Ion channeling (of ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ at 1.0 MeV) has been used to investigate the long-range order in thin-film metastable (GaSb${)}_{1\mathrm{-}\mathrm{x}}$(${\mathrm{Ge}}_2$${)}_{\mathrm{x}}$ alloys as a function of x. Spectra were obtained at a set of incident beam orientations along an angular scan through the [110] direction and parallel to the (11¯0) plane. The asymmetry in the Sb yield was used to follow the phase transformation from the zinc blende structure to the diamond structure.

We have used direct energy transfer to probe the pore morphology of porous silicas. Using rhodamine 6G as the donor and malachite green as the acceptor we show that the maximum length scale probed by this technique is 100 Å. At this scale we clearly demonstrate the relationship between morphology of the pore network and the ‘‘effective fractal dimension’’ d¯ obtained by direct energy transfer. The values of d¯ are shown not to be uniquely associated to a self-similar surface, but rather to the restricted geometries of the porous media.

The melting transition of the two-dimensional stripe-domain phase of bromine layers intercalated into graphite has been investigated by high-resolution x-ray diffraction. The transition is characterized by the progressive vanishing of successive harmonics so that near melting the density is described by a two-dimensional sine wave. Surprisingly, the sine-wave order parameter is best characterized by the power-law form, &, with ${\mathrm{\eta }}_1$ evolving continuously from 0.04 to ∼2.0 through melting.

Inelastic helium-scattering measurements have been used to investigate the influence of oxygen adsorption on the Pt(111) surface phonons. Oxygen was found to form a (1×1) overlayer with nearly unity coverage at elevated surface temperatures. Such an overlayer is shown to induce a remarkable softening of the Pt surface phonons, which is interpreted as a reduction in the Pt attractive interaction arising from sp-electron donation to the oxygen overlayer.

Molecular-dynamics calculations are used to characterize the structure and dynamics of the two solid bilayer phases of the n-alkane tricosane (${\mathrm{C}}_{23}$${\mathrm{H}}_{48}$). In the crystalline orthorhombic phase at 38°C, chains undergo translational, rotational, and torsional motions, but are otherwise essentially all-trans and perfectly ordered with a herringbone packing. By contrast, at 42°C, in the pseudohexagonal (rotator) phase, there is a dramatic increase in longitudinal chain motion, each chain now has four possible orientations and a significant number of conformational defects develop, predominantly at the chain ends.

${\mathrm{He}}^{+}$-ion backscattering studies on the clean W(110) surface at 300 K show that there is less than 2% change in the first interplanar spacing for the clean surface. There is however a large anisotropy in the thermal vibration amplitude for the surface atoms with the component normal to the surface 2.6 times larger than the one-dimensional bulk amplitude while the parallel component in the surface plane shows very little enhancement.

We obtain quasicrystalline structures in Monte Carlo simulations of a simple two-component Lennard-Jones system in two dimensions. The quasicrystal, which shows tenfold symmetry, appears to be an equilibrium state of the system. Although the structure corresponds to tiling of the plane with rhombuses, it is not a Penrose pattern.

Measurements of the radius of gyration ${\mathrm{R}}_{\mathrm{G}}$ and the hydrodynamic radius ${\mathrm{R}}_{\mathrm{H}}$ of colloidal silica aggregates are reported. These aggregates have fractal geometry and ${\mathrm{R}}_{\mathrm{H}}$ is proportional to ${\mathrm{R}}_{\mathrm{G}}$ for 500 Å≤${\mathrm{R}}_{\mathrm{H}}$≤7000 Å, with a ratio ${\mathrm{R}}_{\mathrm{H}}$/${\mathrm{R}}_{\mathrm{G}}$=0.72±0.02. The results are compared with predictions for macromolecules of various shapes. The proportionality of the two radii can be understood with use of the pair correlation function of fractal objects and hydrodynamic interactions on the Oseen level. The value of the ratio remains to be explained.

