Positron-lifetime experiments have been conducted on silicon at temperatures between 300 and 1523 K. A lifetime attributable to positrons annihilating in monovacancies is directly observed above 1450 K. This lifetime has the same value as that associated with monovacancies at low temperature indicating that the character of the monovacancy is essentially independent of temperature. The results yield an activation enthalpy for neutral monovacancy formation of 3.6 ± 0.2 eV. No evidence for divacancy formation could be found.

Using small-angle x-ray scattering, we show that porous silica aerogel has a fractal backbone structure. The observed structure is traced to the underlying chemical (polymerization) and physical (colloid aggregation) growth processes. Comparison of scattering curves for aerogel with silica aggregates confirms this interpretation.

The configuration-coordinate diagram for a model dangling-bond geometry is obtained by ab initio total-energy calculations. The results yield equilibrium geometries for different charge states, thermodynamic and optical transition energies, large Stokes shifts (0.5-0.6 eV), and a negative effective correlation energy (-0.2 ± 0.2 eV). Implications of these results for the current experimental and theoretical understanding are discussed.

The effect of CO chemisorption on the surface magnetism and unfilled electronic structure of Ni(110) is investigated by spin-polarized inverse-photoemission spectroscopy. A saturation in the reduction of the unfilled minority-spin $d$ density of states is observed near 0.5-monolayer CO coverage and attributed to a reduction in the Ni-atom magnetic moments. Transitions into the CO ${\pi }^{*}$ band are also observed with an intensity that increases nearly linearly with coverage. No transference of spin polarization from the Ni substrate to the CO ${\pi }^{*}$ is found.

Time-resolved, selectively excited fluorescence and resonant Raman scattering (RRS) are used to determine the nature of excitons in $\mathrm{Ga}{\mathrm{As}}_x{\mathrm{P}}_{1-x}(x<~0.07)$. All localized excitons, both intrinsic and those bound to isoelectronic nitrogen impurities, have the same $e-h$ exchange splitting into $J=1,2\mathrm{}$ levels. This allows measurement of the density of terminal states and their contribution to RRS by zone-edge $\mathrm{L}{\mathrm{O}}_X$ phonons. An abrupt vanishing of the density of terminal states and this RRS presents firm evidence of the existence of an excitonic mobility edge and determines its position to an accuracy of 1 meV.

When a positive gate voltage is applied to an amorphous-silicon thin-film transistor, electrons become trapped in states close to the silicon-dielectric interface. This is studied by a new technique involving the transient discharge current produced under illumination. It is suggested that the behavior may involve metastable dangling bonds generated within the amorphous silicon as a consequence of the field-effect-induced increase in electron concentration. This constitutes an important new instability mechanism for amorphous-silicon thin-film transistors.

The local-density approximation has been very useful for ground-state calculations. Here an analogous approximation is developed for the ensemble of $M$ lowest states, recently discussed by Theophilou. Algorithms are provided for the exchange-correlation energy, $E_{\mathrm{xc}}^M$, and potential, ${\nu }_{\mathrm{xc}}^M$, of ensembles of slowly varying density, $n\left(r\right)\mathrm{}$. These quantities are highly nonlocal functionals of $n\left(r\right)\mathrm{}$ but calculable in terms of properties of uniform, thermal ensembles. Excited-state energies and densities can be obtained.

We show, using a simple local-field-effect theory, that the ratio of the surface conductivity of an overlayer physisorbed on a crystalline surface to its surface conductivity in the absence of the local-field effect is proportional to the square of the local field produced by the substrate, and is therefore very sensitive to the position of the adsorbate. The application of this result to the optical determination of adsorption sites is analyzed.

We sketch the proof of an existence theorem for a time-dependent density-functional theory on the set of $N$-representable densities, extending the constrained-search formulation of Levy into the time-dependent domain, and add a brief comment on practical issues.

We present a new Monte Carlo algorithm for self-avoiding surfaces in a grand canonical ensemble. The topology of the surfaces is kept planar but the boundary is allowed to fluctuate during the simulation. The measured critical exponents $\nu =0.504\mathrm{}\pm 0.016$, $\theta =1.48\mathrm{}\pm 0.17$ indicate that this model belongs to the universality class of branched polymers in three dimensions.

