Showing papers in "Reports on Progress in Physics in 1978"

Bell's theorem. Experimental tests and implications

Abstract: Bell's theorem represents a significant advance in understanding the conceptual foundations of quantum mechanics. The theorem shows that essentially all local theories of natural phenomena that are formulated within the framework of realism may be tested using a single experimental arrangement. Moreover, the predictions by those theories must significantly differ from those by quantum mechanics. Experimental results evidently refute the theorem's predictions for these theories and favour those of quantum mechanics. The conclusions are philosophically startling: either one must totally abandon the realistic philosophy of most working scientists, or dramatically revise out concept of space-time.

Non-linear properties of thermal convection

Abstract: Thermal convection in a layer heated form below is an exemplary case for the study of non-linear fluid dynamics and the transition to turbulence. An outline is given of the present knowledge of the simplest realisation of convection in a layer of fluid satisfying the Oberbeck-Boussinesq approximation. Non-linear properties such as the dependence of the heat transport on Rayleigh and Prandtl numbers and the stability properties of convection rolls are emphasised in the discussion. Whenever possible, theoretical results are compared with experimental observations. A section on convection in rotating systems has been included, but the influence of other additional physical effects such as magnetic fields, side wall geometry, etc., has not been considered.

Principles of dynamic nuclear polarisation

Abstract: After a general introduction, two sections are devoted to a survey of the theories of spin temperature and of nuclear relaxation in solids. This is followed by a short section on the so-called well-resolved solid effect, and a much more detailed one on DNP by thermal mixing in the non-linear low spin-temperature domain. One then analyses the various methods of measurement of the nuclear polarisation, as well as an indirect method of detecting the electronic resonance based on the existence of large nuclear polarisations. The final section describes briefly several applications of dynamic nuclear polarisation.

Decay theory of unstable quantum systems

Abstract: The present review article is aimed at a clear formulation of the basic problematics of the decay and at a description of the various recent attempts to solve this delicate problem, illustrating both their successes and limitations. The organisation of the article is as follows. After a short summary of the classical description of the decay, a detailed study of the quantum non-decay probability is given. Then the peculiar dynamical situation leading to the formation of an unstable system is identified and the formal and physical aspects of the process are discussed. A detailed analysis is then made of the preparation procedure of an unstable system which is shown to amount to a localisation of the decay fragments within a distance of the order of the range of the forces acting between them. After this the discussion of the general properties of the non-decay probability at all times is completed.

Electron spin resonance in the study of diamond

Abstract: The role of electron spin resonance in the study of both natural and synthetic diamond is reviewed. A brief survey of the physical significance of the constants in the spin Hamiltonian, as well as experimental technique, is given. The various nitrogen centres are discussed treating exchange-interaction, Jahn-Teller and relaxation effects associated with these centres. The spin Hamiltonian parameters of these centres are tabled and the results are discussed within the framework of the defect molecule approach. In conclusion, the correlation between optical effects and the ESR measurements in the case of four defect centres are discussed in some detail as this seems to be a powerful method of testing the various models suggested for the observed defects.

Magnetic monopoles in gauge field theories

Abstract: An account is given of the new insight into the theory of magnetic monopoles originating from the work of 't Hooft (1976) and Polyakov (1974). Their magnetic monopole, associated with the conventional electromagnetic gauge group U(1), occurs as a finite-energy smooth soliton solution to an SU(2) gauge theory. A precise picture of its internal structure, the values of its magnetic charge and its mass are obtained. These new developments bring together previously unrelated fields of study, namely the Dirac monopole (with point structure) and the Sine-Gordon soliton in two-dimensional space-time. Properties of more general monopoles, associated with large gauge groups now thought to be relevant in physics, are discussed. Based on this new viewpoint, conjectures can be made about a future quantum theory of monopoles.

Thermodynamics of black holes

Abstract: Black holes appear to conform to a very straightforward generalisation of standard laboratory thermodynamics. This generalised theory is examined in detail, and some concrete results are presented. The thermodynamic connection is based on Hawking's application of quantum theory to black holes, and the quantum aspects are described in detail from several standpoints, both heuristic and otherwise. The precise mechanism by which the black hole produces thermal radiation, its nature and origin, and the energetics of back-reaction on the hole are reviewed. The thermal states of quantum holes are also treated using the theory of thermal Green functions, and the entropy of the hole is shown to be related to the loss of information about the quantum states hidden behind the event horizon. Some related topics such as accelerated mirrors and observers in Minkowski space, super-radiance from rotating holes and the thermodynamics of general self-gravitating systems are also briefly discussed.

