Showing papers on "Random phase approximation published in 1985"
TL;DR: It is argued that the modes contribute to the cooling of the quark-gluon plasma by being analogous to the fluctuations of the order parameter in a superconductor above the critical point.
Abstract: Fluctuations of the order parameter of chiral transition in a hot and dense quark gas are examined in the random-phase approximation with the use of a QCD-motivated effective Lagrangian. We show that there arise soft modes having a large strength and a narrow width above the critical temperature, which are analogous to the fluctuations of the order parameter in a superconductor above the critical point. It is argued that the modes contribute to the cooling of the quark-gluon plasma.
235 citations
01 Jul 1985-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this paper, the response of an atom to an imposed harmonic perturbation is determined by applying the time-dependent variational principle to a multiconfiguration wave function constructed from Dirac orbitais.
Abstract: Multiconfiguration relativistic random-phase approximation has been developed to describe excitations of an atomic system having a multiconfiguration ground state. The response of such an atom to an imposed harmonic perturbation is determined by applying the time-dependent variational principle to a multiconfiguration wave function constructed from Dirac orbitais. Terms in the wave function independent of the external field lead to the multiconfiguration Dirac-Fock description of the ground state. Terms proportional to the external field lead to a multiconfiguration generalization of the relativistic random-phase approximation. Application of the multiconfiguration relativistic random-phase approximation theory to resonance transitions in Mg- and Zn-like ions are presented.
56 citations
TL;DR: In this article, the nuclear response functions for quasielastic electron scattering are investigated in the random phase approximation for the relativistic lagrangian of the σω model.
45 citations
TL;DR: A model of spin-polarized fermions hopping on a two-dimensional lattice with a nearest-neighbor interaction V is studied, and it is concluded that a method suitable for the weak-coupling regime is needed to complete the phase diagram.
Abstract: A model of spin-polarized fermions hopping on a two-dimensional lattice with a nearest-neighbor interaction V is studied. Random-phase-approximation calculations predict that the half-filled system undergoes a density-wave transition for positive values of V, an odd-angular-momentum pairing transition for small negative V, and a condensation-phase-separation transition for more negative values of V. The classical lattice-gas Ising limit matches onto the density-wave transition for Vg0 and the condensation transition for Vl0. A strong-coupling expansion in powers of the ratio of single-fermion transfer-matrix element t to the two-body interaction V provides the leading corrections to the Ising limit. In order to explore the intermediate-coupling regime, fermion Monte Carlo calculations were carried out and various Green's functions characterizing the quantum correlations evaluated. With use of finite-size scaling techniques, the density-wave and condensation phase boundaries were followed into the intermediate-coupling regime. At these transitions, measured quantities scaled well with the usual Ising indices. Unfortunately, the weak-coupling regime lies beyond the reach of these simulations, and we conclude that a method suitable for the weak-coupling regime is needed to complete the phase diagram.
42 citations
40 citations
TL;DR: In this paper, the authors apply the quantum dynamical Monte Carlo technique to the calculation of the complex time correlation function of the flux through a SQUID, and compare the results with those of three representative theories: a random phase approximation (RPA), an effective adiabatic approximation (EA), and an optimized Random Phase approximation (ORPA).
Abstract: We apply the quantum dynamical Monte Carlo technique to the calculation of the complex time correlation function of the flux through a SQUID, and compare the results with those of three representative theories: a random phase approximation (RPA), an effective adiabatic approximation (EA), and an optimized random phase approximation (ORPA). The RPA quickly becomes unreliable as the damping is increased beyond a negligible amount. For small coupling, the variational information contained in the EA theory results in superior predictions of initial decay rates, but for moderate coupling the inherent nonergodicity of the theory leads to incorrect results for the long time behavior of the correlation function. The ORPA fails to renormalize the initial frequency properly, but succeeds in predicting the correct qualitative behavior even at moderately large values of the damping. This illustrates the power of the QDMC as a benchmark against which to test approximations, and as a bridge between their regions of applicability.
