R. Lai

Nonlinear nanoscale localization of magnetic excitations in atomic lattices

Abstract Reviewed here is the nonlinear intrinsic localization expected for large amplitude spin waves in a variety of magnetically ordered lattices. Both static and dynamic properties of intrinsic localized spin wave gap modes and resonant modes are surveyed in detail. The modulational instability of extended nonlinear spin waves is discussed as a mechanism for dynamical localization of spin waves in homogeneous magnetic lattices. The interest in this particular nonlinear dynamics area stems from the realization that some localized vibrations in perfectly periodic but nonintegrable lattices can be stabilized by lattice discreteness. However, in this rapidly growing area in nonlinear condensed matter research the experimental identification of intrinsic localized modes is yet to be demonstrated. To this end the study of spin lattice models has definite advantages over those previously presented for vibrational models both because of the importance of intrasite and intersite nonlinear interaction terms and because the dissipation of spin waves in magnetic materials is weak compared to that of lattice vibrations in crystals. Thus, both from the theoretical and the experimental points of view, nonlinear magnetic systems may provide more tractable candidates for the investigation of intrinsic localized modes which display nanoscale dimensions as well as for the future exploration of the quantum properties of such excitations.

On using the coherent far IR radiation produced by a charged-particle bunch to determine its shape: I Analysis

Abstract Because a short bunch of relativistic charged particles produces characteristic far infrared radiation when appropriately perturbed, the resulting spectrum can be related to the bunch form factor to provide information on the longitudinal shape. An important question which we address here regards the accuracy of the shape determined from such a spectroscopic measurement. Once the frequency dependence of the intensity of the emitted radiation has been obtained, there are two analysis methods which have been used to produce the longitudinal shape. Both make use of extrapolation into frequency regions where data is not available. One approach relies on the assumption that the bunch is symmetric so that a cosine Fourier transform can be used to find the shape. In the second approach, which we have proposed, a Kramers-Kronig relation is applied to the spectral form-factor data to find the minimal phase and then the asymmetric bunch shape is determined from the complete Fourier transform. By studying a variety of possible symmetric bunch shapes and extrapolations we have been able to identify the source of possible errors inherent in this phase determination process. For all reasonable shaped bunches and extrapolations we find that the actual phase is well represented by the minimal phase obtained from the Kramers-Kronig analysis. A straightforward extension illustrates how spectral measurements at different angles with respect to the beam trajectory may be used to define the 3-D bunch shape.

On using the coherent far IR radiation produced by a charged particle bunch to determine its shape — II Measurement with synchrotron and transition radiation

Abstract The coherent radiation spectrum from mm long relativistic electron bunches perturbed in different ways is investigated with a large aperture Michelson spectrometer. From these data we are able to determine the structure and asymmetric shape of the longitudinal electron density of short relativistic electron bunches on a submillimeter length scale by utilizing the Kramers-Kronig analysis technique described previously. Both synchrotron and transition radiation have been used successfully to determine the bunch shape. We further demonstrate how the influence of the LINAC tuning on bunch width and bunch asymmetry can be determined by using this technique.

Intrinsic localized spin wave modes in easy-axis antiferromagnetic chains

Two types of intrinsic localized spin wave modes are found in perfect antiferromagnetic chains of classical spins with on-site easy-axis anisotropy: the amplitude is either double or single peaked. In the small spin deviation limit, both types become identical envelope solitons. The degree of localization increases as either the maximum spin deviation or the ratio of the anisotropy constant to the exchange coupling constant increase. However, only the single peaked intrinsic localized mode is stable with regard to a noise perturbation.

Determination of bunch asymmetry from coherent radiation in the frequency domain

The coherent far infrared radiation induced from relativistic electron bunches of millimeter and submillimeter length provides a novel way to characterize the bunch shape, and we review this approach here. Although only the intensity of the coherent spectrum is measured, it is measured over the entire frequency range. Because of the completeness of this spectral measurement, it is possible to extract both the amplitude and the phase information of the radiating source by applying a Kramers-Kronig relation to the spectral form factor to find the minimal phase. The bunch shape is determined from the complete Fourier transform. One potential problem is the uniqueness of the phase. For all reasonably shaped bunches we have shown that the actual phase is well represented by the minimal phase obtained from the Kramers-Kronig analysis, hence the method can be used to identify the bunch asymmetry. This technique has been used to analyze the shapes of the submillimeter-long electron bunches at the Cornell linear accelerator. {copyright} {ital 1996 American Institute of Physics.}

Mie computational test of the extinction cross-section sum rules and optical moments for large dielectric spheres and shells

Abstract Numerically determined extinction cross-section spectra of large spherical particles containing spectral and geometrical inhomogeneities are used to test the general nature of the extinction cross-section sum rules and optical moments. The results are independent of the scattering details.

Studies of the modulational instability of antiferromagnetic spin waves in 1D

Abstract For antiferromagnetism spin chains the modulational instability of extended nonlinear spin waves has been investigated both analytically within the framework of linear stability analysis and also numerically by means of molecular dynamics simulations. For the particular case of on-site easy-axis anisotropy treated here linear stability analysis predicts the instability region and the growth rates of modulation satellites. Our numerical simulations demonstrate that the analytical predictions correctly describe the onset of instability. For long time scales when the instability is fully developed the linear stability analysis fails and the time evolution of the modulated spin waves can show both regular and chaotic behavior.

Vibrational dynamics associated with multiple elastic configurations in KI: ag +

Abstract A systematic experimental investigation of the fcc lattice-defect system KI: Ag + shows that the thermally activated disappearance of the strengths of the IR and Raman active vibrational modes and the appearance of rf dispersion and frequency-shifted UV electronic transitions are associated with the Ag + ion moving from an on-center to an off-center configuration. The possibility that an impurity/intrinsic-localized-mode complex could dissociate to produce the rapid temperature dependence has been tested but the absence of a concentration dependence for the normalized vibrational mode strengths with temperature appears to rule out this trapping model. A jump-rotational-diffusion model can describe the observed results in terms of two elastic configurations separated from each other by a large energy barrier. The impurity jumps between the configurations with temperature dependent dwell times. The physics behind multiple elastic configurations existing at a single point defect site is still unclear.

Sum rules and optical moments for a coarse scattering medium

The frequency-dependent extinction cross section of an ellipsoidal particle is examined from the point of view of sum rules and optical moments. For particles of any size a second-moment representation generates a characteristic extinction cross-section frequency that depends only on the plasma frequency, the dc dielectric constant of the bulk material, and the particle shape and is independent of the order or the disorder inside the object. In addition, this characteristic extinction frequency has the same value as the characteristic absorption frequency previously found for the small-particle Rayleigh limit. To illustrate these intrinsic features, numerical calculations are carried out on spherical shells and on silicon spheres. A spherical particle with fractal structure is used to demonstrate that the characteristic frequency can depend on a special internal structure of the particle. Finally, the characteristic frequency of a dust of randomly oriented ellipsoids is determined. Since the characteristic extinction frequency is independent of particle size, size distributions cannot influence its value.

Novel temperature dependence of the vibrational relaxation of SH− in a two-state hopping system

Abstract Vibrational saturation measurements of the SH − molecule doped in KBr are performed from 1.7 to 78 K. If a single relaxation time is fit to the saturation curves, it is found to increase with increasing temperature, in contrast to theoretical expectations. However, KBr:SH − has recently been shown to possess two distinct elastic configurations between which interconfigurational hopping has been identified. If the vibrational lifetimes are different in these two states, an increasing effective decay time for the system as a whole can be obtained even when the two individual times remain constant or even decrease with rising temperature.