Showing papers on "Random phase approximation published in 1989"
TL;DR: The random-phase approximation for a gas of particles obeying (1/2) fractional statistics, in the context of Feynman perturbation theory performed in the fermion representation, is shown to yield a gauge-invariant Meissner effect with full screening in the ground state, a coherence length comparable with the interparticle spacing, and a linearly dispersing undamped collective mode.
Abstract: The random-phase approximation for a gas of particles obeying (1/2) fractional statistics, in the context of Feynman perturbation theory performed in the fermion representation, is shown to yield a gauge-invariant Meissner effect with full screening in the ground state, a coherence length comparable with the interparticle spacing, and a linearly dispersing undamped collective mode.
203 citations
TL;DR: The theory treats Coulomb and Fr\"ohlich interactions on an equal footing and includes all effects (within the RPA) of Fermi statistics, Landau damping, plasmon-phonon mode coupling, phonon self-energy correction, and dynamical screening.
Abstract: We calculate quasiparticle properties of a weakly polar two-dimensional electron gas by taking into account both electron-electron Coulomb and electron--optical-phonon Fr\"ohlich interactions. Electronic self-energy is calculated exactly in the leading order of the total effective dynamical interaction. The total effective dynamical interaction is obtained within the random-phase approximation (RPA) by starting from the total bare interaction, which includes both Coulomb and Fr\"ohlich interactions and then by screening it with all the bubble diagrams. Our theory thus treats Coulomb and Fr\"ohlich interactions on an equal footing and includes all effects (within the RPA) of Fermi statistics, Landau damping, plasmon-phonon mode coupling, phonon self-energy correction, and dynamical screening. We include finite-thickness effects in actual GaAs-based semiconductor microstructures by considering electron- and phonon- (``slab modes'') confinement effects. Some of our interesting results are (1) Coulomb and Fr\"ohlich interaction effects are nonmultiplicative, and the actual many-body correction for an electron gas is substantially different from the one-polaron result; (2) there are interesting and observable plasmon- and phonon-induced satellite structures in the low-energy side of the electronic spectral function and the density of states; (3) at low electron densities phonons tend to screen Coulomb interaction, whereas at large electron densities electrons screen the Fr\"ohlich interactions; (4) our calculated effective mass and inelastic-scattering length are consistent with the available experimental results.
78 citations
TL;DR: A calculation of the self-energy of an electron from the screened exchange energy using the /ital GW/GAMMA approximation are similar to those of the random-phase approximation.
Abstract: A calculation is presented of the self-energy of an electron from the screened exchange energy. Vertex corrections are included. They have a large effect upon the predictions of the width of the occupied energy bands. For aluminum the band-width is predicted to be 0.7 eV wider in the /ital GW/approximation compared to /ital GW/GAMMA which includes vertex corrections. Results using the /ital GW/GAMMA approximation are similar to those of the random-phase approximation.
64 citations
TL;DR: In this paper, a polarization propagator based theory for calculation of the diamagnetic part of the magnetic susceptibility is presented, where the paramagnetic term also may be obtained from the propagator and it is thus possible to compute both contributions at the same level of approximation, e.g., in RPA, SOPPA, and CCPPA.
Abstract: A polarization propagator based theory for calculation of the diamagnetic part of the magnetic susceptibility is presented. Since the paramagnetic term also may be obtained from the propagator it is thus possible to compute both contributions at the same level of approximation, e.g., in RPA, SOPPA, and CCPPA. Contrary to other commonly used procedures the present method gives a total result for the susceptibility, which—even in a limited basis set—is independent of the origin chosen for the vector potential.
60 citations
TL;DR: In this article, a series of approximations for the influence of many-body effects on the optical spectra of laser-excited two-and three-dimensional semiconductors is described.
Abstract: A series of approximations is described that considerably simplifies the calculation of the influence of many-body effects on the optical spectra of laser-excited two- and three-dimensional semiconductors. The approximations are compared with numerical evaluations of the full random phase approximation theory. They are shown to give excellent results above the Mott density.
56 citations
TL;DR: Inclusion of exchange and correlation effects in the dielectric matrix is shown to decrease the bandwidth relative to the random-phase-approximation result by a significant amount, and local-field effects are found to have little effect on the quasiparticle band structure of these simple metals.
