Search Results (14 total)

The weak dissipation of spin waves in magnetic materials makes antiferromagnets realistic candidates for the experimental observation of intrinsic localized modes. Our molecular dynamics simulations with different antiferromagnets reveal that the lowest lying uniform spin wave mode of the antiferromagnet (C₂H₅NH₃)₂CuCl₄ is unstable and that it can be driven sufficiently hard with a 1 Oe cw microwave field to generate intrinsic localized spin wave modes.

The modulational instability of extended nonlinear spin waves in antiferromagnetic chains with on-site easy-axis anisotropy has been investigated both analytically in the frame of linear-stability analysis and numerically by means of molecular-dynamics simulations. The 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 modulated nonlinear spin waves can evolve into localized excitations. We explore the possibility of generating intrinsic localized spin-wave modes from extended spin waves through modulational instability and find that both discreteness and strong nonlinearity seem to be essential for the creation of long-lived localized excitations. The addition of weak dissipation is found to impose a finite amplitude threshold even for infinite chains.

Localization of lattice vibrations is investigated for a soft zone-center optic mode overlapping the acoustic spectrum of an anharmonic one-dimensional diatomic lattice. An intrinsic resonant mode is found below the plane-wave optic spectrum. Using numerical methods the frequency and eigenvector of the intrinsic resonant mode has been measured as a function of its amplitude and the optic-mode frequency. Molecular-dynamics simulations show that these localized modes are stable.

The longitudinal electron density in millimeter long relativistic electron bunches is determined from the coherent transition radiation spectrum using a Kramers-Kronig minimal phase analysis technique. The bunch shape is also measured by superimposing a position-dependent energy slew on the bunch and then energy analyzing the beam. The results show that the minimal phase analysis correctly describes the asymmetric electron density distribution.

We demonstrate analytically and numerically that the simplest isotropic ferromagnetic one-dimensional system to show intrinsic localized spin-wave resonances requires only isotropic exchange interactions between first and second neighbors. Our numerical simulations indicate that the lifetime of these intrinsic localized spin-wave resonances depends on the mode parity, the maximum spin deviation, and the relative strength of the next-nearest-neighbor–to–nearest-neighbor exchange interactions. In the small amplitude limit translating localized spin-wave resonances behave like solitons but for larger amplitudes they are scattered by the discreteness of the lattice and decay by radiating plane spin-wave modes.

We find that a stationary intrinsic localized spin-wave resonance can exist and is long lived within the linear spin-wave spectrum for a perfect one-dimensional antiferromagnetic chain of classical spins with single-ion easy-plane anisotropy. Numerical simulation studies demonstrate that the excitation is stable with regard to a noise perturbation. The resonance is infrared active and its frequency decreases with increasing maximum spin deviation.

The dynamical properties of intrinsic localized spin-wave modes (ILSM's) in perfect antiferromagnetic chains of classical spins with on-site easy-axis anisotropy are investigated. Only ILSM's of odd parity exist below the plane-wave magnon frequency band. The degree of localization increases as either the maximum spin deviation or the ratio of the anisotropy constant to the exchange coupling constant, D/J, increases. Traveling ILSM's can also be excited; however, they become pinned for large D/J.

The reorientational spectra in the mid- and far infrared of OH^-, OD^-, SH^-, SD^-, SeH^-, and TeH^- in a variety of different alkali halides are studied. All of these systems have ∼300 cm^-1 modes due to librations of the diatomics about their centers of mass. A surprising change in the dynamics of these librational modes with increasing temperature is discovered: their absorption strengths are observed to disappear by ∼150 K in the sodium-chloride-structure hosts and reappear in the millimeter-wave region in the forms of generalized-Debye spectra. This is in striking contrast to what is expected on the basis of the estimated orientational barrier heights using the known librational frequencies. It also contrasts with the cesium-chloride-structure salts, where the librational strengths of the molecules are conserved up to RT. The temperature-dependent variations of the fcc systems can be explained using a thermally activated jump-rotational-diffusion model wherein each defect hops with temperature-dependent dwell times between two distinct elastic configurations of the lattice-defect system: one involving librational and the second supporting diffusive rotational states. © 1996 The American Physical Society.

We demonstrate that the jump-rotational-diffusion model originally developed in the context of quasielastic neutron scattering from high temperature liquids can describe the two-state behavior observed in the low temperature dynamics of some molecular and point defects in crystals. The unusual temperature dependences of the IR spectra of both diatomic chalcogen hydrides in alkali halides and KI:Ag⁺ can be explained in terms of two elastic configurations between which the impurities jump with temperature-dependent dwell times.

The coherent far-infrared radiation induced from relativistic electron bunches of submillimeter length provides a way to characterize the bunch shape. Once the spectrum of the bunch form factor is measured, one can apply the Kramers-Kronig relations to the spectral form factor to find the minimal phase and then calculate the bunch shape from the complete Fourier transform. One potential problem is the uniqueness of the phase so determined since, in general, the phase shift associated with the Blaschke product must also be included. Here we examine a variety of possible asymmetric bunch shapes in order to identify any errors inherent in this approach.

The shapes of submillimeter long electron bunches at the Cornell linear accelerator have been determined by measuring the coherent far-ir synchrotron and transition radiation spectrum produced by the charge distribution. With the aid of a Kramers-Kronig analysis of the spectral data, we show that the longitudinal bunch shape including the asymmetry can be accurately determined.

Since perturbed relativistic charged particle bunches of millimeter or submillimeter size emit coherently in the far infrared frequency region, there is growing interest in using this spectrum to obtain precise information about the bunch form factor. It is described here how the maximal information, including bunch asymmetry, can be extracted from such measurements.

We propose a theoretical frame to handle trapping problems in phase-modulation fluorometry. Applying operator manipulations, we arrive at a ‘‘perturbation’’ expansion in a straightforward and systematic manner. Furthermore, we show that the numerical implementation of this approach can be enhanced by the reduction technique. Following this approach, we carry out some numerical investigations and the results appear to be instructive. Finally, we point out how to generalize the method to the cases of several fluorescence species and differential phase fluorometry.

Theoretical treatment of resonance radiation trapping under steady-state conditions, particularly of quenching experiments, is presented. Applying operator manipulations, Holstein’s equation of radiation trapping [Phys. Rev. 72, 1212 (1947)] is solved by a perturbation expansion, which can be physically interpreted in terms of multiple scatterings. The resulting expression shows that in a steady-state quenching experiment radiation trapping invalidates the relation of the Stern-Volmer type except when the amount of the buffer gas is small. In the limit of low buffer-gas pressure, a simple expression is given for the effective lifetime in the Stern-Volmer relation: the average time of the detected resonant photons remaining within the enclosure. While it is true that this suggested approach is perturbative in nature, the asymptotic behavior of the high-order expansion coefficients, however, enables us to reduce the summation over infinitely many alternatives to some leading terms. Such a reduction procedure helps curtail the computation time and improve the precision. Finally, some numerical illustrations of our suggested ideas are presented, which shows this approach is a good tool to treat the steady-state trapping effect.