We employ the recently established formalism for the calculation of the dielectric response function for strongly coupled Coulomb liquids to obtain the dispersion of the plasmon mode in a two-dimensional one-component plasma in the strong-coupling domain. This formalism is based on the physical picture of particles being quasilocalized at strong coupling. The analytical and numerical calculations are carried out over a range of liquid-state coupling parameters up to Γ=${\mathit{e}}^2$ √πn /${\mathit{k}}_{\mathit{B}}$T=120 and for arbitrary wave numbers. When the slow thermal migration of the quasisites (around which the particles are localized) is neglected, the plasmon dispersion is oscillatory and the oscillations become more pronounced with increasing Γ. When the coupling is very strong (Γ=120, e.g.), the distance to the first minimum in ω(k) and the spacing between successive minima approaches ${\mathit{K}}_0$=3.3/a, the lattice spacing in the reciprocal lattice. The ‘‘direct’’ thermal effects due to the slow migration are represented by a phenomenological modification of the dielectric function. This modification leaves the plasmon dispersion almost entirely unaffected up to ka≊1. For ka>1, however, changes in the dispersion due to the direct thermal motion are as follows: (i) the dispersion is no longer oscillatory; ω(k) rises from zero to a maximum and then cuts off beyond that at a value k=${\mathit{k}}_{\max }$(Γ), which approaches an asymptotic limit ${\mathit{k}}_{\max }$(${\mathrm{\Gamma }}_{\mathit{m}}$) close to ${\mathit{K}}_0$ as Γ approaches ${\mathrm{\Gamma }}_{\mathit{m}}$=137±15; (ii) the plasmon frequency is increased especially at the lower coupling values where one expects the thermal motion to play a more significant role; (iii) the dispersion exhibits two branches: the upper branch corresponds to the plasmon mode and the heavily damped lower soundlike branch is already identified in random-phase-approximation calculations.

We compare our theoretical results with data available from molecular-dynamics (MD) simulations: the agreement between theory and MD data becomes more and more favorable, as it should, with increasing Γ. At Γ=50, the agreement is very good indeed.

A fast and numerically stable technique has been developed for the numerical simulation of parallel-plate rf glow discharges, operating at frequencies exceeding ∼10 MHz. In such discharges the ion concentrations do not vary on the time scale of the rf cycle. Thus the transport equations for charged particles can be decoupled into a set of time-averaged and time-dependent equations. Rather than follow the evolution of the discharge following initiation, a solution of the time-averaged equations provides the steady-state solution directly. Once the time-averaged solution is known, a solution of the time-dependent equations yields the modulation of the time-dependent variables about their time-averaged value. Decoupling of the equations also reduces the number of variables at each step, thereby enabling optimization of matrix solutions for the system of nonlinear equations. Finally, good initial guesses are provided by solutions of simplified models. Electron kinetics and transport data are obtained by solving the zero-dimensional Boltzmann equation for electrons in conjunction with a model for the kinetics of excited states. Simulations have been performed for discharges in Ar and ${\mathrm{SF}}_6$. A typical case for an argon discharge is executed in 6 min on a Micro Vax II Computer as compared to twice as many minutes on a Cray computer for similar models reported by other workers. Results of the simulations are in agreement with those reported elsewhere and with measurements made using a tuned Langmuir probe.

An investigation is made of the relativistic effect on the nonlinear stimulated scattering of large-amplitude laser radiation in the presence of the self-generated magnetic field in a laser-produced plasma. The relativistic Vlasov equation expressed in terms of the guiding-center coordinates has been solved to obtain the response of the magnetized plasma electrons. It is noted that the extreme relativistic effect increases the growth rates of the stimulated scattering by a factor of c/${\mathit{v}}_{\mathit{e}}$, where c and ${\mathit{v}}_{\mathit{e}}$ are the free-space speed of light and thermal velocity of the plasma electrons.

