In a thermal-convection box the boundaries parallel to the convection rolls are replaced by ramps, i.e., smooth variations in the Rayleigh number from supercritical to subcritical values. Such a geometry selects a small wavelength band. The wavelength selected by the left and right ramps are not necessarily the same—resulting in a wavelength gradient. This gradient leads to forced phase diffusion: The convection pattern drifts from the short-wavelength end to the ramp selecting the longer wavelength. An experimental observation of this phenomenon is presented.

The propagation of light in a disordered medium with purely elastic scattering is considered. The intensity correlation functions for the diffusely reflected and transmitted light from a slab are shown to have long-range power-law behavior. This arises from the interference of the underlying wave fields of the diffusive modes of the light intensity. This interference also gives rise to anomalous fluctuations in the transmission coefficient analogous to conductance fluctuations in metals.

We describe experiments that demonstrate a new method for producing high-power microwave emission. The unstable oscillations of a virtual cathode which forms when a magnetized relativistic electron beam is injected into a circular waveguide generate the microwave radiation. In contrast to previous virtual cathode microwave generation techniques, electrons in the waveguide cannot be reflected back into the diode. Using this technique, we have produced 1.4 GW at 3.9 GHz with several hundred megawatts radiated in harmonic radiation.

Harmonics of plasma waves excited by ${\mathrm{CO}}_2$-laser–induced stimulated Raman scattering have been observed by frequency- and wave-number-resolved ruby Thomson scattering. Measurements of their relative amplitudes agreed with predictions of nonlinear warm-plasma wave-steepening theory up to the maximum observed amplitude of the fundamental component, n${̃}_1$/$n_0$=16%.

A metastable c(2×2) structure is formed by atomic oxygen on Pd(100) under conditions of slow diffusion combined with rapid adsorption. We propose that formation of the c(2×2) under these conditions must be controlled by characteristics of the dissociative adsorption event itself.

Two-dimensional random surfaces are constructed by the mapping in d-dimensional space of a triangular network (of linear size L) with hierarchical bond structure. A relation between static properties of such surfaces and the resistance exponent, ζ, of two-dimensional inhomogeneous structures allows us to show that free surfaces have an average radius of gyration ξ∼${\mathrm{L}}^{\mathrm{\zeta }/2\mathrm{}}$ for L→∞. When a self-avoidance constraint is imposed, a Flory argument gives ξ∼${\mathrm{L}}^{2/\mathrm{D}}$ with D=(2+d)/(2+(1/2ζ), and an upper critical dimension dc=8/ζ. Specific examples are discussed where exact, nontrivial values of ξ can be predicted by renormalization and scaling arguments. Universal, as well as nonuniversal, asymptotic regimes occur.

The roughness of the (110) surface of face-centered-cubic copper has been measured between 200 and 700 °C, by x-ray diffraction. The changes observed are reversible and suggest a roughening temperature of ${\mathrm{T}}_{\mathrm{R}}$≃600 °C. This value is compared with simple theoretical estimates and found to be in satisfactory agreement.

The rainbow effect in ion channeling (in very thin crystals) is considered. In the analysis the results of catastrophe theory are used. The calculations were performed for 10-MeV ${\mathrm{H}}^{+}$ ions and the 〈100〉 channel of a 1000-Å-thick Au crystal.

Temperature- and polarization-dependent surface extended x-ray absorption fine-structure measurements from higher-neighbor shells are used to study the anisotropy of surface vibrational amplitudes. In Cu(100)c(2×2)-Cl, mean square displacements of both Cl and surface Cu atoms are found to be approximately twice as large within the surface as along the normal. This anisotropy is opposite to that reported in recent experimental and theoretical studies of other systems.

Measurements of the ordering kinetics for the p(2×1) chemisorbed layer on W(110) have been made by LEED angular-profile analysis at coverages at which the p(2×1) sturcture exists as a single phase and coexists with disordered lattice gas. It is shown that the growth of ordered domains proceeds with a power-law time dependence, consistent with calculations, and obeys a scaling relationship. In the coexistence region, a bimodal size distribution is observed.

