Spectrum Of Elementary Excitations Research Articles

Effective field and spin-wave excitation in a narrow-band Hubbard magnet

Within the framework of strongly correlated Hubbard model the Green function of transverse spin components is derived. In the first nonvanishing approximation with respect to the inverse effective radius of interaction all contributions to the self-energy were determined. The spectrum and damping of elementary excitations is analyzed for the ferromagnetic phase. At the temperature T=0 and hole concentration c⪡1 the spin-wave stiffness constant D includes additional term which is proportional to c 4/3. It differs from the result D∼ c when the effective “dressing” of the interaction line is neglected. In order to find the temperature dependence of the susceptibility, self-consistent equations for mean spin and chemical potential have been solved numerically. The results for the temperature dependence of the magnetization are consistent with experimental values.

Single and Collective Electron Excitations in the Solid C60F18

The electronic structure of the solid polycrystalline fluorinated fullerene C60F18 has been investigated by reflection electron‐energy‐loss spectroscopy for the first time. The elementary excitation spectrum of the solid C60F18 was compared with the respective spectrum of the solid C60, and an explanation of their significant similarity in the range of π→π* transitions was suggested.

Stability and dynamics of vortex clusters in nonrotated Bose-Einstein condensates

We study stationary clusters of vortices and antivortices in dilute pancake-shaped Bose-Einstein condensates confined in nonrotating harmonic traps. Previous theoretical results on the stability properties of these topologically nontrivial excited states are seemingly contradicting. We clarify this situation by a systematic stability analysis. The energetic and dynamic stability of the clusters is determined from the corresponding elementary excitation spectra obtained by solving the Bogoliubov equations. Furthermore, we study the temporal evolution of the dynamically unstable clusters. The stability of the clusters and the characteristics of their destabilizing modes only depend on the effective strength of the interactions between particles and the trap anisotropy. For certain values of these parameters, there exist several dynamical instabilities, but we show that there are also regions in which some of the clusters are dynamically stable. Moreover, we observe that the dynamical instability of the clusters does not always imply their structural instability, and that for some dynamically unstable states annihilation of the vortices is followed by their regeneration, and revival of the cluster.

Magnon mechanism of defect reactions in solids

A phenomenological theory is developed that describes the effect of a magnetic field on the defect reactions in a solid. The theory is based on a concept regarding the lattice magnetism according to which a defect induces a magnetoactive (magnon) branch in the spectrum of elementary excitations of a crystal lacking a magnetic structure in the absence of defects. The probability of a defect complex disintegrating in a magnetic field is calculated in terms of the magnon mechanism of the reaction.

Crystals with sublattices belonging to the cubic system and special features of their elementary excitation spectra

Genesis of spectra of elementary excitations of crystals comprising sublattices belonging to the cubic system is investigated. Using the methods of group theory, the Brillouin sublattice zones are transformed into the Brillouin crystal zones, and degeneration is established caused by the convolution of branches of the spectrum. The degeneration is removed with allowance for hybridization of sublattice states. The special features of the band spectra of some crystals belonging to the cubic system are discussed.

The magnetoelastic mechanism of singlet phase formation in a two-dimensional quantum antiferromagnet

A model describing the second-order phase transition with respect to the magnetoelastic coupling parameter from the antiferromagnetic (AFM) to the singlet state in a two-dimensional quantum magnet on a square lattice is proposed. The spectrum of elementary excitations in the singlet and AFM phases is calculated using an atomic representation, and the evolution of transverse and longitudinal branches of this spectrum is studied in the vicinity of the transition point. It is established that the AFM to singlet phase transition is related to softening of the longitudinal branch of oscillations. In the singlet phase, the gap plays the role of a parameter characterizing the distance to the phase transition point. It is shown that the spectrum of transverse oscillations in the AFM phase corresponds to the Goldstone boson. Based on an analysis of the stability of the spectrum of elementary excitations, a phase diagram is constructed that determines the regions of the existence of phases with plaquette-deformed lattices.

Magnetic structure and elementary excitation spectra of copper metaborate

The calculation of spin-wave spectra of the easy-plane model of CuB 2O 4 copper metaborate has been made. The coincidence of the spectra periods along the tetragonal c -axis for the quasi-one-dimensional chains and the three-dimensional subsystem points out the ferromagnetic order inside the chains with antiferromagnetic inter-chain ordering. The exchange interaction constants were found from the comparison with the inelastic neutron scattering data.

Dynamic structure factor of the one-dimensional Bose gas near the Tonks-Girardeau limit

While the 1D Bose gas appears to exhibit superfluid response under certain conditions, it fails the Landau criterion according to the elementary excitation spectrum calculated by Lieb. The apparent riddle is solved by calculating the dynamic structure factor of the Lieb-Liniger 1D Bose gas. A pseudopotential Hamiltonian in the fermionic representation is used to derive a Hartree-Fock operator, which turns out to be well-behaved and local. The Random-Phase approximation for the dynamic structure factor based on this derivation is calculated analytically and is expected to be valid at least up to first order in $1/\gamma$, where $\gamma$ is the dimensionless interaction strength of the model. The dynamic structure factor in this approximation clearly indicates a crossover behavior from the non-superfluid Tonks to the superfluid weakly-interacting regime, which should be observable by Bragg scattering in current experiments.

