Concept Of Quasiparticles Research Articles

Physics. Quasi-particles survive--for now.

The concept of quasi-electrons is central to understanding the physical behavior of solids, but the behavior of some materials, especially copper oxide superconductors, seems to be incompatible with this concept. Manganese oxides, the more conventional cousins of the cuprates, have been studied intensively to test the quasi-particle concept. As Keimer explains in his Perspective, the manganites seemed to behave even more strangely than the cuprates, but a report by Chuang et al. now comes to the rescue of the quasi-particles. The case remains open for the cuprates.

High-resolution UV-photoemission of solids: success, limitations and the future

Abstract Numerous studies have demonstrated that a gratifying agreement is found between UV-photoemission data and single-particle band calculations. However, many-body mechanisms can markedly modify this simple analysis and require an interpretation in terms of quasi-particles. This aspect will be illustrated by photoemission data deeply influenced by two types of many-body interaction: the very strong 4f electron correlation in heavy fermion systems and the important electron–phonon interaction modifying the surface state of Be(0001). Moreover, a complete breakdown of the quasi-particle concept occurs in one-dimensional (1D) metals and spectral functions calculations within the Tomonaga–Luttinger liquid formalism are required. Photoemission studies devoted to quasi-1D metals confirm the predicted absence of Fermi step but systematically fail to reveal the collective spin and charge excitations (spinons and holons). We have tackled this problem on single-atom metallic Au chains prepared on a vicinal surface of Si(111) and we observe an explicit hint of separated spin and charge excitations. These different examples illustrate the fact that the photoemission process is not setting any limit to the observation of very low-energy excitations but that the crucial problems arise from the quality and stability of the surfaces probed at low temperature.

Transparency of12Cfor protons

Existing cross-section data for $1p$-shell knockout in the reaction ${}^{12}\mathrm{C}{(e,e}^{\ensuremath{'}}{p)}^{11}\mathrm{B}$ ---as obtained under different kinematic conditions---are shown to be mutually consistent, apart from a recent measurement performed in Mainz. New data have been collected at the Amsterdam Pulse Stretcher that confirm the normalization of the older measurements. An analysis of the world's ${}^{12}\mathrm{C}{(e,e}^{\ensuremath{'}}{p)}^{11}\mathrm{B}$ data has yielded precise values of the spectroscopic factor for $1p$-shell and $1s$-shell knockout from ${}^{12}\mathrm{C}.$ These values have been used to evaluate the transparency of the ${}^{12}\mathrm{C}$ nucleus for $1p$-shell and $1s$-shell protons separately on the basis of recent high-energy ${}^{12}\mathrm{C}{(e,e}^{\ensuremath{'}}{p)}^{11}\mathrm{B}$ data taken at a four-momentum transfer squared ${Q}^{2}$ of 1.1 $(\mathrm{GeV}{/c)}^{2}.$ As the resulting average value of the nuclear transparency, $0.81\ifmmode\pm\else\textpm\fi{}0.04,$ is considerably higher than the value obtained from previous analyses and theoretical estimates, the high ${Q}^{2}$ data were used instead for an independent determination of the spectroscopic strength for $1p+1s$ knockout. Combining these results with the low ${Q}^{2}$ data the spectroscopic factors appear to be momentum-transfer dependent. Possible explanations of these surprising results in terms of reaction-mechanism effects or a possible breakdown of the quasiparticle concept at high ${Q}^{2}$ are discussed as well.

Model of high-Tcsuperconductivity driven by the intrinsic Kondo-type interaction: Effective phonon scheme

We have investigated whether electron-hole composite quasiparticles in ${\mathrm{CuO}}_{2}$ planes can drive the hole-doped high-${T}_{c}$ superconductivity. Using this quasiparticle concept, we derive a Kondo-type interaction mediated by the effective Gell-Mann and Breuckner-type phonon. It is shown that this interaction opens a superconducting gap near the Fermi level. We explain the resistivities in the a-b plane and also along the c axis using the same concept of superexchange interactions. We have also found that the measured magnetic susceptibility and nuclear-spin relaxation rates can be explained using the present theoretical scheme in a consistent way.

