Showing papers on "Random phase approximation published in 1991"
TL;DR: In the 1D quantum limit the intrasubband plasmon displays the linear dispersion characteristic of 1D free-electron behavior and collective 1D intersubband excitations appear as dispersion-less and have a negligible shift from the single-particle energy.
Abstract: The energy and the wave-vector dispersion of single-particle and collective excitations of the one-dimensional (1D) electron gas in GaAs quantum wires have been determined by resonant inelastic light scattering. In the 1D quantum limit the intrasubband plasmon displays the linear dispersion characteristic of 1D free-electron behavior. Quantitative agreement is found with calculations based on the random-phase approximation. In contrast, collective 1D intersubband excitations appear as dispersion-less and have a negligible shift from the single-particle energy
234 citations
TL;DR: In this article, a network of circle maps is investigated. Successive transitions are found among coherent, ordered, partially ordered (PO), and turbulent phases, whose dynamics consist of quasistationary high-dimensional chaotic motion, low-dimensional attractor ruins, and switching among them.
129 citations
TL;DR: Etude theorique du spectre des excitations elementaires dans des systemes electroniques unidimensionnels varies, en absence de champ magnetique, montrent la decomposition du pic de resonance due a l'interaction coulombienne entre electrons situes dansdes couches differentes.
Abstract: We study theoretically the elementary excitation spectrum in various one-dimensional electron systems in the absence of a magnetic field. We first calculate the elementary excitations in a single quantum wire under the random-phase approximation. We find that the intersubband collective excitation frequency can be 5--6.5 times higher than the corresponding single-particle excitation energy due to a large depolarization shift. Next, we calculate the plasmon excitation energy of a double-layered quantum-wire system. Our result shows that the resonance peak splits due to the Coulomb interaction between electrons in different layers. We also study the elementary excitations in one-dimensional lateral quantum-wire superlattices. We include the Coulomb interaction between electrons in different wires and allow tunneling between neighboring wires. Finally, we calculate the spectral weights of the elementary excitations of both a single quantum wire and quantum-wire superlattices for various parameter values. We compare our results with recent experiments and find good agreement.
113 citations
TL;DR: In this article, a systematic study of nuclear properties with Skyrme forces is presented, including nuclear ground state properties with Hartree-Fock (HF) and self-consistent semiclassical approaches.
Abstract: The author presents a systematic study of nuclear properties with Skyrme forces. The properties studied include: (i) nuclear ground state properties with Hartree-Fock (HF) and self-consistent semiclassical approaches; (ii) properties of giant resonances with a random phase approximation (RPA) and sum rule approach; (iii) microscopic nucleon-nucleus optical potentials and related quantities with the mass operator method; and (iv) nucleus-nucleus optical potentials and fusion barriers with the energy density formalism.
65 citations
TL;DR: In this paper, the half-life of the 0+ → 0+ neutrinoless double beta decay for 76Ge, 82Se, 86Kr, 114Cd, 128, 130Te and 134, 136Xe and the upper limit for the effective neutrino mass of 3.0 eV was deduced from available experimental data.
53 citations
TL;DR: In this paper, it has been argued theoretically and numerically that resonant particle mediated mode coupling effects actually play an important role, and that the statistically averaged effect of this mode coupling is a zeroth-order increase in the growth rate.
Abstract: The use of the random phase approximation in quasilinear theory has been controversial for some time. For the bump‐on‐tail instability this approximation leads to the neglect of mode coupling effects mediated by the resonant particles. Recently, it has been argued theoretically and numerically that resonant particle mediated mode coupling effects actually play an important role, and that the statistically averaged effect of this mode coupling is a zeroth‐order increase in the growth rate. The quasilinear theory of the interaction between a warm beam and a slow wave structure is formally identical to the quasilinear theory of the interaction between a warm beam and a plasma in the weak beam limit. Strong mode coupling effects have been experimentally observed when a weak warm beam interacts with waves on a slow wave structure. When a statistical average is done over the mode coupling, however, the predicted zeroth‐order increase in the growth rate is not observed.
47 citations
TL;DR: In this paper, the neutrino scattering from a dense degenerate gas of electrons, protons and neutrons is investigated, where correlations induced by the Coulomb interactions of the electrons and protons are considered.
41 citations
TL;DR: In this paper, the authors applied functional integral techniques to the problem of level densities for interacting Fermi systems and derived a mean-field approximation of the partition function with a high level of accuracy.
39 citations
TL;DR: By Monte Carlo simulation of a three-dimensional lattice model for a symmetric diblock copolymer melt, this paper showed that significant deviations from Gaussian copolymers conformational behaviour set in far above the order-disorder transition.