Electron and atomic structure of polymers: Implication for nuclear tracks formation

A review of experimental and theoretical activities in a study of an electron and atomic structure of polymers is presented. Polymers having conjugated π -electron systems were studied experimentally in different aggregate states within the temperature interval 4.2– 300 K using spectrophotometric and spectroluminescence methods. Special attention has been paid to oxidization processes in polymers caused by ionizing radiation. The theoretical approach developed in Ukraine for electron and atomic structure calculations is based on the theory of multiple scattering of elementary excitations (electrons and phonons). This approach provides a possibility to estimate energy spectra of elementary excitations, parameters of interatomic correlations, electron density distributions, absorption and emission (photoluminescence) spectra. Study of the fluorescence spectra provides a possibility to estimate the electron structure as well as the local characteristics of latent tracks induced by nuclear particles. Dependencies of the shape of photoluminescence bands can be investigated experimentally for different kinds of nuclear particles within a wide energy range. Application of combined experimental and theoretical approach of investigation of solid-state nuclear track detector (SSNTD) physical properties, and determination of nuclear particle characteristics (charge, atomic number, etc.) are discussed.

Phase transitions in a ferromagnet with biquadratic exchange and hexagonal single-ion anisotropy

The phase states and the spectra of elementary excitations of a ferromagnet with a biquadratic exchange interaction and a spin-2 magnetic ion are investigated. The phase diagrams of the system are constructed for different relationships among the material constants. It is shown that the phase diagram is substantially altered at large values of the single-ion anisotropy constants and biquadratic exchange constant.

Low-temperature thermodynamics of one-dimensional Wigner glass on a highly disordered host lattice

The low-temperature thermodynamics of one-dimensional (1D) Wigner glass on a disordered lattice (WGDOHL), which comes to existence when the inter-electron distances exceed noticeably the inter-site ones, has been studied. An efficient computer procedure, based on the presentation of the statistical sum as a product of random transfer-matrixes, has been developed for calculations of thermodynamic characteristics of the system under consideration. The lattice structures were varied from completely chaotic up to strictly regular ones. It has been established that for any degree of disorder the entropy and heat capacity of the system tend to zero linearly as the temperature is reduced. It is shown that the spectrum of elementary excitations has gapless structure. An instability of 1D WGODHL has been revealed: under conditions of vanishingly small disordering of the lattice, the long-range order in 1D WGODHL is broken by frustrations that are 1D analogs of the frustrations in two- and three-dimensional spin glasses.

Low‐temperature thermodynamic properties of one‐dimensional disordered electron lattice system

AbstractThe ground state and low‐temperature thermodynamic properties of a one‐dimensional Generalized Wigner crystal on disordered host‐lattice have been studied. It is established that the spectrum of elementary excitations has a gapless structure. An instability of the ground state with respect to arbitrary small disordering of host‐lattice is revealed. This instability leads to long‐range correlations breaking. An influence of long‐range action of the inter‐particle repulsion potential on thermodynamic properties is studied. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

THERMODYNAMICS OF LOW-DENSITY ELECTRON GAS ON HIGHLY DISORDERED ONE-DIMENSIONAL HOST LATTICE

The low-temperature thermodynamics of a one-dimensional electron gas on a disordered lattice, which comes to existence when the inter-electron distances exceed noticeably the inter-site ones, has been studied. An efficient computer procedure, based on the presentation of the partition function as a product of random transfer-matrixes, has been developed for calculations of thermodynamic characteristics of the system under consideration. The lattice structures were varied from completely chaotic up to the strictly regular one. It has been established that for any degree of disorder the entropy and heat capacity of the system tend to zero linearly as the temperature is reduced. The conclusion about the gapless character of the elementary excitations spectrum has been made. An instability of one-dimensional electron gas on a disordered lattice has been revealed: under conditions of vanishingly small disordering of the lattice, the long-range order in the systems under consideration is broken by frustrations that are one-dimensional analogues of the frustrations in two- and three-dimensional spin glasses.

Low-temperature thermodynamic properties of a one-dimensional disordered electron lattice system

A new method of low-temperature thermodynamic calculation of a one-dimensional generalized Wigner crystal on a disordered host-lattice is proposed. This method is based on the system statistical sum presentation in terms of modified transfer-matrixes. A gapless structure of the elementary excitations spectrum of the system under consideration is found.

