Spectrum Of Elementary Excitations Research Articles

Effect of electron irradiation on the spectra of elementary excitations in C60 fullerite

Electron irradiation produces changes in the spectra of elementary excitations of C60 fullerite, which are manifested by decrease in the π-plasmon energy, bandgap width, and energies of the HOMO-LUMO and other molecular transitions, smoothening of the corresponding spectral features, and significant growth in the quasicontinuous low-energy background intensity, the latter being indicative of an increase in the conductivity. The observed “red shifts” are related to collectivization of a part of the π electrons, the formation of chemical bonds between adjacent molecules (polymerization), and the corresponding increase in the proportion of sp3-hybridized electrons. Characteristic electron energy loss (EEL) spectra of an intact fullerite sample non-perturbed by the polymerization process were measured. The EEL spectra of fullerite exhibit a multipole structure due to the (σ + π)-plasmon and reveal an exciton feature which is highly sensitive with respect to electron irradiation and can be used to characterize the initial fullerite structure and to indicate the polymerization onset.

Thermodynamics of a trapped Bose condensate with negative scattering length

We study the Bose–Einstein condensation (BEC) for a system of 7Li atoms, which have negative scattering length (attractive interaction), confined in a harmonic potential. Within the Bogoliubov and Popov approximations, we numerically calculate the density profile for both condensate and non-condensate fractions and the spectrum of elementary excitations. In particular, we analyze the temperature and number-of-boson dependence of these quantities and evaluate the BEC transition temperature T BEC. We calculate the loss rate for inelastic two- and three-body collisions. We find that the total loss rate is strongly dependent on the density profile of the condensate, but this density profile does not appreciably change by increasing the thermal fraction. Moreover, we study, using the quasi-classical Popov approximation, the temperature dependence of the critical number N c of condensed bosons, for which there is the collapse of the condensate. There are different regimes as a function of the total number N of atoms. For N<N c the condensate is always metastable but for N>N c the condensate is metastable only for temperatures that exceed a critical value T c.

REFERENCE APPROACH IN THEORY OF PSEUDOSPIN SYSTEMS

For theoretical description of pseudospin systems with essential short-range and long-range interactions we use the method based on calculations of the free energy functional with taking into account the short-range interactions within the reference approach in cluster approximation. We propose a consistent formulation of the cluster expansion method for quantum pseudospin systems. We develop a method allowing one to obtain within the cluster approximation an Ornstein-Zernike type equation for reference cumulant Green function of an arbitrary order. In the two-particle cluster approximation we derived an explicit expression for pair temperature cumulant Green function of the reference system. In the cluster random phase approximation we calculated and studied thermodynamic characteristics, elementary excitation spectrum, and integral intensities of the Ising model in transverse field.

Reorientation phase transition in temperature in a two-dimensional ferromagnet taking magnetoelasticity into account

Elementary excitation spectra are investigated in a thin ferromagnetic film. The temperatures of stability of phase transitions “easy axis”–angular phase and “easy plane”–angular phase are obtained. It is shown that the reasons behind the formation of the angular phase is the presence of magnetoelastic interaction. The Curie temperature of the system under investigation is determined.

Spectrum of elementary and collective excitations in the dimerizedS=1/2Heisenberg chain with frustration

We have studied the low-energy excitation spectrum of a dimerized and frustrated antiferromagnetic Heisenberg chain. We use an analytic approach, based on a description of the excitations as triplets above a strong-coupling singlet ground state. The quasiparticle spectrum is calculated by treating the excitations as a dilute Bose gas with infinite on-site repulsion. Additional singlet $(S=0)$ and triplet $(S=1)$ modes are found as two-particle bound states of the elementary triplets. We have also calculated the contributions of the elementary and collective excitations into the spin structure factor. Our results are in excellent agreement with exact diagonalizations and dimer series expansions data as long as the dimerization parameter \ensuremath{\delta} is not too small $(\ensuremath{\delta}&gt;0.1),$ i.e., while the elementary triplets can be treated as localized objects.

