Showing papers on "Random phase approximation published in 1978"
TL;DR: In this paper, the authors used renormalized random-phase approximation (RPA) to describe the nuclear excitations and found that the difficulty in fitting the data may lie in the splittings rather than in the absolute values of the transition energies.
87 citations
TL;DR: In this article, the low-lying and high-lying Kπ = 0+, 1+ and 2+ states of the deformed rare earth region are studied in the framework of the quasi-particle random phase approximation.
65 citations
TL;DR: In this paper, the linear response of a nucleus in an external oscillating field has been studied in the framework of TDHF theory and a method to obtain RPA sum rules from the TDHF polarizability has been explicitly developed.
28 citations
TL;DR: The theory of dilute magnetic alloys with 4d and 5d transition metals as host is reviewed in this article, where the effect of the local environment on the formation of local moments is considered within a generalized Landau approach and from a more microscopic point of view.
28 citations
TL;DR: In this article, different isovectors and isoscalar giant resonances (dipole, monopole and quadrupole) have been studied by evaluating energy-weighted and cubic energyweighted sum rules.
23 citations
01 Jan 1978
20 citations
TL;DR: In this paper, a simple one-dimensional model hamiltonian was used to calculate the polyene band structure in the Hartree-Fock approximation, and the generalised linear response function was calculated in the random phase approximation.
20 citations
TL;DR: In this article, a description of collective electronic excitation states (called miniexcitons) for simulations of temperature dependent ESR line shapes of triplet molecular aggregates embedded in ordered matrices is presented.
Abstract: We introduce a description by quasistationary collective electronic excitation states (recently called also miniexcitons) for simulations of temperature dependent ESR line shapes of triplet molecular aggregates embedded in ordered matrices. We put emphasis on the fact that in organic molecules, where the triplet spins are strongly bound to the orbital motion, the sites’ spin oscillate in phase; this electronic coherence, being caused by the electronic part of the resonance exchange quantum interaction, cannot be termed a short memory process. It is emphasized on the fact that the time dependent stochastic models are based essentially on the random phase approximation. The latter is shown to suppress the collective response of the sites and, therefore, to be of little use for simulations at low temperatures. We use a total effective hamiltonian which allows us to calculate a spin hamiltonian for each collective electronic excitation. With the help of this spin hamiltonian, we perform actual ESR line shapes simulations for triplet pairs and finite chains of molecules obtained for system N h 8/N d 8 and DBN. We discuss the variation of the linewidth with the size of the aggregate and show that extreme narrowing at very low temperatures is not due to exciton motion, but to the renormalization of the electronic spin density on the size of the aggregate. Then we discuss the temperature effect as a consequence of thermal transitions among the exciton states of the aggregate and derive from comparison with experiments a high temperature limit relaxation rate U° (T). Our simulations allow insight into the spin perturbation mechanism associated with the temperature.
17 citations
16 citations
TL;DR: In this article, the pair correlation function is calculated in the random phase approximation for the infinite barrier model of a metal surface and the interaction energy density is calculated, and is given very accurately by the local density approximation.
16 citations
TL;DR: In this article, the equivalence of several recent results on the spin susceptibility of the electron gas to the calculation of Brueckner and Sawada using the random-phase approximation (RPA) is reported.
Abstract: The equivalence of several recent results on the spin susceptibility of the electron gas to the calculation of Brueckner and Sawada using the random-phase approximation (RPA) is reported. An error is pointed out in the exact high-density expansion of this quantity and a formula presented which is exact in the high-density limit and also accurately describes the RPA result at metallic densities.
TL;DR: In this article, the generalized random phase approximation with exchange permit is used to describe satisfactorily the experimental data available on Xe photoionization in the energy region ω ≈ Z Ry and do not confirm the recent prediction of the statistical model on the existence of a specific atomic collective dipole level.
TL;DR: In this article, the coordinate-space formalism for rotation and time-reversal invariant interactions is derived and explicit expressions for the interaction strengths and transformation coefficients are given for the Skyrme interactions.
Abstract: Based on the conventional particle-hole representation of the random-phase-approximation eigenvalue equation, it is shown that the coordinate-space formalism can be derived directly and is valid generally for rotation and time-reversal invariant interactions. Explicit expressions are given for the interaction strengths and transformation coefficients in the case of Skyrme interactions.
TL;DR: In this paper, the excitation energies and transition amplitudes of even N = Z sd shell nuclei were predicted for the OSRPA equations and the results were employed in the calculation of electron scattering form factors.
TL;DR: In this paper, the ground state properties of the electron-hole system in a strong magnetic field are investigated throughout the whole density range using a variational formalism which takes account of the EH pairing, explicity derived are the momentum distribution, the electronhole pair amplitude and the single-particle excitation spectrum.
