Showing papers on "Random phase approximation published in 1977"

Screening Effect and Quantum Transport in a Silicon Inversion Layer in Strong Magnetic Fields

Abstract: The broadening of Landau levels and the transport quantities, such as the transverse and Hall conductivity and cyclotron resonance linewidth, are calculated in an inversion layer on the (100) surface of p -type Si at zero temperature. Main scattering mechanisms are assumed to be charged impurity scattering and surface roughness scattering. A new expression for the surface roughness scattering is obtained. The self-consistent Born approximation is employed for the effect of scattering, and the random phase approximation for the screening. Because of the singular density of states the screening depends on the position of the Fermi level and becomes weak when it lies at the tail region of each Landau level. Scattering potentials become strong and of slowly varying type in this case. Except such a special case, overall features agree with the results obtained for short-ranged scatterers.

The correlation energy in the random phase approximation: Intermolecular forces between closed‐shell systems

Abstract: A new expression for the correlation energy within the random phase approximation (RPA) is presented. It has the following properties: it is (1) size consistent, (2) invariant to unitary transformations of degenerate orbitals, (3) correct to second order in perturbation theory, and (4) when applied to a supermolecule comprised of two interacting closed‐shells, it describes the dispersive part of the interaction at the coupled Hartree–Fock (HF) level, i.e., the van der Waals’ coefficient extracted from its long‐range behavior is identical to that obtained from the Casimir–Polder expression using the dynamic coupled Hartree–Fock polarizabilities of the isolated systems. This expression, which requires only particle–hole two‐electron integrals for its evaluation, is expected to yield considerably more accurate potential energy curves between closed‐shell systems than second‐order Moller–Plesset perturbation theory which, as is shown, describes dispersion forces at the less accurate uncoupled HF level. In add...

Ab initio calculations of oscillator and rotatory strengths in the random‐phase approximation: Twisted mono‐olefins

Abstract: Ab initio (STO‐nG) computations of ordinary and rotatory intensities of low‐lying electronic transitions are presented for twisted ethylene and twisted trans‐2‐butene in the random‐phase approximation (RPA). The intensities are computed in both dipole length and dipole velocity forms, as well as the mixed form for the oscillator strength, and the convergence of these formally equivalent results is examined in the RPA and several other methods for constructing the electronic excitation: the virtual orbital, or single‐transition, approximation (STA), the monoexcited configuration‐interaction, or Tamm–Dancoff, approximation (TDA), and one version of the higher RPA (HRPA). We show that the RPA has consistent advantages over the TDA for calculation of CD as well as ordinary intensities. Our computations confirm that a localized, ethylenic chromophore is indeed adequate to account for the low‐lying CD spectrum in mono‐olefins. Further, even with minimal valence‐shell basis sets, our RPA rotatory strengths agree...

Spin susceptibilities and the theory of itinerant-electron antiferromagnetism

Abstract: This article investigates the equilibrium states of antiferromagnetic itinerant-electron systems in the Hartree-Fock approximation. As a result, the spin susceptibilities are determined in the random phase approximation. The lowlying collective excitations are then obtained by finding the poles of these susceptibilities. We start by giving a brief review of the Hartree-Fock procedure and by indicating how the susceptibilities are obtained. The density matrix approach, where the ground state is interpreted as that minimizing the energy, is used throughout. Using an effective Coulomb interaction of the Hubbard type we consider two distinct systems: a one-band system with an incommensurate spin density wave in its ground state, and a many-band simply commensurate model for f.c.c. manganese. The first of these is such that the band structure and resulting susceptibilities can be obtained explicitly. The spin-wave energies and wave-vectors are found by a careful, small energy and momentum transfer, ex...

Fine structure of the magnetic dipole states in /sup 208/Pb

Abstract: The splitting of the magnetic dipole states in $^{208}\mathrm{Pb}$ due to the admixture of two-particle, two-hole configurations is calculated. The qualitative structure of a one-particle, onehole calculation remains but about 30% of the $M1$ strength is distributed over many levels above 8 MeV. The influence of the spin-dependent part of the interaction on the results is critically discussed.

