Spectrum Of Elementary Excitations Research Articles

The soliton solutions of ϕ6field theory at finite temperature

The pair-cutoff real-time Green function method with the coherent state approach is used to investigate the (1+1)-dimensional ϕ6 field theory. The topological and non-topological soliton solution as well as their elementary excitation spectra at finite temperature are found. Two critical temperatures, one corresponding to the topological soliton disappearance and the other to the symmetry breaking restoration, are given.

Elementary electronic excitations in a quasi-two-dimensional electron gas.

The complete elementary excitation spectrum for a quasi-two-dimensional electron gas is calculated within the random-phase approximation for both single and multilayer systems. Specific predictions are made for the direct observation via inelastic light scattering of a number of new modes and mode-coupling effects.

Spectrum of Excitations of an Easy‐Plane Ferromagnet (S = 3/2) in a Magnetic Field

AbstractThe quantum spectrum of elementary excitations of easy‐plane ferromagnet with S = 3/2 in a magnetic field is calculated by the diagram technique for Hubbard operators. It is established that the collective excitation spectrum in the low‐temperature region consists of one longitudinal and two transverse branches. The limiting cases of strong and weak anisotropy are analysed in detail. In the region of small values of quasimomentum the intervals are determined in whose limits the ω(q) dependence is quadratic and linear in quasimomentum modulus.

Theory of glass dynamics: The low-lying modes of anharmonic materials.

We study the lowest-lying excited states of arbitrary anharmonic solids (crystalline, glassy, or amorphous). Defining the harmonic (``skeleton'') lattice underlying any such anharmonic medium, we show that its spectrum of elementary excitations provides a rigorous upper bound to that of the anharmonic medium. We then calculate upper bounds to that spectrum, in terms of an isotropic, homogeneous, harmonic medium.

ABSENCE OF A GAP IN THE EXCITATION SPECTRUM (PHONONS) OF ANHARMONIC SOLIDS

Acoustic phonons are the suitably quantized low-lying normal modes of elastic solids. Their energies ε(k) are given by ε(k)=ħω(k), where the frequencies ω(k) are proportional to k (k=wavevector, or inverse wavelength) and vanish as w=sk (s=speed of sound) in the limit k→0. Here we prove a similar result for a reasonably general class of anharmonic solids, applicable even to such solids as the various He4 phases, H2 molecular solid, etc., which are in the extreme quantum limit. We show that the spectrum of elementary excitations in the harmonic solid provides an upper bound to the spectrum of elementary excitations in the similar anharmonic solid having the same ground state interatomic spring constantsK.

Spin-wave gap in uniaxial spin glasses.

The spectrum of elementary excitations (``spin waves'') is computed within the Sherrington-Kirkpatrick model for spin glasses with single-ion uniaxial anisotropy. It is shown that in the longitudinal spin-glass phase (when only spin components along the ``easy axis'' are frozen) the spin-wave spectrum has a gap which vanishes, as the anisotropy is reduced, at a nonzero value of the anisotropy, namely at the critical value for the onset of transverse spin-glass freezing. The gap is computed as a function of the anisotropy.

Triplet Correlation Effects in the Maxon-Roton Spectrum of Superfluid 4He

Using Brillouin-Wigner perturbation theory up to the second order, it was shown that triplet correlations play an important role in the spectrum of elementary excitations of liquid 4He as well as in the ground state. Their influence is increased with the increase in density. These effects for Lennard-Jones potential, with respect to Jastrow's correlations, are from 16 to 20% in maxon and 15 to 26% in roton part of the spectrum. It was also found that at the particle densities (in 1028 m-3) beyond 2.46 for MDD-2 and 2.8 for Lennard-Jones the energy of a maxon is greater than the double roton energy. The effects of different potentials were also studied. It was shown that Bruch-McGee's MDD-2 potential leads to the smallest value of the roton gap.

A Scaling Approximation of Quasi-Particle Interaction in Helium II

The thermodynamic variables are expressed in terms of the elementary excitation spectrum obtained from the combined correlation-basis-function and canonical transformation method. A scaling factor accounting for the quasi-particle interaction is logically introduced. By adjusting its value we obtain good results to the thermo-dynamic variables for superfluid helium 4.

