A new phenomenon is reported in which light satisfying a Bragg-type resonance condition gets strongly reflected by a weak coarse-period (tens of microns) superlattice that is superimposed on a fine-period (submicron) stratified medium. A couple-wave theory is developed to describe this interaction.

By means of the Poincaré-sphere representation, new exact solutions for the evolution of the polarization in anisotropic crystals exhibiting a third-order nonlinearity and subjected to a dc electric field are found. This allows us to discover the existence of bifurcations of the nonlinear eigenpolarizations. A stability analysis of the eigenpolarizations shows how the intrinsic instabilities evolve in the interaction between self-induced and dc-field–induced birefringence.

A general analytical solution for the temporal (or spatial) evolution of electromagnetic cascading in the context of beat-wave generation of intense plasma waves is given. The amplitude of the nonlinear beat plasmon can be solved separately, after which the solution of the whole electromagnetic cascade can be constructed in terms of Bessel functions. Implications of the analytical model for the beat-wave acceleration, plasma heating, and laser fusion are discussed.

Scanning tunneling microscopy is used to confirm the π-bonded chain structure of the Si(111)2×1 surface. Both the amplitude and voltage dependence of the vertical corrugation exclude the buckling model for the structure of this surface. Spectroscopic measurements of the tunneling current versus voltage identify a band gap for the 2×1 surface states. Spatial images of disorder-related states are obtained by tunneling at energies inside of this band gap.

The energy exchange process between atomic hydrogen gas and liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ has been considered at low temperatures. It is found that previous calculations have substantially underestimated sticking and Kapitza conductance in this system.

Dynamic behavior of ultrafine particles of gold around 20 Å in size was examined at the level of the atomic resolution by an electron microscope equipped with a real-time video recording system. In real time, it was observed that the shape of the particles changed continually through an internal transformation from a single crystal to a twinned crystal, and vice versa. The transformations were induced to some extent by the irradiation of the electron beam. They took place abruptly in less than 0.1 sec.

Commensurate, incommensurate, and incommensurate-rotated Xe monolayers are shown to exist on Pt(111), i.e., on a close-packed ‘‘smooth’’ metal surface. The existence of a commensurate-incommensurate transition induced by varying temperature at constant coverage is experimentally demonstrated (${\mathrm{T}}_{\mathrm{IC}=57\mathrm{}–}$59 K). The use of high-resolution He diffraction enables the investigation of structures and transitions of 2D phases irrespective of the nature of the substrate and without any damaging of the adsorbed phase.

The equilibrium roughening of perfect, clean, crystalline surfaces is described in terms of thermally created atomic steps in the frame of a lattice model (loop gas). If the bending energies for convex and concave step corners are different, the ‘‘particle-hole’’ symmetry is destroyed and the sharp infinite-order roughening transition disappears. Instead, a rather smooth crossover from a smooth to a rough surface occurs. A simple model illustrates this result, which is expected to be important for the crystal-growth process and for surface-catalytic reactions.

We have used a graphite fiber as a substrate to study the kinetics of desorption and sticking of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ for temperatures between 15 and 1.6 K. The sticking coefficient on the clean surface is 0.026, and is independent of coverage from 0 to 1 layer. A simple phenomenological model is used to show that the desorption rates and sticking coefficient we observe are consistent with previous thermodynamic measurements.

An empirical interatomic potential for covalent systems is proposed, incorporating bond order in an intuitive way. The potential has the form of a Morse pair potential, but with the bond-strength parameter depending upon local environment. A model for Si accurately describes bonding and geometry for may structures, including highly rebonded surfaces.

Aqueous solutions of tobacco mosaic virus are ideal systems for visualizing ‘‘molecular’’ distributions in the nematic phase by freeze-fracture electron microscopy. The order parameter in well aligned regions is measured to be greater than 0.9. Twist deformations appear to be most common, confirming that the twist elastic constant is smaller than either the splay or bend constants. Edge and screw disclinations are observed in tobacco mosaic virus nematics in configurations predicted by continuum theory. Disclination cores have been observed for the first time.

