Percolation in a concentration gradient has been carried out for the cubic lattice. In contradistinction with the two-dimensional case, the frontier of the infinite cluster extends over a macroscopic range of concentrations. In that range almost all occupied sites belong to the frontier which has dimension three. This defines an ideally porous material. The ratio of the local concentrations of the frontier and its external perimeter is equal to the ratio of the local overall concentrations of occupied and empty sites.

Phonon dispersion curves for the [110] 〈1¯10〉 ${\mathrm{TA}}_2$ branch in ${\mathrm{Ni}}_{\mathrm{x}}$${\mathrm{Al}}_{100\mathrm{-}\mathrm{x}}$ (50<x<65) reveal a pronounced minimum whose position in q space is composition dependent but whose frequency does not change appreciably with temperature. In the same q range, substantial elastic scattering also appears which is associated with localized embryos of a low-temperature phase. The diffuse scattering from the elastic displacement field about these embryos is highly anisotropic, giving rise to the striated strain contrast (tweed) in the electron microscope image. A common electronic origin of these effects is proposed.

A model combining diffusion and electric current field is used to simulate patterns in electrochemical deposition. The patterns generated are similar to those obtained in experiments. We find that the crossover from fractal to dendritic is a function of the ratio of the product of electrolyte concentration and applied voltage to the self-diffusion coefficient of ions. For low values of the ratio the fractal dimension of the clusters, is about 1.60–1.68 which is close to that of diffusion-limited aggregates.

Absorption of microwave power by superconductor-normal-superconductor junctions with square-well pair potentials at low temperatures is calculated to occur in sharp steps whenever the microwave energy is an integer multiple of the minimum energy required to excite quasiparticles from the ground state into bound Andreev states.

We have been able to produce conditions for doubly resonant Raman scattering where both incident and scattered light are simultaneously in resonance with an electronic interband transition, by fine tuning of the energy separation between light- and heavy-hole valence-band states using uniaxial stress. Under these conditions the Raman tensor for scattering by phonons becomes strongly asymmetric and forbidden scattering by TO phonons is also observed. The ability to approach the double resonance in a continuous manner allows direct comparison between experiment and theory and leads to a better understanding of these novel phenomena.

Vacuum-emission and carrier-separation techniques have been used to make the first direct observation of ballistic electron transport in silicon dioxide, and the first direct observation in any material of the interaction of ballistic electrons with single phonons of the lattice. With very thin oxides, total ballistic transport of the electrons is observed for voltages of ≲1 V dropped across the remaining oxide portion after tunneling. For voltage drops >1 V, a transition from the ballistic to the steady-state regime is seen. Monte Carlo simulations are used to predict the observed experimental behavior including quantum mechanical interference effects and phonon-induced sidebands in the electron distributions.

Photoluminescence emission at twice the fundamental band-gap energy is observed in GaSb-AlSb multiple quantum-well structures for well thicknesses below 42 Å, whereas it is not observed for larger thicknesses. The intensity of the emission increases dramatically with decreasing well width below this value. The appearance of these novel features is traced to changes in the character of the band-edge wave functions due to quantum-well confinement.

Electron-capture spectroscopy has been used to investigate long-ranged and short-ranged two-dimensional magnetic order at free (uncoated) surfaces of homogeneous (island-free), ultrathin fcc γ-Fe(111)(1×1) films pseudomorphically deposited on Cu(111) substrate crystals. At 298 K, the electron-spin polarization at the surface of a four-monolayer thin fcc Fe film amounts to +16%, directly revealing the existence of long-ranged ferromagnetic order. There is evidence that short-ranged ferromagnetic order still exists at 400 K at free surfaces of two and one-monolayer thin fcc Fe films.

A dynamic high-temperature series expansion is used to study the three-dimensional random-bond Ising model as a function of temperature and concentration of ferromagnetic bonds. The phase boundaries for the paramagnetic, the ferromagnetic, and the spin-glass phases are determined. We discuss the static and dynamic critical behavior and verify the dynamic scaling hypothesis at the spin-glass transition.

Piscosecond time-resolved absorption studies of the midgap absorption associated with photogenerated charged solitons in trans-polyacetylene are reported. Both direct photogeneration of charged-soliton pairs and neutral- to charged-soliton conversion are observed. The former are created by intrachain absorption and decay on a subpicosecond time scale. The latter are formed following interchain photoexcitation and are the dominant population of charged solitons at times greater than 20 ps.