Abstract: The quasiparticle excitation spectrum is calculated for the nearly-free-electron metals Li, Na, and Al by evaluation of the electron self-energy operator within the GW approximation and a generalized plasmon-pole model. The calculated quasiparticle energies for Na and Al are in excellent agreement with angle-resolved photoemission experiments. For Na in particular, the occupied-band width is significantly narrower than the free-electron value, as found in experiment. Inclusion of exchange and correlation effects in the dielectric matrix is shown to decrease the bandwidth relative to the random-phase-approximation result by a significant amount. Local-field effects, reflected in the off-diagonal elements of the dielectric matrix, are found to have little effect on the quasiparticle band structure of these simple metals.
55 citations
TL;DR: Using the once and thrice energy-weighted moments of the random-phase-approximation strength function, compact expressions for the average energy of surface collective oscillations of clusters and spheres of metal atoms are derived.
Abstract: Using the once and thrice energy-weighted moments of the random-phase-approximation strength function, we have derived compact expressions for the average energy of surface collective oscillations of clusters and spheres of metal atoms. The L =0 volume mode has also been studied. We have carried out quantal and semiclassical calculations for Na and Ag systems in the sphericaljellium approximation. We present a rather thorough discussion of surface diffuseness and quantal size effects on the resonance energies.
42 citations
TL;DR: Using the resonant random phase approximation the escape widths of the multipole giant resonances in /sup 208/Pb are calculated and reasonable agreement with available experimental data is obtained.
Abstract: Particle-hole excitations are studied in terms of a single-particle representation consisting of bound states and resonant solutions (Gamow resonances) of a ``realistic'' Woods-Saxon potential. Within this representation a particle-hole resonant random phase approximation is developed. Using the resonant random phase approximation the escape widths of the multipole giant resonances in $^{208}\mathrm{Pb}$ are calculated and reasonable agreement with available experimental data is obtained.
42 citations
TL;DR: In this paper, the anharmonicities of the two-phonon spectrum of 40 Ca were calculated by using a boson expansion approach, and the deviations from harmonicity were found to be very small, both for the low-lying phonons and giant resonances.
35 citations
TL;DR: The longitudinal response function for quasielastic electron scattering from /sup 12/C and /sup 40/Ca at a momentum transfer q = 550 MeV is calculated in a relativistic random-phase approximation (RPA).
Abstract: The longitudinal response function for quasielastic electron scattering from /sup 12/C and /sup 40/Ca at a momentum transfer q = 550 MeV is calculated in a relativistic random-phase approximation (RPA). The particle-hole response is calculated exactly in the finite system while vacuum-polarization effects are included in a local-density approximation. The RPA result with vacuum polarization is about 25% below the Hartree response and comparable to Saclay data.
35 citations
TL;DR: Isoscalar giant resonances in finite nuclei are studied in a relativistic random-phase approximation approach and it is found that monopole and quadrupole modes exhibit a collective character.
Abstract: Isoscalar giant resonances in finite nuclei are studied in a relativistic random-phase approximation approach. The model is self-consistent in the sense that one set of coupling constants generates the Dirac-Hartree single-particle spectrum and the residual particle-hole interaction. The relativistic random-phase approximation is used to calculate response functions of multipolarity L = 0, 2, 3, and 4 in light and medium nuclei. It is found that monopole and quadrupole modes exhibit a collective character. The peak energies are overestimated, but not as much as one might think if the bulk properties (compression modulus, effective mass) were the only relevant quantities.
TL;DR: A version of the random-phase approximation for the description of nuclear excitations which is a consistent extension of the QHD1 mean-field theory of the ground states of doubly magic nuclei, which includes correlations induced by the isoscalar {sigma} and {omega} mesons of Q HD1.
Abstract: We present a version of the random-phase approximation for the description of nuclear excitations which is a consistent extension of the QHD1 mean-field theory of the ground states of doubly magic nuclei. This approach includes correlations induced by the isoscalar {sigma} and {omega} mesons of QHD1. Our method employs a nonspectral single particle propagator in such a way that we avoid any basis truncation and automatically include the escape widths implied by the theory. Our calculations yield exactly conserved random-phase approximation transition currents as well as correct treatment of spuriosity for 1{sup {minus}}T=0 excitations. Because of the flexibility of our numerical method, we can treat discrete excitations, giant resonances, and the continuum response in general---including quasielastic scattering---in a unified way. We compare our results with experimental ({ital e},{ital e}{prime}) form factors for various discrete excitations in {sup 12}C, {sup 16}O, {sup 40}Ca, and {sup 48}Ca as well as with the quasielastic Coulomb response functions for {sup 12}C and {sup 40}Ca. Agreement with transition charge densities is typically quite good and in some cases superior to comparable nonrelativistic random-phase approximation calculations. Transition current densities are less well described. The question of sum rules in the relativistic random-phase approximation is also addressed.
