Study of the effects of confinement in the collective excitations of liquid deuterium

The spectrum of collective excitations of liquid deuterium in the bulk and confined within a MCM-41 matrix having pores of an average diameter of 2.4 nm has been studied by means of neutron scattering and molecular dynamics simulations. The main effects of confining liquid D₂ consist in a shift of the characteristic frequencies associated with damped density oscillations to higher energies. A strong decrease in diffusivity is also observed upon confinement.     

Determination of the short-wavelength propagation threshold in the collective excitations of liquid ammonia

The dynamics structure factor S(Q,E) of liquid ammonia l-NH3 at T = 200 K and at its vapor pressure has been measured by inelastic x-ray scattering (IXS) in the 1-15 nm(-1) momentum transfer ( Q) range. Contrary to previous IXS studies on other associated liquids and glasses, in l-NH3 a large inelastic signal is observed up to Q = 15 nm(-1). This, enabling S(Q,E) measurements as a function of Q at constant E transfer, allows us to demonstrate experimentally the transition from a propagating dynamics regime, where the acoustic excitation energy linearly disperses with Q, to a high-Q regime, where it is no longer possible to observe a dominant excitation in the S(Q,E).     

Quantum effects on liquid dynamics as evidenced by the presence of well-defined collective excitations in liquid para-hydrogen

The origin of the well-defined collective excitations found in liquid para-H2 by recent experiments is investigated. The persistence of their relatively long lifetimes down to microscopic scales is well accounted for by calculations carried out by means of path-integral-centroid molecular dynamics. In contrast only overdamped excitations are found in calculations carried within the classical limit. The results provide fully quantitative evidence of quantum effects on the dynamics of a simple liquid.     

Theory of collective excitations in simple liquids

We present a parameter-free theory of the collective excitations in simple liquids such as liquid metals or rare gases. The theory is based on the mode-coupling theory (MCT), which has been previously applied successfully for explaining the liquid-to glass transition. The only input is the liquid structure factor. We achieve good agreement both for the liquid dispersion (maximum of the longitudinal current spectrum) and width (damping) with experimental findings. The time-dependent memory function predicted by MCT has a two-step exponential decay as previously found in computer simulations. Furthermore MCT predicts a scaling of the liquid dispersion with the effective hard-sphere diameter of the materials. This scaling is obeyed by the available experimental data.     

Multistep variational approach to collective excitations

We discuss a multistep variational approach to collective excitations. The approach is developed in a boson formalism (bosons representing particle-hole excitations) and based on an iterative sequence of diagonalizations in subspaces of the full boson space. Purpose of these diagonalizations is that of searching for the best approximation of the ground state of the system. The procedure also leads us to define a set of excited states and, at the same time, of operators which generate these states as a result of their action on the ground state. We examine the cases in which these operators carry one-particle one-hole and up to two-particle two-hole excitations. We also explore the possibility of associating bosons to Tamm-Dancoff excitations and of describing the spectrum in terms of only a selected group of these. Tests within an exactly solvable three-level model are provided.     

Fluid-dynamical description of nuclear collective excitations

The role of antisymmetric tensor fields in the gauging of groups is related to theorems on cohomology theory, and Cartan integrable systems are discussed. Examples are given. Various possibilities to gauge d = 11 supergravity by decontracting its underlying group are considered. In particular the simple supergroups Osp (1 | 64) and SU(32 | 1) yield a negative result, but a certain non-semisimple supergroup containing Osp (1 | 32) is proposed as a viable candidate. The corresponding action would no longer contain the 3-index photon A_μν?, but instead a second spin $\text{3}\text{2}$ field η_μ and boson fields $\text{B}{}_{\mathrm{\mu }}{}^{\mathrm{a1a2}}$ and $\text{B}{}_{\mathrm{\mu }}{}^{\mathrm{a1\dots a5}}$. A first order formalism for d = 11 is presented. It is to be used for an improved form of dimensional reduction.     

Optically-driven cooling for collective atomic excitations

We explore how to cool collective atomic excitations in an optically-driven three-level atomic ensemble, which may be described by a model of two coupled harmonic oscillators (HOs) with a time-dependent coupling. Moreover, the model of two coupled HOs is further generalized to address the resolved sideband cooling issues, where the lower-frequency HO can be cooled whenever the cooling process dominates over the heating one during the sideband transitions. Unusually, due to the absence of the heating process, the optimal result for cooling collective excitations in an atomic ensemble could break the standard resolved sideband cooling limit for general models of two coupled HOs.     