Effects of biquadratic exchange on the spectrum of elementary excitations in spin ladders

We investigate the influence of biquadratic exchange interactions on the low-lying excitations of a S=1/2-ladder using perturbation theory, numerical diagonalization of finite systems and exact results for ladders with matrix product ground states. We consider in particular the combination of biquadratic exchange interactions corresponding to ring exchange on the basic ladder plaquette. We find that a moderate amount of ring exchange reduces the spin gap substantially and makes equal bilinear exchange on legs and rungs consistent with experimentally observed spectra.

Arithmetic properties of spectra produced by Farey hierarchies of approximants

We discuss the consequences of the hierarchical nature of series of approximants of aperiodic crystals on their diffraction patterns and spectra of elementary excitations. We show how a linear form defined on Z3 can be used to order Bragg reflections in diffraction patterns according to their amplitudes, and gaps in spectra of elementary excitations according to their widths, for all the structures in the hierarchy. Bragg peaks amplitudes and gap widths are projective functions on P(Z3), recursively defined on 2D Farey sets (generalization of Farey series).

Sum Rules and Density Response of A Two-Dimensional Charged Bose Fluid

The density response of a two-dimensional charged Bose fluid at absolute zero temperature is studied beyond the random-phase approximation by including the many-body correlation effects. the correlations are accounted for through a static local-field correction obtained by satisfying in an approximate way the third frequency-moment sum rule of the density response function. the static structure factor, the pair-correlation function, the elementary excitation spectrum, and the ground-state energy are calculated over a wide range of boson number density. Wherever available, the results are compared with the recent diffusion Monte Carlo data and the calculation based upon the theory of Singwi et al. We also present a comparison between the results of electron and charged Bose fluids for pair-correlation function and elementary excitation spectrum.

The role of pair correlations in the formation of the ground state and the elementary excitation spectrum in a superfluid Bose liquid (A Review)

The paradoxes and disparities in the contemporary microscopic theory of superfluid helium (He–II) are discussed along with possible ways of resolving them by taking pair correlations of He4 atoms into consideration. It is shown that most paradoxes are associated with the commonly accepted initial assumption concerning the dominating role of single-particle Bose condensate (SPBC) in the quantum microstructure of the superfluid component ρs. The existence of intensive SPBC leads to a strong hybridization of the elementary excitation branches and to a common dispersion law for all boson branches, which is identified with the quasiparticle spectrum E(p) observed experimentally from slow neutron scattering in liquid helium. However, the stability of this spectrum during a transition through the λ-point and the large value of the gap in the vicinity of the “rotonic” minimum contradict both the Landau theoretical criterion of superfluidity and the small value of experimentally measured critical velocity. At the same time, a strong interaction between particles in the Bose liquid He4 strongly suppresses the SPBC which amounts to less than 1% of all He4 atoms and hence cannot be the main constituent of the superfluid component, unlike the case of a weakly nonideal Bose gas. Moreover, for a quite strong attraction between particles in a certain region of the momentum space, bound pairs of bosons can be formed in the superfluid Bose liquid, and a coherent pair condensate (CPC) analogous to the Cooper pair condensate in superconductors may appear. Such a strong CPC may completely suppress the weak SPBC. In this case, the one-particle spectrum ε(p) of elementary excitations does not hybridize with the collective (two-particle) spectrum and does not appear in the structure of the dynamic form factor S(p,ε), i.e., does not coincide with the spectrum measured from neutron scattering. The dispersion of one-particle spectrum is defined by the momentum dependence of the pair order parameter Ψ̃(p) and may have a minimum or a point of inflection at p≠0. This peculiarity in the one-particle spectrum of a Bose liquid with CPC but without SPBC vanishes together with Ψ̃(p) at the temperature Tc=Tλ of the phase transition from the superfluid to the normal state (unlike the rotonic minimum in the collective spectrum), while the corresponding critical velocity vc=min[ε(p)/p] vanishes at the λ-point in accordance with the Landau criterion and the experimental data. The assumption that the strong “Cooper-like” CPC is responsible for the quantum structure of the superfluid component ρs is confirmed indirectly by the successful application of the Justrow approximation (based on strong pair correlations) for describing the properties of liquid He4 and quantum liquid mixtures He3–4He on one hand, and by an anomalously large effective mass of He3 impurity atoms in He4, which is approximately equal to total mass of He3 and He4 atoms, thus pointing to the existence of helium atoms in superfluid liquid He–II. The value of the superfluid velocity circulation quantum in the Onsager–Feynman vortices in a Bose liquid with CPC but without SPBC is discussed as well as the critical velocities of superfluid He4 in ultrathin films and channels in which the creation and motion of quantum vortices are ruled out, and the quasiparticle spectrum undergoes dimensional quantization.

