Laboratory experiments and numerical simulations have been conducted for plane channel flow with a streamwise-periodic array of cylinders. The primary transition in this open flow occurs as a convective rather than absolute instability and leads to traveling-wave packets, which advect out of the system. The ordered secondary state is characteristic of closed flows, in contrast with other open flows where the primary transition often leads directly to turbulence.

Spatial dark-soliton (SDS) stripes are experimentally found in the transverse plane of a laser beam that passes through a rectilinear diffraction screen before propagating in a self-defocusing nonlinear material. Materials with different mechanisms of nonlinearity manifest the same qualitative results. The SDS nature of the observed phenomenon is verified by numerical simulations of the (2+1)-D nonlinear Schrödinger equation, analytical solutions for the (1+1)-D case, and their comparison with experimental data.

We derive an integrable model of coupled equations for the description of the transient evolution of small-amplitude ion acoustic waves interacting with the electromagnetic waves. We obtain then a universal behavior of the acoustic wave driven by a strong ponderomotive force: The asymptotic solution consists of two supersonic localized singular coherent structures following a sonic wave front and asymptotically colliding at the sound speed. The reflected electromagnetic wave presents singularities in the same regions as the acoustic wave.

We report very accurate simulations of the velocity autocorrelation function of a tagged particle in a two-dimensional lattice-gas cellular automaton. We observe, for the first time, that the hydrodynamic long-time tail of the velocity autocorrelation function decays significantly faster than ${\mathit{t}}^{\mathrm{-}1}$. The simulation results are compatible with the predictions of self-consistent mode-coupling theory. However, observation of a true (t √lnt ${)}^{\mathrm{-}1}$ decay, which is predicted by the latter theory as t→∞, would require simulations that are 15 orders of magnitude longer than is currently feasible.

Polymer concentration correlations in a network swollen by a solvent are studied by small-angle neutron scattering. The network is made by statistical cross-linking of long chains in solution. At small scattering vectors, the intensity strongly increases upon swelling. Upon stretching, unusually oriented isointensity lines (‘‘butterflies’’) are observed. These effects are well described by a model of fractal heterogeneities of cross-linking giving rise to nonhomogeneous swelling or deformation.

The structure of monolayers of ${\mathrm{CH}}_3$(${\mathrm{CH}}_2$${)}_{22}$COOH on water, along ∼21 °C isotherms between 18 and 35 Å ${}_{}{}^2{}_{}{}^{}$/molecule, has been studied with glancing-angle x-ray diffraction and specular reflection. Following an incremental area change, in some regions of the isotherm, the diffraction pattern evolves until the surface pressure π is relaxing ≲0.01 dyn/cm h. Reentrant behavior between tilted and nontilted phases is observed. Phase coexistence is indicated by differences between the deposited area/molecule and the area of the x-ray-determined unit cell.

The kinetics of the deposition or adsorption of large spheres onto a solid surface is investigated theoretically. Unlike previous models, the interaction between the adsorbed and the bulk particles is considered explicitly. It is shown that in the asymptotic regime (large coverage) the coverage approaches its saturation value according to the power law ρ(∞)-ρ(t)∼${\mathit{t}}^{\mathrm{-}2/3}$, the exponent of which is different from that predicted by the random sequential adsorption model.

We report the development of cellular mixing at a vertical interface between miscible solutions with a single diffusing species. For the binary systems studied, this patterned convection occurs only when the interface is driven. Experimental results support the interpretation of this hydrodynamic instability as a surface-tension effect. We relate this convective instability, and viscous fingering between fluids with small relative viscosities, to the macroscopic morphology of electrodeposits and precipitates.

On the (1×1) Ir(110) surface the diffusion of single Ir adatoms is two dimensional; i.e., an adatom can diffuse along the [11¯0] surface channel by atomic hopping as well as across the channel by atomic replacement. The energy needed for an Ir adatom to hop along the channel is 0.80±0.04 eV, or 0.09 eV larger than needed to replace a lattice atom in the cross-channel jump. In contrast, Ir adatom diffusion on the (1×2)-reconstructed surface, with double-space [11¯0] channels, is one dimensional; i.e., an adatom can only hop along these channels. The activation energy, 0.86±0.03 eV, is higher than that for either cross-channel or along-the-channel diffusion on the nonreconstructed surface.

A study of the linear and nonlinear optical behavior of recently realized crystalline organic semiconductor quantum wells is reported. Using the Davydov Hamiltonian, we find analytical solutions for the optical response function, and we predict the existence of intrinsic optical bistability and two-beam-coupling energy transfer in these materials.

