A Monte Carlo simulation of the formation of regular facet patterns during catalytic CO oxidation on Pt(110) has been conducted. The simulation was based on the elementary steps of the surface reaction, the CO-induced 1×1⇄1×2 phase transition, and the enhancement of ${\mathrm{O}}_2$ adsorption at step sites. The model can reproduce the formation of sawtoothlike facet patterns with a spacing of ∼100 to 200 Å. The facets represent a Turing structure caused by the coupling of kinetic instabilities with the mass transport of Pt atoms.

We study the change of quantum spectra under two types of perturbations. One of them corresponds to the breaking of classical integrability and amounts to a crossover from level clustering to level repulsion. The second type of perturbation breaks time-reversal invariance; under conditions of classical chaos the degree of level repulsion then grows from linear to quadratic. To characterize the spectral changes we propose, for each type of transition, a distribution of nearest-neighbor spacings. Both of these ‘‘generalized Wigner surmises’’ are rigorous for suitable ensembles of 2×2 matrices but prove reliable for dynamical systems with many levels.

We have used microprobe RHEED and a convex curved substrate to study the terrace-width dependence of equilibrium surface structures on vicinal Si(001). We observe a transition from double to single domains of the 2×1 reconstruction in periodic [110] step arrays that occurs gradually over a range of miscut angle from 0° to 5°, and is independent of temperature over the measured range of 500–800 °C. Possible mechanisms for this transition and its relation to a transition in step heights are described.

We investigate by molecular-dynamics simulation the time evolution of a collision cascade in a condensed Ar solid, initiated by a 1-keV Ar atom. The particle density and velocity in the collision-cascade volume, the kinetic- and potential-energy distributions of the recoiling Ar atoms, and the energy distribution of sputtered particles are monitored. The high pressure building up in the core of the cascade drives the material into the cascade periphery and towards the surface. The surface erupts and hundreds of atoms are emitted, leaving behind a huge crater.

A computer-simulation method is described which yields the ground state and the low-energy excited states of a system of interacting localized particles in disordered media. The method is applied to electrons in Anderson-localized systems, when Coulomb interactions are important. The results are discussed in connection with some long-standing controversies about properties of electrons in a Coulomb gap.

The fine structure in the 0.61-eV photoluminescence band from the deep defect EL2 in semi-insulating GaAs has been studied under uniaxial stress and magnetic field. The results show no deviation from full ${\mathit{T}}_{\mathit{d}}$ symmetry and hence support the isolated-arsenic-antisite model of EL2. Measurements of both the shift due to the hydrostatic component of the stress as well as the g factor of the final state of the transition confirm the identification of this photoluminescence with EL2.

High-resolution temperature-dependent photoemission data from Na 2p core levels reveal substantially larger phonon broadening in the first atomic layer of Na(110) than in the bulk. We show that the enhanced width is due primarily to the excitation of relatively soft phonon modes perpendicular to the surface. Soft surface-phonon modes also account for previously reported but uninterpreted broadening of transition-metal surface-atom core levels.

Entropy generation during an ultrarelativistic nuclear collision is discussed. It is shown that the maximum entropy densities attained are limited by the time during which the collision takes place. This provides a strong upper limit on the maximum entropy density attained in a collision of given multiplicity density.

The effect of the 2×1 reconstruction on the core-electron binding energies of the outermost Si(100) layers has been determined using high-resolution photoemission data. A previously unobserved 190-meV crystal-field splitting is resolved for the up-atoms of the asymmetric surface dimers, whose average core-level shift is -400 meV. The signal from the down-atoms is clearly identified and has a shift of +220 meV. These new findings indicate a charge flow of ∼0.05e from the subsurface to the surface layers, with a substantially larger difference of ∼0.34e between the up-atoms and down-atoms in the dimer.

We report oscillations in diffracted electron intensities during ion bombardment of Si(100) by 200- and 250-eV Xe, both alone and with sequential and simultaneous epitaxy. Analysis of the phase and frequency of the oscillations shows that, to first order, ion bombardment ‘‘undoes’’ previous epitaxy and cancels or partially cancels simultaneous deposition. Surprisingly, the phase relationship of growth and sputtering is both antisymmetric and linear, indicating that the ion-induced oscillations are dominated by simple, vacancy-mediated, layer-by-layer sputtering.

