Sawtooth oscillations (STO) in the Ohmically heated WT-3 tokamak are strongly modified or suppressed by localized-electron cyclotron resonance heating (ECH) near the q=1 surface, where q refers to the safety factor. The efect of ECH is much stronger when it is applied on the high-field side as compared to the low-field side. Complete suppression of the STO is achieved for the duration of the ECH, in most cases, when it is applied on the high-field side of a low-density plasma, provided the ECH power exceeds a threshold value. The STO stabilization is attributed to a modification of the current-density profile by hot electrons generated by ECH, which reduces the shear in the q=1 region.

We present a new mean-field theory of diffusion-limited aggregation (DLA). We apply our approach to calculate the ensemble-averaged structure seen in recent experiments in a two-dimensional channel geometry. Our method explains the similarity between the average DLA occupancy and the Saffman-Taylor finger pattern.

The scaling behavior of open dissipative systems including the Kim-Kosterlitz exponents for interfacial growth, and the current fluctuations in a flowing sandpile are derived using a generalization of the arguments applied by Kolmogorov to the inertial range of turbulence. The approach may be considered a nonequilibrium equivalent to Flory theory.

We report on x-ray scattering studies of the nematic to smectic-A transition in 4-cyano-4’-octylbiphenyl under nonequilibrium shear-flow conditions. As the transition is approached, the interplay between the viscous frictional and the flow-induced-fluctuation forces on the nematic director leads to a series of regimes whose occurrence results from the divergence in one of the viscosities due to the critical slowing down of the smectic-A order-parameter fluctuations. The experiments demonstrate that, under flow, synchrotron x-ray-diffraction techniques provide a powerful structural probe of steady-state dynamical behavior.

Measurements of the dielectric permittivity are reported for solid solutions of antiferroelectric betaine phosphate and ferroelectric betaine phosphite at frequencies ${10}^{\mathrm{-}2}$ Hz<ν<${10}^0$Hz and temperatures 2 K<T<300 K. In these compounds the inorganic components are linked by hydrogen bonds to one-dimensional chains. The mixed crystal with a phosphate concentration x=0.95 orders antiferroelectrically at ${\mathit{T}}_{\mathit{c}}$=79 K. For T>${\mathit{T}}_{\mathit{c}}$ the dielectric response is dominated by ac and dc conduction. The 40% sample exhibits pure relaxational behavior indicative of a transition into an orientational glass state.

The diffusion of atomic hydrogen on the Si(111)7×7 surface has been investigated experimentally by means of optical second-harmonic diffraction from a submonolayer grating of adsorbed hydrogen. With submicron grating periods, we are able to measure surface diffusivities D<${10}^{\mathrm{-}14}$ ${\mathrm{cm}}^2$ ${\mathrm{s}}^{\mathrm{-}1}$. We find a thermally activated diffusion process with a barrier of ${\mathit{E}}_{\mathit{a}}$=1.5±0.2 eV and a preexponential factor ${\mathit{D}}_0$ of ${10}^{\mathrm{-}3}$ ${\mathrm{cm}}^2$ ${\mathrm{s}}^{\mathrm{-}1}$.

The migration of Si on Si(001) has been investigated by analyzing the number density of islands formed during deposition using scanning tunneling microscopy. The activation energy and prefactor for diffusion in the fast direction, which is along the surface dimer rows, are found to be 0.67±0.08 eV and ∼${10}^{\mathrm{-}3}$ ${\mathrm{cm}}^2$/sec, respectively.

The Ge(111) surface has been reported to undergo a disordering phase transition at approximately 1050 K. Our synchrotron x-ray diffraction study demonstrates that there is, instead, a progressive disordering of the topmost layers, which retain long-range order up to at least 1150 K. Neither diffuse scattering associated with roughening or surface melting nor effects due to surface incommensurability are observed. A model involving a proliferation of surface vacancies gives a consistent description of the data.

The effective charge on a surface atom gives both the force on the atom in terms of an external, unscreened field, and the change in the surface dipole contribution to the work function when the atom is displaced. It is shown that this can give rise to a surface instability if the effective charge is big enough. In the case of O adsorbed on Cu(001) in the fourfold hollow site, calculations give an effective charge of -0.9‖e‖, which means that the ‘‘ideal’’ c(2×2) structure at 0.5-monolayer coverage is unstable. This provides an explanation for the reconstruction to the (√2 ×2 √2 )R45° phase which is observed.