Abstract: By Monte Carlo simulation of a three-dimensional lattice model for a symmetric diblock copolymer melt, we show that significant deviations from Gaussian copolymer conformational behaviour set in far above the order-disorder transition. This gradual stretching of the copolymers with increasing interaction strength, accompanied by the emergence of a collective scale characterizing compositional fluctuations, implies a failure of the random phase approximation (RPA) throughout the disordered phase.
36 citations
TL;DR: La forme parametree des corrections de champ local est introduite pour construire la fonction dielectrique et la susceptibilite de spin non locale decrivant les proprietes dielectriques d'un liquide electronique degenere paramagnetique a 2 dimensions.
Abstract: The parametrized form of local-field corrections is introduced to construct the dielectric function and the nonlocal spin susceptibility that describe the dielectric properties of a paramagnetic degenerate electron liquid in two dimensions beyond the random-phase approximation (RPA). The parameters that take into account the spin-parallel and spin-antiparallel electron-electron interactions at short distances are determined in such a way that the response functions correctly give the Monte Carlo values for the compressibility and the spin susceptibility, respectively, in the long-wavelength limit. The local-field correction thus obtained is compared with that in the Hubbard approximation; the static dielectric function and the spin susceptibility are also compared with those in the RPA. It is found that the sign of the static dielectric function depends on both the dimensionless coupling constant and the wave number, which is a feature markedly different from the RPA results.
35 citations
TL;DR: In this paper, the decay rates of two-neutrino ββ decay were calculated in a fully number-projected quasiparticle random-phase approximation, based on the restoration of particle-number symmetries both at the level of qua-particle and phonon excitations.
TL;DR: In this paper, a detailed review of numerical calculations that involve a single hole in the antiferromagnetic phase and are based on the Monte Carlo method and the exact diagonalization of small clusters is presented.
Abstract: Current ideas on the interaction between magnetic and superconducting states in strongly magnetized systems are discussed in terms of the Hubbard model and its limiting case in the form of the t – J model. Two approaches to the problem are compared, namely, those of weak and strong Coulomb repulsion, i.e., U W and U W, respectively, where U is the repulsion and W the electronic band width. The dynamic magnetic susceptibility of the system is analyzed in both cases, and different types of magnetic instability are identified. Spin fluctuations that grow near the instability boundaries of the paramagnetic phase give rise to Cooper instability. The role of longitudinal and transverse spin fluctuations in the evolution of the superconducting state in a magnetically ordered phase is also investigated. Particular attention is devoted to the two-dimensional model near half-filling. Analytic studies based on the generalized random phase approximation are presented. In addition, a detailed review is given of numerical calculations that involve a single hole in the antiferromagnetic phase and are based on the Monte Carlo method and the exact diagonalization of small clusters. The problem of two interacting holes is also examined. Such studies may provide the conceptual basis for magnetic (correlational) mechanisms of high-Tc superconductivity in copper oxide compounds.
TL;DR: In this article, the quadratic response function (QRF) is evaluated within the random phase approximation (RPA), to compute frequency-dependent first hyperpolarizabilities β(ω,ω).
Abstract: The quadratic response function (QRF) is evaluated within the random phase approximation (RPA), to compute frequency‐dependent first hyperpolarizabilities β(ω,ω). The method treats electron correlation consistent through first order, so the computed values are equivalent to coupled‐perturbed Hartree–Fock (CPHF) results. The QRF is obtained by solving systems of linear equations, thus circumventing the RPA eigenvalue problem. The QRF equation of motion is used to develop hyperpolarizability identities in the dipole length and mixed‐velocity representations. The two forms of β are equivalent at the RPA level, and provide a useful measure of completeness of basis. The method is applied to the hyperpolarizability of HF and H2O. It is found that basis sets used in previous studies were not saturated for all β components, and that basis sets which satisfy length–velocity sum rules for linear response properties are not sufficient for agreement of quadratic response properties. The calculated dispersion ratios are in good agreement with experimental measurement, indicating that dispersion effects are properly described by frequency‐dependent calculations in the RPA at field energies which are small compared to vertical excitation energies.
TL;DR: In this paper, the influence of spin fluctuations on the formation of a superconducting state is investigated in the three-dimensional Hubbard model with a strong Coulomb interaction (U>>t) ((t-J) model.