TL;DR: In this article, the authors reported the first calculations of vibrational rotational strength of chiral molecules from two nuclear shielding tensors, γλα β(0) and ξλαβ(0), at the SCF level using the random phase approximation.
Abstract: It was recently shown that vibrational rotational strengths of chiral molecules can be expressed in terms of two nuclear shielding tensors, γλαβ(0) and ξλαβ(0). We report the first calculations of vibrational rotational strengths from these nuclear shielding tensors. The molecule studied is NHDT. Calculations are carried out at the SCF level using the random phase approximation. γλαβ(0) is calculated using length, momentum and force representations. ξλαβ(0) is calculated using angular momentum and torque representations. A range of basis sets, both ‘‘conventional’’ and ‘‘polarized’’ in type, are employed. Gauge and origin dependence are also investigated.
TL;DR: In this paper, the total scattering rate of a single energetic electron in the conduction band of GaAs in the presence of a non-equilibrium electron-hole plasma was calculated using different approximations for the screening effects of the free carriers.
Abstract: The total scattering rate of a single energetic electron in the conduction band of GaAs in the presence of a non-equilibrium electron-hole plasma is calculated using different approximations for the screening effects of the free carriers. The best agreement with recent experimental results is obtained when full dynamic screening is taken account of (exactly within the random phase approximation), including the lattice polarizability.
TL;DR: Etude de la masse de resonance cyclotron d'un gaz d'electrons quasi-bidimensionnel dans un puits quantique simple d'AsGa, en fonction de l'intensite du champ magnetique applique, de the densite electronique, and of the largeur du puits.
Abstract: The cyclotron-resonance mass of a quasi-two-dimensional electron gas in a single GaAs quantum well is investigated as a function of the magnetic field strength, the electron density, and the quantum-well width. The screening of the electron-phonon interaction is taken into account within the static random-phase approximation. The theoretical results are compared with available experimental data.
TL;DR: The usefulness of the Lanczos method is illustrated by applying it to calculate the 2{sup +} strength functions for {sup 16}O, {sup 40}Ca, and {sup 208}Pb in the giant quadupole resonance region.
Abstract: A method is presented for random-phase-approximation calculations of nuclear response in the continuum. It utilizes the Lanczos method for solving the basic inhomogeneous coupled-channel integral equation. The method can provide information on both the direct and statistical decay components. We illustrate the usefulness of the method by applying it to calculate the 2{sup +} strength functions for {sup 16}O, {sup 40}Ca, and {sup 208}Pb in the giant quadupole resonance region. It is shown that the calculations reproduce the observed peak positions and widths very well.
TL;DR: On considere un modele d'un bloc de jellium polarisable a temperature finie avec des charges de surface qui flechissent les bandes vers le bas: les vibrations du reseau sont negligees.
Abstract: We discuss plasmon and phonon modes in the accumulation layer of a model semiconductor with the aid of calculations based on a nonlocal description of dielectric response in the random-phase approximation. The first model we consider is that of a polarizable jellium slab at finite temperature with surface charges that bend the bands downward; lattice vibrations are ignored. Choosing model parameters appropriate to lightly doped InAs(110) at room temperature, we obtain intersubband as well as intrasubband plasmons and discuss their dispersion relations, localization, and line shapes. Evidence for two-dimensional and ``acoustic'' plasmons is presented: dispersion relations that have square-root and linear behavior at long wavelengths, respectively. All plasmon modes are strongly damped when their dispersion curves enter the single-particle continua. When the dynamical response of the lattice is included in the model, we obtain coupled plasmon-phonon modes or ``plasmarons'' and study their dispersion and line shapes. Our results confirm and extend the interpretation of high-resolution electron-energy-loss spectroscopy data reported recently for InAs(110) exposed to atomic hydrogen.
TL;DR: In this article, a theory of excitations dans un systeme de fermions avec des fluctuations quantiques importantes qu'on peut decrire par la resonnance des determinants de Slater non-orthogonaux, c'esta-dire par la theorie de Hartree-Fock resonnante, precedemment developpee.
Abstract: On developpe une theorie des excitations dans un systeme de fermions avec des fluctuations quantiques importantes qu'on peut decrire par la resonnance des determinants de Slater non-orthogonaux avec des structures de correlation differentes, c'est-a-dire par la theorie de Hartree-Fock resonnante, precedemment developpee. On obtient une nouvelle approximation pour les excitations a partir des petites fluctuations dependantes du temps de l'etat fondamental de Hartree-Fock resonnant d'une maniere similaire a l'obtention de Hartree-Fock dependante du temps dans l'approximation de la phase aleatoire (RPA). On l'appelle RPA resonnant
TL;DR: In this article, it was shown that the partial photoionization cross sections of outer subshells of atomic Eu in the giant resonance region are determined by the action of the 4d-electron excitations.
