comparison of different fission theories and models. The
proposed observables are chosen from fission barriers, spontaneous fission
lifetimes, fission yield characteristics, and fission isomer excitation
energies.
Journal of Physics G Nuclear and Particle Physics 02/2015; 42(7). DOI:10.1088/0954-3899/42/7/077001 · 2.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We calculate the energy deposition by very short laser pulses in SiO_2
(alpha-quartz) with a view to establishing systematics for predicting damage
and nanoparticle production. The theoretical framework is time-dependent
density functional theory, implemented by the real-time method in a multiscale
representation. For the most realistic simulations we employ a meta-GGA
Kohn-Sham potential similar to that of Becke and Johnson. We find that the
deposited energy in the medium can be accurately modeled as a function of the
local electromagnetic pulse fluence. The energy-deposition function can in turn
be quite well fitted to the strong-field Keldysh formula for a range of
intensities from below the melting threshold to well beyond the ablation
threshold. We find reasonable agreement between the damage threshold and the
energy required to melt the substrate. The ablation threshold estimated by the
energy to convert the substrate to an atomic fluid is higher than the
measurement, indicating significance of nonthermal nature of the process. A
fair agreement is found for the depth of the ablation.
[Show abstract][Hide abstract] ABSTRACT: The ground state octupole correlations energies obtained with the D1M variant
of the Gogny force are analyzed in detail. First we consider the correlation
energy gained at the mean field level by allowing reflection symmetry breaking.
Next we consider the energy gain coming from symmetry (parity) restoration and
finally we analyze the ground state correlation energy after configuration
mixing with axially symmetric octupole states. We find that these correlations
do not significantly affect the trends of binding energies and systematics near
closed shells. In particular, the too-large shell gaps predicted by
self-consistent mean field models are not altered by the correlations.
[Show abstract][Hide abstract] ABSTRACT: Deformation, a key concept in our understanding of heavy nuclei, is based on
a mean-field description that breaks the rotational invariance of the nuclear
many-body Hamiltonian. We present a method to analyze nuclear deformations at
finite temperature in a framework that preserves rotational invariance. The
auxiliary-field Monte-Carlo method is used to generate the statistical ensemble
and calculate the probability distribution associated with the quadrupole
operator. Applying the technique to nuclei in the rare-earth region, we
identify model-independent signatures of deformation and find that deformation
effects persist to higher temperatures than the spherical-to-deformed shape
phase-transition temperature of mean-field theory.
[Show abstract][Hide abstract] ABSTRACT: We derive equations of motion for calculating the near-edge x-ray absorption spectrum in molecules and condensed matter, based on a two-determinant approximation and Dirac's variational principle. The theory provides an exact solution for the linear response when the Hamiltonian or energy functional has only diagonal interactions in some basis. We numerically solve the equations to compare with the Mahan-Nozières-De Dominicis theory of the edge singularity in metallic conductors. Our extracted power-law exponents are similar to those of the analytic theory, but are not in quantitative agreement. The calculational method can be readily generalized to treat Kohn-Sham Hamiltonians with electron-electron interactions derived from correlation-exchange potentials.
Physical Review B 01/2014; 89(7). DOI:10.1103/PhysRevB.89.075135 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We calculate the dielectric response of crystalline silicon following irradiation by a high-intensity laser pulse, modeling the dynamics by the time-dependent Kohn-Sham equations in the presence of the laser field. Pump-probe measurements of the response are numerically simulated by including both pump and probe externals fields in the simulation. As expected, the excited silicon shows features of an electron-hole plasma of nonequilibrium phase in its response, characterized by a negative divergence in the real part of the dielectric function at small frequencies. The response to the probe pulse depends on the polarization of the pump pulse. We also find that the imaginary part of the dielectric function can be negative, particularly for the parallel polarization of pump and probe fields. We compare the calculated response with a simple Drude model. The real part of the dielectric function is well fitted by the model, treating the effective mass as a fitting parameter while taking electron density from the calculation. The fitted effective masses are consistent with carrier-band dispersions.
