A nonequilibrium molecular-dynamics method is proposed to simulate the behavior of simple fluids under high shear rates. A thermostat that makes no assumptions regarding the form of the streaming velocity of the fluid is introduced. Simulations are carried out for a Weeks-Chandler-Andersen dense liquid. With the form of the streaming velocity partially or wholly constrained, we find evidence for a reentrant solid or string phase, but the phase disappears if constraints are removed. Small-angle neutron-scattering data from a sheared colloidal suspension are presented. There is no evidence of a steady-state shear-induced structure in the suspension.

Negativity of the conditional Lyapunov exponents is a necessary and in many cases a sufficient condition for the occurrence of synchronization between a chaotic drive and the response subsystem. Treating one of the variables of coupled logistic maps as the drive, our numerical simulations show that the negativity of the conditional Lyapunov exponents does not always guarantee synchronization and, additionally, the domain of initial conditions for the drive variables needs to be specified in which case the synchronization occurs. Additionally, an example showing the pitfalls of the numerical study of synchronization when the conditional Lyapunov exponent is positive is presented.

The numerical solution of a one-dimensional Fokker-Planck equation by means of the split operator method of Feit, Fleck, and Steiger [J. Comput. Phys. 47, 412 (1982)] is used to analyze the time-correlation function of a stochastic variable in a bistable potential. We find the relaxation to be well described by an exponential law. The rate contains a dynamical renormalization term arising from a memory kernel, which is essential to describe the dynamics.

I demonstrate stochastic resonance in a discrete-state, continuous-time random walk in one dimension. The probabilities for the walker to step to the right or to the left depend on an external field. The field varies in space and time such that the walker feels a bistable, periodically modulated random potential. Unbiased random forces act on the walker in addition to the field. When the field is strong relative to the unbiased forces, the walker is confined to a single well, and a power spectrum shows low noise levels and little power at the modulation frequency. When the field is weak relative to the unbiased forces, the walker moves erratically between the wells, and a power spectrum is noisy. For intermediate field strengths, the walker moves periodically from one well to another, and the power at the modulation frequency increases dramatically with respect to the noise level. In this respect, the behavior of the discrete-state model is analogous to the standard, continuous-state Langevin models.

We consider the effects of disorder in tethered membranes created by random polymerization of fluid films. The randomness quenched in upon polymerization is found to be irrelvant, implying that such membranes exhibit properties essentially identical to those of homogeneous tethered membranes.

The low-frequency vibrations of freely suspended smectic liquid-crystal films are analyzed. The characteristic frequencies for very thin films can be used for the determination of the smectic-liquid-crystal–vapor surface tension. In thick films one can also determine from the vibrational frequencies the compressional elastic constant B for smectic liquid crystals.

Discrete quadrupole focusing systems are subject to an electromagnetic instability wherein the growing transverse motion of the beam interacts with the quadrupole field and the ${\mathrm{TE}}_{11}$ waveguide mode. We study this via three-dimensional particle simulation. We find evidence of the instability, with growth rates and frequencies in reasonable agreement with theory. The instability is found to saturate at a low amplitude, however, and has no discernable effect on the macroscopic properties of the beam.

We investigate the diffusion in the action variable when the frequency of an integrable isochronous map is modulated. Purely stochastic, hyperbolic, or periodic deterministic modulations are considered. The diffusion coefficient in the invariant for the unperturbed map is exactly determined and shown to be nonzero, except in the last case, when the modulation is smooth and nonzero, due to the presence of topological barriers.

For the n-dimensional nonlinear wave equations ${\mathrm{\partial }}_{\mathrm{\alpha }}$${\mathrm{\partial }}_{\mathrm{\alpha }}$cphi=${\mathit{dF}}_{\mathit{i}}$(cphi)/dcphi, i=1,2, . . ., a kind of general soliton solution is obtained. It contains some interesting specific solutions, such as N multiple solitons and the quadric solitons. The discussion of their properties shows that there exist some motional sphere solitons with a definite radius and some quadric breathers which have various shapes.

