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Showing papers on "Random phase approximation published in 2006"


Journal ArticleDOI
TL;DR: In this paper, the polarization of graphene is calculated exactly within the random phase approximation for arbitrary frequency, wavevector and doping, and the dynamical polarization for low q and arbitrary ω is employed to calculate the dispersion relation and the decay rate of plasmons and acoustic phonons.
Abstract: The polarization of graphene is calculated exactly within the random phase approximation for arbitrary frequency, wavevector and doping. At finite doping, the static susceptibility saturates to a constant value for low momenta. At q = 2kF it has a discontinuity only in the second derivative. In the presence of a charged impurity this results in Friedel oscillations which decay with the same power law as the Thomas–Fermi contribution, the latter being always dominant. The spin density oscillations in the presence of a magnetic impurity are also calculated. The dynamical polarization for low q and arbitrary ω is employed to calculate the dispersion relation and the decay rate of plasmons and acoustic phonons as a function of doping. The low screening of graphene, combined with the absence of a gap, leads to a significant stiffening of the longitudinal acoustic lattice vibrations.

953 citations


Journal ArticleDOI
TL;DR: In this paper, the current status of the incompressibility coefficient of symmetric nuclear matter was determined from experimental data on isoscalar giant monopole and dipole resonances (compression modes) in nuclei, by employing the microscopic theory based on the random phase approximation (RPA).
Abstract: Accurate assessment of the value of the incompressibility coefficient, K, of symmetric nuclear matter, which is directly related to the curvature of the equation of state (EOS), is needed to extend our knowledge of the EOS in the vicinity of the saturation point. We review the current status of K as determined from experimental data on isoscalar giant monopole and dipole resonances (compression modes) in nuclei, by employing the microscopic theory based on the random-phase approximation (RPA).

230 citations


Journal ArticleDOI
TL;DR: Self-consistent results confirm substantially the results for Si and other semiconductors obtained perturbatively and extend the conclusion to LiF and Ar, a wide-gap insulator and a noble-gas solid.
Abstract: Theoretically the Kohn-Sham band gap differs from the exact quasiparticle energy gap by the derivative discontinuity of the exchange-correlation functional. In practice for semiconductors and insulators the band gap calculated within any local or semilocal density approximations underestimates severely the experimental energy gap. On the other hand, calculations with an “exact” exchange potential derived from many-body perturbation theory via the optimized effective potential suggest that improving the exchange-correlation potential approximation can yield a reasonable agreement between the Kohn-Sham band gap and the experimental gap. The results in this work show that this is not the case. In fact, we add to the exact exchange the correlation that corresponds to the dynamical random phase approximation screening in the GW approximation. This accurate exchange-correlation potential provides band structures similar to the local density approximation with the corresponding derivative discontinuity that contributes 30%–50% to the energy gap. Our self-consistent results confirm substantially the results for Si and other semiconductors obtained perturbatively R. W. Godby et al., Phys. Rev. B 36, 6497 1987 and extend the conclusion to LiF and Ar, a wide-gap insulator and a noble-gas solid. © 2006 American Institute of Physics. DOI: 10.1063/1.2189226

153 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of medium polarization on pairing in neutron and nuclear matter were studied and the screening potential was calculated in the RPA limit, suitably renormalized to cure the low density mechanical instability of nuclear matter.
Abstract: Effects of medium polarization are studied for ${}^{1}{S}_{0}$ pairing in neutron and nuclear matter. The screening potential is calculated in the RPA limit, suitably renormalized to cure the low density mechanical instability of nuclear matter. The self-energy corrections are consistently included resulting in a strong depletion of the Fermi surface. All medium effects are calculated based on the Brueckner theory. The ${}^{1}{S}_{0}$ gap is determined from the generalized gap equation. The self-energy corrections always lead to a quenching of the gap, which is enhanced by the screening effect of the pairing potential in neutron matter, whereas it is almost completely compensated by the antiscreening effect in nuclear matter.

