Showing papers on "Random phase approximation published in 1995"
TL;DR: An efficient scheme for evaluating the quasiparticle corrections to local-density-approximation (LDA) band structures within the GW approximation is reported and the static dielectric matrix of the Si(001) surface is fully calculated within the random phase approximation (RPA).
Abstract: We report an efficient scheme for evaluating the quasiparticle corrections to local-density-approximation (LDA) band structures within the GW approximation. In this scheme, the GW self-energy corrections are evaluated in a sufficiently flexible Gaussian orbital basis set instead of using plane-wave Fourier representations of the relevant two-point functions. It turns out that this set has to include orbitals up to f-type symmetry, when in the LDA calculations Gaussian orbitals up to d-type symmetry are needed for convergence. For bulk Si, both schemes yield virtually identical quasiparticle band structures and the demand on computer time is roughly the same. For the Si(001)-(2\ifmmode\times\else\texttimes\fi{}1) surface, the GW Gaussian orbital scheme is a factor of 5 faster. In our calculations for Si(001)-(2\ifmmode\times\else\texttimes\fi{}1) the dynamic dielectric matrix is obtained by applying a plasmon-pole approximation. The static dielectric matrix of the Si(001) surface is fully calculated within the random phase approximation (RPA). In addition, we have performed quasiparticle surface band-structure calculations employing two model dielectric matrices. Our respective results are compared with those obtained employing the full RPA dielectric matrix as well as with results of previous calculations by other authors which were based on model dielectric matrices.
186 citations
TL;DR: The present formalism promotes the Pauli exclusion principle and avoids the collapse of the double-beta-decay matrix elements and is applied to the double beta decay of Mo.
Abstract: A self-consistent method of treating excitations of the proton-neutron quasiparticle random-phase approximation is presented. The non-self-consistent methods violate the Pauli exclusion principle and lead to an eventual collapse of the ground state. This behavior renders a reliable calculation of the nuclear matrix elements, relevant for the prediction of double-beta-decay half-lives, difficult. The present formalism promotes the Pauli exclusion principle and avoids the collapse of the double-beta-decay matrix elements. We have applied this formalism to the double beta decay of ${}^{100}$Mo.
169 citations
TL;DR: The theory of the rotatory strength tensor describing circular dichroism (CD) of electronic excitations in oriented molecules is summarized in length and velocity formulations, and an ab initio implementation of the calculation of this tensor and its electric dipole and electric quadrupole components in the random phase approximation is presented in this article.
66 citations
TL;DR: Cubic response functions in the random phase approximation have been derived and their use for computations of static and dynamic second hyperpolarizabilities is demonstrated.
50 citations
TL;DR: In this paper, the cross sections for muon-neutrino and muonantine-antitrino charged-current reactions were calculated in the RPA model and the branching ratios for the main decay channels of these reactions were determined.
Abstract: Motivated by a recent experiment at LAMPF we calculate cross sections for muon-neutrino and muon-antineutrino charged-current reactions on $^{12}\mathrm{C}$ within the continuum RPA model. We also determine the branching ratios for the main decay channels of these reactions by application of a statistical model.
50 citations
TL;DR: In this article, the electronic excitation energies of alkali excess clusters Na2Cl, Na3Cl2, Na4Cl3, Na6Cl5, Na14Cl13, and Na18Cl17 were investigated using configuration interaction singles and RPA (random phase approximation).
Abstract: Electronic excitation energies of alkali‐excess clusters Na2Cl, Na3Cl2, Na4Cl3, Na6Cl5, Na14Cl13, and Na18Cl17 are investigated using CIS (configuration interaction singles) and RPA (random phase approximation). The accuracy of these approximations is established for Na2Cl by equation‐of‐motion coupled‐cluster singles and doubles calculations and by comparison to experimental results. The mode of localization of the excess electron is decisive for the electronic excitation energy. No cluster‐size dependence of the excitation energy is found. The direct UHF‐CIS (unrestricted Hartree–Fock‐CIS) and UHF‐RPA implementation within the program package TURBOMOLE is briefly described in the appendix.
45 citations
TL;DR: In this article, the random phase approximation (RPA) and the lattice fluid model are combined to formulate expressions for scattering, the spinodal, and the interaction parameter from compressible polymer mixtures.