Plasma effects on the spontaneous emission of synchrotron radiation from weakly relativistic electrons

Abstract: A method for computing the spectral emissivity of spontaneous synchrotron radiation is discussed. The Klimontovich formalism in plasma kinetic theory is adopted in which an ensemble average of the microscopically emitted power is considered. The present method clarifies the meaning of the random phase approximation which is imposed in several existing theories of synchrotron radiation. Both the effects of dielectric polarization and two‐particle correlations are included in the present discussion. The theory is applied to the case of a plasma in thermal equilibrium, for which it is shown that the effect of pair correlations on the emissivity vanishes. On the other hand, the effect of dielectric polarization is studied numerically for a wide range of parameters.

Photosorption for sodium and magnesium atoms in the random-phase approximation with exchange

Abstract: The photoexcitation oscillator strengths and the photoionization cross sections for sodium and magnesium atoms are calculated in the random-phase approximation with exchange. The results obtained are in good agreement with other many-body calculations and with experimental data. A method of taking interchannel correlations into account is presented, and these correlations for 3s to nu p, 2p to nu d, 2p to nu s and 2s to nu p transitions are found to be important.

Surface plasma oscillations in layer metal particles

Abstract: Light scattering cross section and positions of two surface plasma resonances in a layer metal particle containing dielectric foreign nucleus are calculated within the random phase approximation. The results obtained provide a possible explanation of contradictions in size dependence of surface plasma resonance position [1, 2] and are able to explain two peak structure of photoabsorption cross section of small silver particle observed in [1].

Surface Plasmons in Thin Metal Films, Circular Cylinders, and Spherical Voids

Abstract: The influence of the size effect on the frequencies of the surface plasmons (SP) in thin films, cylinders, and spherical voids is studied within the random phase approximation. The cross sections for excitation of SP by fast electrons and X-rays and the photoabsorption cross sections are calculated. Es wird der Einflus des Size-Effekts auf die Frequenzen der Oberflachen-Plasmonen (SP) in dunnen Schichten, Zylindern und spharischen Hohlraumen in der “random-phase”-Naherung untersucht. Die Anregungsquerschnitte der SP durch schnelle Elektronen und Rontgenstrahlen sowie die Photoabsorptionsquerschnitte werden berechnet.

Application of the relativistic random-phase approximation to the photoionisation of atoms

Abstract: Exploratory calculations of atomic photoionisation cross sections including both relativistic and correlation effects are undertaken using the relativistic random-phase approximation (RRPA). In these preliminary calculations the authors examine the photoionisation of He and Be atoms. The cross sections and asymmetry parameters for these light elements agree precisely with non-relativistic RPA values. Elastic phaseshifts for the e-+He+ and e-+Be+ systems are obtained and agree with non-relativistic calculations in the 1P eigenchannel; the 3P phaseshifts have no counterpart in non-relativistic theory.

Thermodynamic properties of ferromagnets with uniaxial and biaxial anisotropy

Abstract: The thermodynamic properties of spin-one Heisenberg ferromagnets with uniaxial and biaxial anisotropy are investigated in the molecular field approximation (MFA) and random phase approximation (RPA). In MFA a full phase diagram comprising three lines of bicritical points, is obtained. Using the standard-basis operator method, the collective excitation spectrum is studied in detail; moreover the softening of the excitations at the phase boundaries is discussed. A self-consistent version of RPA with emphasis on the role of kinematic restrictions with regard to the standard-basis operators is analyzed. Moreover, the phase transitions are considered in the Ising model, where a line of tricritical points occurs, and the planar model, where a line of bicritical points, two lines of tricritical points and a line of triple points, occur.

Coupling of single particle and collective modes in a one-component plasma (theory applicable to liquids)

Abstract: Recent computer simulation studies of the one-component plasma have indicated that for large values of the plasma parameter the single-particle motion is strongly influenced by the density fluctuations. By making use of a form of the random phase approximation an expression is derived for the memory function associated with velocity correlation in which the coupling of the single particle and 'plasmon' modes is clearly exposed. Satisfactory quantitative agreement is obtained with the recent molecular dynamics studies for both the memory function and frequency spectrum.