Antidot structures have been prepared by etching arrays of 100-nm holes into a two-dimensional electron gas of ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$As quantum wells. In the far-infrared response we observe the unique collective excitation spectrum of antidots. It consists of a high-frequency branch which starts, in a magnetic field B, with a negative B dispersion and then increases in frequency with B. A second low-frequency branch corresponds at high B to edge magnetoplasmons which circulate around the holes. For small B this branch approaches the cyclotron frequency, where the electrons perform classical cyclotron orbits around the holes.

We present evidence for the first observation of localization corrections to the thermoelectric coefficient η relating current flow to applied temperature gradient, using the two-dimensional electron gas of a silicon-on-sapphire inversion layer as a test system. We observe an increase in the magnitude of η in a weak, perpendicular magnetic field, which we explain in terms of the suppression of quantum interference effects by the field. Good agreement is found between theory and experiment.

Amplification of echo signals in ordered magnetic materials was postulated by many authors in the middle sixties. However, theoretical estimates of amplification were invariably several orders of magnitude higher than observed in ferrimagnetic materials. We have reformulated the equation of motion to include the nonlocal dipole-dipole interaction to describe the dynamic response of an echo. Our estimates of amplification compare reasonably well with measured values.

The nonlinear current-voltage (I-V) characteristics are discussed in terms of thermally activated vortex-antivortex pairs in the ${\mathrm{CuO}}_2$ planes. The interaction energy of the two vortices in a pair grows linearly with their separation due to the interplane coupling. This linear interaction gives rise to a critical current which decreases linearly with the temperature. The I-V characteristic has the form V∼I(I-${\mathit{I}}_{\mathit{c}}$${)}^{\mathrm{\alpha }\left(\mathit{T}\right)\mathrm{-}1}$. Comparisons with experiments on Y-Ba-Cu-O are discussed. We suggest that the I-V data in the case of a magnetic field may be associated with a thermal vortex-unbinding transition.

Water chemisorbs molecularly on Ni(110) at 180 K producing a c(2×2) overlayer, at 0.5 ML (ML=monolayer=1.15×${10}^{15}$ molecules ${\mathrm{cm}}^{\mathrm{-}2}$). This water is undetectable by ir methods which indicates the absence of clustering (and, hence, no intermolecular H bonding) in the first 0.5 ML. Population of a second ‘‘icelike’’ layer, which is nearly saturated after an additional 0.5 ML, makes the first 0.5 ML ir active and produces strong H bonding interactions. The plane of the chemisorbed water, which is originally parallel to the surface, is reoriented towards the surface normal on filling of the second layer.

When processes not conserving the double occupation number are inhibited, one obtains a model like the Kivelson-Schrieffer-Su-Heeger extension of the Hubbard model. We solve this model exactly in any number of dimensions. The system free energy is given for any fixed filling. The ground-state energy and double-occupancy order parameter are discussed for d=2.

It is argued that any quantum probability should correspond to a ratio between the number of counts (positive results) in some measurement and the number of copies of the physical system initially prepared. Then, the proofs of Bell’s theorem are criticized on the grounds that the probabilities used to show a violation of the Bell inequalities do not fulfill that condition. A hidden-variables model is proposed which reproduces the results of the optical experimental tests of the inequalities, even with perfect polarizers and detectors.

We describe the first experimental demonstration of dual recycling, an optical system which should improve considerably the performance of proposed laser-interferometric gravitational-wave detectors. The results were in excellent agreement with predictions: An enhancement of the signal-to-noise ratio by a factor of 7 was observed.

Previous models of cycltron-line features in the spectra of cosmic γ-ray bursts have assumed that the lines form in a thin, static atmosphere. Here we show that the observed absorption lines may in fact form in a radiation-diriven plasma wind expelled at relativistic velocities from the surface of a neutron star.

Axions of mass 3–8 eV should have a cosmological abundance of about 50 ${\mathrm{cm}}^{\mathrm{-}3}$ and reside in rich clusters of galaxies. Their decays to two photons will produce a line at a wavelength ${\lambda }_{\mathit{a}}$∼3100–8300 Å (width Δλ/λ∼${10}^{\mathrm{-}2}$). We have searched unsuccessfully for such a feature in the intergalactic light of three rich clusters, closing this ‘‘window,’’ and leaving open only the window from ${10}^{\mathrm{-}6}$ to ${10}^{\mathrm{-}3}$ eV. This implies that if the axion exists, it likely comprises the dark matter. Our flux limits are of relevance to other relics whose decays produce monoenergetic photons.