A general relationship is established between wave-packet evolution and quantization in classically integrable systems. Because of wave-packet spreading, one cannot simply take the Fourier transform of the time evolution of a wave packet. Instead, one must propagate the wave packet using the actions as Hamiltonians. The energy eigenvalues which result are the Einstein-Brillouin-Keller values, and new forms for the eigenfunctions appear. These are free of caustic singularities, and represent averages of wave packets over the invariant torus.

We investigate the current-mass ratios of the light quarks by fitting the squares of meson masses to second order in chiral-symmetry breaking, determining corrections to Weinberg's first-order values: $\frac{m_u}{m_d}=0.56\mathrm{}$, $\frac{m_s}{m_d}=20.1\mathrm{}$. We find that to this order, $\frac{m_s}{m_d}$ is a known function of $\frac{m_u}{m_d}$. The values of the quark-mass ratios can be constrained by limiting the size of second-order corrections to the squares of meson masses. We find that for specific values of presently unmeasured phenomenological parameters one can have a massless $u$ quark. In that case 30% of the squares of meson masses arise from operators second order in chiral-symmetry breaking.

The electric and magnetic form factors for the nucleon and the $N-\Delta$ transition are computed in the semiclassical approximation to the Skyrme model and compared with phenomenological dipole parametrizations. A model-independent relation between the magnetic form factors is obtained. The transverse electric transition form factor is identically zero in this approximation, while the longitudinal form factor is nonzero.

Using a generating function with both commuting and anticommuting variables, we calculate perturbatively the violation of the principle of detailed balance due to breaking of time-reversal symmetry in a statistical nuclear reaction. We apply this result to the best data and establish upper bounds for the strength of, and the spreading width associated with, the symmetry-breaking part of the Hamiltonian. We argue that the spreading width is the fundamental parameter of the theory. A way of further improving the upper bounds is suggested.

Inner-shell excitation and ionization processes in relativistic collisions of very heavy ions are treated by a nonperturbative method for the first time. The time-dependent Dirac equation is solved by a finite-difference method for the scattering of ${\mathrm{U}}^{92+}$ on ${\mathrm{U}}^{91+}$ at a value of $E_{\mathrm{lab}}$ corresponding to 1 GeV/amu and zero impact parameter. The $K$-shell-ionization probabilities are compared with those resulting from first-order perturbation theory.

The present work reports the first absolute photoionization cross-section measurements for the open-shell atom chlorine. Measurements were made from the ${}_{}{}^1{}_{}{}^{}\mathrm{S}$ ionization threshold at 16.4 eV to 75 eV. Agreement with several calculations is good at the higher photon energies. We have observed a Cooper minimum at about 44 eV as predicted by theory. However, near threshold the various calculations differ considerably with each other and with the present results.

We describe a new method for measuring oscillator strengths for transitions between excited atomic states. The oscillator strength is determined from changes in the angular distribution or polarization of fluorescent light induced by the ac Stark effect of a laser pulse. The method can be thought of as a variant of the hook method in which the roles of atoms and photons have been interchanged.

A new approach to the velocity selection problem is presented. It establishes a relation between the existence of propagating steady states as a function of a parameter and a certain inhomogeneous linear problem, the solvability of which determines the set of allowed velocities. The method is illustrated on the example of a geometrical model of solidification. It is used to explain analytically the fact that at large velocity the Saffman-Taylor "fingers" have width close to ½ and to predict the scaling exponent for the dependence of the finger width on velocity.

We present an analytic approach to the problem of predicting the widths of fingers in a Hele-Shaw cell. Our analysis is based on the WKB technique developed recently for dealing with the effects of surface tension in the problem of dendritic solidification. We find that the relation between the dimensionless width $\lambda$ and the dimensionless group of parameters containing the surface tension, $\nu$, has the form $\lambda -\frac{1}{2}\sim {\nu }^{\frac{2}{3}}$ in the limit of small $\nu$.

Among all problems of pattern selection the one posed by the Saffman-Taylor finger is yet unsolved, although numerics and experiments indicate the very simple property that the relative width of the finger tends to ½ in the low-surface-tension limit. We explain this by performing an expansion beyond all orders leading to the formulation of a nonlinear eigenvalue problem with a discrete set of solutions. In particular we predict that the relative width of the finger tends to ½ as the surface tension to the power ⅔.