Theory of systems with random Ising bond interactions in the paramagnetic phase

Abstract: A theory of the thermodynamic properties (spin correlation functions, susceptibility and specific heat) in the paramagnetic phase of Ising systems with various forms of random bond arrangement is developed. It represents an extension of the random phase approximation (RPA). The difference equation for the spin correlation functions which has random coefficients is solved using the coherent potential approximation (CPA), and the various parameters calculated self-consistently. Results for the quenched dilute bond case are in good agreement with other theories. For a truncated Gaussian distribution of bond strengths it is found that the transition temperature and other properties are fairly universal functions of the mean square of the distribution. The transition to a ferromagnetic phase disappears when the concentration of negative exchange bonds becomes large enough, apparently because of spin glass formation. Results for various binary distributions are also given.

Magnetic excitations in ferromagnets and antiferromagnets within the local exchange approximation

Abstract: An expression for the spin wave stiffness constant D is obtained within Callaway and Wang's [1] scheme for calculating the transverse spin susceptibility χ( q , ω) of a ferromagnet. The method is related to the random phase approximation and it is shown that the results are greatly simplified by neglecting local field effects. This approximation yields much simpler expressions for χ( q , ω) in the ferromagnetic, antiferromagnetic or ferrimagnetic case than are obtained in previous multiband schemes.

Microscopic Lagrangian description of warm plasmas: 3. Nonlinear wave‐particle interaction

Abstract: The averaged-Lagrangian method is applied to nonlinear wave-particle interactions in an infinite, homogeneous, magnetic-field-free plasma. The specific example of Langmuir waves is considered, and the combined effects of four-wave interactions and wave-particle interactions are treated. It is demonstrated how the latter lead to diffusion in velocity space, and the quasilinear diffusion equation is derived. The analysis is generalized to the random phase approximation. The paper concludes with a summary of the method as applied in Parts 1-3 of the paper.

Local Field Correction and Acoustic Sum Rule for Si and Ge

Abstract: The microscopic inverse dielectric matrix in the limit of a long wavelength is calculated in the random phase approximation for Si and Ge by rushing the realistic band models. It is shown that, if a proper form of the pseudopotential is employed, the acoustic sum rule can be substantially satisfied without any adjustable parameter, and the local field correction to the static dielectric constant can not be neglected.

A self-consistent theory for exchange and correlation effects in an electron gas

Abstract: A self-consistent theory for electron correlations formulated so as to satisfy both the compressibility and spin susceptibility sum rules within the framework of the so-called generalised random phase approximation, is used for studying numerically correlation effects on electron gas properties at metallic densities. The physical implication of the self-consistency requirement in the theory is discussed from the diagrammatic viewpoint of perturbation theory.

Spin Waves in Itinerant Electron Antiferromagnet

Abstract: In the Skater model for itinerant electron antiferromagnets, spin wave spectrum is calculated by taking into account long range Coulomb interactions. The method of normal mode is used in the random phase approximation in order to get various modes of spin waves, i.e., acoustical and polar spin waves. It is shown that polar spin waves exist when the inter-atomic Coulomb interaction is sufficiently strong. Furthermore, the velocity of the acoustical spin wave is shown to be reduced by the inter-atomic Coulomb and exchange interactions. The magnitudes of these reductions are numerically estimated for an antiferromagnet with b.c.c. structure. It is shown that the observed result of the spin wave velocity for Cr is explained by these numerical results.

Theory of Fluctuations in Quasi-One-Dimensional Electron-Phonon Systems

Abstract: The effect of fluctuations on the Peierls transition in quasi-one-dimensional systems is investigated within a renormalizecl random phase approximation in the half-filled tight-binding model. The fluctuations are divided into two parts; thermal fluctuations and zero point fluctuations. It is shown that the effect of the thermal fluctuations is always dominant even if the transition temperature is extremely low.

Derivation of Random Phase Approximation on the Basis of the Generalized Schwinger Representation of the Fermion-Pairs

Abstract: A derivation of the random phase approximation is given, which is hased on the gener­ alized Schwinger representation of the fermion-pairs. The method is made of two stages: First, the truncation of the boson space is clone, and secondly comes the separation o£ boson operators into static and fluctuating parts. The pairing vibration in the super phase and the so-called quadrupole oscillation are treated by the method, which leads to the same results as those of RPA. Then the relations of these bosons to on"'s appearing in RPA are clarified.