128 citations


Journal ArticleDOI
TL;DR: In imaginary potentials and gradual attenuation by a mask function, absorbing bounds turn out to provide a reliable alternative to fully fledged continuum random phase approximation calculations, although rather large absorbing bounds are required to simulate reliably well continuum conditions.
Abstract: Absorbing boundary conditions are often employed in time-dependent mean-field calculations to cope with the problem of emitted particles which would otherwise return back onto the system and falsify the dynamical evolution We scrutinize two widely used methods, imaginary potentials and gradual attenuation by a mask function To that end, we consider breathing oscillations of a $^{16}\mathrm{O}$ nucleus computed on a radial one-dimensional grid in coordinate space The most critical test case is the computation of resonance spectra in the (linear) domain of small amplitude motion Absorbing bounds turn out to provide a reliable alternative to fully fledged continuum random phase approximation (RPA) calculations, although rather large absorbing bounds are required to simulate reliably well continuum conditions We also investigate the computation of observables in the nonlinear domain This regime turns out to be less demanding Smaller absorbing margin suffice to achieve the wanted absorption effect

78 citations


Journal ArticleDOI
TL;DR: In this article, the Hartree-Fock and generalized random phase approximations are derived and the dynamic polarizability is calculated for spinless fermions with weak $p$-wave interactions.
Abstract: Correlation functions related to the dynamic density response of the one-dimensional Bose gas in the model of Lieb and Liniger are calculated. An exact Bose-Fermi mapping is used to work in a fermionic representation with a pseudopotential Hamiltonian. The Hartree-Fock and generalized random phase approximations are derived and the dynamic polarizability is calculated. The results are valid to first order in $1∕\ensuremath{\gamma}$, where $\ensuremath{\gamma}$ is Lieb-Liniger coupling parameter. Approximations for the dynamic and static structure factor at finite temperature are presented. The results preclude superfluidity at any finite temperature in the large-$\ensuremath{\gamma}$ regime due to the Landau criterion. Due to the exact Bose-Fermi duality, the results apply for spinless fermions with weak $p$-wave interactions as well as for strongly interacting bosons.

53 citations


Journal ArticleDOI
TL;DR: In this article, the screening properties of the two-dimensional electron gas with Rashba spin-orbit coupling were studied and the required polarization operator was calculated in an analytic form without any approximations.
Abstract: We study screening properties of the two-dimensional electron gas with Rashba spin-orbit coupling. Calculating the dielectric function within the random phase approximation, we describe the new features of screening induced by spin-orbit coupling, which are the extension of the region of particle-hole excitations and the spin-orbit-induced suppression of collective modes. The required polarization operator is calculated in an analytic form without any approximations. Carefully deriving its static limit, we prove the absence of a small-$q$ anomaly at zero frequency. On the basis of our results at finite frequencies we establish the new boundaries of the particle-hole continuum and calculate the SO-induced lifetime of collective modes such as plasmons and longitudinal optical phonons. According to our estimates, these effects can be resolved in inelastic Raman scattering. We evaluate the experimentally measurable dynamic structure factor and establish the range of parameters where the described phenomena are mostly pronounced.

45 citations


Journal ArticleDOI
TL;DR: In this article, the nuclear matrix elements of the neutrinoless double-beta decay (0νβ β) of most nuclei with known 2νββ-decay rates are systematically evaluated using the Quasiparticle Random Phase Approximation and Renormalized QRPA (RQRPA).
Abstract: The nuclear matrix elements M 0ν of the neutrinoless double-beta decay (0νβ β) of most nuclei with known 2νββ-decay rates are systematically evaluated using the Quasiparticle Random Phase Approximation (QRPA) and Renormalized QRPA (RQRPA). The experimental 2νβ β-decay rate is used to adjust the most relevant parameter, the strength of the particle-particle interaction. With such procedure the M 0ν values become essentially independent of single-particle basis size, the axial vector quenching factor, etc. Theoretical arguments in favor of the adopted way of determining the interaction parameters are presented. It is suggested that most of the spread among the published M 0ν ’s can be ascribed to the choices of implicit and explicit parameters, inherent to the QRPA method.