The Dynamic Structure Factor of 4He beyond the Roton Minimum

The dynamic structure factor S(\({\vec q}\), ω) of 4He is studied at zero temperature in the momentum region at and above the roton minimum by field-theoretical methods. The model is derived from the Gavoret-Nozieres (GN) two particle propagator by introducing the concept of quasiparticles. In this way a connection between the field theory of GN and the phenomenological models of Zawadowski-Ruvalds-Solana (ZRS) type is obtained. An improved expression for the dynamic structure factor within ZRS-like models is found. Numerical results for S(\({\vec q}\), ω) are presented for momentum and energy independent effective interactions between quasiparticles.

Noninstantaneous collisions and two concepts of quasiparticles

The kinetic theory recently implemented in heavy ion reactions combines a nonlocal and noninstantaneous picture of binary collisions with quasiparticle features. We show that the noninstantaneous description is compatible with the spectral concept of quasiparticles while the commonly used variational concept is consistent only with instantaneous collisions. The rearrangement energy, by which the variational concept surpasses the spectral one, is shown to be covered by a medium effect on noninstantaneous collisions.

Kondo-lattice in an applied magnetic field: spin-split masses and metamagnetism

The concept of heavy quasiparticles in the Kondo-lattice limit of the periodic Anderson model leads in a natural manner to the spin-split effective masses and metamagnetism. The spin-minority electrons become very heavy, whereas the spin-majority particles approach the band limit as the system reaches magnetic saturation.

Generalization of a Fermi Liquid to a Liquid with Fractional Exclusion Statistics in Arbitrary Dimensions: Theory of a Haldane Liquid

In 1957 Landau [1] introduced the famous concept of quasiparticles with Fermi-Dirac statistics (FDS) in a Fermi liquid in order to describe electrons in a metal where the interaction between electrons was believed to be very significant, as a generalization of the Sommerfeld theory for a degenerate Fermi gas [2]. This description was later justified by Luttinger [3]. Recently Haldane [4] introduced the concept of fractional exclusion statistics (FES) which quasiparticles in strongly interacting systems in arbitrary dimensions might satisfy as a generalization of Pauli’s exclusion principle. Wu [5] first formulated quantum statistical mechanics (QSM) in the state representation and derived the distribution function for an ideal gas with FES as a generalization of the FD and the Bose-Einstein (BE) distribution functions. [Throughout this Letter the FES is referred to as the Haldane-Wu statistics (HWS) such as FDS and BES, while a gas or liquid with HWS is called a Haldane gas or liquid such as a gas or liquid with FDS (BES) was called a Fermi (Bose) gas or liquid.] This concept has played a very important role to understand strongly interacting system such as the TomonagaLuttinger model (TLM) [6], the Calogero-Sutherland model (CSM) [7], and the Haldane-Shastry model (HSM) [8] in one dimension and the fractional quantum Hall effect in two dimensions [9]. Especially, notable is that Dasnieres de Veigy and Ouvry [10] revealed a deep connection between the FES and the fractional quantum Hall system, while Bernard and Wu [11] and Isakov [12] found the one between the FES and the CSM. More recently, QSM formulation has been developed further by Nayak and Wiczek [13], Isakov, Arovas, Myrheim, and Polychronakos [14], and the author [15]. Here, the QSM formulation allows us to evaluate the equation of state for an ideal gas with HWS in arbitrary dimensions with obtaining all the exact cluster coefficients in the cluster expansion [14,15]. However, interacting quasiparticles with HWS in arbitrary dimensions have never been investigated yet, and therefore the concept of a Haldane liquid is still missing. In this Letter I will explore this concept as a generalization of the Landau’s Fermi liquid theory and generalize the Luttinger’s theorem for a Fermi liquid to that for a Haldane liquid.

