Showing papers on "Random phase approximation published in 1994"
01 Jan 1994
TL;DR: In this paper, the authors discuss the conditions under which the linearized (Cahn-like) theory of spinodal decomposition holds for block copolymer melts, where chains may stretch out in a dumbbell-like shape even in disordered phase, before the microphase separation transition.
Abstract: The classical concepts about unmixing of polymer blends (Flory-Huggins theory) and about mesophase ordering in block copolymers (Leibler's theory) are briefly reviewed and their validity is discussed in the light of recent experiments, computer simulations and other theoretical concepts. It is emphasized that close to the critical point of unmixing non-classical critical exponents of the Ising universality class are observed, in contrast to the classical mean-field exponents implied by the Flory-Huggins theory. The temperature range of this non-mean-field behavior can be understood by Ginzburg criteria. The latter are also useful to discuss the conditions under which the linearized (Cahn-like) theory of spinodal decomposition holds. While Flory-Huggins theory predicts correctly that the critical value of the Flory χ-parameter scales with chain length N (for symmetrical mixtures) χc ∝ 1/N, it strongly overestimates the prefactor and its use for fitting experimental data yields spurious concentration dependence. Also the chain radii depend on both χ and the composition of the mixture, thus invalidating the random phase approximation (RPA). Particular strong deviations from the RPA are predicted for block copolymer melts, where chains may stretch out in a dumbbell-like shape even in the disordered phase, before the microphase separation transition is approached. This review concludes with an outlook on interfacial phenomena and surface effects on these systems and other open problems in this field.
247 citations
TL;DR: In this article, the two-neutrino double-beta decay to excited final states is discussed in the framework of the quasiparticle random-phase approximation (QRPA), where the excited states are assumed to have the structure of one or two QRPA phonons.
57 citations
TL;DR: The continuum random phase approximation is used to describe the muon capture on 12C,16O, and 40Ca to reproduce the experimental total capture rates to better than 10% using the free nucleon weak form factors and two different residual interactions.
Abstract: We use the continuum random phase approximation to describe the muon capture on 12C,16O, and 40Ca. We reproduce the experimental total capture rates on these nuclei to better than 10% using the free nucleon weak form factors and two different residual interactions. However, the calculated rates for the same residual interactions are significantly lower than the data if the in-medium quenching of the axial-vector coupling constant is employed.
41 citations
TL;DR: In this paper, the random phase approximation (RPA) was used to compute the thermodynamics and electrostatics of chemically linked polyelectrolytes chains into block copolymer molecules.
Abstract: We study binary polyelectrolytes in the melt state and in concentrated solutions using the random phase approximation (RPA) We compute the thermodynamics and electrostatics of chemically linked polyelectrolytes chains into block copolymer molecules (copolyelectrolytes) Polyelectrolytes blends and copolyelectrolytes in the presence of free ions have Debye–Huckel‐type effective monomer–monomer interactions, even when the polymer chains are not charged Copolyelectrolyte of chemically linked chains of opposite charge in the absence of counterions, have ion–ion effective interactions characteristic of a dielectric medium, contrary to the case of polyelectrolyte blends where these effective interactions are Debye–Huckel type, even on the absence of free ions The dielectric constant in such a diblock copolymer melt is proportional to the square of the degree of polymerization Np In the reference state (without interactions) RPA assumes random walk statistics, which are nearly unperturbed in the dielectric;
38 citations
TL;DR: This work finds a smooth evolution of the Anderson mode for weak coupling into the Bogoliubov sound mode for hard-core bosons and compute the collective-mode spectrum using a generalized random-phase-approximation analysis within the equations-of-motion formalism.
Abstract: We study the evolution of the ground state and the excitation spectrum of the two- and three-dimensional attractive (negative-U) Hubbard model as the system evolves from a Cooper-pair regime for U\ensuremath{\ll}t, to a composite boson regime for U\ensuremath{\gg}t. Our work is motivated by the observation that the high-temperature superconductors, with their short coherence lengths and unusual normal-state properties, may be in an intermediate coupling regime between these two limits. A mean-field analysis of pairing, suitably generalized to account for a shift in the chemical potential, is known to be able to describe the ground-state crossover as a function of U/t. We compute the collective-mode spectrum using a generalized random-phase-approximation analysis within the equations-of-motion formalism. We find a smooth evolution of the Anderson mode for weak coupling into the Bogoliubov sound mode for hard-core bosons. We then include a long-range Coulomb interaction and show that it leads to a plasmon which again evolves smoothly from weak to strong coupling.