34 citations
TL;DR: It is shown that the use of the random-phase approximation to determine the response of a metal surface to a static uniform external field in conjunction with a local-density description of exchange and correlation in the ground state violates the exact force sum rule.
Abstract: It is shown that the use of the random-phase approximation (RPA) to determine the response of a metal surface to a static uniform external field in conjunction with a local-density description of exchange and correlation in the ground state violates the exact force sum rule. As a result, the static image plane is located considerably closer to the surface and the external field is less effectively screened than in a consistent treatment of exchange and correlation in both ground state and response. In addition, this implementation of the RPA violates the so-called surface f-sum rule, giving a too low number of electrons available for excitation near a metal surface. As a consequence of these violations, the first and first-inverse frequency moment of the imaginary part of the dynamic induced charge as well as its small-frequency behavior are incorrectly described.
31 citations
TL;DR: The theoretical cross sections calculated withrandom phase approximation wave functions, which provide the lower limit of spin-isospin strength, describe the data rather well at all scattering angles, while the random phase approximation +..delta.. cross sections underestimate the data at forward angles.
Abstract: A microscopic analysis of the complete forward angle spectra of the /sup 90/Zr(p,n) reaction is presented for an incident energy of 200 MeV. It is shown that the whole spectra up to an excitation energy of E/sub x/ = 70 MeV are the result of correlated one-particle--one-hole (1p1h) spin-isospin transitions only. The spectra reflect, therefore, the linear spin-isospin response of the target nucleus to the probing (p,n) field. Two different cross section calculations are performed: one with usual random phase approximation wave functions and one with generalized random phase approximation wave functions which include ..delta.. isobar degrees of freedom explicitly. We find that the theoretical cross sections calculated with random phase approximation wave functions, which provide the lower limit of spin-isospin strength, describe the data rather well at all scattering angles, while the random phase approximation +..delta.. cross sections underestimate the data at forward angles. In this connection we discuss the quenching of the spin-isospin strength in detail.
27 citations
TL;DR: In this paper, a set of finite-temperature RPA equations is obtained within the framework of the quasiparticle-phonon nuclear model using a separable multipole interaction.
Abstract: A set of finite-temperature RPA equations is obtained within the framework of the quasiparticle-phonon nuclear model using a separable multipole interaction. The calculations have been performed for the isovector dipole and isoscalar quadrupole modes in 58Ni. The author finds that the centroids of the giant resonances undergo a bending in the critical temperature region where the collapse of the pairing gap takes place.
19 citations
TL;DR: The projected random phase approximation (PRPA) as discussed by the authors is derived from a generator coordinate ansatz, which allows the calculation of excited states in the region of phase transitions, where conventional RPA breaks down.
18 citations
Metz1
TL;DR: In this paper, an analytical form of the direct correlation function is proposed to determine the structure factor of liquid alkali metals in the OCP reference system, assuming that the interionic interaction can be modeled by the effective pair potential in the model potential approach.
TL;DR: In this article, an optimized random phase approximation from an expansion of the free energy in terms of the generating functional was derived for a quantum mechanical tunneling system coupled to a Gaussian bath.
Abstract: We derive an optimized random phase approximation from an expansion of the free energy in terms of the generating functional. We apply the theory to a quantum mechanical tunneling system coupled to a Gaussian bath. The theory reproduces both short‐time behavior and long‐time averages of (real time) correlation functions, and compares favorably with both nonoptimized RPA and the cumulant expansion.
TL;DR: In this article, the stopping power of the degenerate electron liquid at metallic densities in the dielectric formalism was calculated and the strong Coulomb-coupling effects beyond the random phase approximation were taken into account through the static and dynamic local field corrections.
