Spectrum Of Elementary Excitations Research Articles

One-dimensional frustrated Heisenberg antiferromagnet of spin 1/2 in a strong magnetic field.

The properties of the one-dimensional antiferromagnet of spin 1/2 in a strong magnetic field h with nearest- and next-nearest-neighbor interactions are studied in terms of the equivalent Bose-gas problem. We find that below the first critical field ${\mathit{h}}_{\mathit{c}\mathit{u}}$ the phase with a gap in the elementary excitations spectrum and corresponding plateau in the magnetization curve should exist due to the two-particle attraction in the vicinity of the commensurate wave number ${\mathit{p}}_{0\mathit{u}}$=\ensuremath{\pi}. In the interval between the first and second critical fields ${\mathit{h}}_{\mathit{c}\mathit{u}}$h${\mathit{h}}_{\mathit{c}\mathit{f}}$, the system behaves as the one-dimensional Bose gas with particle repulsion and the magnetization has a typical square-root dependence on field.

On a structure of superfluid helium-4 elementary excitation spectrum

New experimental data of the liquid helium inelastic neutron scattering are reported. With the help of two Gaussian fitting procedure it was shown that for wave vectors q>q 0=0.48& -1 , the sharp phonon-maxon-roton peak consists of two modes, thus, elementary excitation spectrum possesses a complex structure. We can assume that one of these modes belongs to the neutron scattering as on the usual, “normal” fluid, while the other mode being connected with the liquid excitations induced by the presence of the bose-condensate.

Dynamics of quantum films

From no assumptions other than a microscopic many-body Hamiltonian, we calculate the elementary excitation spectrum of mono- and double-layer helium films. Essential to quantitative predictions is the inclusion of time-dependent short-range correlations of the liquid.

Intersubband Elementary Excitations of Quasi-Two-Dimensional Electron Systems in Quantum-Well

Theoretical study is made for the dispersion relation and for the spectra of intersubband elementary excitations in the quasi-two dimensional electron gas of GaAs/AlGaAs quantum well. Density-density correlation functions appearing in expressions for transition probabilities are calculated by means of the perturbation expansion with respect to the Coulomb interactions between electrons in the conduction subbands. From the calculated results, at long-wavelength limit the effects of the attractive Coulomb interactions between electrons and holes on the energy shift of the collective excitations from the single-particle excitations are shown to be comparable to the depolarization effects. The peak positions of the collective charge- and spin-density excitations in spectra are in qualitative agreement with the recent experimental findings.

Elementary excitations in ferroelectromagnetics with “nonfrozen” orbital momentum

Abstract Spectrum and character of elementary excitations in uniaxial ferroelectric ferromagnetics with a magnetic moment of orbital nature have been studied in the cases of parallel and mutually perpendicular orientations of equilibrium electric and magnetic dipole moments. Unusual high-frequency excitations, i.e. oscillations of the magnetic moment with change of its magnitude and precession of the electric polarization vector are predicted.

First viscosity of dilute3He-4He mixtures below 0.6 K

Starting with the Boltzmann transport equation, the first viscosity of dilute3He-4He mixtures for various3He concentrations x is evaluated up to around T ≅ 0.6 K by including the contribution from three-phonon processes (3PP) in the anomalous elementary excitation spectrum of liquid4He. Due to 3PP, the characteristic time τη for3He viscosity at high temperatures, i.e., T⩾2TF where TF is the3He Fermi temperature, is evaluated as 5 × 10−12/xT, which is smaller than the value estimated by Rosenbaum et al. This is interpolated with τη in the degenerate (quantum) region, T≪TF. The obtained viscosities are in better agreement with experimental results than those of Baym and Saam, whose theory does not include 3PP. However, at very low concentrations there exists a discrepancy between the present theory and experiments, so that an alternate treatment should be considered.

Muonic-molecule formation in a high-density D-T system: Application to a solid phase.

We discuss the resonant formation mechanism of muonic molecular ions in a high-density D-T system, putting emphasis on a condensed-matter effect. After generally formulating the problem, we apply the formulation to a solid phase of an admixture of ${\mathrm{D}}_{2}$, DT, and ${\mathrm{T}}_{2}$. The spectral intensity distribution of a whole system proves to be essential in understanding the above-mentioned effect. Namely the numerically calculated distribution exhibits the presence not only of a Lorentzian peak centered at a conventional, slightly shifted resonance energy but also of a broad peak lying over an energy domain that extends from the resonance energy with a scope comparable to the bandwidth of the elementary excitation spectra of the solid. The profile of the spectral distribution suggests the possibility that the condensed matter should take part in the quasiresonant reaction as an absorber of excess energy in the initial state. In fact, the formation rate at low temperatures turns out to be enhanced dramatically in the presence of subthreshold resonance levels, as compared with Vesman's mechanism [Sov. Phys. JETP Lett. 5, 91 (1967)], i.e., molecular formation in a free space. The numerical calculation of formation rates is carried out for a target system involving ortho-, para-${\mathrm{D}}_{2}$, or their thermal admixture.

