Collective excitations in low-density 2D electron systems

We report the observation of sharp plasmon and magnetoplasmon modes in ultra-low-density 2D electron systems. Well defined dispersions for the modes are observed at densities as low as 1.1×10⁹ cm⁻² and with excitation wave vectors as large as 1.2×10⁵ cm⁻¹. Interestingly, both modes are found to be more easily measured in low-density systems than in high-density systems. The strength of the light scattering cross-sections at low density suggests potential applications to the study of quantum phase transitions at large r_s.     

Collective electron excitations in 3D graphite clusters

Group-theoretical analysis and subsequent quantum-chemical calculations based on the molecular orbital method applied to a cyclic model of 3D semimetallic graphite lead to a multiplet of spectroscopic combinations of Slater determinants. The transition energies ΔE between terms of the multiplet are interpreted as the energies of collective electron mesoscopic excitations ?ω in the entire set of electron states characterizing the metal-type conductivity of a cluster. The estimate ?ω～0.2ΔE(N ₀/1000)^2/3 is obtained for a cluster consisting of N ₀ primitive cells. Depending on the thermal processing, N ₀=(0.3–20)×10⁶ in pyrolytic graphite, and accordingly ?ω～(10–150) eV. In the case when the energy cannot be determined accurately, methods permitting the variation of an excitation over a wide range (such as the spectroscopy of synchrotron radiation absorption and the characteristic energy losses of charged particles) appear to be the most promising.     

Solitary excitations in one-dimensional ferromagnetic spin chains with biquadratic exchange interaction

By using the traveling wave method, the solutions of the elliptic function wave and the solitary wave are obtained in a ferromagnetic spin chain with a biquadratic exchange interaction, a single ion anisotropic interaction and an anisotropic nearest neighbour interaction. The effects of the biquadratic exchange interaction and the single ion anisotropic interaction on the properties (width, peak and stability) of the soliton are investigated. It is also found that the effects vary with the strengths of these interactions.     

Calculation of the average values of the Zeeman and dipole—dipole spin energies of a paramagnet at low temperatures

For average values of the Hamiltonians of the Zeeman energy and the dipole-dipole spin interaction energy, a series is obtained representing the expansion of these averages in powers of the inverse temperature of the dipole-dipole interaction reservoir. The average values of specific operators appearing in the series as coefficients of the series are calculated from the complete density matrix of the system. These coefficients essentially are the low-temperature moments of resonance absorption lines of the spin system.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 76–78, September, 1991.     

Collective singlet excitations and evolution of raman spectral weights in the 2D spin dimer compound SrCu2(BO3)(2)

Raman light scattering of the two-dimensional quantum spin system SrCu2(BO3)(2) shows a rich structure in the magnetic excitation spectrum, including several well-defined bound state modes at low temperature, and a scattering continuum and quasielastic light scattering contributions at high temperature. The key to the understanding of the unique features of SrCu2(BO3)(2) is the presence of strong interactions between well-localized triplet excitations in the network of orthogonal spin dimers realized in this compound.     

Dynamics of spin fluctuations in the Heisenberg paramagnet

The lowest-energy poles of the dynamic spin propagator for the Heisenberg model at T ? T_c are estimated using a gaussian approximation for the second-order memory function. The poles are connected with the observed maxima of the neutron scattering function at large momentum transfer.     

Collective excitations in superconductors

The excitations connected with quantum condensation of a fermion gas are calculated quantitatively along the lines of a previous paper. We find exact agreement with earlier results: excitations of the phonon type in the absence of Coulomb interaction, strong indications for their nonexistence in the presence of this interaction.     

Localized spin excitations in magnets with triangular spin ordering

Some types of localized spin excitations in hexagonal antiferromagnets with triangular spin ordering are investigated. The character of relaxation of these spin excitations is studied.     

Collective excitations in superconductors with structural transition

In this paper we have found new low-frequency collective excitations in superconductors with structural transformation. The connection between our results and the experiments on Raman scattering in 2H-NbSe₂ cDw- superconductor is discussed.     

Configuration interaction study of single and double dipole plasmon excitations in Na8

We carry out a microscopic analysis of the ground and excited states of the Na₈ metal cluster within the jellium model. We perform a series of configuration interaction calculations on a Hartree–Fock basis and construct eigenstates of the Hamiltonian which carry up to 4-particle 4-hole components. Based on the analysis of the dipole transition strengths, we single out those states which can be interpreted as the collective dipole plasmon and its double excitations. These modes are found to possess a high degree of harmonicity, deviations from the harmonic limit remaining, however, of the order of 10%.     

