Showing papers in "Reviews of Modern Physics in 1982"

Electronic properties of two-dimensional systems

Abstract: 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.

The Potts model

Abstract: 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.

Strongly coupled plasmas: high-density classical plasmas and degenerate electron liquids

Abstract: Classical and degenerate, strongly coupled plasmas are approached by computer simulations, analytic theories, and variational methods. Thermodynamic properties predicted in those various approaches are compared and examined; possibilities of Wigner crystallization and other instabilities are investigated. Salient features in the static and dynamic correlations are reviewed. Screening effects are analyzed and applied to calculations of the enhancement factor of the thermonuclear reaction rate and the electric resistivity in dense plasmas. Theoretical predictions on the dynamic properties are compared with the x ray and electron scattering data for the metallic electrons.

Theory of pulsar magnetospheres

Abstract: 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 (\ensuremath{\sim}${10}^{15}$ g/cc) and in strong magnetic fields (\ensuremath{\sim}${10}^{12}$ 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.

Einstein Gravity as a Symmetry Breaking Effect in Quantum Field Theory

Abstract: This article gives a pedagogical review of recent work in which the Einstein-Hilbert gravitational action is obtained as a symmetry-breaking effect in quantum field theory. Particular emphasis is placed on the case of renormalizable field theories with dynamical scale-invariance breaking, in which the induced gravitational effective action is finite and calculable. A functional integral formulation is used throughout, and a detailed analysis is given of the role of dimensional regularization in extracting finite answers from formally quadratically divergent integrals. Expressions are derived for the induced gravitational constant, for the induced cosmological constant, and for quantized matter theories on a background manifold, and a strategy is outlined for computing the induced constants in the case of an SU(n) gauge theory. By use of the background field method, the formalism is extended to the case in which the metric is also quantized, yielding a derivation of the semiclassical Einstein equations as an approximation to quantum gravity, as well as general formulas for the induced (or renormalized) gravitational and cosmological constants.

The mean-field theory of nuclear structure and dynamics

Abstract: The physical and theoretical foundations are presented for the mean-field theory of nuclear structure and dynamics. Salient features of the many-body theory of stationary states are reviewed to motivate the time-dependent mean-field approximation. The time-dependent Hartree-Fock approximation and its limitations are discussed and general theoretical formulations are presented which yield time-dependent mean-field equations in lowest approximation and provide suitable frameworks for overcoming various conceptual and practical limitations of the mean-field theory. Particular emphasis is placed on recent developments utilizing functional integral techniques to obtain a quantum mean-field theory applicable to quantized eigen-states, spontaneous fission, the nuclear partition function, and scattering problems. Applications to a number of simple, idealized systems are presented to verify the approximations for solvable problems and to elucidate the essential features of mean-field dynamics. Finally, calculations utilizing moderately realistic geometries and interactions are reviewed which address heavy-ion collisions, fusion, strongly damped collisions, and fission.

Large N limits as classical mechanics

Abstract: 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 $N=\ensuremath{\infty}$ theory. "Solving" the $N=\ensuremath{\infty}$ 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.

The effect of neutral nonresonant collisions on atomic spectral lines

Abstract: The effect of neutral nonresonant collisions on the shape, shift, and intensity of atomic spectral-line profiles is reviewed. A general treatment for the study of an atomic spectral line is developed by establishing reasonable assumptions about the relevant collision processes and by finding an expression for the Fourier transform of the line profile. The authors look at parallel developments of other methods of calculation, consider special limits of practical interest, and illustrate numerical evaluations of complete line profiles. Interatomic potentials for use in line-profile calculations are described, and the problems imposed by nonadditivity and nonadiabaticity are also noted. The observation of line profiles by both conventional and tunable-laser techniques is surveyed. Representative experimental measurements of the widths and shifts of collision-broadened spectral line cores are tabulated, and the phenomena of satellites, oscillations, and power-law behavior of line wings are compared with theoretical expectations. The use of experimental results for the determination of excited atom-atom interactions, the prediction of collision broadening in stellar atmospheres, and the effect of foreign gases on laboratory standard wavelengths are also discussed.