The electrical characterisation of semiconductors

Abstract: An important aspect of the large expansion in the development and production of solid-state devices has been the demand for more sophisticated techniques for determining the electrical properties of semiconductors, especially silicon and the III-V compounds. A very wide range of measurement techniques now exists and it is the purpose of this article to review those techniques which are in widespread use or which show promise for future application, and at a time for continuing innovation in this area, to indicate present trends and material problems which may arise in the near future.

Approaches to nuclear friction

Abstract: This review summarises the present knowledge of experimental information on frictional effects in nuclear reactions and describes the theoretical approaches in detail. The experimental evidence for nuclear friction stems from reactions where large-scale collective motion is involved, that is fission, deep inelastic heavy-ion scattering and giant excitations. The various theories and models employed are based on the concept that friction, damping and viscosity in nuclei are due to an irreversible flow of energy (and/or linear or angular momentum) from collective to intrinsic motion.

Magnetic properties of the actinide elements and their metallic compounds

Abstract: A review is given of the magnetic properties of metallic actinides, and of the relationship between their properties and their electronic structures. It is shown that the 5f electrons of light actinides often form into bands, rather than exist as localised electrons as is usually the case for the 4f electrons of metallic rare earths. The 5f bands form due to 5f-5f wavefunction overlap or to hybridisation with s, p or d electrons. As a result of the band formation, a wide variety of magnetic phenomena may occur. These range from non-magnetic transition-metal behaviour, to spin fluctuations and itinerant magnetism, as well as localised 5f electron magnetism.

Nuclear Coulomb energies

Abstract: The present status of the theory of Coulomb displacement energies, Delta Ec, of analogue states in nuclei (mirror nuclei in particular) is reviewed. In recent years, a considerable amount of investigation has been carried out on the problem of evaluating Delta Ec in order to resolve the persistent discrepancy between theory and experiment over a wide range of nuclei. The calculated values are found to be approximately 7% smaller than the experimental values. This discrepancy is known as the Okamoto-Nolen-Schiffer anomaly. The authors review in this work all possible correction terms which contribute more than 10 keV to Coulomb displacement energies, and also discuss and suggest the (possible) solution of the Coulomb energy problem in nuclei.

Incoherent scatter of radio waves from the ionosphere

Abstract: Systematic studies of the upper atmosphere by the so-called incoherent-scatter technique have now been conducted at a limited number of sites for more than a decade. The article reviews the history of the development of the technique, the basic theory involved and the atmospheric parameters which have been successfully measured, together with a summary of existing facilities and current developments.

Mechanisms of continental drift

Abstract: The authors review the evidence for continental drift, the mechanism by which it occurs, and the visible effects of the process.

Electron tunnelling spectroscopy

Abstract: A basic consequence of quantum mechanics makes possible measurable electrical currents between two conductors separated by a sufficiently thin uniform ( approximately 20 AA) insulator. The energy-spectroscopic information that can be, and has been, derived from careful measurement of the current-voltage (I-V) relation in such experiments is the subject of the present review. Giaever in 1960 demonstrated that when one member of the junction is superconducting, the differential conductance dI/dV as a function of bias energy eV directly measures the density of quasiparticle excitations of the superconductor. A second type of energy spectroscopy, that of inelastic excitation thresholds observed in the second derivative d2I/dV2, was demonstrated initially by Esaki (1958) in phonon spectroscopy of semiconductor tunnel diodes. Finally, the present state of the field is assessed and some guesses are hazarded as to those areas most likely to lead to important future advances.

Application of pulse radiolysis methods to study the reactions and structure of biomolecules

Abstract: To study reactions occurring on timescales faster than approximately 10-4-10-3 s, simple mixing of reagents is too slow and other methods of initiating reactions have to be used. Pulse radiolysis is a recent method of initiating fast reactions, and this review emphasises the versatility of this technique for studying events occurring between 10-11 and 102 s after energy absorption. The extent and timescale of radiation damage in dilute aqueous solutions are outlined and the facile manipulation of experimental conditions to isolate a single reactive species is illustrated. The basic requirements for the radiation pulse and detection system are discussed. The experimental techniques which have been used to monitor fast reactions following pulse radiolysis include optical absorption spectroscopy and light scattering, and time-resolved electrical conductivity, polarography, and electron spin resonance spectroscopy. Some applications of pulse radiolysis to study fast reactions of biologically important molecules are illustrated, including the kinetics and thermodynamics of redox processes and electron-transfer reactions, enzymic processes and structural effects which are revealed by physical or kinetic properties.

