Quartz undergoes a gradual transition with pressure from a crystalline to an amorphous state. The mechanism for this pressure-induced transition is not well understood. By combining new experimental pressure data and recently developed theoretical methods, we have determined the structural properties of α-quartz near and above this transition. We suggest that highly unfavorable interpolyhedral anion-anion distances, which occur at the transition pressure, play a key role in driving the transition.

The clean Au(111) surface is found, using x-ray diffraction, to reconstruct between 300 and 1250 K with two distinct phases. Below 865 K, the surface structure consists of an equiilbrium density of kinks between rotationally equivalent domains, each of which is itself composed of a sequence of uniaxial discommensurations. Above 880 K, the arrangement of kinks and discommensurations is disordered, which results in an isotropic compression of the reconstructed layer. The phase transition between the two structures is first order.

The nucleation density ${\mathit{N}}_{\mathit{x}}$(T) of Ag islands on the 2-monolayer-thick silver intermediate layer on W(110) is measured by UHV scanning electron microscopy and compared with kinetic nucleation equations. This yields a well-determined value of ${\mathit{E}}_{\mathit{d}}$+2${\mathit{E}}_{\mathit{b}}$=0.65±0.03 eV, with pair-binding energy ${\mathit{E}}_{\mathit{b}}$=0.25±0.05, diffusion energy ${\mathit{E}}_{\mathit{d}}$=0.15±0.10, and adsorption energy ${\mathit{E}}_{\mathit{a}}$=2.2±0.1 eV. Values for Ag/Ag(111) are calculated in effective-medium theory, giving ${\mathit{E}}_{\mathit{a}}$=2.23, ${\mathit{E}}_{\mathit{b}}$=0.28, ${\mathit{E}}_{\mathit{d}}$=0.12, and ${\mathit{E}}_{\mathit{d}}$+2${\mathit{E}}_{\mathit{b}}$=0.68 eV. Observations on vicinal and impure surfaces are reported, and implications for defect binding energies discussed.

An Ising-model description of the L$1_0$ structure in fcc AB binary alloys with antiferromagnetic nearest-neighbor coupling (${\mathit{J}}_{\mathrm{nn}}$<0) and ferromagnetic next-nearest-neighbor interaction (${\mathit{J}}_{\mathrm{nnn}}$>0) leads to frustration in the bulk but a stable c(2×2) superstructure for a free (010) surface. Using Monte Carlo simulations with ${\mathit{J}}_{\mathrm{nnn}}$/${\mathit{J}}_{\mathrm{nn}}$=-0.05, we show that the surface orders at higher tmeperature than does the bulk. As the temperature is lowered towards the first-order bulk transition, surface-induced ordering occurs, thus producing the first confirmation of the prediction by Lipowsky.

A scanning-tunneling-microscopy study of the structures produced by the adsorption of atomic H on the Si(100) surface is presented. Moderate exposures at room temperature (RT) produced the 2×1 monohydride phase. Saturation exposures at 370 K produced the mixed 3×1 phase which, following an additional RT exposure, slowly reverted to the bulklike 1×1 dihydride phase. Saturation exposures at RT produced the 1×1 dihydride phase indicating that the 3×1 phase is a H-induced reconstruction which exists only under special conditions. The etching of the Si(100) surface by H atoms was also observed and its implication for saturation coverage and the species present on the surface are discussed.

A monovacancy defect is observed by positron-lifetime and Doppler-broadening experiments in semi-insulating GaAs after photoquenching the EL2 defects. The monovacancy exhibits the same thermal and optical recoveries as the metastable state of EL2 and its concentration is proportional to that of the EL2 defect. It is concluded that the metastable state of the EL2 defect involves the monovacancy. The results indiacte that the monovacancy or its immediate surroundings is negatively charged.

We report the first experimental observations of magnetic-field-independent corrections to the conductivity of a weakly interacting 2D electron gas on solid on ${\mathrm{H}}_2$. These incoherent corrections, which are substantially larger than the weak-localization terms at high helium-gas densities, are estimated via an electron-trapping model. The model has essentially no adjustable parameters and gives a self-consistent extrapolation between the weak- and strong-disorder limits. In the regime in which trapping is dominant we observe a residual electron mobility ${\mathrm{\mu }}_{\mathrm{res}}$∼${\mathit{T}}^{\mathrm{-}1}$ associated with a lower quantum limit of diffusion.

Interference among Bragg beams and effects of polarization-state mixing in a multiple-beam diffraction process leads to a new method of determining noncentrosymmetric phases when an elliptically polarized x-ray beam is used. This idea is proposed for the first time, based on the perturbation theory of scattering, and is demonstrated by an experiment on a GaAs single crystal.

Fermionic squeezing is derived in analogy to squeezed light, but with two distinct features: (i) Intramode squeezing can be achieved by noise attenuation in the particle sector at the cost of noise amplification in the antiparticle sector (or vice versa). Multimode squeezing requires the presence of at least two field modes, one field mode acting similarly as the antiparticle sector in intramode squeezing. (ii) Due to the invariance of the operator algebra under generalized Bogoliubov-Valatin transformations, squeezing is characterized by trigonometric functions. Possibilities for an experimental realization are discussed.