Sum-rule approaches to the elementary excitation spectrum of interacting gases of finite bosons in harmonic potentials

Based on the corrected sum rules and generalized virial identities, we derive an expression for all modes of excitation spectrum of interacting Bose gases at finite atom numbers in axially anisotropic potentials, in terms of the N-body ground state average. Using the variational Gaussian calculation for the ground-state wave function, its explicit analytic formulas are obtained. These results show clearly the dependence of excitation spectrum on the interaction strength parameter $p = \sqrt{2/\pi}({a_{\rm sc}}/{a_{\rm ho}})(N-1)$ and trap geometry parameter $ \lambda $ for the system with N = 1 through $N\rightarrow \infty $ . For $ \lambda = 0$ and 1 the dependences have simple and intuitive physical interpretations. We compare the low-lying excitation spectra with the existing numerical results and make quantitative predications for future experiments and numerical simulation for higher-lying excitation modes.

A density-functional approach to fermionization in the 1D Bose gas

A time-dependent Kohn–Sham scheme for 1D bosons with contact interaction is derived based on a model of spinor fermions. This model is specifically designed for the study of the strong interaction regime close to the Tonks gas. It allows us to treat the transition from the strongly interacting Tonks–Girardeau to the weakly interacting quasicondensate regime and provides an intuitive picture of the extent of fermionization in the system. An adiabatic local-density approximation is devised for the study of time-dependent processes. This scheme is shown to yield not only accurate ground-state properties but also overall features of the elementary excitation spectrum, which is described exactly in the Tonks-gas limit.

The Δ chain with different base–base and base–vertex interactions

On the basis of the recently determined crystallographic structure of the delafossiteY CuO2.5, we argue that the Cu–O network has nearly independentΔ chains, with however different interactions between thes = 1/2 spins, owing to the different angles, distances and coordinations of the Cu ions. Althoughband-structure calculations are still lacking, motivated by this observation we study herethe sawtooth lattice for different ratios of the base–base and base–vertex interactions,Jbb/Jbv. By exact diagonalization and extrapolation to the infinite-size limit, weshow that the elementary excitation spectrum is the same for total spinsStot = 0 and 1,but not for Stot = 2, and has a gap only in the interval . The gap, dispersionless for Jbb = Jbv, acquires increasing k-dependence as the ratio Jbb/Jbv moves away from unity, with the minimum energy excitations fork = 0 (k = π)when Jbb/Jbv<1 (Jbb/Jbv>1). Finally, we show that the gap closure is related to the instability of the dimers in theground state as the difference between the interactions increases.

On the possibility of simultaneous spiral and superfluid ordering in a Fermi liquid

A study is made of one of the possible forms of ordering of Fermi systems—superfluid spiral ordering, wherein not only the phase invariance of the state is broken but so are the translational invariance and the invariance with respect to spin rotations. A general method of studying superfluid spiral ordering is formulated on the basis of the Fermi-liquid approach to the consideration of superfluid states. For a one-component Fermi system we obtain the self-consistency equations for four order parameters and the temperature of the simultaneous phase transition to the superfluid and spiral states. The system of equations is investigated in the case of two nonzero order parameters. The transition temperature and the energy gap in the spectrum of elementary fermionic excitations are obtained as functions of the parameter of the spiral. The region of values of the spiral parameter in which the spiral superfluid ordering can exist is determined. The correlation function of the spins in the presence of spiral ordering is investigated.

Bose-Einstein condensates in 1D optical lattices

We discuss the Bloch-state solutions of the stationary Gross-Pitaevskii equation and of the Bogoliubov equations for a Bose-Einstein condensate in the presence of a one-dimensional optical lattice. The results for the compressibility, effective mass and velocity of sound are analysed as a function of the lattice depth and of the strength of the two-body interaction. The band structure of the spectrum of elementary excitations is compared with the one exhibited by the stationary solutions (``Bloch bands''). Moreover, the numerical calculations are compared with the analytic predictions of the tight binding approximation. We also discuss the role of quantum fluctuations and show that the condensate exhibits 3D, 2D or 1D features depending on the lattice depth and on the number of particles occupying each potential well. We finally show how, using a local density approximation, our results can be applied to study the behaviour of the gas in the presence of harmonic trapping.

The thermodynamic properties and special features of spectra of elementary excitations of unstable valence Sm-and Ce-based compounds

The experimental data on the spectra of elementary excitations measured by inelastic neutron scattering and on the heat capacity and the coefficient of thermal expansion are used to analyze the correlation between the spectral characteristics of the electron and phonon subsystems and the special features of the temperature dependence of the thermodynamic properties of a number of unstable valence Sm-and Ce-based compounds. The anomalous behavior of the thermodynamic properties of these compounds is defined by the special features of their phonon and electron (4f and conduction electrons) spectra. The rearrangement of the 4f-electron spectrum as a result of temperature variation plays a decisive part in the formation of temperature dependences of the heat capacity and the coefficient of thermal expansion of unstable valence systems.