Electric fields can cause structural changes of the blue phases (BP) that appears in chiral liquid crystals. In BP I and BP II, electrostrictive effects cause a shift of the Bragg reflection wavelengths with little intensity change. In BP III, for systems with negative dielectric anisotropy, there is a strong increase in the selective reflection intensity in addition to electrostriction. We have measured the time response of these effects to on-off modulated electric fields for a material with negative dielectric anisotropy. We observe that the response times for BP I and BP II electrostriction are ∼10 s, while the response times for BP III peak intensification are ∼10 ms. We discuss the results in terms of models for these mechanisms and the as yet unknown structure of BP III.

The pretransitional behavior in the isotropic and reentrant-isotropic phases near a uniaxial nematic phase in the potassium laurate–decanol–${\mathrm{D}}_2$O system has been studied by light scattering and magnetically induced birefringence. The light-scattering measurements show mean-field behavior at both isotropic transitions and strong angular dependences from which a bare correlation length of 22 Å has been determined for both isotropic phases. The magnetic-field effects, however, depart from mean-field behavior, and, in conjunction with the light-scattering data, reveal a temperature-dependent magnetic anisotropy and furnish evidence for a change in micellar shape.

A phenomenological theory of polar structures in chiral biaxial liquid crystals is constructed exploiting the properties of a symmetric and traceless tensor order parameter field ${\mathit{Q}}_{\mathrm{\alpha }\mathrm{\beta }}$(r) and of a polar field ${\mathit{P}}_{\mathrm{\alpha }}$(r). Full advantage is taken of the symmetry of the order parameters by systematic use of the method of integrity bases, which allows us to establish an expansion of the most general SO(3)-invariant free-energy density to arbitrary powers in the components ${\mathit{Q}}_{\mathrm{\alpha }\mathrm{\beta }}$ and ${\mathit{P}}_{\mathrm{\alpha }}$. A coordinate-independent parametrization of the invariants is introduced that yields a classification of local polar structures and some predictions about possible topologies of phase diagrams without the necessity of performing numerical calculations. As one prominent result, the theory predicts a polar, chiral biaxial state that exists due to a piezoelectric coupling of a chiral biaxial tensor field and the polarization field and which disappears if tensor is uniaxial. We then provide a general theory of flexopolarization in biaxial systems. A general biaxial system is described by 12 fundamental flexopolarization modes. Special cases, obtained by imposing symmetry restrictions to the tensor field Q, reduce the number of modes. Finally, the theory is applied to chiral phases. Simple polar chiral structures including cholesteric and smectic-${\mathit{C}}^{\mathrm{*}}$ liquid crystals are analyzed. In particular, it is shown that if the smectic-${\mathit{C}}^{\mathrm{*}}$ phase is stabilized due to the piezoelectric coupling between Q, P, and a density wave, then it must be described as a biaxial uniform spiral with at least two nonvanishing commensurate harmonics. The minimization of the quadratic part of the Landau–de Gennes energy supplemented by (flexo)polarization terms may give rise to incommensurate two- or three-dimensional polar structures that can be stabilized by cubic terms.

The Brownian motion random-walk model of a polymer gives unphysical results for the case of a charged polymer in an electric field. To avoid these difficulties we use two stochastic processes in which the finiteness of the monomer size is retained. For a continuum model we use Kac’s telegrapher process. The relation of this to the Brownian motion picture corresponds to the relation between a Poisson process and its corresponding Wiener process. In both cases idealized and unrealistic properties of the Wiener process are avoided. Explicit results in any dimension are obtained by going over to a completely discrete process. By both methods, and in contrast to Brownian motion predictions, physically reasonable O(${\mathit{N}}^2$) dependence is found for the mean-squared extension of the size-N polymer. We also examine the breakdown of the Brownian motion approach by considering the effect of the electric field on the usual limiting process by which the discrete model becomes Brownian motion.

