G. F. Bertsch

University of Washington Seattle, Seattle, Washington, United States

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Publications (364)1401.51 Total impact

  • Source
    Y. Alhassid · G. F. Bertsch · C. N. Gilbreth · H. Nakada · C. Özen
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    ABSTRACT: The configuration-interaction 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 mid-mass 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 model-independent 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.
    Preview · Article · Jan 2016
  • Y. Alhassid · G. F. Bertsch · C. N. Gilbreth · H. Nakada
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    ABSTRACT: We assess the accuracy of finite-temperature mean-field 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 finite-temperature Hartree-Fock and Hartree-Fock-Bogoliubov 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 well-controlled statistical errors. The main weak points in the mean-field treatments are seen to be: (i) the extraction of number-projected 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 saddle-point approximation to extract the number-projected densities is not a significant source of error compared to other errors inherent to the mean-field theory. We also present an alternative formulation of the saddle-point approximation that makes direct use of an approximate particle-number projection and avoids computing the usual three-dimensional Jacobian of the saddle-point integration. We find that the pairing condensate is less amenable to approximate particle-number projection methods due to the explicit violation of particle-number conservation in the pairing condensate. Nevertheless, the Hartree-Fock-Bogoliubov 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.
    No preview · Article · Dec 2015
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    ABSTRACT: Irradiation of intense and ultrashort laser pulses on dielectric surface is calculated in real-time using first-principles time-dependent density functional theory. It is found that calculated energy distribution transferred from laser pulse to electrons in dielectrics explains measured threshold and depth of laser-induced damage.
    No preview · Article · May 2015
  • Source
    G. F. Bertsch · W. Loveland · W. Nazarewicz · P. Talou
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    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.
    Full-text · Article · Feb 2015 · Journal of Physics G Nuclear and Particle Physics
  • Source
    S. A. Sato · K. Yabana · Y. Shinohara · T. Otobe · K. M. Lee · G. F. Bertsch
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    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.
    Full-text · Article · Dec 2014 · Physical Review B
  • Source
    Luis M. Robledo · George F. Bertsch
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    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.
    Full-text · Article · Aug 2014
  • Source
    Y. Alhassid · C. N. Gilbreth · G. F. Bertsch
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    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.
    Preview · Article · Aug 2014 · Physical Review Letters
  • G. F. Bertsch · A. J. Lee
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    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.
    No preview · Article · Jan 2014 · Physical Review B
  • Source
    S. A. Sato · K. Yabana · Y. Shinohara · T. Otobe · G. F. Bertsch
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    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.
    Full-text · Article · Jan 2014 · Physical Review B
  • Source
    G. F. Bertsch · L. M. Robledo
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    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.
    Full-text · Article · Jan 2014 · Computer Physics Communications
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    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.
    No preview · Conference Paper · Jan 2014
  • Source
    L. M. Robledo · R. N. Bernard · G. F. Bertsch
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    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.
    Full-text · Article · Oct 2013 · Physical Review C
  • K-M Lee · K Yabana · G F Bertsch

    No preview · Dataset · Aug 2013
  • Source
    G. F. Bertsch · A. Lee
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    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.
    Preview · Article · Jun 2013
  • K-M Lee · K Yabana · G F Bertsch

    No preview · Dataset · May 2013
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    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.
    No preview · Article · Mar 2013
  • Y Shinohara · S A Sato · K Yabana · J-I Iwata · T Otobe · G F Bertsch
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    ABSTRACT: There is no abstract available for this article.
    No preview · Article · Jan 2013 · The Journal of Chemical Physics
  • Source
    Y. Shinohara · S.A. Sato · K. Yabana · J.-I. Iwata · T. Otobe · G.F. Bertsch

    Full-text · Dataset · Dec 2012
  • Source
    L. M. Robledo · R. Bernard · G. F. Bertsch
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    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.
    Full-text · Article · Oct 2012 · Physical Review C
  • Source
    Y. Shinohara · S.A. Sato · K. Yabana · J.-I. Iwata · T. Otobe · G.F. Bertsch
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    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.
    Full-text · Article · Aug 2012 · The Journal of Chemical Physics

Publication Stats

17k Citations
1,401.51 Total Impact Points

Institutions

  • 1993-2015
    • University of Washington Seattle
      • • Institute for Nuclear Theory
      • • Department of Physics
      Seattle, Washington, United States
  • 2001
    • Tohoku University
      • Department of Physics
      Sendai, Kagoshima-ken, Japan
  • 1971-1998
    • Michigan State University
      • Department of Physics and Astronomy
      East Lansing, MI, United States
  • 1985-1986
    • 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
  • 1981-1982
    • 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