Elementary-excitation spectrum of a weakly interacting bose system

We study the elementary-excitation spectrum of a system of bosons interacting with a spherical-shell potential function from a theory combining the advantages of the method of correlated-basis-function and canonical transformation. By fitting physical parameters with those of liquid helium four into our derived equations, the computed results agree reasonably well with the experimental ones obtained from neutron scattering.

Effect of an electric field on the energy spectrum and hydrodynamics of superfluid helium

A strong electric field leads to anisotropy of the spectrum of elementary excitations of superfluid helium and affects the hydrodynamics, reducing the superfluid flow perpendicular to the field.

Complete integrability of the supersymmetric model (cos ?)2

Complete integrability of the supersymmetric, two-dimensional “sine-Gordon” model of field theory within the framework of the Hamiltonian interpretation of the method of the inverse problem is proved. The classical r-matrix of the model is computed, and its equivalence to the r-matrix the Grassmann Thirring model is established. Variables of “creation-annihilation” type are constructed, and the spectrum of elementary excitations of the system is obtained.

Low-temperature thermodynamics of Heisenberg helimagnets

The authors present a new variational approach suitable for studying the temperature-dependent properties of Heisenberg helimagnets. This problem brings intriguing difficulties; indeed a direct extension to helimagnets of the techniques which are successful for collinear systems, provides unsatisfactory results. Recent calculations concerning quantum contributions to the elementary excitation spectrum at zero temperature have pointed out that magnon-magnon cubic interactions, which are neglected in standard variational methods, are essential to prevent an obvious violation of the Goldstone theorem. Thus the authors modify the variational approach in order to take such interactions into account and we obtain the temperature dependences of the helix wave-vector and the spin wave spectrum which at vanishing temperature substantiate previous perturbative calculations. The phase diagram of the ANNNH model is obtained on the basis of the authors' theory.

Flux sensitivity of a piecewise normal and superconducting metal loop.

We consider a loop composed of a superconducting segment and a normal segment with an Aharonov-Bohm flux through the hole of the loop. The normal segment is assumed to be long compared to the superconducting coherence length \ensuremath{\xi} but short compared to a mean inelastic diffusion length. The elementary excitation spectrum of the ground state of this loop is periodic with period hc/2e as long as the superconducting segment is larger than \ensuremath{\xi}. If the superconducting segment length becomes of the order of \ensuremath{\xi}, quasiparticles can tunnel through the superconducting gap and give rise to an excitation spectrum which is periodic with period hc/e. .AE

Dielectric response of one-dimensional semiconductor quantum wires

We develop a non-singular theory for the dielectric response of one dimensional quantum wires as occuring, for example, in semiconductor quantum well and metal-insulator-semiconductor inversion layer systems. The theory includes collisional broadening and realistic wavefunction effects. The dynamical response of a lateral two-dimensional superlattice (made from a periodic array of such quantum wires) is calculated to obtain the elementary excitation spectrum. The plasmon spectrum in the superlattice forms a band, and the characteristic one-dimensional to two-dimensional plasmon transition is seen as the superlattice period is decreased.

Dielectric response of one-dimensional semiconductor quantum wires

Abstract We develop a non-singular theory for the dielectric response of one dimensional quantum wires as occuring, for example, in semiconductor quantum well and metal-insulator-semiconductor inversion layer systems. The theory includes collisional broadening and realistic wavefunction effects. The dynamical response of a lateral two-dimensional superlattice (made from a periodic array of such quantum wires) is calculated to obtain the elementary excitation spectrum. The plasmon spectrum in the superlattice forms a band, and the characteristic one-dimensional to two-dimensional plasmon transition is seen as the superlattice period is decreased.

Finite-size study of the one-dimensional spin-(1/2) dimerized Heisenberg chain.

