Recent advances in the study of solid surfaces and thin films using variable-energy positron beams are reviewed. In the first part the authors discuss the process of positron moderation and technical aspects of positron beam production and application. The second part is (roughly) organized in sections that apply to increasing time scales appropriate to the positron-solid interaction. These are (a) first encounter and scattering effects, (b) energy loss and stopping profiles, (c) diffusion of thermalized positrons, (d) positron-surface interactions, and (e) studies of defects near surfaces and interfaces. The review is written with more emphasis on the most recent measurements and interpretations than on the chronology of various developments.

Experimental and theoretical data for dielectric functions, x-ray absorption coefficients, and generalized oscillator strengths needed for a description of the energy-loss spectrum of fast charged particles in solid silicon are given. Theories used to calculate spectra of total energy loss ("straggling spectra") are described. The convolution method is used to calculate straggling functions for thin silicon absorbers. They are compared with those obtained from other theories (Landau). For relativistic particles ($\gamma >100\mathrm{}$), the Vavilov-Shulek theories give incorrect functions for absorbers of thicknesses $t<\mathrm{}1\mathrm{}$ mm. The conversion of energy-loss spectra into ionization spectra is discussed, and the latter are compared with experimental functions. Good agreement is found between calculated and observed functions for electrons, mesons, protons, and their antiparticles and for $\alpha$ particles. From this agreement, the error ($1\sigma$) of the theoretical values of the most probable energy loss ${\Delta }_p$ and the full width at half maximum, $w$, is estimated to be less than ±1%.

This review gives a detailed overview of the current status of our understanding of the nucleon-nucleon forces. The authors review the known long-range meson exchange forces and explain how these forces originate from an underlying quark model constrained by chiral symmetry, a symmetry that is very well satisfied in low-energy nuclear phenomena. These effective meson exchange forces describe the large-impact-parameter nucleon-nucleon scattering. The authors show how the small-impact-parameter nucleon-nucleon scattering can be explained by the quark structure of the nucleons and why this quark model is successful in reproducing the energy dependence of the "measured" $S$- and $P$- wave nucleon-nucleon phase shifts.

An account is given of a theoretical approach to surface structure and reactivity that is within the framework of solid-state theory, yet strives for chemical ways of interpretation. One begins from highly delocalized band structures, but introduces interpretational tools (density-of-states decompositions, crystal orbital overlap populations) that allow a tracing of local, chemical acts. It is quite feasible to construct interaction diagrams for surfaces, and to make frontier orbital arguments, just as for molecules. There are some interesting ways in which the surface-adsorbate interaction differs from simple molecular binding—in particular, in the way that two-orbital four-electron and zero-electron interactions can turn into bonding. The surface and bulk acting as a reservoir of electrons or holes at the Fermi level are important in this context. Chemisorption emerges as a compromise in a continuum of bonding whose extremes are dissociative adsorption and surface reconstruction.

Fluctuations in resistivity with an approximately $\frac{1}{f}$ spectrum are found in most conducting materials. Some of the detailed mechanisms, which turn out to be quite diverse, are explored in this review. Several new techniques are explained. Various theories are critically examined. Some connections with other widespread phenomena, such as highly nonexponential dielectric relaxations, are briefly investigated.

This article presents a unified formulation and review of an extensive class of radiation effects and devices based on free or quasifree electrons. The effects and devices reviewed include slow-wave radiators [such as Čerenkov, Smith-Purcell, and TWT (traveling-wave tube) effects and devices], periodic bremsstrahlung radiators [such as undulator radiation, magnetic bremsstrahlung FEL's (free-electron lasers), and coherent bremsstrahlung in the crystal lattice], and transverse-binding radiators [such as the CRM (cyclotron resonance maser) and channeling radiation]. Starting from a general quantum-electrodynamic model, both quantum and classical effects and operating regimes of these radiation devices are described. The article provides a unified physical description of the interaction kinematics, and presents equations for the characterization of spontaneous and stimulated radiative emission in these various effects and devices. Universal relations between the spontaneous and stimulated emission parameters are revealed and shown to be related (in the quantum limit) to Einstein relations for atomic radiators and (in the classical limit) to the relations derived by Madey for magnetic bremsstrahlung FEL for on-axis radiative emission. Examples for the application of the formulation are given, estimating the feasibility of channeling radiation x-ray laser and optical regime Smith-Purcell FEL, and deriving the gain equations of magnetic bremsstrahlung FEL and CRM for arbitrary electron propagation direction, structure (wiggler) axis, and radiative emission angle.