Abstract: Ground state properties of the electron-hole system in a strong magnetic field are investigated throughout the whole density range. By use of a variational formalism which takes account of the electron-hole pairing, explicity derived are the momentum distribution, the electron-hole pair amplitude and the single-particle excitation spectrum. The ground state energy is computed not only variationally but also in the random phase approximation (RPA). The result shows that there exist both gas and liquid excitonic phases each of which has characteristic nature of the electron-hole pairing. Qualitative discussions are given as to semimetals and highly excited semiconductors with emphasis on the presence of incoherent excitons.
TL;DR: In this paper, the rotatory intensities of the low-lying electronic transitions of twisted cis-butadiene and planar trans-trans butadiene were computed in the random phase approximation (RPA) method.
TL;DR: The random phase approximation with constraints on particle number and angular momentum is used to describe strongly disturbed bands in the high spin region of well-deformed nuclei as mentioned in this paper, where the particle number is a function of the angular momentum.
TL;DR: In this article, the isotherms and phase diagram in the constant coupling approximation for an Ising metamagnet with various values for the ratio of the ferromagnetic to the antiferromagnetic coupling parameters were computed.
Abstract: We compute the isotherms and phase diagram in the constant coupling approximation for an Ising metamagnet with various values for the ratio of the ferromagnetic to the antiferromagnetic coupling parameters. The constant coupling method is set up entirely with the internal fields as the variation parameter. The search for the tricritical point, both directly and indirectly via the hessian of the internal fields led to the conclusion that this model has no tricritical point, but a critical end point. Comparing our computation with the experimental result for FeBr2, we find that the value for the critical end point lies closer to the experiment than either the molecular field theory or the random phase approximation.
TL;DR: In this paper, an expression for the spin wave stiffness constant in a partially ordered ferromagnetic alloy was derived for a two-sublattice model employing the random phase approximation and the coherent potential approximation.
Abstract: An expression for the spin wave stiffness constant in a partially ordered ferromagnetic alloy is derived for a two-sublattice model employing the random phase approximation and the coherent potential approximation. It is shown by a numerical calculation that the spin wave stiffness constant is very sensitive to the degree of atomic order.
TL;DR: Using the random phase approximation and analytical calculation, this article showed that a higher surface plasma mode can exist near a plane metal-vacuum interface provided the oscillation is localized.
Abstract: Using the random phase approximation and analytical calculation we show that a “higher” surface plasma mode can exist near a plane metal-vacuum interface provided the oscillation is “sufficiently” localized. This conclusion is confirmed for the infinite potential barrier model of the surface where a mode with long-wavelength frequencyω≃ωp in sodium is predicted.
TL;DR: In this article, the two-particle random-phase approximation theory of Vary and Ginocchio is used to generalize the extended basis shell model to include the effect of core correlations.
Abstract: Previous calculations have shown that the accurate prediction of two-nucleon overlap factors requires the inclusion of very large numbers of highly excited configurations. These "extended basis shell model" calculations treat a pair of nucleons added to an inert core. In the present paper, the two-particle random-phase approximation theory of Vary and Ginocchio is used to generalize the extended basis shell model to include the effect of core correlations. Including them is shown to have a small effect on two-nucleon stripping reactions, such as $^{16}\mathrm{O}(t, p)^{18}\mathrm{O}$. However, the random-phase approximation enables us to include the effect of the highly excited single-particle states on the transition amplitudes for the pickup of a pair, e.g., $^{16}\mathrm{O}(p, t)^{14}\mathrm{O}$. Because the admixed states have wave functions which are large in the surface and exterior regions, they result in large enhancements of pickup cross sections. We have also studied the breakdown of the random-phase approximation, when eigenvalues become complex. We provide a proof, for an arbitrary two-body interaction, that the potential strength for which the random-phase approximation eigenvalues become complex, is the minimum strength for which nontrivial solutions to the Hartree-Fock-Bogolyubov equations exist.
TL;DR: In this article, a self-consistent RPA method is presented for N-particle Fermi systems with two-and three-body forces, which corresponds to the equation-of-motion method.
Abstract: A self-consistent RPA method is presented for N-particle Fermi systems with two- and three-body forces. The RPA equations for the generalised 'particle-hole' and 'particle-particle' problem, respectively, are derived by minimising the mean excitation energies defined analogously to mean single-particle energies. This corresponds to the equation-of-motion method. For the generalised 'one-particle/two-particle-one-hole problem' the Green-function scheme is used in order to establish a connection between two possible ways of treating the problem. It is shown that the RPA Hamiltonians have almost the same structure as in the case of pure two-body forces, and that Rowe's general RPA equations are contained as a special case of the treatment.
TL;DR: In this paper, a diagrammatic technique was developed for the estimation of the screened potential of π-electron systems, which was expanded in terms of the polarization propagators which were constructed from either the singlet or triplet vertex parts.
Abstract: A diagrammatic technique was developed for the estimation of the screened potential of π-electron systems. The screened potential was expanded in terms of the polarization propagators which were constructed from either the singlet,\(\hat \Gamma ^1 \), or triplet vertex part,\(\hat \Gamma ^3 \). These vertex parts correspond to the singlet or triplet excitations, respectively, in the Random Phase Approximation (RPA) containing exchange diagrams. The excitation energies were calculated by using the screened potential in the framework of RPA with exchange. The excitation energies of several conjugated molecules with or without a hetero atom are in agreement with the experimental data.