The Heisenberg chain with impurities: crossover from Fermi-liquid to non-Fermi-liquid behaviour

We consider an integrable SU(2)-invariant model consisting of a Heisenberg chain of spins S (the Takhtajan-Babujian model) interacting with a finite concentration c of impurity spins S´. The thermodynamic Bethe-ansatz equations are stated for this model. The ground-state equations are analysed as a function of c, the magnetic field and the coupling parameter (impurity rapidity p0) of the impurities to the lattice. In zero field the ground state is a singlet for finite c, but becomes non-Fermi-liquid-like as c0 for S´<S. Two rapidity bands play a role at T = 0 corresponding to strings of length 2S and 2S´, respectively. The van Hove singularities of the empty bands define two critical fields, Hc(c,p0) and Hs(c,p0), at which the susceptibility diverges. Hc tends to zero as c0 giving rise to a crossover from non-Fermi-liquid behaviour for H>Hc to Fermi-liquid-like behaviour for H<Hc. The spectrum of elementary excitations and the finite-size corrections to the ground-state energy are calculated, and used to discuss the asymptotic behaviour of spin-spin correlation functions for long times and large distances.

Ground-state correlations in a charged Bose quantum wire

We study the ground-state correlations in a charged Bose quantum wire within the self-consistent-field approximation of Singwi, Tosi, Land, and Sjölander. A simple cylindrical model for the quantum wire is used. Static properties (structure factor, pair correlation function, and screened interaction potential), elementary excitation spectra, and ground-state energies are calculated at different boson number densities and wire radii. Our study shows that, in addition to the density of bosons, the wire radius provides an extra control on the strength of the many-body correlations. The correlation effects are found to become increasingly important with decreasing wire radius at a fixed density, and vice versa. The results obtained using the lower-order random-phase approximation are also given. We have compared our results for the screened potential with those for the semiconductor electron quantum wire. It is found that the screened potential for charged bosons is more attractive than that for electrons for rs<8, while the two potentials become essentially the same for rs>8. Therefore, we conclude that the exchange effects associated with the electron statistics act to oppose the overscreening properties of the charged Bose system and are dominated by the Coulomb correlation effects at low carrier densities corresponding to rs>8.

Critical slowing of local fluctuations near the paraelectric-incommensurate phase transition in Rb2ZnCl4

A study is reported of the anomalous broadening of Mn2+ EPR lines on the high-temperature side of the paraelectric-incommensurate phase transition in Rb2ZnCl4 crystals. It is shown that the resonant-line broadening is inhomogeneous and due to the contribution of low-frequency fluctuations corresponding to the central peak in the elementary-excitation spectrum. The data obtained have permitted us to obtain the critical correlation-length exponent ν=0.64±0.02 corresponding to the 3d XY model of Heisenberg.

A fermi disks model for the dynamical response of films of liquid 3He

We develop a model for the dynamical susceptibility of films of liquid 3He adsorbed on substrates. Although the film cannot be regarded as a two-dimensional system in coordinate space, we show that its spectrum of elementary excitations corresponds to that of a collection of two-dimensional Fermi liquids in momentum space, which might be pairwise coupled through the atom–atom interaction. The model is applied to compute the response of the film to density fluctuations and can be straightforwardly extended to investigate the spin susceptibility.