35 citations
TL;DR: In this article, the exotic (μ−, e−) conversion reaction for the 48Ti nucleus is studied in the framework of the quasi-particle random phase approximation (QRPA).
30 citations
TL;DR: In this article, energy gradients and effective one-electron density matrices corresponding to excitation energies calculated with the random phase approximation of the polarization propagator are derived and combined with second-order ground state energies yields final state total energies and their gradients.
Abstract: Energy gradients and effective one‐electron density matrices corresponding to excitation energies calculated with the random phase approximation of the polarization propagator are derived. Combination of these results with second‐order ground state energies yields final state total energies and their gradients. Geometry optimizations and evaluations of one‐electron properties are performed for excited states of formaldehyde.
27 citations
TL;DR: An analysis of collective longitudinal electrostatic plasma excitations in quasi-one-dimensional degenerate plasmas is presented using the dielectric function in the random phase approximation to compute the complete spectrum of the collective excitations, including frequencies and damping or growth rates.
Abstract: An analysis of collective longitudinal electrostatic plasma excitations in quasi-one-dimensional degenerate plasmas is presented using the dielectric function in the random phase approximation. Analytical continuation of the dielectric function into the lower energy half plane allows us to compute the complete spectrum of the collective excitations, including frequencies and damping or growth rates. In contrast to two- and three-dimensional plasmas, a multicomponent quasi-one-dimensional system at zero temperature is found to exhibit one undamped plasmon mode for each component. The conditions for the occurrence of unstable modes are investigated and the influence of temperature and collisions on the results is discussed.
24 citations
TL;DR: In this paper, the transition matrix elements for the 0+ → 0+ double-beta decays are calculated for 48Ca, 76Ge, 82Se, 100Mo, 128Te and 130Te nuclei, using a δ-interaction.
22 citations
TL;DR: In this paper, the rotational strengths of the core electron excitations of the chiral molecule propylene oxide were calculated using the random phase approximation (RPA) method, and the predicted values were small but still in the realm of experimental detectability, having dissymmetry factors g of the order of 10 −3.
22 citations
TL;DR: In this article, the complete dynamics of electrons and nuclei are derived using the time-dependent variational principle (TDVP) and a linear approximation to the evolution equations (the classical harmonic approximation) which corresponds to a generalized random phase approximation (RPA) based on an AGP electronic reference state and which explicitly includes the dynamics of the nuclei, is studied and presented.
Abstract: A formulation of the complete dynamics of electrons and nuclei is presented. The dynamical equations are derived using the time‐dependent variational principle (TDVP). The approximate electronic state vectors are antisymmetrized geminal power (AGP) states parameterized as projected coherent states, while the nuclei are treated as classical point particles. This leads to a formulation of time‐dependent AGP theory that generalizes time‐dependent Hartree–Fock (TDHF) theory and explicitly includes the dynamics of the nuclei. The linear approximation to the evolution equations (the classical harmonic approximation) which corresponds to a generalized random phase approximation (RPA) based on an AGP electronic reference state and which explicitly includes the dynamics of the nuclei, is studied and presented in this paper. The equations are formulated in terms of the primitive nonorthogonal electronic atomic basis thus avoiding any transformation to orthonormal molecular orbitals during the evolution.
TL;DR: It is shown that the GCM succeeds in describing the GMR energies in nuclei and that the empirical breathing-mode energies of heavy nuclei can be reproduced by forces with an incompressibility close to K=300 MeV in the RMF theory.
Abstract: The breathing-mode giant monopole resonance (GMR) is studied within the framework of the relativistic mean-field theory (RMF) using the generator coordinate method (GCM). The constrained incompressibility and the excitation energy of isoscalar giant monopole states are obtained for finite nuclei with various sets of Lagrangian parameters. A comparison is made with the results of nonrelativistic constrained Skyrme Hartree-Fock (HF) calculations and with those from Skyrme random phase approximation (RPA) calculations. In the RMF theory the GCM calculations give a transition density for the breathing mode, which greatly resembles that obtained from the Skyrme HF+RPA approach and also that from the scaling mode of the GMA. From the systematic study of the breathing-mode as a function of the incompressibility in GCM, it is shown that the GCM succeeds in describing the GMR energies in nuclei and that the empirical breathing-mode energies of heavy nuclei can be reproduced by forces with an incompressibility close to K=300 MeV in the RMF theory.