Abstract: We calculate the stopping power of the degenerate electron liquid at metallic densities in the dielectric formalism. The strong Coulomb-coupling effects beyond the random-phase approximation are taken into account through the static and dynamic local-field corrections. It is shown that those strong-coupling and dynamic effects act to enhance the stopping power substantially in the low-velocity regime, leading to an improved agreement with experimental data.
TL;DR: In this article, the authors used softmode theory in the random phase approximation as well as a static molecular-field model to obtain a magnetic ordering between 1 and 2 K with field strengths ranging from 30 to 50 kG.
TL;DR: In the random phase approximation, the effective mass effect is cancelled by the particle-hole correlation due to the σ-meson exchange, and the ω -meson also plays an important role in the response function as discussed by the authors.
TL;DR: In this article, the structure of the liquid simple metals from Li to Pb has been analyzed based on an optimized random phase approximation and on effective interatomic pair potentials derived from pseudopotential perturbation theory.
Abstract: We present calculations of the structure of the liquid alkali-metals, based on the optimized random phase approximation and on effective interatomic pair potentials derived from pseudopotential perturbation theory. In our approach the atomic interactions are decomposed self-consistently into a repulsive short-ranged part and a weak long-range perturbation, the repulsive part being modelled by a hard-sphere reference system via the Weeks-Chandler Andersen functional expansion. We show detailed comparisons with computer-simulation data and with experiment, demonstrating that the optimized random phase approximation with a hard-sphere reference system applies equally well to the liquid alkali metals as to the liquid polyvalent metals. Thus it is a comprehensive description of all liquid simple metals from Li to Pb. The conceptual advantages of this method as compared to other approaches are discussed.
02 Dec 1985
TL;DR: In this paper, a detailed theoretical model for the optical properties of heavily dopQd oxide semiconductors was formulated and applied to calculate their integrated luminous, solar and thermal properties.
Abstract: We formulate a detailed theoretical model for the optical properties of heavily dopQd oxide semiconductors. The complex dielectric function is expressed as e = 1+χVE+χFC+χPH, where the χs are susceptibilities due to valence electrons (VE), free carriers (FC), and polar optical phonons (PH). We obtained χVE by including a wavelength-shifted Urbach tail of the direct semiconductor transition. χFC was calculated from the Gerlach-Grosse theory and accounting for screening of point-like ions within the Random Phase Approximation. Pseudo-potential scattering as well as exchange and correlation effects in the electron gas were considered. χPH was specified through emipirically determined damped Lorentz oscillators. The model was applied to In203:Sn coatings and used to compute their integrated luminous, solar and thermal properties. Optimization of thickness and electron density yielded single coatings on glass with 78% normal solar transmittance and 20% hemispherical thermal emittance.
TL;DR: In this paper, a self-consistent RPA theory with the SK3 interaction is proposed to predict cross sections and asymmetries for the nucleus 16 O (e,e'x) reaction.
25 Mar 1985
TL;DR: In this article, the Coulomb effects on cyclotron resonance in two-dimensional electron systems are investigated based on a self-consistent approach which improves the random-phase approximation.
Abstract: Coulomb effects on cyclotron resonance in two-dimensional electron systems are investigated based on a self-consistent approach which improves the random-phase approximation. The memory function in the dynamic magnetoconductivity depends on the electron density not only through the filling factor v but also on its combination with the dimensionless density parameter r /sub s/ in the form r /sub s/ v to the 3/2 power. The memory function reproduces the data of Wilson, Allen and Tsui for intermediate densities.
TL;DR: In this article, the dispersion equation for acoustic plasmons in a high-density degenerate two-component Fermi gas is given and analyzed, with exact analytic continuation from the upper to the lower half of the complex frequency plane.
Abstract: The dispersion equation for acoustic plasmons in a high-density degenerate two-component Fermi gas is given and analysed. The random phase approximation (RPA) dielectric function is used, with exact analytic continuation from the upper to the lower half of the complex frequency plane. For small wavevector the acoustic plasmon frequency and damping depend on only two parameters, the ratio of Fermi velocities and the ratio of Thomas-Fermi wavevectors. A large mass ratio is not necessary for weak damping.
