Showing papers in "Reports on Progress in Physics in 1971"

Interactions between ions with orbital angular momentum in insulators

Abstract: Until recently most interest in interactions between paramagnetic ions has centred upon ions with quenched orbital angular momentum, for which exchange has the isotropic Heisenberg form JS1.S2 and the major anisotropic terms are due to magnetic dipole-dipole interaction. The interactions for ions with unquenched orbital angular momentum are quite different; exchange is anisotropic, and in addition to exchange and magnetic multipole interactions, there are sometimes sizeable anisotropic contributions from electric multipole interactions and virtual phonon exchange. The qualitative theory of these interactions has been developed and is summarized herein, and the problems in making quantitative theories are discussed. A large number of experimental measurements have been made, mainly on rare earth ions, but several on Co2+, using electron paramagnetic resonance pair spectra and other techniques. The experimental methods and the data obtained are described. The difficulties of interpreting the mass of data are discussed, and the successful identification of particular mechanisms is critically explained. So far the only definite example of electric multipole interaction is for Ce3+ ions in LaCl3 and LaBr3. Virtual phonon exchange is highly likely to be the principal nondipolar interaction in UO2, rare-earth ethyl sulphates and Pr3+ in LaCl3. Most measured nondipolar interactions are due to exchange.

Structural aspects of the ice-water system

Abstract: A review is given of the present state of structural knowledge about the water molecule, small molecular clusters, the various stable and metastable polymorphs of ice and liquid water. Mention is also made of `polywater' and of surface and interfacial structures. The discussion is restricted primarily to structural matters and the emphasis is upon work done since 1965. This review was completed in October 1971.

Electron spin polarization

Abstract: The production and detection of electron polarization are discussed in the context of low-energy collision phenomena. After a short summary of the history of the subject, the formalism of Stokes parameters is reviewed. Physical processes are surveyed in the following sequence: elastic collisions with spin zero targets (Mott scattering); spin exchange in elastic scattering from spin one-half targets; spin-orbit coupling effects and spin exchange in inelastic electron-atom collisions; photoionization; collisional ionization in atom-atom interactions. The phenomena are described in terms of the algebraic formalism developed in the earlier part of the paper and brief descriptions of experiments illustrate the methods used and the results obtained in testing theoretical predictions. This review was completed in November 1971.

Bragg reflection x ray optics

Abstract: After a brief summary of the properties of x rays and of materials in the x ray frequency range, the ray optics and microscopic wavefield optics of Bragg reflection in perfect crystals are reviewed. Particular attention is given to the phase relationships between fields, to the spatial modulation of wavefields and to the role of the structure amplitude in determining the position of wavefields in the crystal lattice. A ray-optical description of x ray interferometers cut from blocks of perfect crystal is developed in some detail and this leads on to a description of some of the many uses of x ray interferometers, from measurements of phase shifts to the characterization of crystal defects and new absolute measurements of lattice spacings. Further developments of x ray optical angle and length standards and of resonators for x ray lasers are discussed in the concluding sections.

Soft x ray spectroscopy and the electronic structure of solids

Abstract: Electronic transition processes accompanying the emission and absorption of soft x radiation are discussed, and elementary band structure relevant to the interpretation of soft x ray spectra is reviewed. The importance of many-body effects is emphasized and the relative importance of Auger and instrumental broadening examined. A full description of experimental techniques is given, including x ray sources, spectrometer and dispersion geometry, detection and recording methods. The correction of spectra, for their interpretation in terms of band structure, is discussed and methods of debroadening spectra are examined. A complete survey is made of experimental spectra for the pure elements, related to their electronic structure; and the role of soft x ray emission and absorption data in the development of the theories of alloys and chemical bonding is described.