Abstract: It is demonstrated that the partial photoionization cross sections of outer subshells of atomic Eu in the giant resonance region are determined by the action of the 4d-electron excitations. The cross section for photoionization of the semifilled 4f7 subshell is also entirely dominated by the interaction with 4d10 electrons.
TL;DR: In this article, the anomalous self-energy part is assumed to be frequency independent but its momentum dependence is determined without any restrictions, and the vertex correction enhances these scatterings to increase the transition temperature by several times.
Abstract: In the two-dimensional Hubbard model with the electron density nearly half-filled, RPA treatments resulted in d -wave superconductivity. We study how a vertex correction affects electronic self-energy part, thereby, superconducting transition temperature. The normal state self-energy part is estimated at the lowest temperature which can be reached numerically. The anomalous self-energy part is assumed to be frequency independent but its momentum dependence is determined without any restrictions. It has a large amplitude near the Fermi line and a d -wave symmetry, since there are strong scatterings near momentum transfers Q =(±π, ±π), lattice constant being taken to be unity. The vertex correction enhances these scatterings to increase the transition temperature by several times. The effective mass is also increased by a few percent. The frequency dependence of the anomalous self-energy part is found to give a small correction.
TL;DR: In this article, the hierarchical reference theory is shown to be an accurate global theory of fluids at least above the critical temperature, where the hierarchy is truncated at the first equation, the one connecting the free energy to the pair correlation function with an Ornstein-Zernike ansatz.
Abstract: We show that the hierarchical reference theory is an accurate global theory of fluids at least above the critical temperatureT
c. The hierarchy is truncated at the first equation, the one connecting the free energy to the pair correlation function, with an Ornstein-Zernike ansatz. In this approximation the theory can be considered as a sophisticated generalization of the optimized random phase approximation which has genuine nonclassical critical exponents and for which scaling is satisfied. We study the system of hard spheres plus the Lennard-Jones attractive well and find a good agreement with measuredPVT, specific heat, correlation length, and structure factor in rare gases. The accuracy of the theory remains very good up to freezing density.
TL;DR: In this paper, the authors evaluate the inelastic electron lifetime in the p regions of III-V semiconductor devices with the aid of the exact temperature-dependent random phase approximation valence band dielectric function.
Abstract: We evaluate the inelastic electron lifetime in the p regions of III‐V semiconductor devices with the aid of the exact temperature‐dependent random phase approximation valence‐band dielectric function. Our results demonstrate that at room temperature the low‐energy electron lifetime decreases markedly while hot‐electron lifetimes are reduced by several tens of percent. We subsequently repeat our calculations with the damped plasmon pole approximation for the dielectric function, obtaining acceptable values with a minimum of computational effort.
TL;DR: In this article, the Kondo picture for the electron-electron attraction due to transfer of pairs of charge carriers between two bands is investigated, and the screening of the Coulomb interaction in a two-band model is considered.
Abstract: The Kondo picture for the electron-electron attraction due to transfer of pairs of charge carriers between two bands is investigated. This interaction may reduce considerably the Coulomb repulsion, leading to enhancement of the superconducting transition temperature and a substantial weakening of the isotope effect. The effect is large when the empty band is narrow and the energy gap between the two bands is small. The screening of the Coulomb interaction in a two-band model is considered in detail using the random-phase approximation, and the dynamic dielectric functions are derived. It is found that the intraband vertex of the empty band is almost unscreened and that of the occupied one has metallic screening. The screening of the pair-transfer vertex is independent of the intraband Coulomb interactions and may even be magnified. The possible relevance to the superconducting oxides is suggested.
TL;DR: In this paper, a strongly excited magnetic-dipole scissors mode was detected in 154 Gd with high-resolution inelastic electron scattering, at an excitation energy of E x = 2.936 MeV and some more weakly excited states in its vicinity.
TL;DR: In this paper, the collective scattering function Scoll, which describes light (neutron-, x-ray) scattering under wavevector configurations, is obtained from Monte Carlo simulations for a symmetrical polymer mixture.