The IBA-1 is reviewed with particular emphasis on the symmetry structure that arises naturally from its inherently algebraic approach. The formulation of the model, in both its algebraic and its numerical aspects, is presented and the basic character of its predictions is discussed and compared with nuclear data. The limitations of the model, and efforts to ameliorate these by appropriate extensions, are also reviewed in some detail. An effort is made to provide a simple, transparent understanding of the basic physics by clarifying the relation of the mathematical structure to underlying physical ideas, and to provide guidance in understanding and carrying out practical calculations.

The authors review the ideas and manifestations of QCD coherence in high-energy reactions producing jets. They suggest experiments involving two-jet production, high-$p_{\perp }\gamma$ production and high-$p_{\perp }W$ production at hadronic colliders as ways to see QCD coherence in stringlike effects. They also suggest a procedure for finding the dip in the inclusive hadron spectrum of jets for those jets produced at hadron colliders. Simple explanations of the various coherence effects in QCD are discussed, as is the idea of local parton-hadron duality.

The event rates and their recognized uncertainties are calculated for eleven solar neutrino experiments using accurate solar models. The same solar models are used to evaluate the frequency spectrum of the $p$ and $g$ oscillation modes of the sun and to compare with existing observations. A numerical table of the characteristics of the standard solar model is presented. Improved values have been calculated for all of the neutrino absorption cross sections evaluating the uncertainties for each neutrino source and detector as well as the best estimates. The neutrino capture rate calculated from the standard solar model for the ${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$ experiment is 7.9(1±0.33) SNU, which spans the total theoretical range; the rate observed by Davis and his associates is (2.0±0.3) SNU. The ratio of the observed to the predicted flux at Earth of neutrinos from ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ decay lies in the range $0\le \left[\frac{\varphi {\left({}_{}{}^8{}_{}{}^{}\mathrm{B}\right)}_{\mathrm{observed}}}{\varphi {\left({}_{}{}^8{}_{}{}^{}\mathrm{B}\right)}_{\mathrm{predicted}}}\right]\le 0.5$. The recent results from the Kamiokande II electron scattering experiment confirm this conclusion. This discrepancy between calculation and observation is the solar neutrino problem. Measurements of the energy spectrum of solar neutrinos can discriminate between suggested solutions of the solar neutrino problem. Nonstandard solar models, many examples of which are also calculated in this paper, preserve the shape of the energy spectrum from individual neutrino sources, whereas most proposed weak-interaction explanations imply altered neutrino energy spectra. Detailed energy spectra of individual neutrino sources are presented as well as a composite solar neutrino spectrum. hep neutrinos from the ${}_{}{}^3{}_{}{}^{}\mathrm{He}+p$ reaction, probe a different region of the solar interior than do ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ neutrinos. Measurements of the very rare but highest-energy hep neutrinos are possible in proposed experiments using electron scattering, ${}_{}{}^2{}_{}{}^{}\mathrm{H}$, and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ detectors. The standard solar model predicts $p$-mode oscillation frequencies that agree to within about 0.5% with the measured frequencies and reproduce well the overall dispersion relation of the modes. However, there are several small but significant discrepancies between the measured and observed frequencies. The complementarity of helioseismology and solar neutrino experiments is demonstrated by constructing a solar model with a drastically altered nuclear energy generation that eliminates entirely the important high-energy ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$ neutrinos, but which affects by less than 0.01% the calculated $p$-mode oscillation frequencies.