We present an analysis based on the extrapolation of finite-size results to the infinite one-dimensional spin-(1/2) dimerized isotropic Heisenberg system for the whole range of the dimerization parameter \ensuremath{\delta} (\ensuremath{\Vert}\ensuremath{\delta}\ensuremath{\Vert}\ensuremath{\le}1) at zero temperature. This system undergoes a transition at \ensuremath{\delta}=0, and a gap opens in the spectrum of elementary excitations. The exponent \ensuremath{\nu}, which characterizes the opening of the gap, is estimated with use of the finite-size results (with size up to N=18). We investigate two finite-size-scaling hypotheses, assuming a pure power-law behavior [case (1)], or taking into account logarithmic corrections [case (2)]. To estimate \ensuremath{\nu}, we use the derivative of the reciprocal of the gap, the derivative of the gap, and the Callan-Symanzik function. We show that the first of these is less affected by finite-size corrections than are the other two. Using it, we have obtained, in case (1), \ensuremath{\nu}=0.71\ifmmode\pm\else\textpm\fi{}0.01, in agreement with previous estimates, and in case (2), \ensuremath{\nu}=0.668\ifmmode\pm\else\textpm\fi{}0.001, in very good agreement with the value \ensuremath{\nu}=(2/3) conjectured by den Nijs. We also show that, far from criticality, the ground-state energy per site may be described by the form \ensuremath{\Vert}\ensuremath{\delta}${\ensuremath{\Vert}}^{x}$ with x=1.34\ifmmode\pm\else\textpm\fi{}0.02. However, results for the derivative of this quantity show a different functional dependence upon \ensuremath{\delta}, at least for \ensuremath{\delta}\ensuremath{\gtrsim}0.4. In fact, both the values of the ground-state energy and its derivative agree with the third-order perturbation theory of Harris, with agreement improving as \ensuremath{\delta} approaches 1.

Correlations of a zero-temperature two-dimensional charged Bose gas with ln(r) interaction.

Some properties of a two-dimensional charged Bose gas interacting with a ln(r) potential are investigated on the basis of a self-consistent-field formalism which includes the short-range correlations between bosons through a local-field correction depending on the pair correlation function. Numerical results for the static structure factor, pair correlation function, spectrum of elementary excitations, and the response of the system to a static impurity charge are presented. Our results show the same qualitative behavior as those of the three-dimensional analog but the correlation effects are somewhat more pronounced in two dimensions.

Surface energy-loss function for the inelastic scattering of electrons from a metal substrate with an overlayer of adsorbed alkali-metal atoms.

We investigate the spectrum of elementary excitations of the conduction electrons associated with an overlayer of sodium atoms adsorbed on a metal surface. We compute the surface energy-loss function for the small-angle backscattering of electrons as function of coverage (up to two full monolayers of sodium atoms are considered). The ground-state problem is dealt with by using Lang's uniform-background model of alkali-metal-atom--metal-surface chemisorption. The dynamical response of the electron gas is treated in the random-phase approximation. Previous theoretical work has, for the most part, assumed that an overlayer of atomic thickness responds as bulk matter. Our microscopic calculation reveals that for one-monolayer coverage the overlayer's response in fact deviates significantly from bulklike behavior. For two-monolayer coverage the response is bulklike, with a prominent collective (plasmon) mode. The quantum-mechanical nature of the overlayer response is displayed in detail. A case is made for the need that an angle-resolved electron-energy-loss experiment be performed, using an s-p-bonded metal, such as aluminum, as the substrate (instead of a transition metal).

The self-consistent theory of elementary excitations in systems with many-branch quasiparticle spectra (ferromagnetic semiconductors)

A unified self-consistent theory of the mutual influence of the electronic and spin subsystems in the s-f model approximation for ferromagnetic semiconductors is developed. The calculations are based on the novel approach of the two-time Green function method. It consists in the introduction of irreducible Green functions (IGF) and the derivation of the exact Dyson equation and the exact self-energy operator. It is shown that the IGF method gives a unified and natural approach for the calculation of the elementary excitation spectrum and damping. The full electronic and magnetic quasiparticle spectra of the s-f model are derived by taking explicitly into account magnon-magnon, electron-magnon and electron-electron scattering processes. The recent Babcenco and Cottam results (1981) follow from this theory in the lowest-order approximation.

Quantum theory of the two-dimensional Bose liquid

The local matter density and current density have proven natural coordinates for the study of the elementary excitation spectrum in the Bose liquid. A fundamental decomposition of the current density into algebraic and topological parts is shown to yield both quasiparticle and pseudoparticle excitations.