The explicit form of the condition of multiparticle correlation-function (CF) weakening in simple equilibrium classical liquids and gases is found. The result, which is a rigorous theorem of the real fluid statistical theory, is based only on the CF’s definition and the fact that the long-range forces determining the asymptotic value are due to the interaction of the equilibrium long-wavelength electromagnetic field with matter. To prove this theorem one does not have to suppose the existence of the microscopic potential energy of the fluid molecule interaction, or any model of it. The correlations weaken more slowly than predicted by the theories based on the microscopic potential energy, and there is no higher-order correlation connection with or decoupling to the lower-order ones. In the dilute gas where the intermolecular potential energy can be used, the n-particle CF asymptotic value is determined by the n-atom potential irreducible part, i.e., the corresponding nonadditive contribution to the potential energy determines the most slowly vanishing part of the n-particle CF.

The frequency-dependent complex viscosity of dispersions of various lipid vesicles was measured. Two clearly distinguishable features in these curves could be observed, suggesting two relaxation processes. The first relaxation time is shown to be dependent on the third power of the vesicle radius. This implies an elasticity of entropic nature, which is the result of the translational ordering of the vesicles that are subjected to shear flow and forced out of their equilibrium distribution. A comparison with linear viscoelastic measurements of dispersions of ‘‘real’’ hard-sphere (silica) particles shows remarkable agreement, and trends are extended. Furthermore, the measurements support theoretical work. For the second relaxation process, a linear dependency of the relaxation time on the radius of the vesicle is observed. In addition, the relaxation time and the effective viscosity were found to be linearly proportional. A combination of various similar theories that treat the dynamics of spherical capsules in terms of capsule wall viscoelasticity proved capable of explaining the dependency of the second relaxation time on the product of radius and effective viscosity. The second relaxation process was found to be pertaining to the surface shear viscoelasticity of the vesicle wall. Thus, from the analysis of the data with theory, the surface shear modulus and the surface shear viscosity of the lipid bilayer could be deduced. Furthermore, a small contribution of the curvature modulus was detected in our measurements, for which an estimate could be given. With the aid of literature data for the curvature modulus, the dilatational modulus, and the present value of the surface shear modulus found in this work, Young’s modulus and the Poisson ratio of the investigated bilayer could be established.

Crystalline dendrites, growing in a two-dimensional diffusion field at small Péclet numbers, are investigated. It is shown that, far from the tip, the distribution in size of the side branches gives them a fractal structure of dimension ${\mathit{d}}_{\mathit{f}}$≊1.58±0.03. In spite of the fluctuations, their overall area is the same as the underlying stable parabola observed at the tip. Similarly, anisotropic diffusion-limited aggregation patterns grown in a strip have a mean occupancy profile with a parabolic tip and a selection mechanism similar to that of stable anomalous Saffman-Taylor fingers.

We determine the schedule for the diffusive evolution of isoconcentration ‘‘surfaces’’ into a semi-infinite region, initially empty of diffusant, for a particular time-invariant boundary condition corresponding to constant diffusant concentration. This result is obtained by exploiting the correspondence between solutions of the Fokker-Planck equation for this boundary-value problem and the known fundamental solution in an unbounded space. We find that the initial evolution is linear, as predicted by us in earlier work, while the long-time behavior is as ${\mathit{t}}^{1/2}$, which is the solution predicted from the diffusion equation over the entire time domain. The transition between these limiting regions is described by a more complicated functional dependence.

The Laplacian roughening model is believed to undergo two interfacial roughening transitions. Upon reducing the temperature, the symmetry of the Hamiltonian is broken in two stages, the first in which orientational order develops, and the second in which the interface becomes completely flat. Within the traditional Coulomb gas picture these two transitions correspond, respectively, to unbinding of the dipole and charge pairs. We show that the orientational transition can be described by a continuous interface Hamiltonian and studied in the context of the sine-Gordon type of renormalization group. The flow equations deduced by the present techniques are, to leading order, equivalent to those for a vector Coulomb gas.