We have observed soft x-ray emission from core excitons in several semiconductors and insulators and find that the exciton intensity is related to its binding energy. We propose an explanation for these excitons and this relationship using a Wannier model. The validity of the Wannier model is further tested by comparing our measured exciton binding energies with predicted values. We conclude that this model appears to be a good starting point in the understanding of core excitons.

The scaling variables in the theory of the interacting disordered system are interpreted in terms of the quasiparticle density of states and diffusion constant, and Fermi-liquid interaction parameters. In the absence of spin-flip or spin-orbit scattering, the scaling theory valid to first order in the disorder is interpreted and a general description of possible scaling scenarios is given.

Bistability effects have been observed in electron paramagnetic resonance spectroscopy. When the sample amounts are varied, different instability regimes are detected. The homogeneous profile of the cavity resonator assumes a characteristic double-well pattern when a proper number of paramagnetic centers is put inside the cavity under magnetic resonance conditions. Saturation is not a requirement for the observation of the effect. Hysteresis loops arise from concurrence of this cavity profile and feedback effects of the spectrometer circuitry. Numerical calculations account well for the observed phenomena.

Using circularly polarized synchrotron radiation and spin-resolved photoemission, we have studied the valence and conduction bands of Xe(111) up to about 18 eV above the valence-band maximum. Direct information about the symmetry of bands and their hybridization is provided by photoelectron spin polarization. Our data also prove the existence of a gap in the unoccupied bands above the fundamental gap. A comparison of our data with calculated band structures shows the importance of an energy-dependent self-energy correction.

When new data for the elastic stiffness tensor of single-polarization, single–wave-vector chromium are combined with volume thermal expansion, the magnetic free energy is found to be a function of reduced temperature, T/$T_{\mathrm{N}}$; the dependence of the Néel temperature $T_{\mathrm{N}}$ on longitudinal strain is much stronger than on shear strain, and is linear in strain and nearly isotropic. Grüneisen parameters defined at and below $T_{\mathrm{N}}$ being roughly constant validates this single-parameter model.

We present a simple model calculation for electron tunneling between a metal probe tip and a metal surface with a chemisorbed molecule. It is shown that under suitable conditions, resonant tunneling via interaction with a molecule vibration can give a decrease of 10% or more in the total tunneling conductance. Adsorbate systems are suggested where our predictions may be tested.

While there is no fundamental reason that fluids should not be ferroelectric the only known ferroelectric fluids are chiral smectic-C liquid crystals. In this paper we argue that a variety of nonchiral layered (smectic) phases composed of properly designed oligomeric or polymeric molecules will also be ferroelectric. These ferroelectric fluids, and the solids formed by quenching them, should be of fundamental and practical interest. The nature of these phases, the design of the relevant molecules, and the experimental techniques for identifying the phases are discussed qualitatively.

The proton-glass phase in ${\mathrm{Rb}}_{0.5}$(NH${)}_{0.5}$${\mathrm{H}}_2$${\mathrm{PO}}_4$ can be completely suppressed by 5 kbar, and the details of this suppression are indicative of the nonequilibrium nature of the glass transition. There is a large hydrogen isotope effect not only on the dynamic glass-transition temperature but also on its pressure derivative. The results emphasize the importance of the proton’s intrabond motion in the freezing dynamics.

We report neutron-scattering measurements with polarization analysis on a large single crystal of FeSi up to 650 K. A strongly q-dependent magnetic scattering is observed around (011) and the intensity follows the unusual shape of the static susceptibility. Surprisingly, the energy-integrated magnetic cross sections are almost q independent. This represents an entirely new kind of magnetism which, overall, supports the picture of temperature-induced moment in FeSi.

A strongly speeded-up decay of a collective nuclear excitation has been observed in pure nuclear resonance diffraction of synchrotron-radiation pulses by a perfect single crystal of antiferromagnetic ${}_{}{}^{57}{}_{}{}^{}\mathrm{FeBo}_3^{}{}_{}{}^{}$. Superimposed appeared a quantum-beat spectrum exhibiting the interference of four hyperfine transitions. The angular dependence of the time spectrum was verified with measurements above, below, and at the exact Bragg position. The off-Bragg measurements reveal sensitively the weak quadrupole interaction in ${}_{}{}^{57}{}_{}{}^{}\mathrm{FeBO}_3^{}{}_{}{}^{}$. .AE