The nuclear optical model

Abstract: The main features of the nuclear optical model are reviewed, with particular attention to current growth points. The determination of the form and strength of the nucleon-nucleus interaction by calculation from the nucleon-nucleon interaction is described and the results compared with those obtained phenomenologically. The potential is applied to analyse single-particle bound states, and it is shown how this can give its isospin and state dependence. Even when it is used simply to calculate the elastic scattering, the optical model must be established in the framework of nuclear reaction theory, and allowance made for the effects of all the reaction channels, and in particular for the compound nucleus processes and the associated fluctuations in the cross sections. The model can then be applied to analyse data on the elastic scattering of nucleons, deuterons, helions, tritons and α particles by nuclei, and this enables its parameters to be determined more precisely and its range of validity to be investigated. The nucleon data is so extensive that it is possible to study several small terms in the potential, in particular the isospin term and the nuclear spin term, in addition to the familiar spin-orbit term. The scattering of the composite particles shows several apparently anomalous features that on investigation yield additional information on nuclear structure. The optical model may be extended to give the inelastic as well as the elastic scattering, and this makes it possible to analyse many apparently complicated phenomena. The application of the results of optical model analyses to the calculation of compound nucleus cross sections is also discussed, together with the use of optical model wavefunctions in calculations of direct reaction cross sections. This review was completed in August 1971.

The metallurgical enhancement of type II superconductors

Abstract: The application of superconductivity to electrical engineering requires the availability of type II superconductors with high critical temperatures, high upper critical fields, high critical current densities, high stability and low AC losses. These properties are related to the composition, metallurgical treatment and structure of superconducting materials. The best results achieved to date are related to the current theories, and, insofar as is possible, predictions are given as to the likely upper limits of these quantities. Methods by which improved materials may be made are indicated, and finally the situation with regard to currently available commercial materials is briefly summarized. This review was completed in October 1971.

Electrical conduction in thin metallic, dielectric and metallic-dielectric films

Abstract: This review is concerned with the structure and resultant electrical conduction properties of three types of film, defined as metallic, dielectric and discontinuous metallic; it is shown that the behaviour of the last one can be discussed as extensions of the first two. The review commences with a brief discussion of the type of structure that can be observed. In the light of this, both metallic and dielectric films are considered in detail and the current limiting phenomena in each discussed. Island-structure films (i.e. discontinuous metallic) are then considered and it is shown how dielectric theory can be extended to give an understanding of the mechanisms by which electrons get from one island to the next. A survey is made of the preparation techniques involved, and also of the devices that depend on electrical conduction. Thus it is shown how practical components such as capacitors, resistors, conductors, and to a certain extent active devices, are dependent on metallic or dielectric conduction processes.

Direct methods in crystallography

Abstract: The basic problem of structure determination in x ray crystallography is described together with the methods available up to 1948 - primarily the Patterson, isomorphous replacement and heavy-atom methods. In 1948 inequality relationships were derived by Harker and Kasper but these could only give a solution for very simple structures. A breakthrough came in 1952 with the introduction of the triple-product sign relationship, which could only be applied to centrosymmetric structures. In 1955 Cochran showed that the general-valued phases for non-centrosymmetric structures were also related. After several years, during which a number of more-or-less involved methods were proposed for applying sign relationships, it was shown by Karle and Karle that a comparatively simple approach, the symbolic addition method, could be applied even to complex structures. A variant of the method is also applicable to non-centrosymmetric structures. A different technique of applying phase relationships, developed by Germain, Main and Woolfson has been fully automated and quite complex structures can now be solved with computer programmes which require as input only unprocessed observed data. Some future trends in the development of direct methods may be seen in present work in the use of inequalities involving determinants of high rank and also in methods which attempt to derive values of certain types of structure-invariant quantities.