The highly correlated planetary states of the helium atom calculated recently by Richter and Wintgen [J. Phys. B 24, L565 1991] are shown to be the sequence of the lowest members of Rydberg series of asymmetrical doubly excited states. The analytically calculated parameters of the Rydberg formula reproduce quantitatively this serial behavior. The importance of the dynamic exchange by the orbital momenta between the electrons is emphasized and is taken into account in the dipole approximation. New properties of the states of this type are predicted.

The hyperfine-structure constants of the hydrogen molecular ion in its ground vibrational state are determined experimentally, by the study of the spin structure of high-L n=27 Rydberg levels of the neutral hydrogen molecule. The results are ${\mathit{b}}_{1\mathit{F}}$=922.940(20), c=128.259(26), d=42.348(29), and f=-0.003(15) MHz.

We have calculated for the ten lowest vibrational levels of ${\mathit{H}}_2^{+}$ the first-order hyperfine and spin-rotation constants with the inclusion in perturbation theory of the first-order coupling of the electron and nuclear motion and have determined the hyperfine transition frequencies. The calculated frequencies agree to better than 175 kHz with all the experimentally determined hyperfine frequencies for the N=1 rotational level and to better than 3 kHz for the N=2 rotational level. Relativistic and radiative effects may account for the remaining discrepancy.

Self-consisent calculations on a number of atoms and ions are carried out with the new local ‘‘Wigner-scaled’’ exchange-correlation functional generated by Zhao, Levy, and Parr: ${\mathit{E}}_{\mathrm{xc}}^{\mathrm{ws}}$[ρ]=-${\mathit{a}}_0$F${\mathrm{\rho }}^{4/3}$ [1-κ${\mathrm{\rho }}^{1/3}$ln(1+1/κ${\mathrm{\rho }}^{1/3}$)]dr, with ${\mathit{a}}_0$=0.932 22 and κ=9.473 62×${10}^{\mathrm{-}3}$. Results appear to be better than those for any known local exchange-correlation functional.

We investigated a reported anomaly in near-threshold pair production, using radioactive sources to measure the γ+Ge→${\mathit{e}}^{+}$+${\mathit{e}}^{\mathrm{-}}$+Ge cross-section at ${\mathit{E}}_{\gamma }$=1063, 1086, 1112, 1173, 1213, 1299, 1332, and 1408 keV. Although the data agree with the theory (numerical calculations based on an exact partial-wave formulation for a screened central potential) at the higher energies, the data lie above the theory at 1063, 1082, and 1112 keV. The discrepancy is reduced by including the final-state Coulomb interaction between the ${\mathit{e}}^{+}$ and ${\mathit{e}}^{\mathrm{-}}$.

Absolute measurements of differential cross sections (DCS’s) for positrons elastically scattered by krypton atoms reveal (1) unexpected resonancelike structures at 25 and 200 eV when the DCS’s at the fixed scattering angles of 30°, 60°, 90°, and 120° are plotted versus energy, and (2) evidence for absorption effects between the elastic and inelastic scattering channels. The resonancelike structures may represent a new type of scattering resonance arising from coupling effects between the elastic and inelastic scattering channels.

Eikonal expressions for differential scattering amplitudes for electronic s-${\mathit{p}}_{\pm 1}$ transitions in heavy-particle collisions are derived. It is shown that differential orientation effects follow from an interplay between phases related to the heavy-particle dynamics and the Coulomb perturbation of the initial and final states. At intermediate energies where semiclassical trajectory calculations give preferred orientation for both capture and excitation we show that in the resulting differential scattering cross sections the orientation tends to vanish for excitation, whereas, it prevails for capture in doubly- and multiply-charged-ion–atom collisions.

Calculations are presented that show pronounced cusps in the rates of higher harmonic emission at intensities where a particular above-threshold multiphoton detachment channel closes.

A pure-state analysis of spontaneous emission of photons by a one-dimensional harmonic oscillator in a spherically symmetric cavity is presented. Using a classical path representation with respect to photon paths the transition between the limiting cases of coupling to a single cavity mode and the continuum limit is discussed analytically.