15 Jun 1994
TL;DR: In this article, the photoionization cross sections and asymmetry parameters of the four main valence ionization processes in H 2 S, corresponding to the formation of respectively 2b 1, 5a 1, 2b 2 and 4a 1 holes, have been calculated in the random phase approximation (RPA).
Abstract: The photoionization cross sections and asymmetry parameters of the four main valence ionization processes in H 2 S, corresponding to the formation of respectively 2b 1 , 5a 1 , 2b 2 and 4a 1 holes, have been calculated in the random phase approximation (RPA). By resorting to a recently proposed computational procedure, based on the K -matrix technique, the RPA equations for the four coupled ionization channels have been projected on a basis set of L 2 functions and solved, at any excitation energy above the ionization threshold, in a way that allows to recover the electronic continuum degeneracy.
TL;DR: In this article, the random phase approximation method is extended to calculate static electrical conductivity of fully ionized plasmas in the presence of a uniform magnetic field, and the components of the corresponding conductivity tenser and other parameters have been obtained for the case of a moderate magnetic field.
Abstract: The random phase approximation method is extended to calculate static electrical conductivity of fully ionized plasmas in the presence of a uniform magnetic field. The components of the corresponding conductivity tenser and other parameters have been obtained for the case of a moderate magnetic field. The results are compared with experimental data available for shock compressed plasmas.
TL;DR: In this article, high-resolution neutron-inelastic-scattering measurements of liquid3He at a temperature of 120 mK and pressures between 0 and 20 bars are analyzed.
Abstract: High-resolution neutron-inelastic-scattering measurements of liquid3He at a temperature of 120 mK and pressures between 0 and 20 bars are analyzed. The improved energy resolution provides for the first time information on the line shape of the spin-fluctuation peak. We find that the low-energy enhancement of the spin fluctuations is stronger than predicted by the paramagnon model (m*/m3 = 1), in particular at high pressures. Also, the spin dependent scattering extends to higher energies than can be accounted for in any simple random-phase-approximation (RPA) model that uses the Lindhard function with an effective mass of about 3 m3. This observation is independent of the interaction potential used in the RPA. However, we show that a simple RPA model where the Lindhard function has an effective mass of 1.9m3 gives a good description of the spin-fluctuation scattering. The zero-sound mode overlaps with both spin fluctuations and multipair excitations, and its energy and line width can only be obtained unambiguously at the smallest wave vectors. For wave vectors larger than 0.6 A−1, the energy and in particular the width of the zero-sound mode depend strongly on the models used for the spin fluctuations and the multipairs. Different damping mechanisms of the zero-sound mode are discussed, and the importance of coherent and incoherent multipairs is illustrated.
TL;DR: In this paper, the effect of compressibility on free volume was discussed in terms of free volume by introducing an extra component to represent small voids, which is similar with the polymer reference site interaction model.
Abstract: The random phase approximation is summarized for compressible polymer blend mixtures in the homogeneous phase region. Similarities with the polymer reference site interaction model are described. The effect of compressibility is also discussed in terms of ‘free volume’ by introducing an extra component to represent ‘small voids’. Variation of the effective Flory-Huggins interaction parameter with composition is found to agree with results obtained using the lattice cluster theory.
TL;DR: In this paper, double and single beta decay were investigated theoretically through quasi-particle random phase approximation (QRPA) which includes neutron-proton pairing as well as neutron-neutron and proton-Proton pairing correlations.
TL;DR: In this article, the outer valence ionization potentials, low-lying electronically excited states and nuclear spin-spin coupling constants of cubane have been investigated at ab initio level by using the inner valence Green function, random phase approximation and equations of motions approaches, respectively, which incorporate the main portion of the electron correlation effects.
TL;DR: In this article, the dielectric screening function epsilon (k, omega ) of the fluid of charged bosom at zero temperature is evaluated in a range of low-to-intermediate coupling strength, in view of recent data on static screening and fluid structure from quantum Monte Carlo methods.
Abstract: The dielectric screening function epsilon (k, omega ) of the fluid of charged bosom at zero temperature is evaluated in a range of low-to-intermediate coupling strength, in view of recent data on static screening and fluid structure from quantum Monte Carlo methods. Correlations beyond the random phase approximation are included within a class of approximations which are well known for the electron fluid, i.e. by introducing a frequency-independent local field factor to be determined through self-consistency requirements connecting various aspects of the physics contained in the dielectric function. The static dielectric function epsilon (k,0) is negative over a range of wavenumbers at all values of the density, leading to oscillatory screening of a foreign charge and to an effective long-range attraction between equi-charged impurities. Quantitative agreement with the Monte Carlo data on static screening is achieved by imposing self-consistency on the compressibility of the fluid in addition to self-consistency on the pair distribution function. Good agreement is also obtained on the pair distribution function and the correlation energy. Within the present class of approximations, the dispersion relation of longitudinal excitations takes the Feynman form, starting at the plasma frequency with a negative dispersion coefficient and going through a minimum before ending at the single-particle recoil frequency.