The formation of magnetic domains in thin epitaxial Co/Au(111) films is investigated. Three-monolayer films are shown to decay into out-of-plane domains of micron size. We find a smooth transition from out-of-plane to in-plane magnetization at a crossover thickness of 4.5 layers and determine that the domain size depends linearly on film thickness below crossover.

Neutron spin-echo and small-angle neutron-scattering techniques were employed to study both the size and the shape fluctuations of the di-2-ethylhexyl sulfosuccinate water-in-oil microemulsion system. We observe a softening of the bending rigidity of the interface with the addition of butanol as cosurfactant. Comparison of both techniques with the theory implies that for the simultaneous description of the shape and size fluctuations both the splay and saddle-splay bending moduli are the key parameters, and the spontaneous curvature has little influence.

Genetic fusion is introduced as a model for evolution. In the fusion two genomes are combined to generate a longer genome. Representing each species by a binary genetic sequence, we introduce a fitness function on the bit sequence. As the evolutionary dynamics, we incoroporate mutation, genetic fusion, and reproduction in proportion to fitness. It is found that genetic fusion leads to the appearance of module-type sequences and duplicated genes. The time necessary to find a sequence with large fitness is largely reduced by the inclusion of genetic fusion, which suggests the application of our algorithm to optimization problems.

Degenerate perturbation theory, without geometrical tools, directly yields vector potentials in the Born-Oppenheimer approximation. The derivation uses only the algebra of the dynamical operators, and the slow variables need not commute.

It is shown that the pair connectedness for percolation on an arbitrary directed graph may be obtained by setting λ=1 in the transmissivity function for a λ-state chiral Potts model, the odd-flow model, and that the pair connectedness of the dual percolation model is expressible in terms of the correlation function of a second chiral Potts model, the odd potential difference model. There is an important distinction between odd and even λ and only those models with odd λ exhibit chirality. The duality relation between the transmissivity and correlation functions leads to a duality relation for pair connectedness of directed percolation theory which extends the result of Dhar, Barna, and Phani for percolation probabilities.

Reflection of microwaves from a cavity is measured in a frequency domain where the underlying classical chaotic scattering leaves a clear mark on the wave dynamics. We check the hypothesis that the fluctuations of the S matrix can be described in terms of parameters characterizing the chaotic classical scatteirng. Absorption of energy in the cavity walls is shown to significantly affect the results, and is linked to time-domain properties of the scattering in a general way. We also show that features whose origin is entirely due to wave dynamics (e.g., the enhancement of the Wigner time delay due to time-reversal symmetry) coexist with other features which characterize the underlying classical dynamics.

We study quantized classically chaotic maps on a toroidal two-diensional phase space. A discrete, topological criterion for phase-space localization is presented. To each eigenfunction an integer is associated, analogous to a quantized Hall conductivity, which when nonzero reflects phase-space delocalization. A model system is studied, and a correspondence between delocalization and chaotic classical dynamics is discussed.

In extended inflation, a new version of inflation where the transition from an inflationary to a radiation-dominated Universe is accomplished by bubble nucleation, bubble collisions supply a potent—and potentially detectable—source of gravitational waves. The energy density in relic gravitons from bubble collisions is expected to be about ${10}^{\mathrm{-}5}$ of closure density. Their characteristic wavelength depends upon the reheating temperature ${\mathit{T}}_{\mathrm{R}\mathrm{H}:}$ λ∼(${10}^4$ cm)[(${10}^{14}$ GeV)/${\mathit{T}}_{\mathrm{RH}}$]. If black holes are produced by bubble collisions, they will evaporate producing shorter-wavelength gravitons.

Vacuum neutrino oscillations with very long wavelengths, comparable to the radius of the Earth’s orbit, can simultaneously explain both the Homestake and Kamiokande-II solar-neutrino counting rates. They imply approximately a 50% reduction in the ${\nu }_{\mathit{e}}$ flux from the p-p process, and seasonal variations in the ν-e scattering rates for ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$ and pep neutrinos, which distinguish this scenario from alternative explanations.

We calculate the path integral on a torus in continuum Liouville theory coupled to conformal matter with c≤1. We find agreement with the results obtained in matrix models, topological gravity, and Korteweg–de Vries hierarchies.

A dramatic reduction in background was achieved in the latest Pacific Northwest Laboratory–University of South Carolina germanium detectors. Two 1.05-kg natural-isotopic-abundance detectors were operated for 1.92 kg yr. The residual spectrum, after straightforward corrections, has a significant region resembling the theoretical spectrum of the two-neutrino ββ decay of ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}$. A fit to the data yields ${\mathit{T}}_{1/2}^{2\nu }$${(}^{76}$Ge)=(1.$1_{\mathrm{-}0.3}^{+0.6\mathrm{}}$)×${10}^{21}$ yr at the 95% C.L., which agrees with shell-model predictions.