42 citations


Journal ArticleDOI
TL;DR: In this paper, the origin of the charge ordering in organic compounds was investigated using an extended Hubbard model, and the authors found that the (3×3) and (3 × 4) charge ordering observed at relatively high temperatures can be considered as a consequence of a cooperation between the Fermi surface nesting, controlled by the hopping integral in the c direction, and electron-electron interactions, where the distant (next nearest neighbor) interactions played an important role.
Abstract: The origin of the charge ordering in organic compounds θ-(BEDT-TTF) 2 X ( X = M M '(SCN) 4 , M =Tl, Rb, Co, M '=Cs, Zn) is studied using an extended Hubbard model. Calculating the charge susceptibility within random phase approximation (RPA), we find that the (3×3)–(3×4) charge ordering observed at relatively high temperatures can be considered as a consequence of a cooperation between the Fermi surface nesting, controlled by the hopping integral in the c direction, and the electron–electron interactions, where the distant (next nearest neighbor) interactions that have not been taken into account in most of the previous studies play an important role. Mean field analysis at T =0 also supports the RPA results, and further shows that in the 3×3 charge ordered state, some portions of the Fermi surface remain ungapped and are nested with a nesting vector close to the modulation wave vector of the horizontal stripe ordering observed at low temperatures in X = M M '(SCN) 4 . We further study the possibility of ...

40 citations


Journal ArticleDOI
TL;DR: In this paper, a self-consistent separable random phase approximation (SRPA) method was proposed for axially symmetric deformed nuclei. But the method is not suitable for the case of deformed isoscalar E2 giant resonances.
Abstract: We formulate the self-consistent separable random phase approximation (SRPA) method and specify it for Skyrme forces with pairing for the case of axially symmetric deformed nuclei. The factorization of the residual interaction allows diagonalization of high-ranking RPA matrices to be avoided, which dramatically reduces the computational expense. This advantage is crucial for the systems with a huge configuration space, first of all for deformed nuclei. SRPA self-consistently takes into account the contributions of both time-even and time-odd Skyrme terms as well as of the Coulomb force and pairing. The method is implemented to describe isovector E1 and isoscalar E2 giant resonances in a representative set of deformed nuclei: $^{154}\mathrm{Sm}$, $^{238}\mathrm{U}$, and $^{254}\mathrm{No}$. Four different Skyrme parameterizations (SkT6, SkM${}^{*}$, SLy6, and SkI3) are employed to explore the dependence of the strength distributions on some basic characteristics of the Skyrme functional and nuclear matter. In particular, we discuss the role of isoscalar and isovector effective masses and their relation to time-odd contributions. The high sensitivity of the right flank of E1 resonance to different Skyrme forces and the related artificial structure effects are analyzed.

38 citations


Journal ArticleDOI
TL;DR: In this article, the excitation energies and electric multipole decay rates of the lowest lying 2{sup +} and 3{sup -} vibrational states in Pb, Sn, and Ni nuclei are calculated following relativistic quasiparticle random-phase approximation formalism based on the relativistically Hartree-Bogoliubov mean field.
Abstract: The excitation energies and electric multipole decay rates of the lowest lying 2{sup +} and 3{sup -} vibrational states in Pb, Sn, and Ni nuclei are calculated following relativistic quasiparticle random-phase approximation formalism based on the relativistic Hartree-Bogoliubov mean field. Two sets of Lagrangian parameters, NL1 and NL3, are used to investigate the effect of the nuclear force. Overall there is good agreement with the available experimental data for a wide range of mass numbers considered here, and the NL3 set seems to be a better choice. However, strictly speaking, these studies point toward the need of a new set of force parameters that could produce more realistic single-particle levels, at least in vicinity of the Fermi surface, of a wide range of nuclear masses.