Glaciers and other large ice masses

Abstract: Substantial progress has been made in recent years with the help of new techniques for the investigation of glaciers and other large ice masses in four major study areas: Flow properties of ice. These can be considered as arising from processes on three distinct scales: (a) lattice-scale processes responsible for the flow characteristics of monocrystals as functions of temperature and stress; (b) crystal-scale processes responsible for the flow characteristics of crystal aggregates and for crystal orientation `fabrics' associated with different stress states and histories; (c) large-scale processes of ice flow and deformation as revealed by vertical, longitudinal and transverse velocity gradients, in the contraction of ice boreholes and tunnels, and through deformations arising when the ice flows over bedrock irregularities. These large-scale processes can be described by relationships of the form dot aboveij = λσ primeij where dot aboveij and σ primeij are the components of the steady-state strain rate and stress deviator tensors and λ is a function of the stress invariants and the properties of the ice. The crystal scale processes have come to be regarded as especially relevant for the understanding of natural ice flow and feature in current laboratory experiments, reviewed in § 2. The large-scale processes form the objective of current field work, reviewed in § 3, where the emphasis is on deep drilling and on the use of radar for measuring both distance and strain along the surface and ice thickness and structure in the vertical. Micrometeorological and isotope studies used to establish current and past glaciological `regimes' are also considered. The dynamics of large ice masses, which is governed by their flow laws and the equations of motion without acceleration terms, which are invariably small compared with those involving the stress deviator σ primeij and the gravitational forces ρgi. Mathematically the problem is to find velocities and displacements by integrating equations of the form partial differential σij/partial differential xi = ρgi with prescribed values of the geometry, velocities and velocity gradients (strain rates) at the boundaries. The boundaries define the three major types of large ice masses, namely glaciers which have rigid boundaries on both sides and at their base, ice shelves which are bounded on two sides but mostly free-floating and hence not subject to shear stresses at the base, and ice caps which are bounded at their base only. Current work in this area, reviewed in § 4, is concerned especially with variations along the direction of flow which account for the detailed surface shapes of large ice masses and permit to deduce their effective flow law parameters. They also are believed to play a crucial role in the processes of glacier sliding and surging which occupy the centre of current discussions on the dynamics of glaciers and ice sheets. For the latter the ice temperature and its drastic effects on ice flow introduce a major complication and create a separate study area. Thermodynamics of polar ice masses. The temperature of large ice masses is controlled by heat conduction in response to boundary heat fluxes and strain heating inside the ice. Both depend on the ice flow in a feedback complicated near the bedrock by possible phase changes. On the other hand the heat fluxes arising at the glacier surface from varied atmospheric processes, while forming a wide study area in their own right, are mostly absorbed in a shallow surface layer penetrated by longer-term `climatic' variations only. Current thermodynamic studies, reviewed in § 5, have the aim of clarifying these effects to the point of filtering out the residue of climatic temperature variations. This is gradually being achieved by matching actual ice temperature profiles, measured in boreholes or estimated from the isotope-depth distribution in ice cores, with theoretical and computer solutions of the temperature conduction equation. Long-term changes in large ice masses. Flow law, dynamic and thermodynamic results can be combined with mass continuity and mass balance considerations to determine how large ice masses react to changes in climatic conditions. Current work, reviewed in § 6, is especially concerned with the present trends of the large polar ice sheets and with the possibilities of modelling glaciers and ice sheets by computer. Although still at an early stage and taxing the capacity of present-generation machines, this approach promises to clarify the complex temperature-velocity feedback operating in polar ice sheets and the melt-water processes in glacier sliding and surging. This makes it possible to outline in the final section (7) the dynamic studies which may be expected to bring further substantial progress in the understanding of glaciers and other large ice masses.

Final-state interactions

Abstract: Final-state interactions are common ground between nuclear and particle physics. They are fruitful because they enable us to infer properties of two (or more)-body systems, or very short-lived `particles' which are otherwise impossible to reach. There are of course numerous difficulties associated with their study, both experimental and theoretical, but considerable understanding has been accomplished in recent years and a survey is presented here. The study of these interactions is also related further to interesting aspects of the two-body interaction not obtainable through elastic scattering, like off-the-energy shell transitions, aspects of quasi-free scattering, and specific three-body effects, such as three-body forces.