A multianode ion chamber with a filling of 95 mol % Ar plus 5 mol % ${\mathrm{CH}}_4$ at an absolute pressure of 3 atm and a drift field of 287 V/cm has been used to measure ionization signals produced by energetic ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$, and ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$ nuclei with ranges up to 0.64 g/${\mathrm{cm}}^2$. Comparing these signals with calculated energy losses we find significant deviations from proportionality for dE/dx≳3×${10}^3$ MeV ${\mathrm{cm}}^2$/g. The fractional deviations are found to be proportional to (dE/dx${)}^{\mathit{m}}$ with m≃2. We attribute these results to columnal recombination and discuss a simple model that is in qualitative agreement with the essential features of the observations.

Density-functional theory (DFT) for finite temperature (T) is used to account for fluctuations of the transition energies in the average-atom model. These fluctuations lead to absorption bands in the bound-bound contribution to the total spectrum. The transition-energy fluctuations are related to the second derivative of the DFT functional Ω[n] with respect to the electron density n(r). In the numerical example (the spectrum and opacity of iron plasma at T=200 eV and solid density), the Thomas-Fermi (TF) form of ${\mathrm{\delta }}^2$Ω[n]/δn(r) δn(r’) is taken. A comparison with a previous approach based on uncorrelated fluctuations around the TF atom clarifies the meaning of that approach. The numerical results indicate that the inclusion of interactions significantly diminishes the absorption bandwidths.

A linearized theory of small perturbations in thermal blooming gives a surprisingly accurate description of the initial evolution of a plane wave propagating through an absorbing fluid medium. In the case of constant absorption and fluid velocity, a sinusoidal perturbation of the optical field grows quasiexponentially at a rate determined by its Fresnel number and the accumulated optical-path difference due to blooming. Perturbations with small transverse length scales grow more rapidly than those with large length scales. The evolution of the optical field and spectrum in the presence of optical turbulence is accurately described. The growth of the small-scale fluctuations eventually leads to a decrease in plane-wave amplitude. This growth puts an ultimate limit on the amount of power that can be propagated through a turbulent medium such as the atmosphere. More complicated cases with varying absorption and velocity profiles can be analyzed using a WKB approximation. Numerical simulations show growth suppression when the velocity varies along the optical path. These predictions from the linearized theory agree well with results from numerical simulations of the full nonlinear system and thus provides a standard for comparing different numerical codes.

We have induced oscillations in the human pupil light reflex using two different kinds of external electronic feedback: smooth negative feedback (SNF) and piecewise constant negative feedback (PCNF). The behavior of the mean amplitude and period at oscillation onset are shown to be in good agreement with a model of this neural system incorporating the external feedback. The critical behavior displayed through amplitude and period fluctuations is different in each case. The observation that amplitude fluctuations are larger (smaller) than period fluctuations for SNF (PCNF) is explained theoretically and by numerical integration of a stochastic delay-differential equation with additive and multiplicative colored noise. We find that both types of noise postpone the Hopf bifurcation in SNF by an amount proportional to the noise intensity and inversely proportional to the correlation time. The implications for analyzing bifurcations in neural systems are discussed.

Coherent inelastic neutron-scattering measurements have been carried out on a hydrogenous single crystal of 9-methyladenine (9-MA), a methylated form of the DNA base, adenine. Several low-lying acoustic modes have been measured. In particular, detailed measurements have been made of the dispersion of the longitudinal-acoustic mode propagating perpendicular to planes parallel, on average, to those of the planar adenine molecules. The sound velocity corresponding to this acoustic mode is similar to that obtained for sound propagation along the DNA helix axis in dry films, but significantly greater than for corresponding velocities obtained from measurements on stacked pyrimidines. The biological relevance of the measurements is discussed in relation to physical properties that may be associated with long stretches of adenine (poly-A) known to exist in certain regions of RNA.

A neural network with variable but state-dependent synaptic strengths is discussed. It is shown that the efficiency in retrieving and storing information is enhanced as compared with the Hopfield model and the usual Hebb memorization rule. A nonopinionated learning capacity is discovered.

In solving the secular equation of a one-dimensional infinite lattice model of poly(dA)-poly(dT), we obtain dispersion relations. [The notation poly(dA)-poly(dT) means that one strand contains only adenine (A) bases, and the other only thymine (T) bases.] We solve the equation for consecutive refinements of the nonbonded force constants based on Raman, Brillouin, and neutron scattering and Fourier-transform infrared experiments in both polynucleotides and random sequence DNA. When these eigenvectors are examined for base roll and propeller twist, such characteristics are found to be dominant in modes around 50 ${\mathrm{cm}}^{\mathrm{-}1}$.

