Dynamical current–current correlation in two-dimensional parabolic Dirac systems

We theoretically investigate the current–current correlation of the two-dimensional (2D) parabolic Dirac system in hexogonal lattice. The analytical expressions of the random phase approximation (RPA) susceptibility, Ruderman–Kittel–Kasuya–Yosida (RKKY) Hamiltonian, and the diamagnetic orbital susceptibility in noninteracting case base on the density–density or current–current correlation function are derived and quantitatively analyzed. In noninteracting case, the dynamical polarization within RPA, and spin transverse susceptibility as well as the RKKY interaction (when close to the half-filling) are related to the current–current response in the 2D parabolic Dirac systems. Both the cases of anisotropic dispersion and isotropic dispersion are discussed.     

First principles study on spin and orbital magnetism in CuFeSb and CuCoSb alloys with C1b structure

In this work the effect of spin–orbit coupling and orbital polarization (OP) corrections on the spin and orbital magnetism of two half-Heusler alloys (CuFeSb and CuCoSb) is investigated by means of local spin density calculations. It is demonstrated that OP corrections predict large orbital moments for Fe and Co individual atoms in the considered compounds. It is found that the Heusler alloys with C1_b structure are potential candidates to show large orbital magnetism.     

Orbital Order and Orbital Excitations in Degenerate Itinerant Electron Systems

We present a theory of orbital ordering in orbital-degenerate itinerant electron systems. The orbital instability in a two-orbital degenerate Hubbard model is investigated in the random phase approximation （RPA）. After demonstrating the criteria for the formation of orbital ordering or the orbital density wave ordering, we find that the orbital and the spin-orbital collective excitation spectra in the ferro-orbital ordered phase exhibit finite gaps. The possible application of the present theory in orbital-ordered 4d compounds is also discussed.     

Strongly enhanced orbital moments and anisotropies of adatoms on the Ag(001) surface

We present ab initio calculations for orbital moments and anisotropy energies of 3d and 5d adatoms on the Ag(001) surface, based on density functional theory, including Brooks' orbital polarization (OP) term, and applying a fully relativistic Korringa-Kohn-Rostoker-Green's function method. In general, we find unusually large orbital moments and anisotropy energies, e.g., in the 3d series, 2.57 mu(B) and +74 meV for Co, and, in the 5d series, 1.78 mu(B) and +42 meV for Os. These magnetic properties are determined mainly by the OP and even exist without spin-orbit coupling.     

Nuclear matter properties in the relativistic random phase approximation

Nuclear matter properties at zero temperature are studied in terms of the relativistic σ-ω model, including the random phase approximation (RPA) contribution. At normal density, the medium polarization adds about −35 MeV of binding energy to the mean field result. The scalar and vector effective interaction strengths are fitted to the nuclear matter saturation conditions under various approximations for the energy functional including the RPA term. The effective mass and bulk modulus are calculated with these parameters. The relative importance of different contributions to the binding energy is analyzed.     

Dominance of the excitation continuum in the longitudinal spectrum of weakly coupled Heisenberg S=1/2 chains

A low-field spin-flop transition in the quasi-one-dimensional antiferromagnet BaCu2Si2O7 is exploited to study the polarization dependence of low-energy magnetic excitations. The measured longitudinal spectrum is best described as a single broad continuum, with no sharp "longitudinal mode," in apparent contradiction with the commonly used chain-mean-field and random phase approximation (MF/RPA) theories. The observed behavior is also quite different than that previously seen in the related KCuF3 material, presumably due to a large difference in the relative strength of interchain interactions. The results highlight the limitations of the chain-MF/RPA approach.     