Regge poles and elementary particles

Abstract: This is a review of Regge theory that is intended both as an introduction to its main features and as a survey of its relation to experimental results on the strong interactions of elementary particles. It includes a survey of the experimental and theoretical background that is required for an understanding of Regge theory. The central part of the review describes the basic ideas of Regge theory and the application of Regge pole models to high-energy collisions of elementary particles. The review concludes with a brief account of extensions of Regge theory that give the Veneziano representation and that give some features of many-particle production in high-energy collisions.

The light of the night sky and the interplanetary medium

Abstract: Four of the components of the light of the night sky are described; the aurora is not included. The early history of star counting to determine the brightness of the integrated starlight is reviewed and suggestions made for improving upon this work and extending it to longer wavelengths. Work on the zodiacal light since 1967 is described in detail. The most recent observations of surface brightness, polarization and the spectrum are given and emphasis is laid on the motions and origin of the interplanetary dust particles. A survey is made of the known emissions of the night airglow with reference to their emitting heights, their variation with time and photochemistry. Particular attention is paid to the problem of the nightglow continuum and the diffuse Lyman-α radiation from the night sky is considered in some detail. Finally, brief mention is made of the diffuse galactic radiation as a possible contributor to the light of the night sky. This review was completed in August 1971.

The theory of weak interactions in particle physics

Abstract: We commence with a summary of the major theoretical ideas which have dominated the field of weak interactions during the last seven years. The main body of the review assumes that the Hamiltonian responsible for weak interactions has the traditional CP-invariant current-current form and that it is a good approximation to work to first order in the weak coupling constant; we then make a quantitative comparison of the resulting theoretical predictions with the experimental data. In the main we find widespread consistency, although the theory lacks predictive power in certain areas where the data seems to be quite simple. The serious inadequacies of the theory are exposed when we consider higher-order terms in the perturbation series; the theory predicts that these diverge unrenormalizably while the data indicates that they are very small. Finally, we review the experimental data on CP-invariant processes and discuss the models which have been advanced to explain this phenomenon.

Theories of star formation

Abstract: A well defined theory of star formation does not yet exist. A serious deficiency therefore remains in current theories of the structure and evolution of stars. Since stars must be forming at the present phase of Galactic evolution, it is pertinent to investigate what conditions favour star formation. Observational evidence for the pre-main sequence phase of stellar evolution is entirely absent apart from that obtained about a single class of proto-stellar objects - the T-Tauri stars. Most theories of star formation require stars to form from the interstellar gas. The existing knowledge of the interstellar gas is not sufficient to define the initial conditions for star formation precisely. However, recent theoretical and observational studies are forcing a radical revision of interstellar concepts. Theoretical studies suggest that interstellar gas clouds may well be transient phenomena and that continuous variation of interstellar conditions may be expected. The observational studies have shown that the previously accepted enumeration of interstellar species may well be far from complete. Four major groups of star formation theory are identified. These deal with formation by collapse under gravity, by random accretion, by condensation, and by processes associated with the activity of galactic nuclei. Considerable attention is given to the theory of collapse under gravity since this is regarded as the major theory of star formation on the ground that its conceptional framework is better defined than that of the others. It is also a theory capable of further specific investigation. It is, however, pointed out that no theory of star formation can be regarded as definitive and that each group of theories has significant contributions to make to a subject where the essential problems - though simple in conception - may not be well posed. The recent numerical investigations of star formation within the context of star formation through collapse under gravity are summarized. These new investigations have shown that one of the basic problems of collapse under gravity (namely the separation of a collapsing region of stellar mass from a very much larger parent cloud or fragmentation) may no longer be a problem in that fragmentation may be inevitable within non-uniform gas clouds collapsing under gravity. The numerical studies have shown that collapse under gravity takes a characteristic form. Such studies have enhanced the prospects for a renewed attack on the problem of collapse under gravity, in view of the possibility of relaxation of the severe constraint imposed by the assumption of spherical symmetry. Numerical studies, by taking account of atomic processes and the detailed structures of gas clouds, seem likely to lead to a clearer definition of the conditions necessary for star formation and to give form to a detailed theory of star formation replacing the more speculative attempts to do so which are reviewed here.