We present experimental evidence which demonstrates for the first time that small particles on substrates sit in deep potential-energy wells and, once floated out of these, can quasimelt between various local minima on shallow morphological free-energy surfaces. The energy needed to nucleate the quasimolten state is found to be orders of magnitude larger than that needed to sustain the state. The particles retain the unstable state for long intervals of time until they find another well on the substrate in which they can form a stable Wulff polyhedron shape.

The highly tilted ${\alpha }_3$ state of Co on Fe(001) has been studied using combined polar and azimuthal x-ray photoelectron diffraction. A more accurate structural picture is derived, with the molecule being tilted at an angle of 55°±2° with respect to the surface normal along 〈100〉 azimuths and probably occupying fourfold hollows. An estimate is also made of the vertical C distance above the Fe surface. The utility of combined θ/φ x-ray photoelectron diffraction measurements for such unusual adsorbate structures is demonstrated.

Activated processes at surfaces such as desorption or sublimation may exhibit a thermal anomaly at the Curie point of a magnetic substrate. We propose to measure this anomaly with an applied magnetic field H, and we predict a decrease in the reaction rate proportional to ${\mathrm{H}}^{\mathrm{x}}$ with x(T>${\mathrm{T}}_{\mathrm{c}}$)=2, x(T<${\mathrm{T}}_{\mathrm{c}}$)=1, and x(T=${\mathrm{T}}_{\mathrm{c}}$)=2/${\mathrm{\delta }}_1$, where ${\mathrm{\delta }}_1$≃1.9 is an exponent for ordinary phase transitions. In the case of a surface transition, or if the substrate is a film with two-dimensional Ising behavior, the anomaly is significantly enhanced.

Experimentally it is well known that diamond and zinc-blende semiconductors show an ‘‘unusual’’ (i.e., negative) thermal expansion at about 100 K. We performed density-functional-theory calculations of thermodynamic potentials (i.e., total energies and entropies) for perfect crystals, to study the temperature dependence of the lattice parameter. The origin of the negative expansion effect is traced back to the entropy contribution of the Gibbs free energy.

An ab initio molecular dynamics simulation of high-temperature proton diffusion in crystalline silicon is presented. This is the first time dynamical effects have been included explicitly in the simulation of this system. We find that the diffusion proceeds via a jumplike mechanism. Because of dynamical effects the diffusion path is substantially different from that inferred from static total-energy calculations. The calculated diffusion coefficient and its temperature dependencne are in good agreement with the available experimental data. It is suggested that scattering experiments may distinguish between different diffusion paths.

We show that the stable structure of a ${\mathrm{CoSi}}_2$/Si(001) interface involves a 2×1 periodic array of Si dimers with bond length 0.23±0.02 nm. We use a novel combination of quantitative transmission-electron diffraction, transmission-electron-microscope (TEM) imaging, and high-resolution TEM to solve the structure and have imaged separate 2×1 and 1×2 domains, ∼100 nm in size, over many μ${\mathrm{m}}^2$ of the interface.

The initial stages of the epitaxial growth on the dimer-reconstructed Si{100} surface is modeled via molecular-dynamics simulations using the Tersoff many-body potential. We report a novel anisotropic spread of surface dimer openings in the direction perpendicular to the original dimer rows. This correlated reaction mechanism dramatically increases the crystal-growth process in one direction as confirmed by recent scanning-tunneling-microscope studies.

In this scanning-tunneling-microscopy study we find that the reaction of Cl with Si(111)-7×7 results in extensive surface modification. On a Cl saturated and annealed surface, most of the adatom layer is stripped away, permitting the complete underlying Si-rest-atom layer to be imaged for the first time. The structure of this layer is shown to correspond closely to that predicted by the dimer-adatom stacking-fault model. We directly observe the presence of the stacking fault, an electronic effect associated with this fault, and different outward surface-normal relaxations in the faulted and unfaulted regions of the cell.

We investigate the quasicrystalline state of a two-dimensional binary alloy in a discrete tiling approximation. Through transfer-matrix calculations we determine the configurational entropy over a range of concentrations. We find that the entropy density is maximized by a state with tenfold symmetry at the quasicrystal concentration. Derivatives of the entropy density at its maximum yield values for the phason elastic constants. Our results confirm the existence of quasi-long-range translational order in equilibrium quasicrystalline alloys and lend support to the random-tiling model of quasicrystals.