Abstract: The random phase approximation (RPA) and the lattice fluid (LF) model are combined to formulate expressions for scattering, the spinodal, and the interaction parameter from compressible polymer mixtures. Effective fields of the RPA are shown to correspond to the interaction parameters of the LF model. A protocol is developed to determine polymer-polymer interaction parameters from scattering data. The formalism is applied successfully to both isotopic and nonisotopic binary polymer blends. It is concluded that some of the unexpected behavior of interaction parameters determined in neutron scattering experiments can be attributed to compressibility effects. The traditional and widely used formula that relates the interaction parameter to a second concentration derivative of an excess free energy is shown to be incorrect for compressible mixtures. Radii of gyration determined from the scattering dependence on wave vector tend to be overestimated unless compressibility is taken into account. It is also shown that the spinodal determined from a scattering experiment that probes a constant volume is identical to one obtained under conditions of constant pressure
41 citations
02 Jun 1995-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this paper, the total and directional components of the mean excitation energy for stopping (I0) and the stopping anisotropy (A) as well as the first moment (I1) of the dipole oscillator strength distribution for 20 small molecules and molecular ions containing 6 to 30 electrons were calculated using the random phase approximation.
Abstract: Using the random phase approximation, we have calculated the total and directional components of the mean excitation energy for stopping (I0) and the stopping anisotropy (A) as well as the first moment (I1) of the dipole oscillator strength distribution for 20 small molecules and molecular ions containing 6 to 30 electrons. We find that for excitations polarized orthogonal (perpendicular) and parallel to the molecular high symmetry axis I0O > I0P for all linear molecules, while I1O < I1P for all molecules other than HF. We note that the Bloch rule holds only for small, compact molecules, and that the Bragg rule should not be used to determine the mean excitation energies of the molecules that we consider here. The stopping anisotropy is positive for all but the C3v molecules, and is largest for the linear, rod-shaped molecules.
28 citations
TL;DR: In this paper, a quasiparticle random phase approximation approach was proposed to reproduce the Nilsson level scheme to a good approximation of the magnetic dipole excitations in even Sm isotopes.
24 citations
TL;DR: In this paper, the temperature sensitivity of the differential gain, threshold carrier density, and radiative current density in 1.3/spl mu/m InP-based strained-layer quantum-well (QW) lasers was analyzed by taking into account the electrostatic deformation in the band profiles and the temperature dependence of the optical dephasing time.
Abstract: By taking into account the electrostatic deformation in the band profiles and the temperature dependence of the optical dephasing time, we study the temperature sensitivity of the differential gain, threshold carrier density, and radiative current density in 1.3-/spl mu/m InP-based strained-layer quantum-well (QW) lasers. Electrostatic deformation is analyzed by the self-consistent numerical calculation of Poisson's equation, the scalar effective-mass equation for the conduction band, and the multiband effective-mass equation for the valence band. The optical dephasing time is then obtained from the intrasubband scattering rates for electrons and holes within the fully dynamic random phase approximation including carrier-carrier and carrier-phonon interactions on an equal basis. It is clarified that the electrostatic band-profile deformation is one of the dominant mechanisms For determining the temperature sensitivity Of the differential gain, while the optical dephasing time has a pronounced influence on the transparent condition at elevated temperatures. We demonstrate that the electrostatic band-profile deformation and the temperature-dependent optical dephasing play essential roles in determining the high-temperature characteristics of InP-based QW lasers. >
23 citations
TL;DR: An expression is obtained for the rest tension of the bosonic relativistic string in terms of the parameters characterizing the microscopic string bit dynamics, namely, an attractive [delta]-function potential in two (transverse) dimensions.
Abstract: We explore the application of approximation schemes from many body physics, including the Hartree-Fock method and random phase approximation (RPA), to the problem of analyzing the low energy excitations of a polymer chain made up of bosonic string bits. We accordingly obtain an expression for the rest tension [ital T][sub 0] of the bosonic relativistic string in terms of the parameters characterizing the microscopic string bit dynamics. We first derive an exact connection between the string tension and a certain correlation function of the many-body string bit system. This connection is made for an arbitrary interaction potential between string bits and relies on an exact dipole sum rule. We then review an earlier calculation by Goldstone of the low energy excitations of a polymer chain using the RPA. We assess the accuracy of the RPA by calculating the first-order corrections. For this purpose we specialize to the unique scale-invariant potential, namely, an attractive [delta]-function potential in two (transverse) dimensions. We find that the corrections are large, and discuss a method for summing the large terms. The corrections to this improved RPA are roughly 15%.