Journal ArticleDOI
TL;DR: Theoretical and experimental study of vibrationally resolved partial photoionization cross sections and angular asymmetry parameter β for the 1σg and 1σu shells of N2 molecule in the region of the σ* shape resonance is reported in this article.
Abstract: Theoretical and experimental study of vibrationally resolved partial photoionization cross sections and angular asymmetry parameter β for the 1σg and 1σu shells of N2 molecule in the region of the σ* shape resonance is reported. The measurements were made at the synchrotron radiation facility SPring-8 in Japan. The calculations in the random phase approximation have been performed using the relaxed core Hartree–Fock wavefunctions with the fractional charge of the ion core equal to 0.7. With its help, the role of interchannel coupling between the closely spaced 1σg and 1σu shells was studied. The experiment demonstrates the existence of a correlational maximum in the 1σu shell photoionization cross section induced by the σ* shape resonance in the 1σg shell. This maximum reveals itself even more clearly in the angular asymmetry parameter β for the v' = 0 and v' = 1 vibrational states of the ion. The calculation in the random phase approximation gives a consistent interpretation of the experimental data.

Journal ArticleDOI
TL;DR: In this paper, the effect of soft collective quadrupole and octupole deformation on the nuclear Schiff moment was investigated in the framework of the quasiparticle random phase approximation with separable quadrupoles and octouples applied to the odd Ra and Rn isotopes.
Abstract: Nuclear forces violating parity and time reversal invariance (P,T-odd) produce P,T-odd nuclear moments, for example, the nuclear Schiff moment. In turn, this moment can induce the electric dipole moment in the atom. The nuclear Schiff moment is predicted to be enhanced in nuclei with static quadru pole and octupole deformation. The analogous suggestion of the enhanced contribution to the Schiff moment from the soft collective quadrupole and octupole vibrations in spherical nuclei is tested in this article in the framework of the quasiparticle random phase approximation with separable quadrupole and octupole forces applied to the odd {sup 217-221}Ra and {sup 217-221}Rn isotopes. In this framework, we confirm the existence of the enhancement effect due to the soft modes, but only in the limit when the frequencies of quadrupole and octupole vibrations are close to zero. According to the QRPA, in realistic cases the enhancement in spherical nuclei is strongly reduced by a small weight of the corresponding ''particle + phonon'' component in a complicated wave function of a soft nucleus. The perspectives of a better description of the structure of heavy soft nuclei are discussed.

Journal ArticleDOI
TL;DR: In this paper, the random phase approximation is used to systematically describe the total muon capture rates on all nuclei where they have been measured, and the experimental values on these nuclei to better than 15% accuracy using the free nucleon weak form factors and residual interactions with a mild A dependency.
Abstract: We use the random phase approximation to systematically describe the total muon capture rates on all nuclei where they have been measured. We reproduce the experimental values on these nuclei to better than 15% accuracy using the free nucleon weak form factors and residual interactions with a mild A dependency. The isospin dependence and the effects associated with shell closures are fairly well reproduced as well. However, the calculated rates for the same residual interactions would be significantly lower than the experimental data if the in-medium quenching of the axial-vector coupling constant were employed to other than the true Gamow-Teller amplitudes. Our calculation thus suggests that no quenching is needed in the description of semileptonic weak processes involving higher multipole transitions and momentum transfer ~mµ, with obvious importance to analogous weak processes.

Journal ArticleDOI
TL;DR: In this paper, the random phase approximation (RPA) long-range correlations are used for building correlated ground states and related one-body density matrix elements for NA scattering off doubly closed-shell nuclei.
Abstract: The random phase approximation (RPA) long-range correlations are known to play a significant role in understanding the depletion of single particle-hole states observed in ($e,{e}^{'}$) and ($e,{e}^{'}p$) measurements. Here the RPA theory, implemented using the D1S force is considered for the specific purpose of building correlated ground states and related one-body density matrix elements. These may be implemented and tested in a fully microscopic optical model for NA scattering off doubly closed-shell nuclei. A method is presented to correct for the correlations overcounting inherent to the RPA formalism. One-body density matrix elements in the uncorrelated (i.e., Hartree-Fock) and correlated (i.e., RPA) ground states are then challenged in proton scattering studies based on the Melbourne microscopic optical model to highlight the role played by the RPA correlations. Agreement between the parameter free scattering predictions and measurements is good for incident proton energies ranging from 200 MeV down to approximately 60 MeV and becomes gradually worse in the lower energy range. Those features point unambiguously to the relevance of the g-matrix method to build microscopic optical model potentials at medium energies, and emphasize the need to include nucleon-phonon coupling, that is, a second-order component of the Feshbach type in the potential at lower energies. Illustrations are given for proton scattering observables measured up to 201 MeV for the $^{16}\mathrm{O}$, $^{40}\mathrm{Ca}$, $^{48}\mathrm{Ca}$, and $^{208}\mathrm{Pb}$ target nuclei.