Physical Review B 01/2014; 89(6). DOI:10.1103/PhysRevB.89.064304 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Levels densities of independent-particle Hamiltonians can be calculated
easily by using the real-time representation of the evolution operator together
with the fast Fourier transform. We describe the method and implement it with a
set of Python programs. Examples are provided for the total and partial levels
densities of a heavy deformed nucleus (Dy-164). The partial level densities
that may be calculated are the projected ones on neutron number, proton number,
azimuthal angular momentum, and parity.
[Show abstract][Hide abstract] ABSTRACT: We investigate mechanisms of coherent phonon generation in time-dependent density-functional theory. It provides intuitive understanding of the generation mechanism as well as its change depending on electric field frequency.
International Conference on Ultrafast Phenomena; 01/2014
[Show abstract][Hide abstract] ABSTRACT: The Gallagher-Moszkowski rule in the spectroscopy of odd-odd nuclei imposes a
new spin constraint on the energy functionals for self-consistent mean field
theory. The commonly used parameterization of the effective three-body
interaction in the Gogny and Skyrme families of energy functionals is
ill-suited to satisfy the spin constraint. In particular, the Gogny
parameterization of the three-body interaction has the opposite spin dependence
to that required by the observed spectra. The two-body part has a correct sign,
but in combination the rule is violated as often as not. We conclude that a new
functional form is needed for the effective three-body interaction that can
take into better account the different spin-isospin channels of the
interaction.
Physical Review C 10/2013; 89(2). DOI:10.1103/PhysRevC.89.021303 · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We derive a time-dependent density functional theory appropriate for
calculating the near-edge X-ray absorption spectrum in molecules and condensed
matter. The basic assumption is to increase the space of many-body wave
functions from one Slater determinant to two. The equations of motion derived
from Dirac's variational principle provide an exact solution for the linear
response when the interaction Hamiltonian has only a core-electron field. The
equations can be solved numerically nearly as easily as the ordinary real-time
time-dependent Kohn-Sham equations. We carry out the solution under conditions
that permit comparison with the expected power-law behavior. Our extracted
power-law exponents are similar to those derived by Nozieres and DeDominicis,
but are not in quantitative agreement. We argue that our calculational method
can be readily generalized to density functionals that take into account the
more general electron-electron interactions that are needed for treating
dynamic effects such as plasmon excitations.
[Show abstract][Hide abstract] ABSTRACT: We have been developing a theoretical framework to describe electron
dynamics in a crystalline solid under an ultrashort laser pulse. We rely
upon the time-dependent density functional theory, solving the
time-dependent Kohn-Sham equation in real-time and real-space. Using our
method, it is possible to describe both linear and nonlinear
light-matter interactions in a unified way. In my presentation, I will
focus on the application to coherent phonon generation, a coherent
atomic oscillation over a macroscopic volume. I will show applications
to two material, semiconductor Si and semimetal Sb. For Si, we have
found that the TDDFT is capable of describe two distinct mechanisms of
the coherent phonon generation. When the laser frequency is below the
direct bandgap, virtual electronic excitation induces impulsive force to
atoms. When the laser frequency is above the gap, real electronic
excitation causes the atomic motion. For Sb, we study the frequency
dependence of the coherent phonon generation and compare our results
with phenomenological theories.
[Show abstract][Hide abstract] ABSTRACT: As part of a program to study odd-A nuclei in the Hartree-Fock-Bogoliubov
(HFB) theory, we have developed a new calculational tool to find the HFB minima
of odd-A nuclei based on the gradient method and using interactions of Gogny's
form. The HFB minimization includes both time-even and time-odd fields in the
energy functional, avoiding the commonly used "filling approximation". Here we
apply the method to calculate neutron pairing gaps in some representative
isotope chains of spherical and deformed nuclei, namely the Z=8,50 and 82
spherical chains and the Z=62 and 92 deformed chains. We find that the gradient
method is quite robust, permitting us to carry out systematic surveys involving
many nuclei. We find that the time-odd field does not have large effect on the
pairing gaps calculated with the Gogny D1S interaction. Typically, adding the
T-odd field as a perturbation increases the pairing gap by ~100 keV, but the
re-minimization brings the gap back down. This outcome is very similar to
results reported for the Skyrme family of nuclear energy density functionals.