Spectrum of elementary excitations of a single domain wall in garnet films with in-plane magnetization

Epitaxial garnet films with in-plane magnetization are studied by microwave technique. Resonances related to different branches of domain wall excitations are observed: a low-frequency Goldstone branch of translational wall vibrations in the MHz band and a high-frequency Gilinskii branch in the GHz band. A strong coupling between neighbouring domain walls is observed, if their distance becomes comparable to the wall thichkness. In this case each resonance is split into two coupled resonant modes, their frequencies being larger at smaller wall distances. Spin wave excitations of a single wall are analyzed by numerical solution of Landau-Lifshitz and magnetostatic equations for the case of bulk orthorhombic ferromagnetic material. Unidirectional Gilinskii branch and asymmetric Goldstone branch are shown to exist in the spectrum of the wall excitations for the case of the propagation direction perpendicular to the magnetization in domains. Unexpected wall excitation spectra are predicted at some values of the anisotropy parameters when the wall becomes unstable. Such spectrum shows a gap in the wave number and a loop in the dispersion of the Goldstone branch.

Paramagnetic labeling as a method for the soft spectroscopy of electronic states

details of both the crystal-field states of the impurity ion and the electronic band spectrum of the metal. A new method for the soft spectroscopy of electronic states based on measurements of the temperature dependence of the width G MM 8 ( T) of transitions between the crystal-field states uM& of a paramagnetic ion implanted in the compound being studied is proposed. To make specific use of this method in neutron and optical spectroscopy, a classification of the types of temperature dependence of the natural relaxation width g M(T) of the levels is devised, and procedures for possible experimental methods are proposed. A nonzero value of the natural relaxation width g G(T) of the crystal-field ground state uG& of an impurity ion at zero temperature is obtained within the proposed self-consistent model, but is beyond the scope of perturbation theory. It is shown that the widely accepted estimate of the characteristic temperature of Kondo systems T*5G G(T50)/2 from the quasielastic scattering width at zero temperature G G(T50)/2 is incorrect in the case of strong relaxation in a system with soft crystal fields. The proposed model is applied to the quantitative analysis of the relaxation of the crystal-field levels of paramagnetic Pr 31 ions implanted in CeAl3 and LaAl3. The results of the calculations are in quantitative agreement with the experimental data. © 1998 American Institute of Physics.@S1063-7761~98!02005-8# The methods that have been developed for studying electronic states in metals ~angle-resolved photoemission spectroscopy; 1 quantum oscillations of the magnetic susceptibility, 2 conductivity, 3 magnetostriction, 4 and elastic moduli 5 associated with the de Haas‐van Alphen effect; infrared spectroscopy; 6 Raman scattering; 7 etc.! provide complementary information regarding the structure of electron spectra. A comparison of the experimental data obtained by different methods with the results of band calculations of the electronic structure provides fairly reliable data on the properties of the compounds studied. The methods for investigating electronic states can be divided into ‘‘hard’’ and ‘‘soft’’ methods. In the case of hard spectroscopy, the influence of the measurement process on the system exceeds the scales W* of the characteristic interactions forming the electronic spectrum of the system ~in Kondo systems W* is of the order of the Kondo temperature TK ; in variable-valence systems W* is of the order of the valence fluctuations !. Therefore, compounds with strong electron correlations, which have low-energy modes in the spectrum of elementary excitations, can be investigated most effectively by soft spectroscopic methods, in which the measurement process does not destroy the eigenstates of the system being investigated. The conditions imposed on spectroscopic measurements by the softness of the elementary excitations in variable-valence and Kondo systems greatly restricts the set of methods that are applicable to the investigation of highly correlated systems. For example, the interpretation of photoemission measurements ~because of the large energy transfers in the measurement process! and data from methods based on de Haas‐van Alphen oscillations ~because of the large magnetic fields, which can destroy the structure of soft excitations! requires a special investigation of the influence of the measurement process on the lowenergy properties of the compound being studied. Therefore, the development of new soft spectroscopic methods for highly correlated electronic systems is an important undertaking. This paper proposes a method for analyzing the electronic structure based on measurements of the temperature dependence of the relaxation of crystal-field levels of an impurity ion which has special properties ~a paramagnetic label! and is implanted in the compound being investigated. A similar idea for investigating semiconductor compounds by an electron paramagnetic resonance technique was proposed