Journal ArticleDOI
TL;DR: In this article, measurements and calculations of the contribution of non-dipole terms in the angular distribution of photoelectrons from the K-shell of randomly oriented N2 molecules are reported.
Abstract: Measurements and calculations of the contribution of the non-dipole terms in the angular distribution of photoelectrons from the K-shell of randomly oriented N2 molecules are reported. The angular distributions have been measured in the plane containing the photon polarization and the photon momentum vectors of linearly polarized radiation. Calculations have been performed in the relaxed core Hartree–Fock approximation with a fractional charge, and many-electron correlations were taken into account in the random phase approximation. Both theory and experiment show that the non-dipole effects are rather small in the photon energy region from the ionization threshold of the K-shell up to about 70 eV above it. From the theory, it follows that the non-dipole terms for the individual 1σg and 1σu shells are considerably large; therefore measurements resolving the contributions of the 1σg and 1σu shells are desirable.

Journal ArticleDOI
TL;DR: In this article, the first-order nondipole terms of the photoelectron angular distribution from randomly oriented nitrogen molecules have been calculated in the framework of density functional theory and by employing a single centre expansion for the bound and continuum wavefunctions.
Abstract: Contributions of the first-order nondipole terms to the photoelectron angular distributions from randomly oriented nitrogen molecules have been calculated in the framework of density functional theory and by employing a single centre expansion for the bound and continuum wavefunctions. Both valence and inner-shell ionizations have been considered in a photon energy range from threshold up to 2000 eV. Our results agree reasonably with calculations based on the random phase approximation and with the experimental data available. Our results for core ionizations confirm the most recent experimental finding of rather small nondipole effects in the near-threshold range. However, nondipole terms turn out to be quite large, even at the threshold, when individual contributions from the 1σ−1g and 1σ−1u ionization channels are considered. Strong interference effects leading to high-energy oscillations in the dipole and nondipole asymmetry parameters are satisfactorily explained within the Cohen–Fano model.

Journal ArticleDOI
TL;DR: In this article, a three-dimensional wave vector analysis of the jellium exchange-correlation (xc) surface energy in the random-phase approximation (RPA) is presented.
Abstract: We report a three-dimensional wave-vector analysis of the jellium exchange-correlation (xc) surface energy in the random-phase approximation (RPA). The RPA accurately describes long-range xc effects which are challenging for semilocal approximations, since it includes the universal small-wave-vector behavior derived by Langreth and Perdew. We use these rigorous RPA calculations for jellium slabs to test RPA versions of nonempirical semilocal density-functional approximations for the xc energy. The local spin density approximation displays canceling errors in the small- and intermediate-wave-vector regions. The Perdew-Burke-Ernzerhof generalized gradient approximation improves the analysis for intermediate wave vectors, but remains too low for small wave vectors (implying too-low jellium xc surface energies). The nonempirical metageneralized gradient approximation of Tao, Perdew, Staroverov, and Scuseria gives a realistic wave-vector analysis, even for small wave vectors or long-range effects. We also study the effects of slab thickness and of short-range corrections to the RPA.