A generalized diffusion-limited aggregation (DLA) with reaction times that has been proposed by Bunde and Miyazima [Phys. Rev. A 38, 2099 (1988)] is considered. Crossover from the DLA to the diffusion-limited self-avoiding walk (DLSAW) is investigated by using the two-parameter position-space renormalization-group method. The crossover exponent and the crossover radius are calculated. The geometrical phase transition between DLA and DLSAW found by Bunde and Miyajima is analyzed by making use of the three-parameter position-space renormalization-group method. A global flow diagram in the three-parameter space is obtained. Above the percolation threshold all the renormalization flows are merged into the DLA point. Below the threshold all the renormalization flows are merged into the DLSAW point. When the reaction time is large, the double-crossover phenomenon occurs below the threshold.

We propose an approximate method for studying steady-state properties and linear stability of the dendritic arrays that are formed in directional solidification of alloys. Our analysis is valid at high growth rates where the primary spacing between dendrites is larger than the velocity-dependent solutal diffusion length. We compute a neutral stability boundary and find that, in the situations where we expect our results to be valid, the experimental data of Somboonsuk, Mason, and Trivedi [Metall. Trans. A 15A, 967 (1984)] lie in the stable region, well away from the boundary.

An analytical theory is developed that describes how a radiation pulse is modified as it propagates through a plasma with arbitrary temporal and spatial density variations. Expressions are derived for the shifts induced in both the frequency and the wave number of the radiation pulse. The possibility of upshifting the frequency of the laser pulse using plasma waves is analyzed. It is found that maximum shifts occur when the phase velocity of the plasma wave is equal to the group velocity of the laser pulse. Nonlinear analysis of laser propagation in underdense plasmas (${\mathrm{\omega }}_{\mathit{p}}^2$/${\mathrm{\omega }}^2$≪1, where ${\mathrm{\omega }}_{\mathit{p}}$ is the plasma frequency and ω is the laser frequency) shows that the frequency shift scales asymptotically as the square root of the propagation distance. Furthermore, it is shown that the laser power remains constant as the pulse propagates through the plasma.

With the use of a new microscopic particle code, ion-ion and ion-electron pair distribution functions are observed for two-component strongly coupled plasmas and compared with theories in detail. The reduction in bremsstrahlung emission from a binary-ionic-mixture plasma is calculated from the pair distribution functions observed. It is found that the reduction in bremsstrahlung emission from a binary-ionic-mixture plasma is approximated by that for a fictitious single-ion-species plasma with a certain ion charge even for the two-component plasma.

We have developed a three-dimensional nonlinear theory for the slow-wave cyclotron instability in a dielectrically loaded waveguide. The efficiency, gain, and bandwidth of the amplifier are calculated from a self-consistent solution of a set of coupled nonlinear differential equations describing the growth of the electromagnetic field and the evolution of the electron trajectories. Calculations show that very broadband amplification with high efficiency is possible in the slow-wave region of the propagating waves. The instantaneous bandwidth (full width at half maximum) near saturation depends on the efficiency (η) of the amplifier. For a cold beam, the bandwidth Δω/ω≊6% at η≊40%, but Δω/ω increases to 30% if the amplifier is operated at an efficiency of 20%. The efficiency is very sensitive to the axial velocity spread of the beam since interaction occurs with waves having large propagation constants. The maximum efficiency drops from 48% to 10% as the velocity spread increases from 0% to 5% but the bandwidth shows a small increase.

We report on the wavelengths and satellite intensity factors of heliumlike nickel and chromium ions undergoing dielectronic recombinations via several high-lying resonance states. We have considered the process 1${\mathit{s}}^2$+εl→(1s2lnl’${)}^{\mathrm{*}}$→1${\mathit{s}}^2$ml’’+γ for n=5, 6, 7, and 8 with all possible values of l and l’, where n is the principal quantum number of the outermost electron of the doubly excited intermediate resonance state. The calculations have been performed in the intermediate-coupling scheme using the multiconfiguration Hartree-Fock model.