TL;DR: In this paper, the magnetic excitations in an itinerant antiferromagnet were investigated by using random phase approximation, and it was shown that electron-hole pair excitations exist in a low frequency and long wave region.
Abstract: The magnetic excitations in an itinerant antiferromagnet are investigated by using random phase approximation. It is shown that electron-hole pair excitations exist in a low frequency and long wave region. The spin waves decay into these modes and have an intrinsic width. Importance of the effect of the anisotropy is also pointed out.
TL;DR: In this article, a theory for collision-induced transitions in the subset of quantum states "ae" is developed, where the full molecular basis is "aeau" and the choice of ae and au is in general arbitrary, and it is shown how to make this separation based on the type of experimental measurement under consideration.
Abstract: A theory for collision‐induced transitions in the subset of quantum states ‖ae〉 is developed, where the full molecular basis is ‖aeau〉. The only assumption is that changes in au occur much more quickly than those in ae. The choice of ae and au is in general arbitrary, and it is shown how to make this separation based on the type of experimental measurement under consideration. For example, an evaluation of an atom–vibrotor problem (i.e., ae=n, au=jmj) is considered. In conjuction with this illustration an analysis of the breathing sphere (BS) approximation is carried out. It is concluded that a modified BS method (i.e., use of the diagonal rotational potential) should be valid when the random phase approximation holds for the rotational scattering amplitude.
TL;DR: In this article, the ARPA in its diagonal form has been employed to compute the linear response function for the molecules LiH, HF, H2O, NH3 and NH3.
TL;DR: In this article, it was shown that taking into account Coulomb interaction in random phase approximation (RPA) may qualitatively change the result of vibronic theory of ferroelectric phase transitions and reduce the softening of the longitudinal acoustic branch of phonon spectrum in a zero-gap semiconductor.
TL;DR: In this article, it was shown that the energy, the wave function and the single-particle density of the ground state are unaffected by the auxiliary potential, and that the singleparticle energies depend on U.
Abstract: Different possibilities in subtracting the centre-of-mass motion from the basic Hamiltonian,a priori with possible different results, motivated the present use of an auxiliary potentialU in the Hartree-Fock approximation. It is shown that the energy, the wave function and the single-particle density of the ground state are unaffected byU. On the contrary the single-particle energies depend onU. Therefore, the auxiliary potential may affect all the methods involving single-particle energies either for the ground state or the excited states. A discussion of its influence on the perturbation expansion, the Tamm-Dancoff and the random phase approximation is presented.
TL;DR: In this article, a quantum 'conserving' approximation evaluation of the Van Hove dynamic liquid structure factor, S(q, omega ), in normal fluids is presented, in terms of the sum of a large number of weighted delta functions of omega which are then widened out until they overlap sufficiently to produce a smooth curve.
Abstract: Considers a quantum 'conserving' approximation evaluation of the Van Hove dynamic liquid structure factor, S(q, omega ), in normal fluids. For a degenerate Fermi liquid the approximation simplifies to the generalised random phase approximation. Providing certain 'stability conditions' are obeyed, the 'conserving' S(q, omega ) will manifestly satisfy the f-sum rule, obey the detailed-balance condition and be positive for all values of omega . Also presents a detailed numerical evaluation of S(q, omega ) (and S(q)) for a model boson system and obtains S(q, omega ) in terms of the sum of a large number of weighted delta functions of omega which are then widened-out until they overlap sufficiently to produce a smooth curve. The results qualitatively resemble the behaviour of many monatomic fluids in so far as well-defined Rayleigh and Brillouin peaks are observed for low q and that for high q, the impulse approximation result is obtained.
01 Dec 1978
TL;DR: In this article, a model based on the approximate separability of particle-hole interaction matrix elements is solved in closed form and found to be in semi-quantitative agreement with experiment for the vibrations of the 4d shell in Xenon.
Abstract: Linear response theory is used to investigate the collective excitation spectra of the outer shells of heavy atoms. A model, based on the approximate separability of particle-hole interaction matrix elements, is solved in closed form and found to be in semi-quantitative agreement with experiment for the vibrations of the 4d shell in Xenon. The separability of these matrix elements in the relevant energy region is shown to be due to a general property of wave functions of atomic potentials. A comparison is also made with full random phase approximation calculations for Xe. It is shown that the schematic model contains all the relevant features to describe the enhanced photo-absorption of this system in the far ultraviolet. The role of anf-wave resonance in the average atomic field is stressed in connection with this enhancement. Detailed agreement between the schematic model and experiment is less good than that obtained from the full random phase calculation, the differences arising due to an approximate treatment of exchange in the former calculation. The total dipole strength in the photo-electric region is approximately the same for both calculations however and in reasonable accord with experiment.