We show that very simple iterative rules for the growth of cells on a two-dimensional lattice can simulate biological-growth phenomena realistically. We discuss random cellular automata models for the growth of fern gametophytes, branching fungi, and leaves, and for shape transformations useful in the study of biological variation and evolution. Although there are interesting analogies between biological and physical growth processes, we stress the uniqueness of biological automata behavior. The computer growth algorithms that successfully mimic observed growth behavior may be helpful in determining the underlying biochemical mechanisms of growth regulation.

We study the dynamics of H-bond motion for a model of a replicating DNA fork. The model contains effects both from the creation of the fork and from self-consistent changes in the H bonds due to the creation of the fork. We use a Morse potential to describe the H-bond interactions. The anharmonic behavior does alter the final H-bond dynamics. The anharmonic aspect increases the H-bond fluctuations by more than a factor of 2 over the harmonic fork results in some frequency regions. We also display the frequency dependence of the H-bond motion and suggest spectral features that can be a signature for the existence of forks in DNA samples.

A refined method for calculating the fractal dimensions of proteins is introduced in this article. The fractal dimensions of 90 proteins covering four structural classes of proteins are established using this method. The relationship between the fractal dimension and tertiary structure of proteins is analyzed. The mean value of the fractal dimension ${\mathit{D}}_2$ for the global structure of proteins is 1.65, which is very close to the theoretical value 5/3 associated with a self-avoiding random walk in three-dimensional Euclidean space.

Growth processes are argued to develop self-organized critical states. This new characterization of growth phenomena yields insight into the origin of fractal pattern formation, and the associated exponents give information on scaling properties beyond that provided by the usual multifractal description. As a major example, the dielectric-breakdown η model is considered. The fractal dimension is estimated to be D=ln3/ln2≃1.585 for η=1. This value is compared with results obtained for different geometries and with values found when lattice effects are present. Also, the limiting cases η→0 and η→∞ are discussed.

The ratio between the bend elastic constant and the anisotropy of diamagnetic susceptibility of a lyotropic mixture of sodium decylsulfate, decanol, and water in the nematic calamitic phase is measured as a function of the temperature. This ratio is obtained by measuring the wavelength of a bend periodic distortion created by magnetic fields. The results are compared to others obtained with different lyotropic mixtures.

The existence of a critical angle for the Brochard-Leger (BL) wall in liquid crystals has been investigated experimentally. The experiment shows that there doubtless exists a critical angle ${\mathrm{\phi }}_{\mathit{c}}$ for the BL wall. When the tilted angle of the external field is larger than ${\mathrm{\phi }}_{\mathit{c}}$, the BL wall becomes unstable and will quickly disappear in the sample. The experiment supports an earlier theoretical analysis [X. Y. Wang, Phys. Rev. A 32, 3126 (1985); Phys. Lett. 112A, 402 (1985)].

An experimental study of dendritic growth of pivalic acid crystals is performed using cells of various thicknesses. It is found that confinement effects become significant for small values of the Péclet number built on the size of the cell, in good agreement with recent theoretical findings.

We study an asymmetric diluted version of the dynamic model for neural networks proposed recently, which explicitly takes into account the existence of several time scales without discretizing the time. The dynamics is neither totally synchronous nor totally asynchronous, and the couplings in the neural networks are asymmetric. These considerations may be regarded as more biologically realistic. We obtain the phase diagram as a function of the temperature ${\mathrm{\epsilon }}^{\mathrm{-}1}$, the capacity α, and the ratio a of the refractory period to the action potential duration.

A recently discussed Hamiltonian approach to dissipation effects is considered that leads to nonlinear equations for the time evolution of the expectation values of some operators. It is shown here that the Heisenberg equations of motion for the coordinate and momentum operators can be exactly solved, allowing a better study of the model.

On the basis of some ideas introduced by Fernández in the preceding paper [Phys. Rev. A 41, 7066 (1990)], an alternative interpretation is given to the widely known nonlinear Hamiltonian of Albrecht, Hasse, and Süssmann.