Abstract: In the three-dimensional Hubbard model with a strong Coulomb interaction (U>>t) ((t-J) model), the influence of spin fluctuations on the formation of a superconducting state is investigated. The system's magnetic susceptibility is calculated in terms of the generalized random-phase approximation, using the diagram technique for Hubbard operators. The authors find the magnetic phase transition lines on the (t/U, n) plane, where n is the electron concentration. Equations similar to those in the strong-coupling theory for a superconductor in the (t-J) model are derived taking into account the spin fluctuations. Equations are obtained for the superconducting transition temperatures Tc for the order parameter of s and d symmetries. Spin fluctuations are shown to suppress the superconductivity near the ferromagnetic instability of the paramagnetic phase for both s and d states. Near the antiferromagnetic instability, a sufficient enhancement of Tc is possible.
TL;DR: In this paper, it was shown that the M1 transition strength in the deformed oscillator model has a dependence on the quadrupole deformation parameter δ which is considerably stronger than linear and varies quadratically with δ in the limit of small deformation.
TL;DR: It is shown that the plasmon-phonon coupling can appreciably shift the dispersion curves, and additional plAsmonlike intersubband modes that exist in regions observable in far-infrared optical spectroscopy are found.
Abstract: The spectrum of the coupled plasmon-phonon collective excitations of a quasi-one-dimensional quantum-well wire is analyzed. Using a three-subband model the electronic polarization function is treated in detail within the random-phase approximation. It is found that, in comparison with the quasi-two-dimensional electron gas, the excitation spectrum of the quasi-one-dimensional electron gas contains additional regions free of Landau damping. The dispersion curves of the collective excitations are calculated and presented in graphical form for three different cases of subband population. It is shown that the plasmon-phonon coupling can appreciably shift the dispersion curves. In the case that more than one subband is populated, we have found additional plasmonlike intersubband modes that exist in regions observable in far-infrared optical spectroscopy.
TL;DR: In this paper, the half-life for the 2 nu beta beta decay transition in 100Mo is calculated by using a conventional proton-neutron quasiparticle random phase approximation method and a recently proposed particle number projected qua-particle random-phase approximation formalism.
Abstract: The half-life for the 2 nu beta beta decay transition in 100Mo is calculated by using a conventional proton-neutron quasiparticle random phase approximation method and a recently proposed particle number projected quasiparticle random phase approximation formalism. The calculations of the relevant matrix elements have been performed by using a realistic effective two-body interaction constructed from the Bonn one-boson exchange potential. Suppression of the 2 nu beta beta decay matrix element is found both in the unprojected and projected models. The collapse of the quasiparticle random phase approximation formalism, induced by renormalized particle-particle interactions, is found to play an important role in dealing with the numerical stability of the results. The particle number projected results are found to be more stable than those corresponding to the unprojected formalism although the particle number projection does not suffice for a complete elimination of spurious ground-state correlations near the collapse of the quasiparticle random phase approximation.
TL;DR: This work investigates many-body corrections to neutrino scattering from electrons and nuclei and finds that at electron densities relevant for supernovae, the total quasielastic {nu}-{ital e} cross section can be reduced by 50%, and elastic neutrinos-nucleus scattering by up to 85%.
Abstract: We investigate many-body corrections to neutrino scattering from electrons and nuclei. The Coulomb correlations of the electrons screen the weak vector neutral current. Electron screening is important only for electron neutrinos (or antineutrinos) and most effective for neutrino energies of several MeV or less. At electron densities relevant for supernovae, the total quasielastic {nu}-{ital e} cross section can be reduced by 50%, and elastic neutrino-nucleus scattering by up to 85%.
TL;DR: In this article, the microphase separation transition in five asymmetric diblock copolymers was studied using small angle X-ray scattering and Leibler's random phase approximation together with a polydispersity correction yields a quantitative description of the scattering profiles.
TL;DR: In this paper, the positions and the transition strengths of giant multipole resonances (including electric, magnetic, Fermi and Gamow-Teller transitions of the spherical nuclei 16O, 40Ca, 48Ca, 90Zr, 208Pb) are calculated using the random phase approximation based on the Hartree-Fock ground states with the new extended Skyrme-Landau interaction - SL1.
TL;DR: In this paper, the influence of quadrupole pair interaction on Kπ = 1+ states in rare earth nuclei using the Quasiparticle Random Phase Approximation was investigated.
Abstract: The influence of a quadrupole pair interaction on Kπ = 1+ states in rare earth nuclei is investigated using the Quasiparticle Random Phase Approximation. Comparison of the obtained results with available experimental data indicates a rather weak quadrupole pair interaction in these nuclei. The calculation of the B(M1) strength is extended to higher energies to account for the recently observed higher lying M1-strength. The differential cross section of inelastic electron scattering is calculated within DWBA for lower lying Kπ = 1+ states and is seen to be consistent with the interpretation that the observed states around 3 MeV are of isovector convection current character and have their main M1-strength coming from natural parity proton components.
