The electronic properties of inversion and accumulation layers at semiconductor-insulator interfaces and of other systems that exhibit two-dimensional or quasi-two-dimensional behavior, such as electrons in semiconductor heterojunctions and superlattices and on liquid helium, are reviewed. Energy levels, transport properties, and optical properties are considered in some detail, especially for electrons at the (100) silicon-silicon dioxide interface. Other systems are discussed more briefly.

This paper discusses the sense in which the large $N$ limits of various quantum theories are equivalent to classical limits. A general method for finding classical limits in arbitrary quantum theories is developed. The method is based on certain assumptions which isolate the minimal structure any quantum theory should possess if it is to have a classical limit. In any theory satisfying these assumptions, one can generate a natural set of generalized coherent states. These coherent states may then be used to construct a classical phase space, derive a classical Hamiltonian, and show that the resulting classical dynamics is equivalent to the limiting form of the original quantum dynamics. This formalism is shown to be applicable to the large $N$ limits of vector models, matrix models, and gauge theories. In every case, one can explicitly derive a classical action which contains the complete physics of the $N=\infty$ theory. "Solving" the $N=\infty$ theory requires minimizing the classical Hamiltonian, and this has been possible only in simple theories. The relation between this approach and other methods which have been proposed for deriving large $N$ limits is discussed in detail.

An overview of the theory of photoelectron angular distributions for atoms is presented. Its features, which are embodied in a single asymmetry parameter $\beta$ in the electric dipole approximation, are examined within the framework of the angular momentum transfer formulation. The $\beta$ parameter is in principle always energy dependent. Within the $\mathrm{LS}$ coupling approximation, however, there are instances, each representing a multitude of particular photoionization processes, in which $\beta$ is an analytically determined constant. The energy dependence of the $\beta$ parameters in such instances is due entirely to spin-orbit and other relativistic interactions. The study of the energy dependence of the $\beta$ parameter in these cases is thus of interest because it spotlights weak-interaction effects which are usually overwhelmed by stronger interactions. We illustrate the general predictions by a detailed consideration of the energy dependence of the $\beta$ parameter for $s$-subshell photoionization processes. It is shown that the asymmetry parameters for atomic $s$ subshells are particularly suitable for distinguishing between purely geometrical effects on the photoelectron angular distribution, resulting from physical conservation laws, and dynamical effects arising from relativistic interactions and electron exchange and correlation. In general, the $\beta$ parameters for $s$ subshells vary with energy; such variation is largest near minima in the cross sections for the corresponding photoelectron channels and in the vicinity of resonances. However, a number of atomic photoionization transitions are identified for which $\beta$ would be a constant (equal to one of the three values 2, $\frac{1}{5}$, or - 1) were it not for relativistic interactions and (in some cases) final-state interchannel coupling and/or initial-state electron correlations. Measurement or calculation of the $\beta$ parameters for such transitions thus provides a sensitive measure of the strength of relativistic interactions as well as of electron correlations.

A review is given of the experimental investigation of high-energy $e^{+}{e}^{-}$ interactions at PETRA, the electronpositron colliding beam accelerator at DESY in Hamburg, Germany. The energy of the machine is such that the strong, electromagnetic, and weak interactions start to play a role in the processes which can be studied. Results are presented on tests of quantum electrodynamics studying $e^{+}{e}^{-}$, ${\mu }^{+}{\mu }^{-}$, and ${\tau }^{+}{\tau }^{-}$. The fermion pair production processes $e^{+}{e}^{-}\to F\overline{F}$, where the $F\overline{F}$ pair stands for either a lepton pair ($e^{+}{e}^{-}$, ${\mu }^{+}{\mu }^{-}$, and ${\tau }^{+}{\tau }^{-}$) or a quark-antiquark pair, are investigated to study the effects of the $Z^{\circ }$ exchange. The measurements of $R$, the ratio of hadronic to pointlike muon pair cross section, the search for new quark flavors, and other new particles are reviewed. Finally the discovery of three-jet events arising from the radiation of hard noncollinear gluons as predicted by quantum chromodynamics and the determinations of the strong coupling constant ${\alpha }_s$ are discussed.

The theory of muonic atoms is a complex and highly developed combination of nuclear physics, atomic physics, and quantum electrodynamics. Perhaps nowhere else in microscopic physics are such diverse branches so intimately intertwined and yet readily available for precise experimental verification or rejection. In the present review we summarize and discuss all of the most important components of muonic atom theory, and show in selected cases how this theory meets experimental measurements.