Positive muons implanted into diamond and zincblende-structured semiconductors often form hydrogen-like paramagnetic muonium (${\mu }^{+}-e^{-}$) states whose characteristics can be investigated with the "muon spin rotation" ($\mu \mathrm{SR}$) technique. In contrast to the case of hydrogen, which is not known to form a paramagnetic state in semiconductors, two coexisting types of muonium states are seen. "Mu" with a large isotropic hyperfine interaction, and "${\mathrm{Mu}}^{*}$" with a small [111]-axially symmetric hyperfine interaction. Both "spectroscopic" properties of these states, such as the electronic $g$ factors and the nuclear hyperfine interactions, and "dynamic" properties, such as their diffusion rates and their rates of interconversion, are accessible with $\mu \mathrm{SR}$. Direct information about the site of the muonium states is available using the channeling effect of the positron from muon decay in a crystalline host. The techniques for probing semiconductors with positive muons are described in this review, and the results they have provided to date are critically discussed. The considerable amount of theoretical work that has been invested in microscopic models of Mu and ${\mathrm{Mu}}^{*}$ is also summarized.

A comprehensive account is given of the macroscopic and microscopic physical properties of chromium (and where appropriate those of its dilute alloys) that relate to its antiferromagnetism. Neutron scattering is treated in great detail, first in the historical introduction, then as an experimental probe of both the magnetic structure and the excitations of the incommensurate spin-density-wave state and (with the assistance of x rays) of the concomitant charge-density wave and strain wave. Neutron scattering is considered as a tool to explore not only the disappearance of long-range order with increasing temperature through the growth of excitations as the weak first-order Néel transition is approached, but also the persistence of these spin fluctuations well into the paramagnetic state—processes that are still little understood. The article surveys, without mathematical details, model systems designed to reproduce the magnetic and thermodynamic properties of Cr. The energy-band structure calculations are given a more comprehensive review. Special attention is paid to calculations of the wave-vector-dependent susceptibility that reproduce the observed wave vector of the spin-density wave, and to a recent finite-temperature calculation that gives almost the right Néel temperature. The review of Fermi-surface studies emphasizes those designed to relate the spin-density wave vector (and its pressure dependence) to the nesting vector of the Fermi surface. An account is given of the spectroscopic determination of the energy gap(s), whose theoretical analysis is still unclear, and of experiments aimed at determining physical properties that throw light on the origin of the weak first-order Néel transition. The article describes the use of magnetic anomalies in the elastic moduli to determine the volume dependence of the exchange interaction responsible for antiferromagnetism in Cr. The experimental features of the spin-flip transition are reviewed, although a theory of this phenomenon is wanting. The experimental study of microscopic structure by the use of hyperfine-interaction properties is surveyed. An account is given of both experimental and theoretical studies of the surface of Cr and of Cr films and sandwiches. Finally, "technical antiferromagnetism" is discussed: the effect of severe internal strain in producing a commensurate antiferromagnetic state, wave-vector Q domains, polarization S domains (for which the experimental evidence is scanty), and ultrasonic attenuation as a tool to study them.

For a decade now the subject of the nature of the two-dimensional melting transition has remained controversial. An elegant theory based on the unbinding of pairs of crystal defects suggested that two-dimensional solids might melt by a transition sequence involving two continuous transitions separated by a novel, nearest-neighbor-bond-orientationally ordered fluid—the hexatic phase. Competing theories predict that the transition is of the usual first-order type observed in three-dimensional systems. This paper is a critical review of the current status of research into the problem of two-dimensional melting, with an emphasis on computer simulations. An attempt is made to point out unresolved issues pertaining to this fascinating and still open question.