Collective excitations, pressure, and compressibility in multilayer systems.

We present a simple method to calculate the spectrum of elementary collective excitations in semiconductor superlattices. It is based on a variational many-body calculation with the hypernetted-chain approximation and the transformation of the structure functions and other quantities depending on the lattice layer indices into Bloch sums. In this method, we can calculate the excitation spectrum for electron and electron-hole superlattices and other more complicated structures. We also calculate the pressure and compressibility in a system of arbitrarily many electron (or hole) gas layers.

Antiferromagnetic and spiral phases in a t-t'-J model.

We present a study of a [ital t]-[ital t][prime]-[ital J] model on a square lattice, where [ital t][prime] denotes hopping along the diagonal, within a suitable 1/[ital N] expansion. The [ital T]=0 phase diagram consists of conventional ferromagnetic and incommensurate ([ital Q],[ital Q]) and ([ital Q],0) spiral phases, in addition to an unconventional antiferromagnetic (AF) phase characterized by the usual ([pi],[pi]), i.e., Neel modulation of the spin configuration and an unusual ([pi],[minus][pi]) phase modulation. We obtain the spectrum of elementary excitations for all these phases and explicitly display the associated Goldstone-mode structure. A detailed analysis of the AF phase including leading quantum-fluctuation effects shows that it remains locally stable at small finite doping, in agreement with the observed behavior in doped antiferromagnets, as well as globally stable against phase separation for sufficiently large [ital t][prime]. A characteristic feature of this AF phase is an [ital anisotropy] in the velocities of its elementary excitations which is suppressed, however, at half-filling. Finally, the spin-spin dynamical structure factor in the AF phase is found to exhibit a double peak at finite doping, which reflects hybridization between the relevant transverse (magnonlike) and longitudinal (holonlike) modes, one of the peaks also being suppressed at half-filling.

Spectrum of elementary excitations of magnetic domain wall

The interaction processes of translation mode and flexural domain wall excitations in yttrium iron garnet single crystals are investigated by magneto-optic and induction techniques. The moving wall magnon spectrum characteristics are determined. The flexural mode eigen frequencies ν r are found to asymmetrically depend on the wall velocity V: ν r( V)≠ν r(- V). In the same time the wall flexural waves themselves are established to influence on its translation motion.

The q-Deformed SU(2) Quantum Antiferromagnetic Model

In this paper we set up a q-analogous Holstein-Primakoff transformation (HPT) and obtain the q-deformed spin-wave solutions. We apply a mean-field treatment that is arranged to preserve some nonlinearity of the Hamiltonian, and the results turn out to be interesting, e.g., a gapful spectrum of the elementary excitation spectrum is found, which is different from the spin-wave results for the conventionial model, and the specific heat is therefore modified greatly. The properties of the Néel state and the ground state are investigated.

Perturbation Approach for the Localization Transition in the Dissipative Two-State System11The project supported in part by Shanghai Science Foundation through Grant No. BA03812

Analysis of a dissipative two-state system at zero temperature shows that the model Hamiltonian may be exactly reduced to a modified quantum sine-Gordon model, which describes the effective interactions between the low-frequency phonons under coupling with a tunneling system. Directly considering the infrared divergence encountered in the conventional perturbation treatment, we have developed a new perturbation approach for the effective Hamiltonian, and derived the exact critical conditions for the localization transition. In the critical regime, a gap will be opened near zero momentum in the elementary excitation spectrum of the low-frequency phonons, and the corresponding ground state wave function is found to be a pairing quasi-particle state, analogous to the BCS superconducting state.

Logarithmic corrections in the one-dimensional Hubbard model with attraction

The free energy in the vicinity of a half-filled band for the one-dimensional Hubbard model with attraction is calculated as a function of density (chemical potential). The expansion of the elementary excitation energies for a lattice of finite length N is accomplished with an accuracy of the order (N2 ln N)−1. An explicit expression for the free energy and the elementary excitation spectrum as functions of external field or of volume N is necessary for studying the asymptotic behavior of correlation functions.

Magnetoexcitons in finite-size semiconductor quantum wells

The spectrum of specific elementary excitations in a many-electron system, i.e., magnetoexcitons, is investigated theoretically. It is shown that in finite-size quantum wells there exist additional branches of magnetoexcitons in comparison with ideal two-dimensional systems. The new branches differ from the known branches in their dispersion laws. The general features of the spectrum for all types of magnetoexciton are obtained.

Temperature variation of the elementary excitation spectrum of thin liquid-4He films.