Ideal magnetohydrodynamic theory of magnetic fusion systems

Abstract: Ideal magnetohydrodynamic theory and its application to magnetic fusion systems are reviewed. The review begins with a description and derivation of the model as well as a discussion of the region of validity. Next, the general properties are derived. These are valid for arbitrary geometry and demonstrate the inherently sound physical foundation of the model. The equilibrium behavior of the currently most promising toroidal magnetic fusion concepts are then discussed in detail. Finally, the stability of such equilibria is investigated. Included are discussions of the general stability properties of arbitrary magnetic geometries and of detailed applications to those concepts of current fusion interest.

Standard solar models and the uncertainties in predicted capture rates of solar neutrinos

Abstract: caused by a specified uncertainty in any of the parameters is evaluated with the aid of a series of standard solar models that were constructed for this purpose; the results are expressed in terms of the logarithmic partial derivative of each flux with respect to each parameter. The effects on the neutrino fluxes of changing individual parameters by large amounts can usually be estimated to satisfactory accuracy by making use of the tabulated partial derivatives. An overall "effective 3o. level of uncertainty" is defined using the requirement that the true value should lie within the estimated range unless someone has made a mistake. Effective 3o. levels of uncertainty, as well as best estimates, are determined for the following possible detectors of solar neutrinos: H, Li, Cl, 'Ga, Br, 'Br, Mo, Mo, ' In, and electronneutrino scattering. The most important sources of uncertainty in the predicted capture rates are identified and discussed for each detector separately. For the Cl detector, the predicted capture rate is 7.6+3.3 (effective 3o. errors) SNU. The measured production rate is (Cleveland, Davis, and Rowjley, 1981) 2. 1+0.3 SNU ( lo. error). For a 'Ga detector, the expected capture rate is 106(1+o08) SNU (also effective 3o. errors). The relatively small uncertainty quoted for the Ga detector is a direct result of the fact that 'Ga is primarily sensitive to neutrinos from the basic proton-proton reaction, the rate of which is determined largely by the observed solar luminosity. The Caltech and Munster measured values for the cross-section factor for the reaction He(a, y) Be are inconsistent with each other. The capture rates quoted above were obtained using the Caltech value for the cross-section factor. If the Munster value is used instead, then the predicted capture rate for the Cl experiment is 4.95+ 2. 1 SNU (effective 3o. errors) and, for the 'Ga experiment, 96.7 (1 ~&o8} SNU (effective 3o. errors). In order for the bestestimate value to agree with the observation of Davis (1978) of 2 SNU for the Cl experiment, the cross-section factor S34 (0) would have to be reduced by about 15o. to less than the Caltech value, i.e. to 7o. less than the Munster value. The characteristics of the standard solar model, constructed with the best available nuclear parameters, solar opacity, and equation of state, are presented in detail. The computational methods by which this and similar models were obtained are also described brieAy. The primordial helium abundance inferred with the aid of standard solar models is Y = 0.25+ 0.01. The complementary relation between observations of solar neutrinos and of the normal modes of oscillation of the sun is examined. It is shown that the splitting of the observed large-n, small-l, p-mode (five minute) oscillations of the sun primarily originates in the outer ten percent of the solar mass, while the neutrinos from B beta decay originate primarily in the inner five percent of the solar mass. The solar luminosity, and the flux of neutrinos from the proton-proton reaction, come mostly from an intermediate region.

The energy levels of muonic atoms

Abstract: 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.

Transport properties of stochastic Lorentz models

Abstract: 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 ${t}^{\ensuremath{-}\frac{3}{2}}$ 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.

Photoelectron angular distributions: energy dependence for s subshells

Abstract: An overview of the theory of photoelectron angular distributions for atoms is presented. Its features, which are embodied in a single asymmetry parameter $\ensuremath{\beta}$ in the electric dipole approximation, are examined within the framework of the angular momentum transfer formulation. The $\ensuremath{\beta}$ 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 $\ensuremath{\beta}$ is an analytically determined constant. The energy dependence of the $\ensuremath{\beta}$ parameters in such instances is due entirely to spin-orbit and other relativistic interactions. The study of the energy dependence of the $\ensuremath{\beta}$ 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 $\ensuremath{\beta}$ 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 $\ensuremath{\beta}$ 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 $\ensuremath{\beta}$ 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 $\ensuremath{\beta}$ parameters for such transitions thus provides a sensitive measure of the strength of relativistic interactions as well as of electron correlations.