TL;DR: In this article, the collective electronic excitations in quasi-one-dimensional cylindrical quantum-well wires are studied theoretically using a two-subband model and the dispersion curves of the intra- and inter-band plasmons in the random-phase approximation for the cases of one and two occupied subbands are calculated.
Abstract: The collective electronic excitations in quasi-one-dimensional cylindrical quantum-well wires are studied theoretically. Using a two-subband model the dispersion curves of the intra- and intersubband plasmons in the random-phase approximation for the cases of one and two occupied subbands are calculated. The influence of the image forces on the intra- and intersubband plasmon is studied in detail. New explicit analytical expressions for the dispersion relations, valid in a wide range of the wave vector are derived. It is shown that the additional intrasubband plasmon, arising if two subbands are occupied, has for small wave vectors a linear dispersion and is independent of the dielectric screening of the quantum-well wire. The intersubband plasmon is split in two branches, one with a positive and one with a negative dispersion. The electric dipole moment of the collective excitations is calculated and the selection rules for the coupling of light with the plasmons are derived.
TL;DR: These calculations to the electromagnetic response function of certain fractional quantized Hall states of transformed fermions interacting with a Chern-Simons field are adopted to spherical geometry in order to facilitate comparison with exact diagonalizations performed on finite size systems.
Abstract: Using a well known singular gauge transformation, certain fractional quantized Hall states can be modeled as integer quantized Hall states of transformed fermions interacting with a Chern-Simons field. In previous work we have calculated the electromagnetic response function of these states at arbitrary frequency and wavevector by using the Random Phase Approximation (RPA) in combination with a Landau Fermi Liquid approach. We now adopt these calculations to a spherical geometry in order to facilitate comparison with exact diagonalizations performed on finite size systems.
TL;DR: In this article, the magnetic properties of holmium trifluoride (HoF3) were analyzed within the mean-field/random-phase approximation (RPA) framework.
Abstract: Well defined magnetic excitations in holmium trifluoride are observed in inelastic neutron-scattering experiments, both in the paramagnetic phase at 1.6 K and in the ferrimagnetic phase at 90 mK. The dispersion relations at the two temperatures have been determined along the high-symmetry directions, and the field dependence of the excitations has been studied at 1.6 K. These measurements and the previous studies of the magnetic properties of HoF3 are analysed within the mean-field/random-phase approximation (RPA). The two lowest electronic states of Ho3+ ions are singlets, which are well separated from the remaining levels. The dominating coupling between the ions is the classical dipole interaction, which forces the system to order at Tc=0.53 K. The two-ion coupling is below the threshold value for inducing the ordering of the electronic system; it only occurs because the magnetic susceptibility is enhanced by the hyperfine interaction between the electronic and nuclear moments on the Ho ions. The classical dipole coupling is calculated directly from first principles, whereby the response function is nearly fixed by the macroscopic properties of the system. The calculated response is found to agree accurately with the observations in the paramagnetic phase, whereas some discrepancies occur in the ordered phase. These may indicate that correlation effects beyond the RPA are important or that two-ion (magnetoelastic) quadrupole couplings are present.
TL;DR: In this article, the effective interaction for the spin-singlet Cooper pairs is calculated using the two-dimensional d-p model for high- T c cuprates on the basis of a renormalized perturbation of the Fermi liquid.
Abstract: The effective interaction for the spin-singlet Cooper pairs is calculated using the two-dimensional d-p model for high- T c cuprates on the basis of a renormalized perturbation of the Fermi liquid: first forming the quasiparticles variationally by the Gutzwiller approximation, then treating the residual interactions among quasiparticles by the RPA. In the charge-fluctuation region, where the p hole is moderately doped and the repulsive interaction ( U pd ) between the nearest-neighbor p and d sites is the dominant parameter, the interaction for d xy pairs is attractive, while it is repulsive both for extended-s and d x 2 - y 2 pairs. The on-site repulsive interaction for p electrons ( U p ) is compatible with this tendency, whereas the one for d electrons ( Ũ d ) is incompatible.