Journal ArticleDOI
TL;DR: In this article, the scissors mode 1+ states were systematically investigated within the rotational invariant Quasiparticle Random Phase Approximation (QRPA) for 130-136Ba isotopes.
Abstract: In this study the scissors mode 1+ states are systematically investigated within the rotational invariant Quasiparticle Random Phase Approximation (QRPA) for 130-136Ba isotopes. We consider the 1+ vibrations generated by the isovector spin-spin interactions and the isoscalar and isovector quadrupole-type separable forces restoring the broken symmetry by a deformed mean field according to A.A. Kuliev et al. (Int. J. Mod. Phys. E 9, 249 (2000)). It has been shown that the restoration of the broken rotational symmetry of the Hamiltonian essentially decreases the B(M1) value of the low-lying 1+ states and increases the collectivization of the scissors mode excitations in the spectroscopic energy region. The agreement between the calculated mean excitation energies as well as the summed B(M1) value of the scissors mode excitations and the available experimental data of 134Ba and 136Ba is rather good. A destructive interference between the orbit and spin part of the M1 strength has been found for barium isotopes near the shell closer. For all the nuclei under investigation, the low-lying M1 transitions have ΔK = 1 character as it is the case for the well-deformed nuclei.

Journal ArticleDOI
TL;DR: In this article, a variational calculation of the spin wave excitation spectrum of double-exchange ferromagnets in different dimensions is presented, which can be used to study the intermediate exchange coupling and electron concentration regime relevant to the manganites.
Abstract: We present a variational calculation of the spin wave excitation spectrum of double-exchange ferromagnets in different dimensions. Our theory recovers the random phase approximation and $1∕S$ expansion results as limiting cases and can be used to study the intermediate exchange coupling and electron concentration regime relevant to the manganites. In particular, we treat exactly the long range three-body correlations between a Fermi sea electron-hole pair and a magnon excitation and show that they strongly affect the spin dynamics. The manifestations of these correlations are many-fold. We demonstrate that they significantly decrease the stability of the ferromagnetic phase and the magnon stiffness. We also show that the ferromagnetic state is unstable against spin wave excitations close to the Brillouin zone boundary. As a result, we find a strong softening of the spin wave dispersion as compared to the Heisenberg ferromagnet with the same stiffness within a range of intermediate concentrations. We discuss the possible relevance of our results to experiments in colossal magnetoresistance ferromagnets.

Journal ArticleDOI
TL;DR: In this article, the authors developed the statistical field theory for the study of a simple charge-asymmetric 1:z primitive model (SPM) and showed that the well-known approximations for the free energy, in particular the DH limiting law (DHLL), can be obtained within the framework of this theory.
Abstract: Based on the method of collective variables, we develop the statistical field theory for the study of a simple charge-asymmetric 1:z primitive model (SPM). It is shown that the well-known approximations for the free energy, in particular the DH limiting law (DHLL) and the optimized random phase approximation (ORPA), can be obtained within the framework of this theory. In order to study the gas–liquid critical point of the SPM we propose a method for the calculation of chemical potential conjugate to the total number density which allows us to take into account the higher-order fluctuation effects. As a result, the gas–liquid phase diagrams are calculated for z = 2–4. The results demonstrate a qualitative agreement with Monte Carlo (MC) simulation data: the critical temperature decreases when z increases and the critical density increases rapidly with z.

Journal ArticleDOI
TL;DR: In this paper, a Lorentzian parametrization of the most reliable experimental data on isovector M1 resonances is constructed for seven spherical nuclei, and it is shown that the widths of M 1 resonances are severalfold, sometimes an order of magnitude, smaller than the value of Γ 0 = 4 MeV, which was recommended for all nuclei.
Abstract: The results of the development (“renaissance”) of giant-resonance physics are briefly discussed from the point of view of their application to creating a photonuclear database. It is indicated that part of the recommendations from corresponding libraries of data are not at the level of the present-day status of giant-resonance physics. A Lorentzian parametrization of the most reliable experimental data on isovector M1 resonances is constructed for seven spherical nuclei, and it is shown that the widths of M1 resonances are severalfold, sometimes an order of magnitude, smaller than the value of Γ0 = 4 MeV, which was recommended for all nuclei. The need for microscopically taking into account configurations more complex than those that are included within the standard random-phase approximation or within the quasiparticle random-phase approximation is emphasized. To be more precise, it is necessary to take into account coupling to phonons, since this changes the temperature dependence of the resonance width in relation to that which was used earlier and since, without this, one cannot explain the properties of pygmy dipole resonances in the region of the nucleon binding energy. Our calculations of the average energies of the pygmy dipole resonances in the Ca and Sn isotopes within the microscopic extended theory of finite Fermi systems reveal that the inclusion of coupling to phonons reduces these energies considerably toward the improvement of agreement with experimental data. The idea of creating a library of photonuclear data for unstable nuclei, including fission fragments, on the basis of the extended theory of finite Fermi systems is discussed in connection with the fact that information necessary for fitting the parameters of phenomenological theories is absent or insufficient for such nuclei.