Collective excitations in semiconductor superlattices

Collective excitations in semiconductor superlattices are studied beyond tbe random-phase approximation (RPA). The Singwi, Tosi, Land and Sjölander (STLS) theory, which accounts for exchange and short-range correlations effects through an effective potential depending on the structurc factor, is generalized to the layered electron system described by the model of Visscher and Falicov. The exact numerical solution of the STLS self-consistent equations provides information about intraplane and interplane correlations. The plasmon dispersion curves are evaluated for some typical values of the coupling constant r_s of the electron system and the distance between the planes for GaAs/AlGaAs semiconductor superlattices. For comparison, the RPA and the Hubbard approximation are also considered.     

Slow spin relaxation in a highly polarized cooperative paramagnet

We report measurements of the ac susceptibility of the cooperative paramagnet Tb2Ti2O7 in a strong magnetic field. Our data show the expected saturation maximum in chi(T) and also an unexpected frequency dependence of this peak at low frequencies (<1 Hz), suggesting very slow spin relaxations are occurring. Measurements on samples diluted with nonmagnetic Y3+ or Lu3+ and complementary measurements on pure and diluted Dy2Ti2O7 strongly suggest that the relaxation is associated with dipolar spin correlations, representing unusual cooperative behavior in a paramagnetic system.     

Ergodic theorem for an impurity spin subsystem in a paramagnet

A model for verifying and developing the fundamental ideas underlying the ergodic hypothesis is proposed. The model describes the dynamics of the spin subsystem formed by impurity charges with spin I and a small g factor in a crystal immersed in a strong constant external magnetic field under conditions where the spin system of the nuclei in the crystal is isolated from the other degrees of freedom. The additive integral of motion is the projection of the total spin of the subsystem onto the external field. Attention is focused mainly on the case of I=1/2. It is shown that the ergodic hypothesis holds if the correlation radius is finite in the initial state and that the ergodic hypothesis is violated if the initial state is sharply localized or has global correlation. The nonergodicity of the ⁸Li− ⁶Li spin subsystem, which is a convenient object for experimental investigations of spin dynamics, is revealed. An estimate is obtained for the time for transition from a sharply localized disturbance of the canonical distribution to a quasiequilibrium state. Zh. éksp. Teor. Fiz. 116, 1398–1418 (October 1999)     

Collective excitations in electron-hole bilayers

We report a combined analytic and molecular dynamics analysis of the collective mode spectrum of a bipolar (electron-hole) bilayer in the strong coupling classical limit. A robust, isotropic energy gap is identified in the out-of-phase spectra, generated by the combined effect of correlations and of the excitation of the bound dipoles. In the in-phase spectra we identify longitudinal and transverse acoustic modes wholly maintained by correlations. Strong nonlinear generation of higher harmonics of the fundamental dipole oscillation frequency and the transfer of harmonics between different modes is observed.     

Collective excitations in exciton crystal

Crystal phase of indirect excitons formed by spatially separated electrons and holes in coupled quantum wells is analyzed. The collective mode spectrum of the exciton crystal at zero and nonzero agnetic fields is found. The spectrum consists of two optical and two acoustical modes (transverse and longitudinal in each case). We also study changes of the dipole crystal collective excitations at the transition exciton crystal-electron-hole plasma.     

Collective excitations in145Ba

High-spin levels in¹²⁵Ba have been produced in the¹¹⁶Sn(¹² C, 3n) reaction and studied by in-beam spectroscopic methods. Two strongly populated band structures are observed. The odd-parity one is based on a 7/2^? state and can be explained as the result of the coupling between anh _11/2 neutron-hole and a prolate type triaxial core. The even-parity band, built on a 7/2⁺ state, corresponds to collective excitations associated with a neutron-hole in theg _7/2 shell. Comparisons with heavier odd-A Ba isotopes and discussions are made in the frame work of the triaxial core model. Nuclear Reactions¹¹⁶Sn(¹²C, 3nγ), E=45–55 MeV; measuredσ(E;E _γ,θ),γγ-coinc,γ-γ delay. Enriched target, Ge(Li) detectors.¹²⁵Ba deduced levels,J,π,γ-mixing.     

Collective excitations in molecular complexes

Green's functions are used to derive an equation for the energies of collective excitations in molecular complexes. It is found that the collective branch contains a solution of exciton type. An expression is derived for the energies of the exciton excitations.     

Collective character of spin excitations in a system of Mn2+ spins coupled to a two-dimensional electron gas

We have studied the low energy spin excitations in n-type CdMnTe based dilute magnetic semiconductor quantum wells. For magnetic fields for which the energies for the excitation of free carriers and Mn spins are almost identical, an anomalously large Knight shift is observed. Our findings suggest the existence of a magnetic-field-induced ferromagnetic order in these structures, which is in agreement with recent theoretical predictions [Phys. Rev. Lett. 91, 077202 (2003)]].