The authors have measured, via ac susceptibility $\chi \mathrm{}(T,H)\mathrm{}$, the superconducting and magnetic phase diagram for the $\left({\mathrm{Th}}_{1-x}{\mathrm{Nd}}_x\right){\mathrm{Ru}}_2$ system. For $0.3<x<\mathrm{}0.4\mathrm{}$ reentry and coexistence behavior are observed at low temperatures. The field-dependent properties show that a weakened superconducting state can exist in a frozen spin-glass or cluster-glass state. Here a field memory effect can easily destroy the superconductivity. This indicates that the spin-glass phase consists of very large randomly frozen ferromagnetic clusters.

Raman scattering resonant with quasi-two-dimensional excitons in GaAs-$\left({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\right)\mathrm{As}$ heterostructures shows enhancement profiles in agreement with exciton structure measured in other optical spectra. The profiles have characteristic asymmetries in the incoming and outgoing resonances that are specific to the exciton-optical-phonon interaction in quantum wells. Direct evidence of the degree of exciton confinement appears in the Raman spectra.

The amplitudes for the allowed and the forbidden (intraband Frölich) scattering by LO phonons are shown to interfere near resonances. This fact is illustrated with measurements on a (100) surface near the $E_0+{\Delta }_0$ gap of GaAs.

The x-ray $L$ absorption spectra were measured for the atomic rare-earth elements Ce, Sm, Gd, and Er. From the energy positions of well resolved absorption lines at threshold in comparison to the corresponding solid-state spectra the atom-to-metal shifts of the core-level binding energies have been determined. For Sm and Er which undergo a valence change on formation of the solid the shifts differ by more than 6 eV from those of Ce and Gd which have the same configuration in the atom and the solid.

The absorption constant $K\left(\nu \right)$ and refractive index $n\left(\nu \right)$ have been measured at photon energies of $0.5<~h\nu <~5.5$ eV in liquid ${\mathrm{K}}_x-\mathrm{K}{\mathrm{Cl}}_{1-x}$ solutions for $0.001<~x<~0.05$ and temperatures up to 900°C, approaching the metal-nonmetal transition. The absorption spectra give clear evidence for $F$-center states existing at all $x$ in equilibrium with aggregated centers, probably small metal clusters. The dielectric susceptibility increases with $x$ by almost 2 orders of magnitude. A fit of the dielectric enhancement by a critical form yields a critical exponent, $\zeta =1.4\mathrm{}\pm 0.2$.

A general class of models is introduced which relate the motion of a phase boundary to properties of the local interfacial geometry. These systems can undergo successive destabilizations as they grow, possibly giving rise to nonequilibrium spatial patterns. This formalism has applications to a wide variety of physical problems, especially including dendritic solidification.

Unrestricted lattice random walks in which a unit conductor is placed along each bond traversed are considered. The mean end-to-end resistance is studied as a function of the number of steps in the walk and the spatial dimension. A critical scaling law is found whose exponent is consistently given by four different calculational schemes.

A model for diffusion-controlled aggregation in which growing clusters as well as individual particles are mobile has been investigated. Two versions of the model in which the cluster diffusion coefficient is either size independent or inversely proportional to number of particles (mass) give very similar results. In the limit of low concentration and large system size both models lead to structures with a fractal (Hausdorff) dimensionality of about 1.45-1.5 in two-dimensional lattice-based simulations.

A model describing the process of cluster growth by aggregation of clusters is introduced and investigated with the Monte Carlo method. It is found that the clusters are scale invariant. The fractal dimension of this new class of kinetic critical behavior is $D={\nu }^{-1\mathrm{}}=1.38\mathrm{}\pm 0.06$ in $d=2\mathrm{}$ dimensions as determined by the radii of gyration and the density correlations of the aggregates. The process is compared with other types of kinetic and equilibrium critical phenomena.

New expressions for the availability dissipated in a finite-time endoreversible process are found by use of Weinhold's metric on equilibrium states of a thermodynamic system. In particular, the dissipated availability is given by the square of the length of the corresponding curve, times a mean relaxation time, divided by the total time of the process. The results extend to local thermodynamic equilibrium if instead of length one uses distance (length of the shortest curve) between initial and final states.