Hubbard one-particle Green function in the antiferromagnetic phase

An analytic approach is presented of electronic one-particle spectra of the one-band Hubbard model at half filling in the antiferromagnetic phase. Starting from the strong-coupling regime U{gt}t, a projection technique is used to set up self-consistent coupled equations for the electron Green function, which are valid down to values U{approx}t. The self-consistent equation for the hole propagator is a direct generalization of the one found from the t-J model. This gives further support to the {open_quotes}string{close_quotes} picture, where propagation of holes creates strings of overturned spins with which the holes interact. Hopping of holes (or electrons) with up spin on the down sublattice is also taken into acount, as well as transitions between the lower and upper Hubbard bands. These are shown to change significantly the incoherent part of the t-J model spectra, by smearing out the shake-off peaks, reminiscent of higher bound string states due to multispin scattering. Coherent (quasiparticle) peaks exist at the band edges, on both sides of the insulating gap. With decreasing U the quasiparticle concept loses its meaning for wave vectors at the center of the magnetic Brillouin zone (MBZ). For large values of U the dispersion of the quasiparticle is found to scale withmore » its band width, which is of order J. Extrema are always found at {bold k}=({pi}/2,{pi}/2). The weight of the quasiparticle at this {bold k} value decreases logarithmically with increasing U. In the strong-coupling limit the spectrum tends to be symmetric, i.e., to become an even function of the frequency around the chemical potential, for {ital any wave vector}. For small values of U the dispersion {ital at the edge} of the MBZ flattens away. The spectral function in this regime, for wave vectors away from the edge of the MBZ, is concentrated mainly on one side of the chemical potential. {copyright} {ital 1996} {ital The American Physical Society}« less

Quasiparticle concept for non-uniformly magnetized plasmas

A quasiparticle concept [Sosenko P P and Decyk V K 1993 Physica Scripta 47 258] is generalized for the case of non-uniformly magnetized plasma. Exact and reduced continuity equations for the microscopic density in the quasiparticle phase space are derived, and the nature of quasiparticles is analyzed. The theory is developed for the general case of relativistic particles in electromagnetic fields, besides non-uniform but stationary magnetic fields.

Electrons in a strong magnetic field on a disk

AbstractThe problem of interacting electrons moving under the influence of a strong magnetic field in two dimensions on a finite disk is reconsidered. First, the results of exact diagonalizations for up to N = 9 electrons for Coulomb as well as for a short‐range interaction are used in the search for a peculiar ground state corresponding to filling factor 1/3. Not for the Coulomb, but only for the short‐range interaction, can the 1/3‐state be safely identified amongst the spectra of various filling factors close to 1/3. Second, the propositions of the concept of quasiparticles, as used in the hierarchical theory, are examined in view of the exact results for the disk geometry. Whereas the theory for the quasiholes is in complete accordance with the spectra, for the quasielectrons, finite size corrections make an analysis difficult. For the quasielectron energy, an extrapolation to N→ ∞ is given and compared with the corresponding extrapolations of three different proposals for trial wave functions. While the limiting value for the best trial wave function is very close to the limit of the exact results, the behavior of the finite size corrections of the exact energies and of the trial wave functions, respectively, is qualitatively rather different.

Microscopic description of magnetized plasma: quasiparticle concept

A quasiparticle concept is developed systematically, from first principles, within the context of microscopic description of magnetized plasma. It is argued that the zeroth velocity-gyroangle harmonic of the microscopic particle distribution function under the gyrokinetic change of variables can be taken as a microscopic quasi-particle density in a reduced phase space. The nature of quasiparticles is discussed and equations of their motion are derived within both exact and reduced microscopic descriptions. The reduced one employs explicitly the separation of interesting time scales.

Generalized squeezing, Bogoliubov quasiparticles, and information in two- and three-mode systems.