Many authors have studied the safety of disposal of highly radioactive wastes in excavated cavities beneath the earth. Work has been concentrated in three areas: prediction of future events and processes which could affect waste containment, mathematical modeling of failure scenarios, and estimation of uncertainties in model predictions. The results of past safety assessments are reviewed and compared in this paper. Anything but a very small release of radioactivity from a repository would appear to be quite unlikely; a quantitative evaluation of the probabilities of small releases has not proved possible.

This is a tutorial review on the Potts model aimed at bringing out in an organized fashion the essential and important properties of the standard Potts model. Emphasis is placed on exact and rigorous results, but other aspects of the problem are also described to achieve a unified perspective. Topics reviewed include the mean-field theory, duality relations, series expansions, critical properties, experimental realizations, and the relationship of the Potts model with other lattice-statistical problems.

Diffusion processes are considered for one-dimensional stochastic Lorentz models, consisting of randomly distributed fixed scatterers and one moving light particle. In waiting time Lorentz models the light particle makes instantaneous jumps between scatterers after a stochastically distributed waiting time. In the stochastic Lorentz gas the light particle moves at constant speed and is scattered stochastically at collisions with the scatterers. For the waiting time Lorentz models the Green's function of the diffusion process is calculated exactly. The diffusion coefficient is found to be the same as for a corresponding random walk on a regular lattice, the velocity autocorrelation function exhibits a long-time tail proportional to $t^{-\frac{3}{2}}$ and super Burnett and higher-order transport coefficients are found to diverge. For the stochastic Lorentz gas similar results are found for the diffusion coefficient and the velocity autocorrelation function, but the generalized super Burnett coefficient, as introduced by Alley and Alder, is convergent in this case. For a special case of the waiting time Lorentz models some other aspects are considered, such as periodic boundary conditions, steady-state diffusion and fluctuations of the velocity autocorrelation function about its average value, due to the initial conditions and to the stochastic distribution of scatterers.

Measurements of E2/M1 and E0/E2 multipole mixing ratios of transitions in even-even nuclei have long provided important tests of nuclear models. The experimental data for transitions in even-even nuclei have been critically surveyed to provide the most accurate results for comparisons with theoretical calculations. The theoretical approaches to the calculations of these mixing ratios on the bases of different nuclear models are considered and compared with experimental data. The variations in signs and magnitudes of the E2/M1 mixing ratios from nucleus to nucleus for the same class transitions and within a given nucleus for transitions from different spin states suggest that a microscopic approach is needed to explain the data theoretically. The pairing-plus quadrupole model has achieved the first successes in predicting these variations, primarily in the osmium to platinum region.

There is a wide range of fundamental physical problems directly related to how pulsars function. Some of these are independent of the specific pulsar mechanism. Others relate directly to the physics of the pulsar and already shed some light on the properties of matter at high density (∼${10}^{15}$ g/cc) and in strong magnetic fields (∼${10}^{12}$ G). Pulsars are assumed to be rotating neutron stars surrounded by strong magnetic fields and energetic particles. It is somewhere within this "magnetosphere" that the pulsar action is expected to take place. Currently there has been considerable difficulty in formulating an entirely self-consistent theory of the magnetospheric behavior and there may be rapid revisions in the near future, which is all the more surprising since many of the issues involve "elementary" problems in electromagnetism. One interesting discovery is that charge-separated plasmas apparently can support stable static discontinuities.

The authors review the development of extended x-ray absorption fine structure (EXAFS) within the last decade. Advances in experimental techniques have been largely stimulated by the availability of synchrotron radiation. The theory of EXAFS has also matured to the point where quantitative comparison with experiments can be made. The authors review in some detail the analysis of EXAFS data, starting from the treatment of raw data to the extraction of distances and amplitude information, and they also discuss selected examples of applications of EXAFS chosen to illustrate both the strength and limitations of EXAFS as a structural tool.

Theories of electron scattering by ionized impurities in semiconductors are reviewed. The early foundations based on the Born approximation and their subsequent refinements are discussed thoroughly. The phase-shift method which is not restricted to the Born approximation is also presented. The situation in heavily doped semiconductors is described. The theories are then compared critically with experiments. Finally, conclusions are drawn and some plausible lines of future work are outlined.

The interdisciplinary theoretical and practical problems involved in the development of devices analogous to lasers for generation of coherent radiation in the 6- to 120-keV photon energy range by stimulated emission of recoilless radiation from nuclear isomers are discussed in depth with a comprehensive bibliography.

New and improved data on the photonic decays of the old (noncharm) mesons has appeared within the past six years. It is now possible to put many of the ideas of the past twenty years to an exhaustive test. A review of these developments is given and the implications for our understanding of electromagnetic interactions of mesons are noted.