Electron spectroscopy for atoms, molecules, and condensed matter

Abstract: In my thesis [1], which was presented in 1944, I described some work which I had done to study β decay and internal conversion in radioactive decay by means of two different principles. One of these was based on the semi-circular focusing of electrons in a homogeneous magnetic field, while the other used a big magnetic lens. The first principle could give good resolution but low intensity, and the other just the reverse. I was then looking for a possibility of combining the two good properties into one instrument. The idea was to shape the previously homogeneous magnetic field in such a way that focusing should occur in two directions, instead of only one as in the semi-circular case. It was known that in betatrons the electrons performed oscillatory motions both in the radial and in the axial directions. By putting the angles of period equal for the two oscillations Nils Svartholm and I [2, 3] found a simple condition for the magnetic field form required to give a real electron optical image i.e. we established the two-directional or double focusing principle. It turned out that

E0-E2-M1 multipole admixtures of transitions in even-even nuclei

Abstract: 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.

Nonlinear optics and spectroscopy

Abstract: ing's vector it follows that the light amplitude at the focal spot would reach 108 volts per centimeter, comparable to the electric field internal to the atoms and molecules responsible for the binding of valence electrons. These are literally pulled out of their orbits in multiphoton tunneling processes, and any material will be converted to a highly ionized dense plasma at these flux densities. It is clear that the familiar notion of a linear optical response with a constant index of refraction, that is, an induced polarization proportional to the amplitude of the light field, should be dropped at much less extreme intensities. There is a nonlinearity in the constitutive relationship which may be expanded in terms of a power series in the electric field components

Physical characteristics of astronomical masers

Abstract: The radio line emission of interstellar molecules routinely shows deviations from thermal equilibrium which culminate with strong maser radiation in some sources. Like its laboratory counterpart, the maser radiation is amplified through the effect of induced processes in a region where a population inversion exists and displays many of the basic features of laboratory lasers. This review is intended to explain the basic theory of astronomical masers and to survey the theoretical models which were developed for specific sources.

Supernovae. Part I: The events

Abstract: Since the heroic era of Baade and Zwicky, our understanding of supernovae has advanced in hops and skips rather than steadily. The most recent jump has been into fairly general agreement that observations of Type I's can be interpreted as the manifestation of the decay of about 1M of Ni56 and observations of Type II's as the manifestation of 1051 ergs deposited at the bottom of a supergiant envelope by core bounce as a central neutron star forms. This paper explores the history of these and other ideas of what is going on in supernovae, the presupernova evolution of the parent stars and binary systems, observed properties of the events, and models for them. A later paper (Part II: the aftermath) will address the results of supernovae-their remnants, production of cosmic rays and gamma rays, nucleosynthesis, and galactic evolution-and the future of supernova research. © 1982 The American Physical Society.

Spectroscopy in a new light

Abstract: Scientific spectroscopy really began in Uppsala, Sweden, where Anders Angstrom in 1853 showed that some of the lines in the spectrum of an electric spark come from the metal electrodes and others from the gas between them. Even earlier, Joseph Fraunhofer had charted the dark lines in the spectrum of the sun, and had measured their wavelengths. But it was Angstrom who first identified some of these lines as corresponding to bright lines emitted by particular substances in the spark. Most importantly, he showed the red line of hydrogen, now known as H\a. In subsequent years, Angstrom found several more visible lines from hydrogen, and measured their wavelengths accurately. When W. Huggins and H. W. Vogel succeeded in photographing the spectra of stars in 1880, they found that these visible lines were part of a longer series extending into the ultraviolet. J. J. Balmer 5 in 1885 was able to reproduce the wavelengths of these lines by a formula, which we might write as

The safety of repositories for highly radioactive wastes

Abstract: 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.

Review of e + e - physics at PETRA

Abstract: A review is given of the experimental investigation of high-energy ${e}^{+}{e}^{\ensuremath{-}}$ 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 ${e}^{+}{e}^{\ensuremath{-}}$, ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, and ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$. The fermion pair production processes ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}F\overline{F}$, where the $F\overline{F}$ pair stands for either a lepton pair (${e}^{+}{e}^{\ensuremath{-}}$, ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, and ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$) or a quark-antiquark pair, are investigated to study the effects of the ${Z}^{\ensuremath{\circ}}$ 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 ${\ensuremath{\alpha}}_{s}$ are discussed.