TL;DR: The coupling of elementary excitation modes in hot nuclei is studied in this paper, where the quasiparticle-phonon nuclear model is extended to a finite temperature by using the formalism of the thermofield dynamics.
Abstract: The coupling of elementary excitation modes in hot nuclei is studied For this aim the quasiparticle-phonon nuclear model (QPM) is extended to a finite temperature by using the formalism of the thermofield dynamics First the energies and structures of one-phonon states are calculated in the thermal random phase approximation and then the thermal QPM Hamiltonian HQPM is expressed in terms of thermal quasiparticles and thermal RPA-phonons The equation for the energies taking into account mixing of one-and two-thermal phonon states is derived The expression of the coupling matrix element between thermal phonons is given
TL;DR: In this article, the random phase approximation method is applied to obtain the coefficient of electron thermal conductivity and other kinetic coefficients of fully ionized plasma, in the absence of a magnetic field.
Abstract: The random phase approximation method is applied to obtain the coefficient of electron thermal conductivity and other kinetic coefficients of fully ionized plasma, in the absence of a magnetic field. Calculations have been carried out for a wide range of temperature and electron concentrations, which include the domain in which the plasma is strongly coupled. The domain of validity of the Wiedemann-Franz law has been tested.
TL;DR: In this article, it was shown that Vlasov plasmas with spherical momentum symmetry cannot exhibit longitudinal plasma instabilities, regardless of the monotonic behavior of the distribution function.
Abstract: It is well known that Vlasov plasmas with spherical momentum symmetry cannot exhibit longitudinal plasma instabilities, regardless of the monotonic behavior of the distribution function. It is shown that this property holds for quantum plasmas also, at least within the random phase approximation.
TL;DR: Two approaches to the isobaric analog resonance (IAR) theory within the random phase approximation in the continuum are considered, based on the partial self-consistency condition which is the result of the isospin symmetry of the nuclear Hamiltonian.
Abstract: Two approaches (exact and approximate) to the isobaric analog resonance (IAR) theory within the random phase approximation in the continuum are considered. Both of them are based on the partial self-consistency condition which is the result of the isospin symmetry of the nuclear Hamiltonian. The evaluations of the IAR partial proton widths for near magic nuclei over a wide atomic mass region are performed. The results obtained within the framework of these approaches are compared with each other and with the experimental data. The method of the calculation of the Coulomb correction to the IAR transition density is also given.
TL;DR: In this paper, the multiconfiguration relativistic random-phase approximation theory is applied to photoionization of the valence subshells 5s1/2, 4d3/2 and 4d5/2.
Abstract: The multiconfiguration relativistic random-phase approximation theory is applied to photoionization of the valence subshells 5s1/2, 4d3/2, 4d5/2, 5P1/2 and 5p3/2 of the Sr atom above the 5p3/2 threshold. Electron correlations associated with double-electron excitations and core polarizations have caused drastic variations in subshell cross sections, branching ratios, angular distributions and spin polarizations of the photoelectrons.
TL;DR: In this paper, a theoretical approach to the polarizability of plasma electrons is presented, which takes into account both bound and free electrons, using density functional theory for electrons, random phase approximation for the response of the electron density and cluster expansion in the average over the ion configurations.
Abstract: A theoretical approach to the polarizability of plasma electrons is presented. The formalism takes into account both bound and free electrons. The methods used are: density functional theory for electrons, random phase approximation for the response of the electron density and the cluster expansion in the average over the ion configurations. The first order of the cluster expansion leads to equations for the polarizability of the average atom. A perturbational approach to this polarizability is proposed. The theory may be applied to static polarizability which express the variances in the statistical broadening of the average atom. A numerical example for gold at 750 eV temperature and solid density is discussed.
TL;DR: In this article, the screened Coulomb interaction in real space for two-dimensional electron gases in a plane and in a superlattice was calculated for a single plane and a single super-clique, and the instability of the electron gas is of Kohn-Luttinger type.
Abstract: We calculate the screened Coulomb interaction in real space for two-dimensional electron gases in a plane and in a superlattice We include anomalous screening due to many-body effects via the local-field correction We find that many-body effects lead to a strong attractive intraplane potential The strong attractive part is interpreted as a many-body effect in the low-carrier-density regime Our interpretation for a superlattice is supported by the results for a single plane The instability of the electron gas is of Kohn-Luttinger type For r s > 2 we find that the attraction is much larger than the Fermi energy which should lead to a Bose-Einstein condensation