TL;DR: The present approach to the temperature dependent random-phase approximation method offers a remedy to some of the limitations inherent in other existing formulations of the theory.
Abstract: The T = 0 coordinate-space linear response random-phase approximation method is generalized to finite temperatures. Continuum effects are included through the temperature single-particle Green's functions. The method is utilized to investigate the temperature dependence of isovector electric dipole excitations in /sup 40/Ca. The experimentally observed downward shift of the giant resonance and the spreading of the dipole strength as temperature increases are clearly reflected in the calculated cross sections. The present approach to the temperature dependent random-phase approximation method offers a remedy to some of the limitations inherent in other existing formulations of the theory.
TL;DR: In this paper, the longitudinal dielectric response function of a relativistic gas consisting of charged bosons together with the corresponding antiparticles is obtained in the random phase approximation, and an analysis of the mode structure and damping properties of the plasma is carried out.
Abstract: The longitudinal dielectric response function of a relativistic gas consisting of charged bosons together with the corresponding antiparticles is obtained in the random phase approximation. The theory is renormalized and an analysis of the mode structure and damping properties of the plasma is carried out.
TL;DR: In this article, a lattice dynamics sheme is developed for handling large amplitude librational motions or hindered rotations, anharmonic translational vibrations and translation-rotation coupling in molecular crystals.
Abstract: We have developed a new lattice dynamics sheme for handling large amplitude librational motions or hindered rotations, anharmonic translational vibrations and translation-rotation coupling in molecular crystals. This formalism is based on expanding the intermolecular potential in the molecular displacements and including the cubic and higher terms, while retaining its exact anisotropy. This potential is first used to construct mean field states for the molecular translations and hindered rotations, and next to solve the equations of motion for the crystal in the random phase approximation (RPA), which takes into account the correlations between the molecular motions as well as translation-rotation coupling. It is illustrated that this scheme gives very accurate results for the ordered a and 7 phases of solid nitrogen, while It also yields, for the first time, a quantum dynamical description of the molecular motions in the plastic s phase and a fairly accurate α—s transition temperature.
TL;DR: In this article, a consistent propagator approximation, denoted as the excitation propagator, is introduced, which describes excitations between N-particle states and its approximation has properties required of consistent random phase approximation schemes.
Abstract: A consistent propagator approximation, denoted as the excitation propagator, is introduced. This propagator describes excitations between N-particle states and its approximation has properties required of consistent random phase approximation schemes. Several properties of this propagator are explored when based on a generalized antisymmetrized geminal power wavefunction. How singularities in the metric occur and how to remove them is discussed in detail. The excitation propagator is also contrasted with the principal (polarization) propagator.
TL;DR: In this article, the authors compared the Henderson-Ashcroft mean density approximation (MDA) with the random phase approximation (RPA) and concluded that the MDA still exceeds the RPA.
Abstract: In connection with the recent criticism by Hafner (see Non-Cryst. Solids, vol.61-62, p.175, 1984) of the Henderson-Ashcroft mean density approximation (MDA), the Helmholtz free energy is calculated through a different thermodynamic relation. The resultant expression for the free energy is clearly beyond that of the random phase approximation (RPA). By comparing both theories on the same footing, it is concluded that the MDA still exceeds the RPA.
TL;DR: In this paper, the electron scattering form factor for excitation of the 1+ state of 88Sr at 3.486 MeV has been calculated in the quasiparticle random phase approximation (QRPA).
TL;DR: In this paper, a theoretical analysis of the local field corrections for the dielectric function of a two-dimensional electron gas is presented, and both static and dynamic results are related to other electronic properties.
Abstract: A theoretical analysis of the local field corrections for the dielectric function of a two-dimensional electron gas is presented. Static and dynamic results are presented and related to other electronic properties.