We have observed the creation of hot excitons at high momentum in ${\mathrm{Cd}}_{\mathit{x}}$${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$Te multiple quantum wells and subsequent relaxation by LO-phonon emission to give sharp luminescence lines superimposed on a broader background. The difference between the exciton lifetime and dephasing time in these materials allows definite identification of the sharp lines as due to luminescence rather than resonant Raman scattering. The results are consistent with excitons localized in alloy potential fluctuations.

The dc Josephson current through a constriction in a two-dimensional superconductor-semiconductor-superconductor junction is calculated. It is shown that when the Fermi wavelength is comparable with the width of the constriction, the critical current shows a steplike variation as a function of the width of the constriction; this is reminiscent of the quantization of the normal-state conductance of point contacts in a two-dimensional electron gas.

From measurements of the resistivity down to 70 mK in magnetic fields to 7.5 T of a series of just-metallic, uncompensated, p-type Si:B samples, we have established that the critical exponent which characterizes the approach of the zero-temperature conductivity to the insulating phase changes from ν=0.${65}_{\mathrm{-}0.14}^{+0.05\mathrm{}}$ in zero field to ν=1.$0_{\mathrm{-}0.20}^{+0.10\mathrm{}}$ in a magnetic field of 7.5 T. This is the first clear experimental evidence for a change in universality class at the metal-insulator transition caused by the application of a magnetic field.

Two-fragment (Be-Be, B-B, and C-C) correlation functions were measured at small relative momenta for noncompound fragment emission in ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$${+}^{197}$Au collisions at E/A=35 MeV. Interpretation of the experimental correlation functions in terms of final-state Coulomb-interaction models indicate average fragment-emission times of about 100–200 fm/c.

Oscillations of ferromagnetic to antiferromagnetic exchange coupling between two Fe layers separated by a Cr spacer of linearly increasing thickness were investigated by imaging the magnetic domains with scanning electron microscopy with polarization analysis (SEMPA). Up to six oscillations in the coupling with a period of 10–12 Cr layers were observed, and, in the case of an extremely well ordered Cr interlayer, additional oscillations with a period of 2 Cr layers were observed.

We demonstrate the existence of a coupled elementary excitation involving a phonon and a ${\mathrm{Nd}}^{3+}$ 4f-electron crystal-field excitation in the high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{NdBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. It explains in a natural way the recent observation of a double-peak structure in the Raman spectra as well as in the inelastic neutron scattering spectra near 300 ${\mathrm{cm}}^{\mathrm{-}1}$ at low temperature. Our assignment is supported by measurements of the dependence of this feature on temperature, isotopic substitution, and polarization. A model based on strong coupling of extended phonon states to localized crystal-field excitations describes the observed behavior well.

We study the gap anisotropy and critical temperature of superconductors within Eliashberg theory. We consider a general anisotropy and spectral shape. In the attractive strong-coupling limit, we find a qualitatively new universal result: The gap becomes isotropic. This sets severe constraints on standard strong-coupling theories of high-${\mathit{T}}_{\mathit{c}}$ superconductors. In the weak-coupling limit, the gap is sensitive to anisotropy. We obtain exact results for the critical temperature both in the strong- and the weak-coupling limits. We propose a simple natural interpolation in the intermediate regime.

We have measured the surface impedance of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ single crystals and have evaluated the conductivity ${\mathrm{\sigma }}_1$ at millimeter-wave frequencies. We observe a peak below ${\mathit{T}}_{\mathit{c}}$ which we suggest is due to case-II coherence factors, like the Hebel-Slichter peak observed in the NMR relaxation rate of classical superconductors. A comparison with theory leads to a large gap, suggesting strong coupling.

The cross section for the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(e,e’d${)}^2$H reaction has been measured as a function of the four-momentum transfer q at a missing momentum of 125 MeV/c. The data show that this reaction cannot be described as quasielastic knockout of a deuteron. In a microscopic description of the (e,e’d) cross section, assuming a direct knockout mechanism, the q dependence reflects the p-n relative wave function in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. However, even a calculation in this framework using a relativistic ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ wave function does not describe the data, indicating that other reaction mechanisms play a non-negligible role.

The first observation of laser-induced recombination, performed with merged beams of protons, electrons, and laser photons in an ion storage ring, is reported. The process was used to study the photorecombination spectrum with high resolution. Deviations from earlier theoretical predictions turned out to be due to a weak external electric field and could be well described by a simple extension of the field-free quantum-mechanical theory, based on properties of the classical electron trajectories in the combined external and Coulomb fields.