For a large class of $d$-dimensional disordered systems, we prove that if an appropriately defined finite-size scaling correlation length diverges at a nontrivial value of the disorder with an exponent $\nu$, then $\nu$ must satisfy the bound $\nu \ge \frac{2}{d}$. Given the assumption that such a correlation length can be defined, the result applies to, e.g., percolation, disordered magnets, and Anderson localization, both with and without interactions. For localization, this puts stringent constraints on scaling theories and interpretation of experiments.

We have studied the photoproduction of $\frac{J}{\psi }$ mesons on H, Be, Fe, and Pb targets using real photons at a mean energy of 120 GeV. The $p_T^2$ spectra were used to separate the coherent diffractive signals from the incoherent signals. Parametrizing the per-nucleus cross sections in terms of power-law dependences, $A^{\alpha }$, we find that ${\alpha }_{\mathrm{coh}}=1.40\mathrm{}\pm 0.06\pm 0.04$ for the coherent diffractive signals and ${\alpha }_{\mathrm{incoh}}=0.94\mathrm{}\pm 0.02\pm 0.03$ for the incoherent signals.

In the chiral soliton (Skyrme) model all isoscalar electromagnetic currents, including the exchange currents in nuclei, are topological and uniquely determined by the current of the individual nucleons. This allows us to obtain a model-independent isoscalar exchange-current operator, which is used to study the magnetic form factor of the deuteron. The structure of the deuteron is incorporated by our evaluating the expectation value of the exchange-current operator with respect to the wave function obtained from the Paris potential. The magnetic form factor of the deuteron is dominated by the exchange-current contribution and is in agreement with experimental data in the range $0<\mathrm{}{q}^2<33\mathrm{}$ ${\mathrm{fm}}^2$.

The coincidence cross section of electrodisintegration of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is studied in various kinematic configurations. In solution of the Faddeev equations for the scattering states of the trinucleon system, the effect of final-state interactions is taken exactly into account. It is not negligible and becomes substantial when the process of direct proton knockout is suppressed.

A dispersion-relation approach is developed for deriving the shell-model potential from the optical-model potential, i.e., for extrapolating the mean field from positive towards negative nucleon energies. This method is applied to neutrons in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. The average neutron-nucleus potential is assumed to have a Woods-Saxon shape; its depth, radius, and diffuseness are calculated for nucleon energies with range from -20 to +40 MeV.

The Doppler-shift attenuation method was used to measure lifetimes of yrast states in ${}_{}{}^{166}{}_{}{}^{}\mathrm{Yb}$. Values of $\tau$ range from 0.16(2) ps at ${24}^{+}$. to 0.05(1) ps at ${34}^{+}$. The $B\left(E2\mathrm{}\right)\mathrm{}$ values decrease from 240 to 120 single-particle units in this spin range, indicating a loss of collectivity which likely reflects a change of the nucleus towards triaxial shape. The side-feeding times are consistent with rotational cascades having moments of inertia 25% larger than the yrast sequence or $B\left(E2\mathrm{}\right)\mathrm{}$ values around 60 single-particle units.

The configurations and dynamics of a polymer ring ($R$) inside a strongly crosslinked polymer gel ($G$) are discussed and compared to those of linear ($L$) and branched ($B$) polymers in $G$. $R$ is not topologically connected to $G$ in the sense that the contour line of $R$ can be reduced to a point without crossing any of the chains of $G$. This topological requirement leads to the correspondence between the configurations of $R$ in $G$ and the set of lattice trees (randomly branched polymers). But the dynamics of $R$ in $G$ is much faster than that of entangled $B$, and is similar to the dynamics of entangled $L$.

The Stark splitting in molecular Rydberg states has been observed for the first time in the ${\mathrm{Na}}_2$ molecule. At high electric field, very simple hydrogenic structures appear, corresponding to the field-induced decoupling between electronic and nuclear motion. At low field strengths core interaction and molecular rotation cause departure from this hydrogenic model.

Positrons ionize helium atoms either by impact ionization, resulting in three outgoing particles, or by positronium formation. We determined cross sections for both processes for incident-positron energies ranging from the respective thresholds to 1000 eV. The cross section for impact ionization by positrons (${\sigma }_{\mathrm{ion}}^{+}$) exceeds the corresponding electron cross section (${\sigma }_{\mathrm{ion}}^{-}$) below 500 eV. On the high-energy side of its maximum, the positronium-formation cross section (${\sigma }_{\mathrm{Ps}}$) lies above all theoretical predictions.

Harpooning transitions or long-range electron jumps have been observed in collisions of ${\mathrm{O}}_2^{+}$ at 100-300 eV from Ag(111) leading to negative molecular ${\mathrm{O}}_2^{-}$ ions. It is demonstrated that the ions are formed in nonviolent collisions and with reasonable probability. The origin and relevance of these harpooning transitions are discussed.