Spin–orbit interaction of photons and fine splitting of levels in ring dielectric resonator

We consider eigenmodes of a ring resonator made of a circular dielectric waveguide. Taking into account the polarization corrections, which are responsible for the interaction of polarization and orbital properties of electromagnetic waves (spin–orbit interaction of photons), results in fine splitting of the levels of scalar approximation. The basic features of this fine structure of the levels are similar to that of electron levels in an atom. Namely: (1) sublevels of the fine structure are defined by an additional quantum number: product of helicity of the wave and its orbital moment; (2) for a waveguide with a parabolic profile of the refractive index each level of the scalar approximation splits into N sublevels (N is the principal quantum number), while for any other profile it splits into 2 sublevels; (3) each level of the fine structure remains twice degenerated due to local axial symmetry of the waveguide. Numerical estimations show that the described fine splitting of levels may be observed in optic–fiber ring resonators.     

Coulomb Screening and Plasmon Excitation in Graphene at Finite Temperature

By using the random phase approximation (RPA) in many-body perturbation theory, we calculate the polarization function of the electron gas in graphene at finite temperature. Based on this, we calculate the temperature dependent dielectric function ε(q). The thermal effect on ε(q) in various q regions is discussed. The temperature dependence is found to be quadratic. We also investigate the plasmon dispersion relation at finite temperature, with the zero-temperature relation as a special case. The result is in good agreement with recent experimental data.     

Calculation of lower excited electronic levels of benzene with the Green's function method

The Green's function method for the calculation of orbital energies which was discussed in a preceding paper is applied to the calculation of the lower excited states of the benzene molecule. The Dyson equation for the polarization (particle—hole) propagator is derived with the diagrammatic technique and the secular equation which gives poles of the polarization propagator is solved. The random phase approximation is not used in our calculation. Theoretical results are discussed in comparison with experimental data. Using a model system of two π-electrons, our method is compared with the equations-of-motion method.     

From sum rules to RPA: 3. Optical dipole response in metal clusters

The properties of plasmon resonances in metal clusters are computed within the random phase approximation (RPA), starting from the Kohn-Sham ground state within the jellium model. The paper aims at a systematic survey of the general trends with varying parameters such as electron number, angular momentum, Wigner-Seitz radius etc. To this end we exploit the flexibility of a particular RPA scheme which allows to switch easily between different levels of approximation.     

Effect of Polarization in （e, 2e） Ionization of Argon

The triple differential cross section of the Ar 2p orbital in a highly asymmetric geometry is calculated by using a modified distorted wave Born approximation method. A short-range polarization potential via density-functional theory is included in our calculations. It is shown that polarization potential is particularly important for calculations of the triple differential cross section.     

Theoretical calculation of triple differential cross sections of 3s orbital of argon in coplanar symmetric （e, 2e） reaction

The triple differential cross section for the low-energy electron impact ionization of inner-valence 3s orbital of argon has been calculated using the modified distorted wave Born approximation in coplanar symmetric energy-sharing geometry. Satisfactory agreement between theory and experiraent is achieved when the polarization and post-collisional interaction (PCI) are included in the calculations. It is shown that the polarization and PCI effects play a very important role in the case of argon at low incident energies.     

Theory of magnetic excitations in KCoF3 at finite temperatures

Magnetic excitations in KCoF₃ at finite temperatures are studied by taking into account the residual orbital moment of the Co²⁺ ions. Equations of motion of Green functions related to all excitation operators between the molecular field states of the ⁴T₁ symmetry of the Co²⁺ ions are solved using the RPA approximation, and temperature-dependent dispersion curves are calculated.     

The influence of dynamical correlations in semiconductor plasmas on optical spectra

On the basis of a nonequilibrium Green's function technique we determine a Bethe-Salpeter equation describing the linear response of a semiconductor under plasma excitation. Dynamical correlations entering the effective screenede-h interaction are treated on the basis of the full RPA polarization function. Numerical results for optical gain and absorption spectra are presented for various temperatures and densities. The validity of the commonly used plasmon pole approximation with regard to the lineshape of optical spectra is critically investigated.     

The Landau-Migdal parameters from the Brueckner theory

The zero-order Landau-Migdal parameters are discussed in the framework of the Brueckner-Hartree-Fock approximation with realistic two- and three-body forces. Their usefulness is proved in two applications. First, the structure functions in high-density nuclear matter are calculated within the linear response theory to weak probes and the neutrino mean free path is predicted for neutron stars. Second, the screening of the low-density neutron matter to the neutron pairing is calculated in the RPA without and with induced interaction. The extension of the calculation to symmetric nuclear matter reveals the anti-screening effect of the proton medium polarization.     