Potential n-type dopants in diamond are investigated via ab initio methods. The well-known distortions around the deep donor N are found to arise from the interaction of the N lone pair with a C dangling bond. P, Li, and Na are all shallow dopants, but their solubilities are muh too low for doping via in-diffusion. Li is a relatively fast diffuser, Na is stable up to moderate temperatures, while P should remain immobile even at high temperatures. Na, being an interstitial dopant, is particularly suitable for ion implantation, since there is no need to displace host atoms.

A new structural phase transition in cesium has been found around 72 GPa using high-pressure powder x-ray-diffraction techniques with a diamond-anvil cell and an imaging plate. The crystal structure of the high-pressure phase Cs(VI) is either hexagonal close packed or double hexagonal close packed. The transition from the open-packed Cs(V) structure to the close-packed Cs(VI) structure implies significant influence of the 5p core repulsion to the cohesion of Cs(VI), as well as a possible occurrence of the 5p-5d overlap.

The shear response of a prototypical martensitic system is studied using a simple model incorporating the effects of both anharmonicity and anomalous phonon behavior. Using parameters determined from experimental phonon data, we show that the pseudoelastic and thermoelastic effects characteristic of Ni-Al and many other shape-memory alloys are closely related to the observed phonon anomalies.

Electron-energy-loss spectroscopy has been used to study momentum-dependent excitations of π electrons in β-carotene. These investigations illustrate that the orbital levels of π electrons in finite polyenes and other conjugated oligomers can be mapped out. It turns out that the electronic structure of the π electrons can be described in terms of a quasi band structure with a finite number of momentum values. The results are compared with theoretical calculations based on an effective Su-Schrieffer-Heeger Hamiltonian.

High-resolution x-ray diffraction has resolved the phase-coherence volume of the charge-density wave in electron-irradiated blue bronze. The coherence volume is 3.7×${10}^8$ Å ${}_{}{}^3{}_{}{}^{}$ for defect density n=0.034 at. %. The coherence lengths decrease roughly as ${\mathit{n}}^{\mathrm{-}0.4}$. Results are contrasted with various theories of pinning.

We report observations in the visible region of the spectrum, on a new coupled surface mode, the long-range surface exciton polariton which is supported by a highly absorbing thin layer. Using careful evaporation procedures it has proved possible to produce very thin islandized films of silver which have relative permitivities with real parts much less than their imaginary parts and which therefore satisfy the criterion necessary to support this new type of resonance.

Classical and quantum electron transport in an annulus formed by a two-dimensional electron gas in a perpendicular magnetic field is studied. The transport is driven by an enclosed time-dependent magnetic flux, and is described with a Landauer-Büttiker-type formalism Transport current is generated because the transmission and reflection probabilities of the electrons which are emitted by the reservoirs on the inside and outside of the annulus are modified from their static values. The role of the probes, as well as the effect of scattering between the inside and outside of the annulus, is studued.

When observed through a thick resonant sample the exponential decay of a Mössbauer γ state appears to be considerably accelerated. We show that, by applying a phase change to the emitted γ radiation field, this accelerated decay signal can be regenerated. The amplitude of this novel echo signal can be much higher than the corresponding source intensity. Sample saturation by an intense source is not required. Experimental results with the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer resonance are in good accordance with the theory.

A Monte Carlo algorithm has been used to study the random-anisotropy model with two-component spins on simple cubic lattices, in the strong anisotropy limit. The magnetic susceptibility diverges at a temperature of ${\mathit{T}}_{\mathit{c}}$/J=1.91±0.03, but no singular behavior is observable in the specific heat near ${\mathit{T}}_{\mathit{c}}$. The two-spin correlation function at ${\mathit{T}}_{\mathit{c}}$ is described by a critical exponent η=0.04±0.04. The magnetization of the ground state of a lattice of size L is approximately 1/[0.37 ln(L)+1] per spin. At t=0 the value of ${\mathrm{\eta }}_1$ is -0.34±0.06, which agrees with small-angle neutron-scattering experiments on certain amorphous magnetic alloys.

We consider a generalized version (including anisotropy) of the stochastic one-dimensional cellular automaton studied by Domany and Kinzel. It recovers Wolfram-like deterministic cellular automata as particular cases. The phase diagram presents three (and not two, as previously suggested) phases which were detected through the numerical study of both the order parameter and the sensitivity to initial conditions. The various universality classes are exhibited as well.

A general method to compute precise approximations to invariant tori of Hamiltonian systems is presented. For illustration, a strongly nonlinear example from accelerator theory is treated, in 21/2 degrees of freedom. Accuracy, computation time, and effectiveness near resonances are found to be highly favorable in comparison to previous methods.