TL;DR: In this article, the differential cross section of the one-photon ionization of the valence shell of Li2 and LiH is computed using the static exchange approximation (SEA) and the random phase approximation (RPA).
Abstract: The differential cross section of the one‐photon ionization of the valence shell of Li2 and LiH is computed ab initio adopting the static‐exchange approximation (SEA) and the random phase approximation (RPA). Using large L2 basis sets of polynomial spherical Gaussian‐type orbitals (PSGTO) the matrix elements involving the final states in the electronic continuum are computed by a K‐matrix based technique which also allows to obtain the phase shifts of the partial wave channels. The results reported here suggest that, with the methods proposed, the generally employed GTO’s could advantageously be exploited to evaluate reliable values of the properties of the continuum.
TL;DR: The wake potential, the induced electron density, and the stopping power for a charged particle moving through a strongly coupled two-dimensional electron gas have been investigated within the framework of linear-response dielectric theory.
Abstract: The wake potential, the induced electron density, and the stopping power for a charged particle moving through a strongly coupled two-dimensional electron gas have been investigated within the framework of linear-response dielectric theory. The influence of the exchange-correlation interaction of electrons on the above quantities has been considered by using a local-field-corrected dielectric function.
TL;DR: In this article, the properties of spin-isospin modes are studied in nuclear matter consisting of nucleons and Δ isobars that interact by the exchange of π and ρ mesons.
TL;DR: In this paper, a system of free, non-relativistic electrons at zero temperature and positive density, coupled to an arbitrary, external electromagnetic vector potential, was considered and the effective action was shown to be quadratic in the scaling limit and can be viewed as an integral over the Fermi sphere of (1+1)-dimensional, chiral schwinger models.
Abstract: We consider a system of free, non-relativistic electrons at zero temperature and positive density, coupled to an arbitrary, external electromagnetic vector potential,A. By integrating out the electron degrees of freedom we obtain the effective action forA. We show that, in the scaling limit, this effective action is quadratic inA and can be viewed as an integral over the Fermi sphere of effective actions of (1+1)-dimensional, chiral schwinger models. We use this result to elucidate Luther-Haldane bosonization of systems of non-relativistic electrons. We also study systems of weakly coupled interacting electrons for which the BCS channel is turned off. Using the quadratic dependence of the effective action onA, we show that, in the scaling limit, the RPA yields the dominant contribution.
TL;DR: In this paper, the collective modes are computed of a layered electron gas described by a t-t' tight-binding band, where the electrons experience besides the long-range Coulomb repulsion an on-site Hubbard U repulsion and a nearest-neighbor attractive interaction.
Abstract: The equations of motion of pairlike excitations in the superconducting state are studied for various types of pairing using the random-phase approximation. The collective modes are computed of a layered electron gas described by a t-t' tight-binding band, where the electrons experience besides the long-range Coulomb repulsion an on-site Hubbard U repulsion and a nearest-neighbor attractive interaction. From numerical calculations we see that the collective-mode spectrum now becomes particularly rich. Several branches can occur below the continuum of quasiparticle excitations, corresponding to order-parameter fluctuations of various symmetries of pairing, and collective spin-density fluctuations. From the collective-mode softening near the nesting vectors it is concluded that in the d-wave paired state an instability occurs toward the formation of a spin-density wave.
TL;DR: In this article, the relativistic corrections obtained from the use of the Darwin and mass velocity operators to first order are included at both levels of approximation, and it is shown that the importance of the corrections is smallest in the correlated calculations, as has also been observed for other properties.
Abstract: Using response function methods we report calculations of the dynamic isotropic polarizability of SnH4 and PbH4 and of the relativistic corrections to it in the random phase approximation and at the correlated multiconfigurational linear response level of approximation. All relativistic corrections obtained from the use of the Darwin and mass‐velocity operators to first order are included at both levels of approximation. We find that correlation and relativistic contributions are not even approximately additive for the two molecules. The importance of the relativistic corrections is smallest in the correlated calculations, as has also been observed for other properties. For SnH4 the correlation contribution and the pure relativistic correction are of the same order of magnitude, whereas for PbH4 the relativistic correction becomes more important than the correlation contribution. We report estimated Cauchy moments, obtained from fitting the dispersion of the calculated corrections as a function of ω2. The frequency dependence of the nonrelativistic polarizability is most pronounced at the correlated level, mainly due to lower excitation energies in the multiconfigurational calculations than those in the random phase approximation.