01 Jan 2006
TL;DR: In this paper, the electro resistivity of metallic hydrogen was calculated in the framework of perturbation theory in electronproton interaction using the Kubo linear response theory while using the two-time retarded Green functions method.
Abstract: We calculate the electroresistivity of metallic hydrogen within the framework of perturbation theory in electronproton interaction. To this end we employ the Kubo linear response theory while using the two-time retarded Green functions method to calculate the relaxation time. The expressions for the second and third order contributions are given. To describe the electron subsystem, the random phase approximation is used, allowing for the exchange interactions and correlations in a local field approximation. Thermodynamics of the proton subsystem is assumed to be given by the Percus-Yevick equation. At a given density and temperature the only parameter of the theory is the hard sphere diameter, which is calculated from effective pair ionic interaction. For a completely degenerated electron gas, the latter is determined by the density of the system. The dependence of the second and the third order contributions on the parameters of the theory is investigated. For all densities and temperatures examined here the third order contribution constitutes more than half of the second order term. The corresponding magnitude of resistivity is about 100 � 250µ cm.

Journal ArticleDOI
TL;DR: In this article, a simple extension of the second random-phase approximation (SRPA) was proposed, in which the reference state contains 2 particle -2 hole correlations, and the quality of such an extension was tested by applying it to a solvable three-level model and found to be good.
Abstract: The second random-phase approximation (SRPA) is the simplest and most natural extension of the RPA. It enlarges the space of the elementary modes introduced to describe the collective states by adding 2 particle -2 hole excitations to the 1 particle -1 hole ones of the RPA. In deriving the SRPA equations, use is made, as in the RPA, of the so-called quasi-boson approximation (QBA) where expectation values in the ground state of the system are approximated by their values in the uncorrelated reference state. This, however, has been shown to imply a degree of approximation worse than that in the RPA. It is, therefore, necessary to improve the QBA by considering a reference state which contains some correlations. Having in mind to perform such calculations for realistic systems, we consider a simple extension of the SRPA in which the reference state contains 2 particle - 2 hole correlations. The quality of such an extension is tested by applying it to a solvable three-level model and found to be good.

Journal ArticleDOI
TL;DR: In this paper, the effects of electron-electron collisions on proton electronic stopping in plasmas of any degeneracy were described. But the results were only compared with the classical classical and degenerate approximations.
Abstract: The purpose of the present paper is to describe the effects of electron-electron collisions on proton electronic stopping in plasmas of any degeneracy. Plasma targets are considered fully ionized so electronic stopping is only due to the free electrons. The stopping due to free electrons is obtained from an exact quantum mechanical evaluation in the random phase approximation, which takes into account the degeneracy of the target plasma. The result is compared with common classical and degenerate approximations. Differences are around 30% in some cases which can produce bigger mistakes in further energy deposition and projectile range studies. We focus our analysis on plasmas in the limit of weakly coupled plasmas then electron-electron collisions have to be considered. Differences with the same results without taking into account collisions are more than 50%.

Journal Article
TL;DR: The random phase approximation (RPA) theory of the Bose-Einstein-condensation-Bardeen-Cooper-Schrieffer crossover in an atomic Fermi gas near a Feshbach resonance that includes the relevant two-body atomic physics exactly is presented.
Abstract: We present the random phase approximation (RPA) theory of the Bose-Einstein-condensation-Bardeen-Cooper-Schrieffer crossover in an atomic Fermi gas near a Feshbach resonance that includes the relevant two-body atomic physics exactly. This allows us to determine the probability for the dressed molecules in the Bose-Einstein condensate to be in the closed channel of the Feshbach resonance and to compare with the recent experiments of Partridge et al. [95, 020404 (2005)10.1103/PhysRevLett.95.020404] with , who have measured the same quantity.