RPA calculation of the effective charge in a hartree-fock basis

Using a Hartree-Fock basis the diagrams of the Tamm-Dancoff and random-phase approximations have been summed to calculate the effective charge in the (1s0d) shell. The enhancement of the effective charge found by this summation is much less than in an oscillator basis, in fact in most cases the HF RPA result is smaller than the value obtained from the lowest order core polarization diagram in an oscillator basis. The HF neutron effective charges are thus too small. For protons the enhancement of the bare effective charge in a HF basis must also be taken into account, but for cases involving the 0d_5/2 orbital the final result appears rather small.     

Time-dependent Gutzwiller approximation for the Hubbard model

We develop a time-dependent Gutzwiller approximation (GA) for the Hubbard model analogous to the time-dependent Hartree-Fock (HF) method. This new formalism incorporates ground state correlations of the random phase approximation (RPA) type beyond the GA. Static quantities like ground state energy and double occupancy are in excellent agreement with exact results in one dimension up to moderate coupling and in two dimensions for all couplings. We find a substantial improvement over traditional GA and HF+RPA treatments. Dynamical correlation functions can be computed and are also substantially better than HF+RPA ones and obey well behaved sum rules.     

Phase Separation of Penetrable Core Mixtures

A two-component system of penetrable particles interacting via a gaussian core potential is considered, which may serve as a crude model for binary polymer solutions. The pair structure and thermodynamic properties are calculated within the random phase approximation (RPA) and the hypernetted chain (HNC) integral equation. The analytical RPA predictions are in semi-quantitative agreement with the numerical solutions of the HNC approximation, which itself is very accurate for gaussian core systems. A fluid-fluid phase separation is predicted to occur for a broad range of potential parameters. The pair structure exhibits a nontrivial clustering behaviour of the minority component. Similiar conclusions hold for the related model of parabolic core mixtures, which is frequently used in dissipative particle dynamics (DPD) simulations.     

Some consequences of relativistic effects upon optical spectra

Large spin–orbit interaction produces large orbital magnetic moments in narrow energy bands. Since the orbital character of the wave functions is more important in orbital than in spin magnetism, the limitations of the local spin density approximation become evident. It is possible to keep the orbital dependence of the exchange interactions by using an orbital polarization scheme or by using Hartree–Fock theory with screened Slater integrals for exchange. This leads to an enhancement of the calculated orbital moment when the magnetism is strong. Recently calculated magnetic moments and calculated sum rules for X-ray magnetic circular dichroism in US are described. Received: 23 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Ab initio many-body treatment of the electronic structure of metals

We propose and apply a combination of an ab initio (band-structure) calculation with a many-body treatment including screening effects. We start from a linearized muffin-tin orbital (LMTO) calculation to determine the Bloch functions for the Hartree one-particle Hamiltonian, from which we calculate the static susceptibility and dielectric function within the standard random phase approximation (RPA). From the Bloch functions we obtain maximally localized Wannier functions, using a method proposed by Marzari and Vanderbilt. Within this Wannier basis all relevant one-particle and unscreened and screened Coulomb matrix elements are calculated. This yields a multi-band Hamiltonian in second quantization with ab initio parameters, for which screening has been taken into account within the simplest standard approximation. Then, established methods of many-body theory are used. We apply this concept to a simple metal, namely lithium (Li). Here the maximally localized Wannier functions turn out to be of the sp³-orbital kind. Furthermore, only the on-site contributions of the screened Coulomb matrix elements are relevant, and a generalized, four-band Hubbard model is justified. The screened on-site Coulomb matrix elements are considerably smaller than the band width because of which it is sufficient to calculate the selfenergy in weak-coupling approximation. We compare results obtained within the screened Hartree-Fock approximation (HFA) and within the second-order perturbation theory (SOPT) in the Coulomb matrix elements for Li and find that many-body effects are small but not negligible even for this simple metal.