One of the outstanding problems in crystal physics is the role nonclassical forces play in stabilizing the cesium chloride versus rocksalt structure for ionic solids. By consideration of classical electrostatic and repulsive forces alone, it is not possible to predict accurately which structure is more stable. I demonstrate on the basis of ab initio pseudopotential calculations that the cesium chloride structure is not stabilized solely on the basis of charge transfer effects, or size effects. Rather, nonclassical (s-p nonlocal) effects appear to be crucial in stabilizing the cesium chloride structure.

In this Letter we present a simple theory for the Aharonov-Bohm effect in semiconductor microstructures, assuming ballistic transport. It is shown that in well-designed symmetric structures it may be possible to attain large (approaching 100% in principle) conductance modulation in a magnetic field even if the transverse dimension of the structure is large, the aspect ratio is poor, and $k_B$T exceeds the correlation energy. The theoretical formulation can also be used to describe the electrostatic effect which has not yet been observed.

We test the influence of the quantum size effect on the optical transmission and reflection spectra of films of free-electron metals. We have made measurements on sputtered gold films of thicknesses less than 6 nm. The agreement between theoretical and experimental dependences provides qualitative support for the quantum–size-effect origin of the observed deviation from the classical model.

A proof is given for the Jahn-Teller problem of an orbital doublet (E⊗ε) that the order of the lowest vibronic levels is fixed by the requirement that the vibrational part of the wave function change sign under 2π rotation in the vibrational coordinates. This sign change in turn is a consequence of the sign change in the electronic part of the wave function, a special case of Berry’s geometrical phase. Experimental confirmation of this sign change and thus of Berry’s phase is available from studies of Jahn-Teller defects in crystals that reveal the sequence of these lowest levels.

Using electroreflectance spectroscopy, we have observed new optical transitions near 0.76, 1.25, and 2.31 eV in an ordered strained-layer Ge-Si superlattice. The possible origins of these transitions are discussed in terms of modification of the electronic band structure by the sample structure.

The valence-band photoemission of Ni(110) shows that the two-hole satellite appears at a constant binding energy of 6.0 eV away from the 3p→3d resonance in accord with earlier results, but on resonance it shifts to ∼6.8 eV. Interpretation of this new result in terms of the singlet and triplet satellite states is proposed.

The transmission of synchrotron radiation through magnetized iron at energies above the K-absorption edge shows relative differences for right and left circular polarization of several times ${10}^{\mathrm{-}4}$. The observed spin dependence of the near-edge photoabsorption is proportional to the difference of the spin densities of the unoccupied bands. In the extended absorption region up to 200 eV above the Fermi level a small spin-dependent absorption is observed and thus is expected to give information on the magnetic neighborhood of the absorbing atom.

We consider the problem of discrimination among ensembles of images generated by distortions of a prototype and the addition of noise. As the noise level is increased the discrimination task becomes qualitatively more difficult in that optimal discrimination requires the computation of increasingly longer-ranged correlations or the solution of increasingly difficult optimization problems. These results suggest the use of such image ensembles in probing the computational abilities of the human visual system, and possible theoretical implications of such experiments are discussed.

Destabilization of the period-doubling scenario in Hamiltonian systems due to noise is studied. A renormalization picture for stochastic area-preserving maps is introduced and a new universal characteristic number ${\gamma }_{\mathrm{H}=\mathrm{17.017.}\mathrm{}..}$ for the noise amplitude is found. This factor governs scaling properties of the escape time from stable regions, which is demonstrated numerically as well.

The covariant description of massless bosonic free particles in space-time of any dimensionality which carry arbitrary representations of the Lorentz group is presented.

We argue that the Kaaumlhler geometry of the loops on space-time describes bosonic string theory. The Kaaumlhler potential is the dynamical (field) variable of closed-bosonic-string theory. The equation of motion for this field is that a generalized Ricci tensor vanishes. Loops on flat space constitute a solution only in 26 dimensions.

We show that non-Abelian gauge fields arise in a nongauged quantum system in the adiabatic approximation by working out a model of N-dimensional rotational symmetry. The induced gauge fields are symmetric under N-dimensional rotations accompanied by compensating gauge transformations of the group SO(N).