It is shown that many types of two-mode squeezing are based on reducible representations of the SU(1,1) or SU(2) groups. In these cases it is possible to squeeze independently each irreducible sector and generalize the concept of squeezing. The related concept of Bogoliubov quasiparticles is also generalized. The information transmitted by two-mode states is calculated as a sum of the information in the two modes and the information in the correlation. Special emphasis is given in the effect of squeezing on the correlation between the two modes. Higher correlations in three-mode systems are also studied in the context of information theory.

On a Concept of Quasiparticles with Parastatistics

AbstractA possibility of description of some physical models in solids in terms of quasiparticles with parastatistics is considered. These quasiparticles can be introduced both by Green's generalized second quantization and by means of a nonlinear operator transformation for Bose and/or Fermi operators. Several examples of the nonlinear transformations are given. Some physical properties of the superconducting models are discussed.

The luminescence lineshape of highly doped direct-gap III-V compounds

For the description of photoluminescence spectra of highly doped III-V compounds, the authors present a lineshape model without fitting parameters. Their calculations have been achieved in the framework of Klauder's fifth approximation. Using a screened and factorised Coulomb potential, they have applied this theory for the first time to the analysis of experimental spectra measured on n-InP and n- and p-GaAs. For slightly compensated material, their computations are in good agreement with experiment. On the basis of a quasi-particle concept that influence of doping on the energy dispersion near the band bottom is discussed. It appears that in highly doped direct-gap material any deviations in the corresponding band from the parabolic k dependence usually assumed to be related to high k-values are not the only factor to be considered.

The quasiparticle concept in vibrational–electronic problems in molecules. I. Partitioning of the vibrational–electronic Hamiltonian

AbstractThe partitioning of the vibrational–electronic Hamiltonian is presented. This partitioning is based on a new quasiparticle transformation that is constructed in such a way that the adiabatic approximation is included into the unperturbed Hamiltonian; nonadiabacity, anharmonicity, and electron correlation are treated as perturbations. We also present the second quantization treatment for bosons. The many body perturbation theory expansion for the vibrational–electronic Hamiltonian is suggested. A comparison of this approach is made with gradient techniques.

Charge distribution differences in heavy mass nuclei

The charge density differences between208Pb and206Pb as well as between206Pb and205Tl have been calculated within the generalized theory of finite Fermi systems. Here one describes206Pb and205Tl as correlated two-neutron hole states and two-neutron hole one-proton hole states in208Pb, respectively. We discuss the spectroscopic strength of the 3s-hole and argue that the remaining difference between the theoretical and experimental charge distributions may be connected with the range of validity of Landau's quasi particle concept.

Density functional theory: Approximating quasiparticle density functional

AbstractA constructive approach for deriving the approximating quasiparticle energy density functional is proposed. As a matter of fact, the proposed approach is the direct development of the Kohn–Sham quasiparticle concept and the Levy–Valone approach. The approach presented takes into account a pseudopotential character of the exchange‐correlation part of the density functional and results in a system of functional equations to obtain ground‐state energies of many‐electron systems.

Approximations to Brueckner theory for nucleon-nucleus collisions

The concept of quasiparticles has been very fruitful for the development of theories of many-body systems of strongly interacting particles. A central problem for a quantitative treatment is the interaction between the quasiparticles (the effective interaction between particles). The Brueckner reaction matrix theory and the Jastrow variational approach have been used extensively, especially for nuclei. Our present concern is the collisions between nuclei. An exact calculation of the effective interaction between the nucleons is then extremely complicated. Only the simplest extreme, the nucleon-nucleus optical potential, can be treated fully. Approximate treatments are called for. The present paper investigates some approximations to the Brueckner theory. We are primarily interested in the imaginary part of the one-body (one-nucleon) mean field, as it relates to the damping of collective motion. Of special interest is the increase at low density. Comparisons with Skyrme forces show that they exaggerate this density dependence.

Spectral density approach for the quasiparticle concept in the s-f model (ferromagnetic semiconductors)

Spectral density approach for the quasiparticle concept in the s-f model (ferromagnetic semiconductors)