The authors present a general discussion of spherical, nonrelativistic blastwaves in an astrophysical context. A variety of effects has been included: expansion of the ambient medium, gravitation, and an embedded fluid of clouds capable of exchanging mass, energy, or momentum with the medium. The authors also consider cases of energy injection due either to a central source or to detonations. Cosmological solutions are extensively treated. Most attention is devoted to problems in which it is permissible to assume self-similarity, as in the prototype Sedov-Taylor blastwave. A general virial theorem for blastwaves is derived. For self-similar blastwaves, the radius varies as a power of the time, $R_s\propto t^{\eta }$. The integral properties of the solution are completely specified by two dimensionless numbers measuring the relative importance of thermal and kinetic energy. The authors find certain exact kinematical relations and a variety of analytic approximations to determine these numbers with varying degrees of accuracy. The approximations may be based on assumptions about the internal density distributions (e.g., shell-like), pressure distribution, or velocity distribution. In many cases exact conditions from, for example, boundary conditions or other constraints may be used to determine unspecified parameters. One new set of exact integral constraints has been derived. The various approximation schemes are tested with known solutions. The authors find that for blastwaves in which the flow extends to the origin, the assumption that the internal velocity is linear with radius is reasonably accurate. For blastwaves in which an interior vacuum develops, the equally simple approximation of constant interior velocity is accurate. These lowest-order approximations are shown to give numerical coefficients in the relation $R=\mathrm{const}\times t^{\eta }$ which are accurate to about 1-2%. The higher-order approximations show an accuracy that in some cases equals that obtained, to date, by direct numerical integration. In addition to the new methods presented, the authors have obtained new results for evaporative blastwaves, impeded blastwaves, blastwaves with cloud crushing, bubbles, cosmological blastwaves (self-similar and non-self-similar, radiative and nonradiative), blastwaves in a wind, and detonations. Some of the new results found are exact. Included are the radiative, cosmological self-similar solution, appropriate to the universe ($z>\mathrm{}10\mathrm{}$) when inverse Compton cooling is efficient [$\ln R=\mathrm{const}+\frac{\left(\ln t\right)(15+\sqrt{17})}{24}$], and certain properties of the solutions mentioned above. In a series of appendixes several related issues are treated: energy conservation for multicomponent fluid in an expanding universe; central and edge derivatives of physical quantities in self-similar adiabatic blastwaves; shock jump conditions including energy input (detonations), and a variety of other matters.

This review is devoted to the physics of current-carrying superconductors and normal metals having two or more stable states sustained by Joule self-heating. The creation, propagation, and localization of electrothermal domains and switching waves leading to the transition from one stable state to another in uniform and nonuniform samples are treated in detail. The connection between thermal bistability and hysteresis, dropping and stepped current-voltage characteristics, self-induced oscillations of current and voltage, selfreplication of electrothermal domains, and the formation of periodic and stochastic resistive structures are considered.

Magnetic ions embedded in a nonmagnetic host exhibit a number of unusual low-temperature properties known collectively as the Kondo effect. Conventional small-parameter expansions for the Kondo effect are plagued by infrared divergences, and the theory of dilute magnetic alloys has for many years centered on nonperturbative techniques. This paper reviews a novel perturbative approach to the magnetic alloy problem based on expansion in $\frac{1}{N}$, with $N$ the ionic angular momentum degeneracy. This approach is analogous to perturbative expansions in statistical mechanics and field theory based on an integer-valued parameter (such as the number of spin components or the number of colors). The large-$N$ expansion reproduces the essential features of the Kondo effect at $O\left(1\right)\mathrm{}$ and appears to yield convergent (or asymptotic) expressions for ground-state properties. In contrast with previous nonperturbative approaches, the expansion provides information on dynamic, as well as static, properties. The evidence for convergence of the expansion is reviewed, and large-$N$ calculations are compared with exact results for static properties. A number of independent, but essentially equivalent, approaches to the large-$N$ expansion have been developed during the last five years. These techniques are reviewed pedagogically, and their relative strengths and weaknesses emphasized. A guide to notation in the recent literature is provided.

The past few years have seen a number of studies involving the presence of molecular hydrogen trapped in $a$-Si: H. The main techniques used have been nuclear magnetic resonance, calorimetry, and infrared absorption. The results suggest that ${\mathrm{H}}_2$ is trapped in a number of samples in microbubbles (≈ 10 Å) under high pressure (≈ 2 kbar) at room temperature, solidifies around 25 K, and undergoes an orientational phase transition around 15 K. The data are reviewed here, and the effects that such ${\mathrm{H}}_2$ microbubbles may have on the properties of $a$-Si: H are discussed.

One of the early projects established by CODATA was intended to provide a recommended set of the fundamental physical constants, which are so important to the analysis and interpretation of experimental data in many scientific disciplines. The first "Recommended Consistent Values of the Fundamental Physical Constants" appeared in 1973 and was subsequently adopted by most international and national bodies in their own recommendations. This process has contributed to improved compatibility of scientific and technical data in all fields of science. CODATA presents here a revised version of these recommendations which takes into account the significant advances in metrology that have occurred since the 1973 analysis. "The 1986 adjustment of the fundamental physical constants" represents a 5-year effort involving experts from the major metrological laboratories of the world. It is hoped that this recommended data set will receive the acceptance of the scientific community which was achieved by its predecessor.