The temperature variation of the elementary excitation spectrum of thin liquid${\mathrm{\ensuremath{-}}}^{4}$He films is derived within the ring diagram approximation. This theory is microscopic only in the long-wavelength limit. Using this anomalous spectrum, the specific-heat data adsorbed on Grafoil graphite and the first, second, and third sounds are analyzed. The temperature variation of the phonon spectrum is very negligible for low temperatures. However, with increasing temperature from 0.6--0.7 K to near the vicinity of the two-dimensional transition temperature of 1.21 K, the temperature effect is significant regarding the physical properties of thin liquid${\mathrm{\ensuremath{-}}}^{4}$He films.

Elementary excitations ofC60from the far infrared to the far vacuum ultraviolet studied by high-resolution electron-energy-loss spectroscopy

We report on measurements by high-resolution electron-energy-loss spectroscopy (HREELS) of the excitation spectrum of thin films of ${\mathrm{C}}_{60}$ (containing less than 10% of ${\mathrm{C}}_{70}$) deposited onto Si(100) in ultrahigh vacuum. Most of our spectra have been taken in the specular direction of the reflected beam. By varying the primary-electron energy from 2 to 150 eV, the frequency regions extending from the far ir, through the visible-uv, to the far vuv have been investigated on one single specimen in the same spectrometer with a resolution varying from 10 meV in the ir to 60 meV in the vuv. Since no strict selection rules apply to electron inelastic scattering, the present data contain information on many of the elementary excitations of the ${\mathrm{C}}_{60}$ fullerite film. Several of the spectral features are comparable to the observations of photon, photoelectron, and neutron spectroscopies which address separate parts of the spectrum. Important additional features are revealed by HREELS, including a transition at 1.55 eV which appears to be the lowest one-electron excitation observed in ${\mathrm{C}}_{60}$ and two collective excitations at 6.3 eV (\ensuremath{\pi} plasmons) and 28 eV (\ensuremath{\sigma} plasmons). When possible, the band intensities are discussed with the help of the dielectric theory of EELS. The present data further advance our knowledge of the rich spectrum of elementary vibrational and electronic excitations of the ${\mathrm{C}}_{60}$ fullerite.

Elementary excitations in one-dimensional quantum wires: Exact equivalence between the random-phase approximation and the Tomonaga-Luttinger model.

We show---contrary to the viewpoint that the random-phase approximation (RPA) is progressively worse in lower dimensions---that the simple random-phase approximation provides an exact description for intrasubband plasmon dispersion in one-dimensional quantum wires, by establishing a general equivalence between the RPA and the strongly correlated Tomonaga-Luttinger model for the elementary-excitation spectra in one-dimensional Fermi systems. We also discuss the formal analogy between intrasubband and intersubband collective modes in quantum wires by showing that the one-dimensional intrasubband collective excitations can also be thought of as depolarization-shifted single-particle excitations. Our results explain why recent experimentally observed one-dimensional-plasmon dispersion in GaAs quantum wires can be quantitatively described by the RPA.

Elementary Excitation Spectra in the s-d Model at Finite Temperatures

Elementary excitations in the s-d model are calculated at finite temperatures with the use of the Bethe-ansatz solution. Our formulation of the spectra is based on Yang and Yang's method which has been developed in the one-dimensional interacting boson system. We investigate the temperature dependence of the spin excitation spectra. It is shown that the narrow peak structure develops in the low energy region as the temperature is decreased. At low temperatures, the shape of the spectra is described well by the renormalized resonance level model.

Collective Behaviour of Photoinjected Plasma in Semiconductors

AbstractThe spectrum of elementary excitations in the nonequilibrium state of a semiconductor under high levels of continuous photoexcitation is studied. Resorting to a quantum transport theory based on the nonequilibrium statistical operator method, the dynamic dielectric response function of the carrier system in the far‐from‐equilibrium steady state is derived, to calculate the Raman scattering spectrum and from it to analyze the spectrum of elementary excitations. Besides the expected bands due to quasi‐particle and collective plasma excitations, the presence is demonstrated of two additional bands associated to low energy excitations and with a linear dispersion relation, and hence termed acoustic plasma modes. The analysis of these two bands points to an interpretation of them as due to the collective motion of the electrons (the ones higher in energy) and of the holes (the ones lower in energy) interacting through the screened Coulomb interaction. At low temperature their lifetimes are mainly determined by dissipative effects mediated by the interaction of the carriers with the radiation recombination field.

Period tripling in the bilinear-biquadratic antiferromagnetic S=1 chain.

We study numerically the elementary excitation spectrum of the most general isotropic spin-1 chain with bilinear-biquadratic nearest-neighbor coupling. Using finite-size scaling, a massless phase with a period tripling in the ground state is found to exist in an extended region around the Lai-Sutherland point. The location of the transition from the known valence-bond-like phase to the trimerized phase cannot, however, be given precisely. We also present results on the ground-state two-point correlation function.