The equilibrium quasicrystal phase of a two-dimensional two-component Lennard-Jones atomic system and two different ensembles of random tilings (binary and uncontrained) are analyzed by means of Monte Carlo simulations. We find that the quasicrystal phase in the atomic system is well described by the random-tiling model. Despite the nonzero configurational entropy density, the phason fluctuations are found not to destroy the quasi-long-range translational order in this phase, in agreement with conjectured square-gradient phason elasticity. Nearly identical, temperature-independent, reduced, phason elastic constants are determined in all three cases.

Quantitative measurements of the second-harmonic response of Al(111), Al(100), and polycrystalline Al to 1.06-μm radiation under ultrahigh-vacuum conditions are presented. The results verify theoretical predictions that at low frequencies the second-harmonic response is sensitive to the charge-density profile of the surface region. The effects of band structure are clearly visible in the anisotropic second-harmonic response for appreciable field components parallel to the surface. The anisotropic response can be interpreted to give information about electronic surface states.

I construct a Jordan-Wigner transformation for spin-one-half quantum systems on two-dimensional lattices. I show that the spin-one-half XY (i.e., a hard-core Bose system) is equivalent (on any two-dimensional Bravais lattice) to a system of spinless fermions and gauge fields satisfying the constraint that the gauge flux on a plaquette must be proportional to the spin (particle) density on site. The constraint is enforced by the addition of a Chern-Simons term of strength θ to the Lagrangian of the theory. For the particular value θ=1/2π, the resulting particles are fermions. In general they are anyons. The implications of these results for quantum spin liquids are briefly discussed.

Two-dimensional arrays of very-small-capacitance Josephson junctions have been studied. At low temperatures the arrays show a transition from superconducting to insulating behavior when the ratio of charging energy to Josephson-coupling energy exceeds the value 1. Insulating behavior coincides withthe occurrence of a charging gap inside the BCS gap, with an S-shaped I-V characteristic. This so far unobserved S shape is predicted to arise from macroscopic quantum coherent effects including Bloch oscillations.

Using neutron-scattering techniques, we have studied the temperature dependence of the magnetic excitations at varied energies in a high-quality single crystal of ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$. This crystal exhibits a sharp superconducting transition at 33 K with nearly 100% flux exclusion. For magnetic excitation energies of 9 meV and lower, the intensity drops dramatically with decreasing temperature for temperatures below ∼150 K. However, at 12 MeV the intensity is independent of temperature between 300 and 5 K.

Statistical mechanics is used to describe the observed information processing complexity of nonlinear dynamical systems. We introduce a measure of complexity distinct from and dual to the information theoretic entropies and dimensions. A technique is presented that directly reconstructs minimal equations of motion from the recursive structure of measurement sequences. Application to the period-doubling cascade demonstrates a form of superuniversality that refers only to the entropy and complexity of a data stream.

We present a study of the antiferromagnetic Potts model in two and three dimensions, using a new method of Monte Carlo simulations, which enables us to perform simulations with greatly improved efficiency. Illustrating the method for the three-state model, we have obtained new results for the entropy and critical exponents in two and three dimensions. The low-temperature phase in three dimensions is shown to have long-range order with a finite-size dependence similar to that of the XY model.

A new type of relativistic magnetiosonic soliton, which is electrically charged with a gigavolt potential, is found to exist in a magnetized electron-positron-proton plasma. Relativistic collisionless shocks resultong from such solitons can carry an even larger electric potential at the shock front. GeV electrons and positrons in some active astrophysical sources may be produced due to acceleration by these electric fields.

We consider the interaction of the decay photons of an unstable relic particle species with the microwave background radiation. The radiative decays of these particles delay recombination and serve as an energy source for the resultant plasma. Nonrelativistic Compton scattering by these electrons couples the decay photons to the microwave background, producing submillimeter distortions. If the decay products close the Universe, they must decay with a radiative branching ratio larger than 2.5×${10}^{\mathrm{-}5}$ in order to produce recently observed excess submillimeter background radiation. To be consistent with measurements of the uv background, their mass m≫114 keV and their decay redshift $Z_d$≫5200.

We describe a mechanism for the new solution of the solar neutrino problem with natural anticorrelation with the solar magnetic-field activity in the convection zone. Our mechanism requires the simultaneous presence of the neutrino flavor mixing and the magnetic (or transition) moment as large as μB∼${10}^{\mathrm{-}10}$ (μ in units of Bohr magneton, B in kG).

We compute the cross-section ratio R in $e^{+}$$e^{\mathrm{-}}$ scattering in chiral color theory and provide a new lower limit on the axigluon mass of 65 GeV if ${\Lambda }_{\mathrm{QCD}=100\mathrm{}}$ MeV, or 100 GeV if ${\Lambda }_{\mathrm{QCD}=200\mathrm{}}$ MeV.