TL;DR: In this paper, a spherical symmetry model, containing explicitly nonlocal effects in the electron-ion interaction, is presented to describe the electronic properties of lithium clusters. But the model is not suitable for the case of the nonlocal potential from which the random phase approximation with exact exchange is applied to calculate the optical response.
Abstract: We present a spherical symmetry model, containing explicitly nonlocal effects in the electron-ion interaction, to describe the electronic properties of lithium clusters. We assume either an optimized discrete ionic structure or a jellium structure. The model provides the nonlocal potential from which the random phase approximation with exact exchange is applied to calculate the optical response of Li clusters. The optical response of Li139+ obtained within this model is in good agreement with the measured giant dipole resonance. The same model is used to predict alkali-metal effective masses; the agreement with band structure calculations is emphasized.
TL;DR: In this paper, the authors examined the contributions of odd-parity nuclear operators to the two-neutrino double beta decay 0 + → 0 + amplitude, which come from the P -wave Coulomb corrections to the electron wave functions and the recoil corrections to nuclear currents.
TL;DR: In this paper, a procedure is used to analyze small-angle neutron-scattering (SANS) data from a pressurized polymer blend mixture (deuterated polystyrene/polyvinylmethylether).
Abstract: A procedure is used to analyze small-angle neutron-scattering (SANS) data from a pressurized polymer blend mixture (deuterated polystyrene/polyvinylmethylether). The Lattice-Fluid (LF) equation-of-state model is used along with a compressible random phase approximation (RPA) in order to obtain free volume fractions and intermonomer interaction potentials. Solving the two sets of equations (LF and RPA) self-consistently within the fitting procedure to the SANS data provides an improvement over the familiar incompressible RPA model. In this approach, the free volume fraction is the main varying parameter. Intermonomer interaction potentials were found to depend on pressure (weakly) and temperature (linearly). © 1995 John Wiley & Sons, Inc.
TL;DR: In this article, the effect of external field penetration into the metal on the barrier potential shape and emission current is demonstrated for both isotropic and anisotropic Fermi surfaces.
Abstract: Polarization and barrier potentials for the point charge at a metal–vacuum interface have been calculated in the Thomas–Fermi and random phase approximations taking into account spatial dispersion effects and quantum nature of conduction electrons. The effect of external field penetration into the metal on the barrier potential shape and emission current is demonstrated. Both isotropic (a spherical Fermi surface) and anisotropic (a flat Fermi surface) cases are considered. Comparison between these cases is made for equivalent conditions.
05 May 1995-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: A survey of the theoretical status in understanding some nonlinear aspects of charged particle interactions in matter is presented in this paper, where a diagrammatic analysis of the many-body interactions between a moving particle and the electron gas is made to investigate, within the random-phase approximation, the Barkas correction to the electronic stopping power, the nonlinear wake potential generated by swift ions in matter, and processes involved in multiple excitations of plasmons.
Abstract: A survey is presented of the theoretical status in understanding some nonlinear aspects of charged particle interactions in matter. In particular, a diagrammatic analysis of the many-body interactions between a moving particle and the electron gas is made to investigate, within the random-phase-approximation, the Barkas correction to the electronic stopping power, the nonlinear wake potential generated by swift ions in matter, and processes involved in multiple excitations of plasmons.
TL;DR: In this article, the role of interlayer coupling on the Neel temperature of (1-2-3) compounds is studied by taking the three-dimensional anisotropic Heisenberg antiferromagnetic model.
Abstract: The role of interlayer coupling (J⊥) on the Neel temperature of (1-2-3) compounds is studied by taking the three-dimensional anisotropic Heisenberg antiferromagnetic model. The two-sublattice model of the antiferromagnet is considered. The double-time Green function technique is employed and the higher-order Green functions are decoupled using the Callen decoupling approximation. The Neel temperature (T N ) for YBa 2 Cu 3 O 6+x (1-2-3) compounds as a function of doping concentrations (x) is estimated. The intraplanar coupling strength (J∥) is treated as a constant throughout the numerical calculations. The theoretical results are compared with that of the random phase approximation (RPA) and existing experimental results
TL;DR: An extension of RPA is proposed where structure and energy of the ground state, as well as those of the one-phonon states, are analyzed with regard to the role of the spurious components associated with a violation of the Pauli principle.