TL;DR: In this paper, a range of isotropic and anisotropic spectral moments of the dipole oscillator strength distribution for N 2 in the random phase approximation were calculated and compared to other calculations and to experiment when available, and concluded that the scheme gives reliable results.
Abstract: We calculate a range of isotropic and anisotropic spectral moments of the dipole oscillator strength distribution for N 2 in the random phase approximation. The internuclear dependence of, in particular, S∥(k) is nonnegligible and vibrationally averaged moments are reported. The results are compared to other calculations and to experiment when available, and we conclude that the scheme gives reliable results. Based on our results, we predict a 1O% anisotropy in the stopping power of oriented N 2 molecules
TL;DR: In this article, it was shown that in the limit of small quark mass, the baryon is accurately approximated by a spatially varying chiral rotation of the vacuum wave function, where the chiral angle satisfies the sine-Gordon equation.
TL;DR: It is demonstrated that an expansion in powers of the strength of the interaction, of the free energy at {ital fixed} order parameter, can be used to generate and correct mean-field theories for interacting quantum many-body systems.
Abstract: We demonstrate that an expansion in powers of the strength of the interaction, of the free energy at fixed order parameter, can be used to generate and correct mean-field theories for interacting quantum many-body systems. The first two terms in the expansion generally yield ordinary Hartree-Fock mean-field theory and the next term gives an ``Onsager reaction field'' correction to Hartree-Fock theory. This method can be used to directly generate expansions for inverse susceptibilities. We illustrate the method for the one- and two-dimensional Hubbard model, for which we consider corrections to mean-field theories of antiferromagnetism for the repulsive-U half-filled case and superconductivity in the attractive-U case. These corrections give a quantitative account superior to that of the random-phase approximation (RPA) for the correlation energy at small and intermediate values of U. For susceptibilities, we recover from the first two terms in the expansion the usual RPA results, while higher-order terms give systematic corrections to the RPA susceptibilities. For the case of superconductivity in the repulsive-U Hubbard model, we show that the higher-order terms in the expansion must be considered to determine whether or not an instability exists. We find that there is no superconducting instability in the repulsive-U case, at least towards ordinary singlet or triplet pairing. We also find no evidence for a superconducting instability driven by a coexisting antiferromagnetic order.
TL;DR: In this paper, the two-point dynamic response functions of the Hubbard model at any filling from a slave-boson representation in the paramagnetic saddle point approximation were calculated using a spinrotation invariant form of the representation due to Kotliar and Ruckenstein, for which the saddle point corresponds to Gutzwiller's variational approximation.
Abstract: We calculate the two-point dynamic response functions of the Hubbard model at any filling from a slave-boson representation in the paramagnetic saddle point approximation. We use a spin-rotation invariant form of the representation due to Kotliar and Ruckenstein, for which the saddle point corresponds to Gutzwiller's variational approximation. In addition to the spin and charge susceptibilities, which have the familiar RPA form in the limit of small wavevectorq, the double and single-occupancy response functions are here calculated for the first time.
TL;DR: It is found that in narrow quantum wells vertex corrections may be quantitatively significant, bringing calculated results close to the experimental data, especially for the case of the band-gap renormalization.
Abstract: We calculate theoretically the lifetime, the effective mass, the renormalization factor, and the exchange-correlation-induced band-gap renormalization for quasiparticles in two-dimensional electron systems. In our theory we go beyond the usual random-phase approximation by including many-body vertex corrections of the Hubbard type in the self-energy and the dielectric function of the electron gas. We find that in narrow quantum wells vertex corrections may be quantitatively significant, bringing calculated results close to the experimental data, especially for the case of the band-gap renormalization. In general, vertex correction is found to be more important for narrower quantum wells and for lower electron densities.
TL;DR: In this paper, the field and the specific mass isotope shifts for caesium and thallium were calculated for the Cs 6s 1/2 ground state and the authors obtained a field shift value which they believe is accurate to 3%.
Abstract: Ab initio many-body perturbation calculations are presented for the field and the specific mass isotope shifts for caesium and thallium. The calculations are fully relativistic and incorporate the random phase approximation core screening corrections together with the lowest order correlation corrections. For the specific mass shift certain higher order corrections are also included. For the Cs 6s1/2 ground state the authors obtain a field shift value which they believe is accurate to 3%. For Tl they find relatively poor agreement between the calculated field shift values at this level of approximation. Current experimental data are re-examined in the light of the results.