The magnetic moment of the ${\Sigma }^{-}$ hyperon has been obtained from measurements of fine-structure splittings of x-ray transitions in ${\Sigma }^{-}$ exotic atoms. The use of targets consisting of sheets of Pb or W immersed in liquid hydrogen, and of a tagging technique utilizing the ${\pi }^{+}$ emitted in the reaction $K^{-}p\to {\Sigma }^{-}{\pi }^{+}$, resulted in improved x-ray signal-to-noise ratio by a factor of 15 over that in previous experiments. The present result is $\mu \left({\Sigma }^{-}\right)=(1.111\mathrm{}\pm 0.031\pm 0.011){\mu }_N$.

Spin-dependent asymmetries have been measured in inclusive deep-inelastic scattering of longitudinally polarized electrons by longitudinally polarized protons. Data were obtained at a scattering angle of 10° and for incident energies of 16.2 and 22.7 GeV, which cover the kinematic range $0.18<x<\mathrm{}0.70\mathrm{}$ and $3.5<\mathrm{}{Q}^2<10.0\mathrm{}$ (${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. The present results provide a test of scaling and of the Bjorken and Ellis-Jaffe sum rules and are compared with various models of proton spin structure.

A measurement is presented of the inclusive production of charged $D^{*}$ and neutral $D$ mesons from nonresonant $e^{+}{e}^{-}$ annihilation in the energy region near the $\Upsilon \mathrm{}\left(4S\right)\mathrm{}$ resonance. The momentum distribution shows a large contribution at high momenta as expected for heavy quark production. Comparison of the spectrum with several phenomenological models is made. The relative yields of the $D^{*}$ and $D$ mesons indicate that the charm cross section is dominated by the $D^{*}$ contribution.

In a sample of $B$ mesons from the decay of the $\Upsilon \mathrm{}\left(4S\right)\mathrm{}$ produced in $e^{+}{e}^{-}$ collisions at the Cornell Electron Storage Ring, inclusive protons, antiprotons, lambdas, and antilambdas have been observed. Lower limits to the $B$-to-baryon branching ratios are derived.

Inclusive production of ($D^0$, ${\overline{D}}^0$) and $D^{\pm }$ mesons have been observed in $e^{+}{e}^{-}$ annihilation at 29 GeV. The signals correspond to $R$ values of $R(D^0+{\overline{D}}^0)=3.25\mathrm{}\pm 1.2$ and $R(D^{+}+D^{-})=1.35\mathrm{}\pm 0.6$. $D^{*\pm }$ production is also observed via the process $D^{*+}\to D^0{\pi }^{+}$ and its charge conjugate. The $D$ and $D^{*}$ production rates are compared.

A correction to the soft-photon theorem for bremsstrahlung is derived for problems in which the strong interaction is known at large distances. In studies of a model problem the correction generally results in significant improvement in accuracy over the original theorem.

Boundary conditions on the monopole Dirac equation are, on the one hand, determined by the requirement that the Hamiltonian be Hermitean. On the other, they enforce the conservation of certain charges at the monopole core. Which charges should be conserved is determined by the short-distance physics of the problem and the connection with boundary conditions is worked out for several cases. It is found to be possible to impose physically reasonable conservation-law requirements which can be realized by no Dirac-equation boundary condition!

The Coulombic Hamiltonian of atoms approximately commutes with an $N$-electron operator which is a sum of O(4) Casimir operators. This accurately fixes configuration mixing within the entire $2s-2p$ shell. Several aspects of the stability of the results are discussed.

The large spread in $\gamma$ values (${1-10}^5$) for gigaelectronvolt secondary beams has been used in a novel technique to measure the relativistic effects on inner-shell excitation in Cu and Al and energy loss in Si and Ge. In $K$-shell excitation, a pronounced influence of density effect is found for the first time, in apparent contradiction to earlier measurements, where the lack of density effects has been a great mystery. These problems are explained as target-thickness effects in connection with transition radiation.

The exchange interaction between ${{}_{}{}^3{}_{}{}^{}\Sigma _{\mathrm{g}}^{}{}_{}{}^{}}^{-}$ ${\mathrm{O}}_2$ molecules, as obtained from ab initio calculations for a singlet, triplet, and quintet dimer, can be well represented by the Heisenberg Hamiltonian. The coupling parameter $J$ depends exponentially on the ${\mathrm{O}}_2$-${\mathrm{O}}_2$ distance, with exponents ranging from 3.6 to 4.2 ${\mathrm{\AA }}^{-1\mathrm{}}$ for different ${\mathrm{O}}_2$ orientations. For most orientations the coupling is antiferromagnetic, but for a few $J$ is positive. The different neighbor interactions in $\alpha$ and $\beta$ ${\mathrm{O}}_2$ solids, which determine their magnetic order and properties, have been evaluated.