Atomic diffusion in semiconductors

Abstract: A simple description of atomic diffusion mechanisms in semiconductors is given, and then the meaning of the diffusion coefficient is outlined. Studies of diffusion in the elemental semiconductors Ge and Si are then described, beginning with self-diffusion, followed by impurity diffusion and concentrating the latter on Si in view of its commercial importance. Then the review looks at the diffusion phenomena found when two dopants are diffused sequentially, such as the effect of electric fields generated during diffusion, and the 'push-out' effect, and other interactions. Diffusion in the III-V semiconducting compounds is then briefly described. Finally, some speculation is made into the future importance of diffusion as a means of introducing dopants into semiconductors, in the context of present developments in semiconductor technology.

The measurement of solar magnetic fields

Abstract: Solar activity is basically caused by the interaction between magnetic fields, solar rotation and convective motions. Detailed mapping of the Sun's rapidly varying magnetic field helps one to understand the mechanisms of solar activity. Observations in recent years have revealed unexpected and intriguing properties of solar magnetic fields, the explanation of which has become a challenge to plasma physicists. The review deals primarily with how the Sun's magnetic field is measured, but it also includes a brief review of the present observational picture of the magnetic field, which is needed to understand the problems of how to properly interpret the observations.

The renormalisation group approach to scaling in physics

Abstract: The auth ors review the application of the renormalisation group to many-body problems, with particular emphasis on the scaling behaviour which may arise. An introductory section discusses the difficulties of calculating the many-body effects near a magnetic phase transition, and shows by a simple example how the renormalisation group may be used to predict scaling behaviour. A general discussion of scaling and universality follows, with a review of techniques for calculation in lattice models, and an outline of the approaches in field theory. The range of applications of these techniques is discussed.

Magnetic dipole transitions in atomic and particle physics: ions and psions

Abstract: Reviews the theory of magnetic dipole transitions (M1 transitions) in H and H-like ions, the history of the development of interest in the analogous transition 23S1 to 11S0+ gamma in He and He-like ions is sketched and the corresponding theory is developed. Experimental values for the lifetime of the 23S1 state are now available in the helium isoelectronic sequence. A systematic comparison of the theoretical and experimental values provides a new and stringent test for quantum electrodynamics. The spectacular discovery, in 1974, of the J/ psi and psi ' particles in high-energy proton-nucleus and electron-positron collisions is reviewed and the theoretical background is given for the far-reaching hypothesis that these particles can be regarded as 3S1 bound states of a 'charmed quark' and its antiparticle. The qualitative success of this assumption, resulting from the subsequent discovery of predicted other narrow resonances in the 3-4 GeV region coupled to the J/ psi and psi ' by one-photon emission, is described and the ideas of the so-called 'naive charmonium model' are outlined. The theory of the radiative transitions of bound states of two spin-1/2 particles is then developed in some detail, with special emphasis on magnetic dipole transitions. The theory is applied to the charmonium model and the great sensitivity of the relativistic M1 decays to the form of the relativistic interaction between the quark and the antiquark is illustrated. A brief survey is given of recent attempts to overcome the deficiencies of the model.

Applications of holography

Abstract: The current status of holography and its applications are discussed with some historical background to place the current activity in perspective. Basic types of holograms are described together with their properties. Applications in optical image formation include photography, microscopy, image storage and image replication. Acoustic, X-ray, electron beam and microwave holography are briefly commented upon. The second major area of application is contour generation and interferometry. Finally, the current status of holographic optical elements is assessed.

Evaluating the efficient of imaging processes

Abstract: This review describes the techniques which are used to evaluate the properties of the recorded image. These techniques are described from the viewpoints of Fourier analysis, quantum efficiency, and information theory. An analysis is made of the mechanisms of conventional silver halide processes which control their imaging efficiency and image quality. Some examples of the methods of image evaluation are given for practical problems involving X-ray images, electronographic cameras and image intensifiers in astronomy, and for the electrophotographic process.