Journal ArticleDOI
TL;DR: In this article, a numerical scheme has been proposed to calculate the screened Coulomb potential, profile of charge density with various quantum wire widths and relative renormalization of gap energy.
Abstract: Bandgap renormalization for the geometry of T-shaped quantum wires is calculated as a function of the electron–hole plasma density and quantum wire width in the random phase approximation. Considering a suitable confinement potential, a numerical scheme has been proposed to calculate the screened Coulomb potential, profile of charge density with various quantum wire widths and relative renormalization of gap energy. We will show that carrier concentration, screened confinement potential and relative bandgap renormalization are functions of the ratio of well width in the x and y directions. We also show that increasing temperature leads to more relative renormalization of gap energy.

Journal ArticleDOI
TL;DR: The high precision of the calculations enables us to analyse not only the collective (surface plasmon) modes and their dependence on the film thickness, but also the intra-band electron-hole excitations, and for the first time oscillatory structures due to inter-band transitions.
Abstract: Surface excitation spectra are calculated, including both collective and single-particle modes, and examined in detail. This is achieved by calculating the non-local dielectric function ?p(Q,z,z',?) of the thin jellium film within the random phase approximation (RPA) (using local density approximation wavefunctions which actually takes us beyond the RPA), from which we then derive the spectral function. The high precision of the calculations enables us to analyse not only the collective (surface plasmon) modes and their dependence on the film thickness, but also the intra-band electron?hole excitations, and for the first time oscillatory structures due to inter-band transitions. The spectra are then analysed with special attention to their dependence on the slab thickness, and the periodic peaks observed due to single-particle excitations in the spectra.

Journal ArticleDOI
TL;DR: In this article, a distorted-waves Born approximation (DWBA) was used to compare calculated and experimental cross sections directly and to discuss the giant resonance excitations in K-48 and Co-58 nuclei.
Abstract: We have used the 43 MeV/nucleon primary tritium beam of the AGOR facility with an intensity of 4x10(7) pps and the BBS experimental setup to study the (t,He-3) reaction between 0 degrees and 5 degrees lab angles on C-12, Ca-48, and Ni-58 targets. The standard ray-tracing procedure has allowed us to obtain excitation-energy spectra up to 30 MeV in six angular bins for each residual nucleus, with an average energy resolution of 350 keV. The reaction mechanism has been described in distorted-waves Born approximation (DWBA) using the DWBA98 code. In this approximation, the form factor is treated as a folding of an effective projectile-nucleon interaction with a transition density. The effective projectile-nucleon interaction has been adjusted to reproduce the 0(degrees) cross section of the 1(+) ground state of B-12 populated in the C-12(t,He-3) reaction. We have employed random-phase approximation (RPA) wave functions of excited states to construct the form factor instead of the normal modes wave functions used earlier. This new DWBA+RPA analysis is used to compare calculated and experimental cross sections directly and to discuss the giant resonance excitations in K-48 and Co-58 nuclei.

Journal ArticleDOI
TL;DR: In this paper, the authors generalize the description of a Bose-Einstein condensed gas to allow for a relative velocity between the superfluid and normal fluid, and determine the critical velocity dynamically as the transition point between stable and unstable dynamics.
Abstract: Based on the dielectric formalism in the generalized random phase approximation, we generalize the description of a Bose-Einstein condensed gas to allow for a relative velocity between the superfluid and normal fluid. In this model, we determine the critical velocity dynamically as the transition point between stable and unstable dynamics. Unlike the zero temperature case, at finite temperature the relative critical velocity of a dilute Bose-Einstein gas is lower than the sound velocity. The requirement of Galilean invariance essential to obtain an invariant result necessitates the use of a gapless and conserving approximation.