Comparing the calculated gaps with the experimental ones, we find that the
theoretical errors have both signs implying that the D1S interaction has a
reasonable overall strength. However, we find some systematic deficiencies
comparing spherical and deformed chains and comparing the lighter chains with
the heavier ones. The gaps for heavy spherical nuclei are too high, while those
for deformed nuclei tend to be too low. The calculated gaps of spherical nuclei
show hardly any A-dependence, contrary to the data. Inclusion of the T-odd
component of the interaction does not change these qualitative findings.
Physical Review C 10/2012; 86(6). DOI:10.1103/PhysRevC.86.064313 · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The time-dependent density functional theory (TDDFT) is the leading
computationally feasible theory to treat excitations by strong electromagnetic
fields. Here the theory is applied to coherent optical phonon generation
produced by intense laser pulses. We examine the process in the crystalline
semimetal antimony (Sb), where nonadiabatic coupling is very important. This
material is of particular interest because it exhibits strong phonon coupling
and optical phonons of different symmetries can be observed. The TDDFT is able
to account for a number of qualitative features of the observed coherent
phonons, despite its unsatisfactory performance on reproducing the observed
dielectric functions of Sb. A simple dielectric model for nonadiabatic coherent
phonon generation is also examined and compared with the TDDFT calculations.
The Journal of Chemical Physics 08/2012; 137(22):22A527. DOI:10.1063/1.4739844 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Spectroscopic observables such as electromagnetic transitions strengths can
be related to the properties of the intrinsic mean-field wave function when the
latter are strongly deformed, but the standard rotational formulas break down
when the deformation decreases. Nevertheless there is a well-defined, non-zero,
spherical limit that can be evaluated in terms of overlaps of mean-field
intrinsic deformed wave functions. We examine the transition between the
spherical limit and strongly deformed one for a range of nuclei comparing the
two limiting formulas with exact projection results. We find a simple criterion
for the validity of the rotational formula depending on $<\Delta \vec{J}^2>$,
the mean square fluctuation in the angular momentum of the intrinsic state. We
also propose an interpolation formula which describes the transition strengths
over the entire range of deformations, reducing to the two simple expressions
in the appropriate limits.
Physical Review C 06/2012; 86(5). DOI:10.1103/PhysRevC.86.054306 · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Hartree-Fock-Bogoliubov approximation is very useful for treating both
long- and short-range correlations in finite quantum fermion systems, but it
must be extended in order to describe detailed spectroscopic properties. One
problem is the symmetry-breaking character of the HFB approximation. We present
a general and systematic way to restore symmetries and to extend the
configuration space using pfaffian formulas for the many-body matrix elements.
The advantage of those formulas is that the sign of the matrix elements is
unambiguously determined. It is also helpful to extend the space of
configurations by constraining the HFB solutions in some way. A powerful method
for finding these constrained solutions is the gradient method, based on the
generalized Thouless transformation. The gradient method also preserves the
number parity of the Bogoliubov transformation, which facilitates the
application of the theory to systems with odd particle number.
[Show abstract][Hide abstract] ABSTRACT: We show that long-range correlations for nuclear masses have a significant
effect on the synthesis of heavy elements by the r-process. As calculated by
Delaroche et al. [1], these correlations suppress magic number effects
associated with minor shells. This impacts the calculated abundances before the
third r-process peak (at mass number A~195), where the abundances are low and
form a trough. This trough and the position of the third abundance peak are
strongly affected by the masses of nuclei in the transition region between
deformed and spherical. Based on different astrophysical environments, our
results demonstrate that a microscopic theory of nuclear masses including
correlations naturally smoothens the separation energies, thus reducing the
trough and improving the agreement with observed solar system abundances.