A detailed study of the linear stability and nonlinear wave collapse of Langmuir solitons in a weakly magnetized plasma is performed. An analytical investigation of the linear soliton instability with respect to long-wavelength transverse perturbation versus magnetic field effect is presented. For a more complete understanding of the growth-rate structure, a numerical solution of the eigenvalue problem that corresponds to the model equations is obtained and compared with analytical predictions. Comparison with earlier results is given. Furthermore, numerical results obtained by a direct simulation method in two dimensions are also presented. In a linear regime, detailed agreement with the results of the corresponding eigenvalue problem is found. In the nonlinear regime of the soliton instability all considered cases exhibit a collapse dynamics. Moreover, in the developed, highly nonlinear stage of the soliton collapse, self-similar behavior consistent with a ‘‘weak’’ collapse regime is found.

Modeling calculations of an electron-cyclotron-resonance ion source (ECRIS) are presented. The adopted model is described through an analysis of the impact of ECR heating, transport, and atomic-collision processes on the particle balance in an ECRIS. On the basis of the obtained numerical results, general relationships among externally controllable parameters, and plasma and ion-beam characteristics are derived. Comparison with experimental results that use the electron energy as the only fitting parameter supports the applied model. The predicted general trends provide some basis for future experiments.

The kinetic-energy distributions (KED’s) of ${\mathrm{Pd}}^{+}$, ${\mathrm{H}}_{\mathit{n}}^{+}$, ${\mathrm{D}}_{\mathit{n}}^{+}$ (n=1 to 3), and electrons created in a picosecond laser-produced plasma from palladium metal were studied using a spherical-sector electrostatic energy analyzer followed by a time-of-flight mass spectrometer. High-resolution, Fourier-transform mass spectroscopy showed no ${\mathrm{PdH}}_{\mathit{n}}^{+}$ or ${\mathrm{PdD}}_{\mathit{n}}^{+}$ ions. The total yield of ${\mathrm{Pd}}^{+}$ ions grew linearly with laser power, while the yields of ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ increased exponentially. The yields of ${\mathrm{H}}_2^{+}$ and ${\mathrm{H}}_3^{+}$ increased with laser power density slowly, comparable to the yields of ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ at low laser power density only. The average ion kinetic energy was a linear function of laser power for all species, but with a much higher average kinetic energy for ${\mathrm{Pd}}^{+}$ than for other species at a given laser power. The KED’s for ${\mathrm{Pd}}^{+}$ were dominated by a plasma component with a Maxwellian shape, accompanied by a thermionic component with a kinetic energy in the range of 0–1 eV. The ${\mathrm{H}}^{+}$, ${\mathrm{H}}_2^{+}$, ${\mathrm{H}}_3^{+}$, and ${\mathrm{D}}^{+}$ KED’s were basically Maxwellian, but the origins were offset to positive kinetic energies, with the offset energy increasing linearly with mass. There was no thermal component for the ${\mathrm{H}}_{\mathit{n}}^{+}$ or ${\mathrm{D}}^{+}$ ions.

Nonlinear interaction of electromagnetic waves and acoustic modes in an electron-positron plasma is investigated. The plasma of electrons and positrons is quite plastic so that the imposition of electromagnetic (em) waves causes depression of the plasma and other structural imprints on it through either the nonresonant or resonant interaction. Our theory shows that the nonresonant interaction can lead to the coalescence of photons and collapse of plasma cavity in higher (≥2) dimensions. The resonant interaction, in which the group velocity of em waves is equal to the phase velocity of acoustic waves, is analyzed and a set of basic equations of the system is derived via the reductive perturbation theory. We find new solutions of solitary types: bright solitons, kink solitons, and dark solitons as the solutions to these equations. An implication of the present theory on astrophysical plasma settings is suggested, including the cosmological relativistically hot electron-positron plasma.

We study a quantum rotor with a piecewise potential V(θ). We find that the tunneling rate through horizontal Kolmogorov-Arnold-Moser curves is not less than a power of ħ as ħ→0. The exponent depends on the order of discontinuity in V(θ). This is in contrast with the tunneling rate through a potential barrier, which is of the order exp(-c/ħ).