Sum rules for nuclear collective excitations

Characterizations of the response function and of integral properties of the strength function via a moment expansion are discussed. Sum rule expressions for the moments in the RPA are derived. The validity of these sum rules for both density independent and density dependent interactions is proved. For forces of the Skyrme type, analytic expressions for the plus three energy weighted sum rules are given for isoscalar monopole and quadrupole operators. From these, a close relationship between the monopole and quadrupole energies is shown and their dependence on incompressibility and effective mass is studied. The inverse energy weighted sum rule is computed numerically for the monopole operator, and an upper bound for the width of the monopole resonance is given. Finally the reliability of moments given by the RPA with effective interactions is discussed using simple soluble models for the hamiltonian, and also by comparison with experimental data.     

Interplay of collective excitations in quantum-well intersubband resonances

Intersubband resonances in a semiconductor quantum well (QW) display fascinating features involving various collective excitations such as Fermi-edge singularity (FES) and intersubband plasmon (ISP). Using a density matrix approach, we treated many-body effects such as depolarization, vertex correction, and self-energy consistently for a two-subband system. We found a systematic change in resonance spectra from FES- to ISP-dominated features, as QW width or electron density is varied. Such an interplay between FES and ISP significantly changes both line shape and peak position of the absorption spectrum. We found that a cancellation of FES and ISP undresses the resonant responses and recovers the single-particle features of absorption for semiconductors with a strong nonparabolicity such as InAs, leading to a dramatic broadening of the absorption spectrum.     

Proton-neutron structure of collective excitations in 94Mo

Due to the recent development of radioactive beam production, various direct reaction studies in reversed kinematics have been made to investigate the behavior of the N = 20 shell closure in the neutron-rich region. Coulomb excitation, proton inelastic scattering, and fragmentation of unstable nuclei have been studied with γ-ray detection. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: motobaya@rikkyo.ne.jp     

Retardation effects in damping of nuclear collective excitations

We expand the nonmarkovian collision integral in terms of multipolarities of the distortion of the Fermi surface. It is shown that damping of zero-sound is determined by all multipolarities of the Fermi-surface deformation. For large zero-sound velocity with respect to the Fermi velocity the relaxation time is related to the quadrupole deformation of the Fermi surface. The contributions of collisions to the total widths of the giant multipole resonances are calculated in a semiclassical macroscopic nuclear model.     

Evidence of long-wavelength collective excitations in magnetic superlattices

We report on a mechanism of dynamic dipolar coupling in magnetic superlattices via long-wavelength nonevanescent fields. In the spin excitation spectra of our heterophase stripe structures, such interactions mediate a singlet<-->doublet crossover in the frequency regime driven by the orientation of an external static field. This crossover is a new feature observed in collective behavior of superlattices, though there is some analogy of this phenomenon with birefringence taking place in optical superlattices. We envision applying the collective effects described here in microwave photonic devices.     

First identification of collective excitations in127Ce

Collective bands have been identified for the first time in the nucleus¹²⁷Ce using in- beam spectroscopic techniques. A band developed up to spin 51/2 is proposed to be based on an h_11/2 neutron hole configuration. The observed backbend is produced by the alignment of an h_11/2 proton pair. Another level sequence is very likely generated by ag_7/2 or d_5/2 neutron- hole.     

Neutron investigation of collective excitations in liquid K-Cs alloys: the role of the electron density

The investigation of the ion dynamics in the liquid alloy K52Cs48, which has the same electron density as Rb, was carried out by neutron inelastic scattering. Well defined collective excitations were observed up to the maximum value of momentum transfer of the experiment. A scaling of the dispersion relation data was found to hold between K-Cs alloys, Rb and Cs. The suggested scaling points out the key role of the conduction electron density in the collective dynamics of alkali metals.     

Oscillator strengths for collective excitations in molecular complexes

Collective excitations of exciton type are considered. The oscillator strength of a collective excitation is expressed in terms of the oscillator strengths of the one-electron transitions. The scope for negative absorption is discussed.     

Interfacing collective atomic excitations and single photons

We study the performance and limitations of a coherent interface between collective atomic states and single photons. A quantized spin-wave excitation of an atomic sample inside an optical resonator is prepared probabilistically, stored, and adiabatically converted on demand into a sub-Poissonian photonic excitation of the resonator mode. The measured peak single-quantum conversion efficiency of chi=0.84(11) and its dependence on various parameters are well described by a simple model of the mode geometry and multilevel atomic structure, pointing the way towards implementing high-performance stationary single-photon sources.     

Dynamics of collective excitations in one dimensional superionic conductors

We study the dynamics of collective excitations of a chain of interacting ions in a periodic potential including the effects of discreteness and of thermal damping. We describe a method to discriminate between the various possible cases ranging from soliton like propagation to the hopping of the collective coordinate. The frequency dependent mobility is computed and specific applications to materials are discussed.