The number and population of Ta sites in the charge-density-wave state of 2H-${\mathrm{TaS}}_2$ were determined at 1.5 K in magnetic fields up to 0.7 T by the method of time-differential perturbed angular correlation on ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$. At 0.35 T a field-induced phase transition is observed, indicating for the first time the coexistence of charge- and spin-density waves in a two-dimensional material.

It is demonstrated that the hopping conductivity of a strongly localized system may be significantly reduced by putting it in proximity with a metal. The experimental results on amorphous indium oxide films give evidence for the loss of screening in highly disordered conductors.

At low temperatures (${\mathrm{T}}_{\mathrm{c}}$${\mathrm{theta}}_{\mathrm{D}}$) a new universal transition in the resistivity behavior of strongly coupled superconductors has been demonstrated experimentally: The power-law behavior changes from ${\mathrm{T}}^{\mathrm{n}}$ (n=3–5) to ${\mathrm{T}}^2$ as disorder increases, provided that the electron-phonon coupling λ is sufficiently high. The physics of the effect is probably based on the breakdown of the electron-phonon momentum-conservation law. Universal correlation allows independent estimates of λ.

The principle of the inverse photoelectric effect has been applied to the study of the metal-electrolyte interface. It is shown that charge-transfer-reaction inverse-photoemission spectroscopy is capable of investigating both empty and occupied electronic states in the inter- facial region. Results for Au(111) electrodes in solutions of tetra- propylammonium tetrafluoroborate in acetonitrile, containing either electron-donor or electron-acceptor molecules, are presented. The spectra obtained by this new technique, for both electron and hole injection into the metal, are shown to contain a wealth of detailed information about, for example, interfacial electronic states and molecular reorganization processes.

We report the first x-ray absorption spectra of pulsed-laser irradiated amorphous Si around the Si L edge, recorded with a time resolution of 18 hs. At irradiances above 0.17 J/${\mathrm{cm}}^2$ significant differences are found with the spectrum of amorphous Si. The disappearance of the characteristic Si ${\mathrm{L}}_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ edge structure at 100 eV and the decrease in overall absorption are interpreted in terms of the metallic character of liquid Si and the formation of droplets, respectively.

Frequency-dependent conductivity measurements are reported in the charge-density-wave state of ${\mathrm{Ta}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Nb}}_{\mathrm{x}}$${\mathrm{S}}_3$ alloys at microwave and millimeter-wave frequencies. For increasing impurity concentration, the pinning frequency of the charge-density-wave collective response increases sharply while the damping of the response remains constant. Although both the classical and quantum tunneling models describe the small-amplitude ac response of the pure materials for ω<${\mathrm{\omega }}_{\mathrm{p}}$, our results clearly support a classical oscillator response for ω>${\mathrm{\omega }}_{\mathrm{p}}$ and for the alloys.

The authors report NMR measurements of the ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ resonance in the charge-density-wave (CDW) conductor ${\mathrm{NbSe}}_3$ on aligned, multicrystalline samples. They have observed the resonance with and without current flow in the sample at temperatures above 59 K, where a single incommensurate CDW is present. Results include (1) a demonstration that the CDW is not discommensurate, (2) quantitative measurements of CDW displacements below threshold, and (3) evidence, including current-induced motional narrowing, of CDW motion throughout the entire sample, above the threshold.

We report magnetoresistance measurements in (TMTSF${)}_2$${\mathrm{ReO}}_4$ which give evidence for a magnetic-field–induced transition. We also find Shubnikov–de Haas oscillations above and below the field-induced transition—a result not explained by recent theories of this novel transition.

From photoinduced absorption and light-induced electron-spin resonance, we demonstrate that the dominant photocarriers generated in polythiophene with excitation above the energy gap ($E_g$) are charged bipolarons (spin 0). The observation of bipolarons ($B^{2\pm }$) rather than polarons ($P^{\pm }$) as the dominant photoexciations demonstrates that the Coulomb contribution to the bipolaron energy ($U_B$) is sufficiently small that $P^{\pm }$+$P^{\pm }$→$B^{\pm 2}$. From the analysis of the spectra, we find $U_B$/$E_g$≃0.12. .AE