Abstract: The random-phase approximation (RPA) is examined in a boson formalism and special attention is focused on the problem of the violation of the Pauli principle affecting this theory. A mapping technique is discussed and the boson image of a two-body fermion Hamiltonian is constructed. Within the boson space so defined, an extension of RPA is proposed where structure and energy of the ground state, as well as those of the one-phonon states, are analyzed with regard to the role of the spurious components associated with a violation of the Pauli principle. A multistep minimization procedure for the determination of a better ground state is also examined. Numerical tests are performed within the Lipkin-Meshkov-Glick model.
TL;DR: The spin excitations of model itinerant-electron ferromagnetic and antiferromagnetic ultrathin films are studied theoretically and the applications of SPEELS to the study of magnetic excitations at surfaces is discussed.
Abstract: The spin excitations of model itinerant-electron ferromagnetic and antiferromagnetic ultrathin films are studied theoretically. The discussion is based on the one-band Hubbard model, treated in mean-field theory. A description of the spin excitations is generated by means of the random-phase approximation. We find both the low-lying collective modes (spin waves) and the spectrum of Stoner excitations. We also compute the spin-polarized electron-energy-loss scattering (SPEELS) spectra for our model films. The applications of SPEELS to the study of magnetic excitations at surfaces is discussed.
TL;DR: In this paper, inelastic neutron scattering experiments were performed to determine the dispersion of the crystal field excitation in Nd2CuO4, which allowed a direct determination of the exchange coupling constants between the Nd ions.
Abstract: We performed inelastic neutron scattering experiments to determine the dispersion of the Г6(1)-Г6(2) crystal field excitation in Nd2CuO4. Our results can be very well described within the random phase approximation model. This allows a direct determination of the exchange coupling constants between the Nd ions. The superexchange interactions mediated by the oxygen and copper-oxygen layers are determined to be significantly stronger than the in-plane exchange between the Nd ions.
04 May 1995-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this paper, a diagrammatic analysis of the many-body interactions between a moving ion passing through a solid with the electron gas embedded in a uniform distributed positive background is presented.
Abstract: We have formulated a diagrammatic analysis to study the many-body interactions between a moving ion passing through a solid with the electron gas embedded in a uniform distributed positive background. Using Feynman diagrams, a derivation of the first nonlinear correction (proportional to the second order on the ion charge) to the induced potential and electron density is presented. These magnitudes are evaluated numerically using the random-phase-approximation to describe the shielded interaction.
TL;DR: In this paper, the extended random phase approximation (ERPA) theory is treated within a framework of a projection method, which is used to calculate the main contributions to the nuclear response in electron-scattering reactions.
TL;DR: The Dyson equation for the particle-hole Green's function, including Coulomb exchange matrix elements, has been solved exactly for a two-dimensional electron gas as discussed by the authors, and both static and dynamic dielectric functions have been calculated and compared with normal random-phase approximation and recent quantum Monte Carlo results.
Abstract: The Dyson equation for the particle-hole Green's function, including Coulomb exchange matrix elements, has been solved exactly for a two-dimensional electron gas. Static and dynamic dielectric functions have been calculated and compared with normal random-phase-approximation and recent quantum Monte Carlo results.
TL;DR: In this article, the authors derived the macroscopic dielectric function for a general cubic lattice and showed that the self-polarization corrections occur as a combined effect of intra-atomic exchange interactions between electrons sitting in different orbitals and a finite inter-atomic tunneling.
Abstract: Starting from the random phase approximation for the weakly coupled multiband tightly-bounded electron systems, we calculate the dielectric matrix in terms of intraband and interband transitions. The advantages of this representation with respect to the usual plane-wave decomposition are pointed out. The analysis becomes particularly transparent in the long wavelength limit, after performing the multipole expansion of bare Coulomb matrix elements. For illustration, the collective modes and the macroscopic dielectric function for a general cubic lattice are derived. It is shown that the dielectric instability in conducting narrow band systems proceeds by a common softening of one transverse and one longitudinal mode. Furthermore, the self-polarization corrections which appear in the macroscopic dielectric function for finite band systems, are identified as a combined effect of intra-atomic exchange interactions between electrons sitting in different orbitals and a finite inter-atomic tunneling.