The bipolariton model and the concept of excitonic-biexcitonic mixed modes in semiconductors

An alternative approach to the so called bipolariton concept has been proposed. We have shown that the bipolariton approximation can indeed be used for calculating the elementary excitation spectrum in the photon-exciton-biexciton system in its biexciton part. The bipolariton concept does not provide the correct spectrum in the excitonic-polariton energy part, where the existence of four-particle electron-hole bound states should be considered. In order to go beyond the bipolariton approximation we developed a Green function formalism for the theory of an interacting electron-photon system in which photons, excitons and biexcitons are treated on an equal level. Our approach is a completely microscopic treatment and is based on the assumption that the excitons and biexcitons are two- and four-particle bound states, formed by electrons and holes. The method of Legendre transforms is used to derive a set of nine independent exact equations for the corresponding vertex and Green functions. Knowledge of the solutions of the above equations provide the excitation energies of the quasiparticles (excitonic-biexcitonic polaritons) formed by coupling of photons with excitons and biexcitons. Those elementary excitations manifest themselves as common poles of the photon propagator, two- and four-particle electron-hole Green functions.

Ground state of a double-layer charged Bose system

The ground state of a double-layer charged Bose system is investigated beyond the random-phase approximation by including the many-body correlation effects. The correlations within the layer are accounted for within the self-consistent theory of Singwi, Tosi, Land, and Sj\"olander, while we have neglected the effect of interlayer correlations. The static susceptibility, the elementary excitation spectrum, and the static screened potentials are calculated and their dependence on the boson number density and the layer spacing is examined. Results are compared with the recent work of Tanatar and Das where correlations are not included. It is found that the inclusion of correlation effects combined with the interlayer interactions can significantly alter the nature of the ground state and, in particular, can favor an inhomogeneous charge-density-wave ground state. A comparison is also made in terms of screened potentials with the double-layer electron system that enables one to have a deeper insight into the role played by the exchange and Coulomb correlations separately.

A new variational method in the problem of the spectrum of elementary excitations in an edge-dislocation crystal

A new method based on a combination of the direct variational method and perturbation theory is proposed to calculate approximately the eigenvalues and eigenfunctions of ground state and the states close to it. The new method is applicable to the Schrödinger equation with a potential proportional to the dilatation produced by an edge dislocation. The ground-state energy obtained in this case is lower than the theoretical values obtained in other works. The energy value is obtained for the lowest state for which the eigenfunction is odd in the azimuthal angle. It is assumed that the spectrum may be described only statistically near the condensation point.

High-frequency magnetoelectric effects in ferroelectrics

The spectrum of elementary excitations of a ferroelectric in a magnetic field H parallel and perpendicular to a spontaneous polarization P was calculated. In the case H∥P high-frequency susceptibility and interaction of electron excitations with electromagnetic waves were analyzed. It was shown that magnetic field induces a high-frequency magnetoelectric (ME) susceptibility and ME gyration in reflected and transmitted light. ME gyration means the rotation of polarization plane of light at an angle proportional to the product (PH). For the transmitted light the angle of ME gyration in the case H⊥n (n is a normal to the' surface) depends periodically on the path travelled; and does not depend on the path if H∥n. ME gyration, in contrast to Faraday effect, is compensated at the double (forth and back) travel of light. Indicated ME effects are possible not only in ferroelectrics but also in a dielectric at the presence of constant electric and magnetic fields.

Superfluidity of q-Deformed Bosons with Weak Interaction

Superfluidity of q-deformed bosons with weak interaction is studied. Bogoliubov's method of approximate second quantization is applied to this case. Under the condition p2/m ≫ ([N]/V)U0, the spectrum of elementary excitations is found to be of the roton-type, and hence superfluidity is predicted.