Abstract: The collective scattering function Scoll(
$$\vec q$$
), which describes light (neutron-, x-ray) scattering under wavevector
$$\vec q$$
, is obtained from Monte Carlo simulations for a symmetrical polymer mixture. The polymers are modelled by self-avoiding walks ofN
A=NB=N steps on a simple cubic lattice, where a fractionφ
V of sites is left vacant, and an attractive energye occurs if two neighboring sites are taken by the same kind of monomer. Spinodal curves are estimated from linear extrapolation of S
coll
−1
(0) vs.e/k
B
T, whereT is the temperature. Also the single chain structure factor is obtained and the de Gennes random phase approximation (RPA) can thus be tested. Unexpectedly, strong deviations are found if one species is very dilute. The estimation of an effective Flory-Hugginsχ-parameter from scattering data is also discussed.
TL;DR: On montre que les structures anisotropiques de la section efficace de diffusion ou inelastique de rayons-X par un monocristal Li et d'electrons rapides par des monOCristaux de Na et Al proviennent des etats collectifs de liaison de zone (ZBC5) dependent fortement de la symetrie and de l'intensite du potentiel du cristal.
Abstract: Anisotropic structures in the inelastic-scattering cross section of x rays from single-crystal Li and of fast electrons from single-crystal Na and Al are found to arise from zone-boundary collective states (ZBCS) that, in contrast to the classical plasmon, depend strongly on the symmetry and on the strength of the crystal potential. We show that within the random-phase approximation (RPA), a nearly-free-electron-like two-band model is able to describe these states existing for q along various high-symmetry directions of the crystal. In Li in particular, a 〈200〉 ZBCS appears as a continuation of the plasmon beyond the cutoff wave vector ${q}_{c}$. For many years structures in this (q,\ensuremath{\omega}) regime have been associated with correlation effects beyond the RPA. Furthermore, due to the high sensitivity on the crystal potential, measured ZBCS's yield valuable information on the band structure, which are currently intensively studied by angularly resolved photoemission.
TL;DR: In this article, the authors analyzed electron energy loss spectra for a degenerate GaAs surface with a band bending, where the effect of band bending is taken into account by means of the layer model which introduces a carrier-free depletion layer at the surface.
TL;DR: In this paper, a nonrelativistic random-phase approximation (RPA) method for including correlation corrections in the investigation of atomic transitions is proposed. But care must be taken with the cavity boundary conditions in the relativistic case to avoid the Klein paradox.
Abstract: Publisher Summary The nonrelativistic random-phase approximation (RPA) is a simple method for including correlation corrections in the investigation of atomic transitions. The RPA leads to a fully coupled, multichannel description of the final states. Relativistic effects become important for heavy atoms and for ions with high nuclear charge, leading to an enhanced spin-orbit splitting of the atomic shells. It is therefore of interest to include relativistic effects along with correlation corrections for describing transitions in such systems. Care must be taken with the cavity boundary conditions in the relativistic case to avoid the Klein paradox. To avoid dealing with infinite matrices, a finite HF pseudospectrum obtained by expanding the HF orbitals in terms of a finite number of B-splines is introduced. The low-lying states in this pseudospectrum can be made to agree to any desired level of accuracy with the HF states by exploiting the freedom available in choosing the number and order of the B-splines used to approximate the HF orbitals.
TL;DR: In this article, the small amplitude limit of the Hartree-fock theory and its relation to other theories, such as the random phase approximation (RPA), the second RPA and the shell model, is discussed.
Abstract: The small amplitude limit of the time-dependent density-matrix theory, which is an extension of the time-dependent Hartree-Fock theory, is presented. Its relation to other theories i.e. the random phase approximation (RPA), the second RPA and the shell model is discussed. It is found that the small amplitude limit is a generalization of these theories.
TL;DR: Using the separable Baranger-Kumar Hamiltonian, the Hartree-Fock-Bogoliubov formalism at finite temperature and the random-phase approximation, the response functions for electromagnetic decay are calculated without explicit construction of therandom- phase approximation eigenmodes.
Abstract: We combine the cranked Hartree-Fock-Bogoliubov formalism at finite temperature with the random-phase approximation to calculate electromagnetic transition strengths for E1, M1, and E2 transitions for any value of energy and spin of rare-earth nuclei. Our calculations are free of spurious contributions because we successfully remove all Goldstone modes from the random-phase approximation calculations. Using the separable Baranger-Kumar Hamiltonian, we can calculate the response functions for electromagnetic decay without explicit construction of the random-phase approximation eigenmodes. We report numerical results on the transitional nucleus $^{156}\mathrm{Er}$, which is prolate or oblate for small or high spin, respectively, and discuss them.