Publications (364)1401.51 Total impact
 [Show abstract] [Hide abstract]
ABSTRACT: The configurationinteraction shell model approach provides an attractive framework for the calculation of nuclear level densities in the presence of correlations, but the large dimensionality of the model space has hindered its application in midmass and heavy nuclei. The shell model Monte Carlo (SMMC) method permits calculations in model spaces that are many orders of magnitude larger than spaces that can be treated by conventional diagonalization methods. We discuss recent progress in the SMMC approach to level densities, and in particular the calculation of level densities in heavy nuclei. We calculate the distribution of the axial quadrupole operator in the laboratory frame at finite temperature and demonstrate that it is a modelindependent signature of deformation in the rotational invariant framework of the shell model. We propose a method to use these distributions for calculating level densities as a function of intrinsic deformation.  [Show abstract] [Hide abstract]
ABSTRACT: We assess the accuracy of finitetemperature meanfield theory using as a standard the Hamiltonian and model space of the shell model Monte Carlo calculations. Two examples are considered: the nucleus $^{162}$Dy, representing a heavy deformed nucleus, and $^{148}$Sm, representing a nearby heavy spherical nucleus with strong pairing correlations. The errors inherent in the finitetemperature HartreeFock and HartreeFockBogoliubov approximations are analyzed by comparing the entropies of the grand canonical and canonical ensembles, as well as the level density at the neutron resonance threshold, with shell model Monte Carlo (SMMC) calculations, which are accurate up to wellcontrolled statistical errors. The main weak points in the meanfield treatments are seen to be: (i) the extraction of numberprojected densities from the grand canonical ensembles, and (ii) the symmetry breaking by deformation or by the pairing condensate. In the absence of a pairing condensate, we confirm that the usual saddlepoint approximation to extract the numberprojected densities is not a significant source of error compared to other errors inherent to the meanfield theory. We also present an alternative formulation of the saddlepoint approximation that makes direct use of an approximate particlenumber projection and avoids computing the usual threedimensional Jacobian of the saddlepoint integration. We find that the pairing condensate is less amenable to approximate particlenumber projection methods due to the explicit violation of particlenumber conservation in the pairing condensate. Nevertheless, the HartreeFockBogoliubov theory is accurate to less than one unit of entropy for $^{148}$Sm at the neutron threshold energy, which is above the pairing phase transition.  [Show abstract] [Hide abstract]
ABSTRACT: Irradiation of intense and ultrashort laser pulses on dielectric surface is calculated in realtime using firstprinciples timedependent density functional theory. It is found that calculated energy distribution transferred from laser pulse to electrons in dielectrics explains measured threshold and depth of laserinduced damage. 
Article: Benchmarking Nuclear Fission Theory
[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.  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the energy deposition by very short laser pulses in SiO_2 (alphaquartz) with a view to establishing systematics for predicting damage and nanoparticle production. The theoretical framework is timedependent density functional theory, implemented by the realtime method in a multiscale representation. For the most realistic simulations we employ a metaGGA KohnSham 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 energydeposition function can in turn be quite well fitted to the strongfield 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 toolarge shell gaps predicted by selfconsistent 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 meanfield description that breaks the rotational invariance of the nuclear manybody Hamiltonian. We present a method to analyze nuclear deformations at finite temperature in a framework that preserves rotational invariance. The auxiliaryfield MonteCarlo 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 rareearth region, we identify modelindependent signatures of deformation and find that deformation effects persist to higher temperatures than the sphericaltodeformed shape phasetransition temperature of meanfield theory.  [Show abstract] [Hide abstract]
ABSTRACT: We derive equations of motion for calculating the nearedge xray absorption spectrum in molecules and condensed matter, based on a twodeterminant 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 MahanNozièresDe Dominicis theory of the edge singularity in metallic conductors. Our extracted powerlaw exponents are similar to those of the analytic theory, but are not in quantitative agreement. The calculational method can be readily generalized to treat KohnSham Hamiltonians with electronelectron interactions derived from correlationexchange potentials.  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the dielectric response of crystalline silicon following irradiation by a highintensity laser pulse, modeling the dynamics by the timedependent KohnSham equations in the presence of the laser field. Pumpprobe 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 electronhole 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 carrierband dispersions.  [Show abstract] [Hide abstract]
ABSTRACT: Levels densities of independentparticle Hamiltonians can be calculated easily by using the realtime 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 (Dy164). The partial level densities that may be calculated are the projected ones on neutron number, proton number, azimuthal angular momentum, and parity. 
Conference Paper: Abinitio theoretical approach to coherent phonon generation in solids
[Show abstract] [Hide abstract]
ABSTRACT: We investigate mechanisms of coherent phonon generation in timedependent densityfunctional theory. It provides intuitive understanding of the generation mechanism as well as its change depending on electric field frequency.  [Show abstract] [Hide abstract]
ABSTRACT: The GallagherMoszkowski rule in the spectroscopy of oddodd nuclei imposes a new spin constraint on the energy functionals for selfconsistent mean field theory. The commonly used parameterization of the effective threebody interaction in the Gogny and Skyrme families of energy functionals is illsuited to satisfy the spin constraint. In particular, the Gogny parameterization of the threebody interaction has the opposite spin dependence to that required by the observed spectra. The twobody 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 threebody interaction that can take into better account the different spinisospin channels of the interaction. 
Dataset: JChemPhys.134.144106
 [Show abstract] [Hide abstract]
ABSTRACT: We derive a timedependent density functional theory appropriate for calculating the nearedge Xray absorption spectrum in molecules and condensed matter. The basic assumption is to increase the space of manybody 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 coreelectron field. The equations can be solved numerically nearly as easily as the ordinary realtime timedependent KohnSham equations. We carry out the solution under conditions that permit comparison with the expected powerlaw behavior. Our extracted powerlaw 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 electronelectron interactions that are needed for treating dynamic effects such as plasmon excitations. 
Dataset: JChemPhys.134.144106
 [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 timedependent density functional theory, solving the timedependent KohnSham equation in realtime and realspace. Using our method, it is possible to describe both linear and nonlinear lightmatter 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: There is no abstract available for this article. 
 [Show abstract] [Hide abstract]
ABSTRACT: As part of a program to study oddA nuclei in the HartreeFockBogoliubov (HFB) theory, we have developed a new calculational tool to find the HFB minima of oddA nuclei based on the gradient method and using interactions of Gogny's form. The HFB minimization includes both timeeven and timeodd 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 timeodd field does not have large effect on the pairing gaps calculated with the Gogny D1S interaction. Typically, adding the Todd field as a perturbation increases the pairing gap by ~100 keV, but the reminimization 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 Adependence, contrary to the data. Inclusion of the Todd component of the interaction does not change these qualitative findings.  [Show abstract] [Hide abstract]
ABSTRACT: The timedependent 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.
Publication Stats
17k  Citations  
1,401.51  Total Impact Points  
Top Journals
 Physical Review C (104)
 Physics Letters B (37)
 Nuclear Physics A (32)
 Physical Review Letters (20)
 Physical Review Letters (18)
Institutions

19932015

University of Washington Seattle
 • Institute for Nuclear Theory
 • Department of Physics
Seattle, Washington, United States


2001

Tohoku University
 Department of Physics
Sendai, Kagoshimaken, Japan


19711998

Michigan State University
 Department of Physics and Astronomy
East Lansing, MI, United States


19851986

The University of Tennessee Medical Center at Knoxville
Knoxville, Tennessee, United States 
Joint Institute for Heavy Ion Research
Oak Ridge, Tennessee, United States 
University of Tennessee
Knoxville, Tennessee, United States


19811982

University of California, Santa Barbara
 Kavli Institute for Theoretical Physics
Santa Barbara, CA, United States 
State of Michigan
Lansing, Michigan, United States


1973

Tel Aviv University
 Department of Physics and Astronomy
Tell Afif, Tel Aviv, Israel


1966

IT University of Copenhagen
København, Capital Region, Denmark