The quark model

Abstract: The quark model is reviewed in a version which includes three colours and at least five quark flavours. Methods are discussed which are applicable to any number of flavours, provided the underlying symmetry is SU(n). The relation between quark bound states and hadron spectroscopy is discussed, and sum rules are given for baryon magnetic moments and hadron mass splittings. Quark-parton models of high-energy hadron-hadron and lepton-hadron scattering are treated. It is concluded that the quark model has been on the whole very successful in accounting for the properties and interactions of hadrons in a qualitative way.

Electron paramagnetic resonance from actinide elements

Abstract: Investigations of actinide ions using electron paramagnetic resonance (EPR) techniques have resulted in the acquisition of new information pertinent to the electronic and nuclear properties of these synthetically produced elements and their ions. The special experimental problems and concomitant techniques which have been employed in acquiring this new information are presented, along with more general information regarding those aspects of actinide technology which are applicable to disciplines other than electron resonance. The results of electron resonance investigations of the actinides are discussed and examined, with emphasis on comparisons between these results and those obtained for rare-earth ions. Problems which remain unresolved are presented and suggestions for possible future experiments are considered.

Gas dynamics with relaxation effects

Abstract: The way in which gas-phase chemical reactions and the excitation of internal modes of molecular energy storage can be collectively described as relaxation processes is explained. A set of equations which is suitable for a general account of gas flows with relaxation processes is exhibited. The dissipative-dispersive character of acoustic waves and the exploitation of the acoustic approximation in studies of flow problems are described. The effects of a non-equilibrium atmosphere on acoustic wave propagation, and the consequent possibility of exponential amplification of disturbances, is considered with reference to explosion and combustion phenomena, as well as a possible link with upper-atmosphere gravity waves. The analysis and explanation of shock-wave structure is briefly discussed using Hugoniot curves and Rayleigh lines. Gas flow in convergent-divergent nozzles, and multiphase flow phenomena are also considered.

Meteorological applications of radar

Abstract: Radar has been used by meteorologists for 30 years or so but it is only during the latter half of this period that the full measure of its versatility has come to be recognised. Operationally important techniques have been or are being developed to identify and track severe storms, to provide warning of tornadoes, to measure and forecast rainfall quantitatively, and to measure winds, turbulence and wind shear. At the same time research meteorologists are using specialised radar techniques to investigate many poorly understood aspects of atmospheric behaviour. This review highlights the strengths and limitations of radar as a tool for observing the atmosphere and attempts to provide a balanced view of its many applications in meteorology.

Materials for low-cost solar cells

Abstract: Photovoltaic materials are reviewed with regard to their possible use in systems that could provide very large amounts of electric power from the Sun before the end of the century. The key is taken to be the cost of the solar cells which are considered to be presented about two orders of magnitude too high. Only silicon, in single crystal or ribbon form, or CdS in thin-film form are thought to be sufficiently developed to permit their possible large-scale exploitation by the last decade of the century. Silicon is considered to have the advantage over CdS at present for large-scale use because of the higher performance levels and the broader existing technology base. CdS thin films are considered to have greater potential if selected improvements can be effected in design and performance, because of lower projected cost and the ease of automating manufacture.

Biophysics of photosynthesis

Abstract: A general review of the more physical aspects of photosynthesis is given. The subject is developed with reference to a time scale covering the various steps between photon capture and the appearance of the first stabilised chemical products. Details of the energy and electron transfer processes are considered and there is discussion of both the efficiency and mechanism for energy conversion.

A review of selected topics in magnetospheric physics

Abstract: These topics are: the magnetopause (topology and microscopic theories of structure, dynamical effects due to interplanetary magnetic fields and solar-wind pressure changes), the plasmasphere (topology, theories of origin, time variations, plasmaphysical processes), the proton radiation belt (origin, theory of particle motion in the geomagnetic field using invariants), wave-particle interaction (simplified theory, experimental verification), the electron radiation belt (origin, transport, losses), excitation of waves (velocity space instabilities, applications and experimental verification), aurorae (origin, plasmaphysical effects associated with barium cloud releases in the ionosphere), the plasmamantle and the plasmasheet (topology, possible connection between the two phenomena), and finally a brief overview of magnetospheres in the Solar System which have been accessible to 'in situ' investigation using space probes.

Recent developments in the spectroscopy of small molecules

Abstract: A short review is given of the techniques in which the spectrum of the molecule is simplified for easier interpretation, or recorded with a higher sensitivity of resolution than in usual absorption or emission spectroscopy. The second part of this article is devoted to techniques in which not only the energy of the molecular states are measured but also other characteristics, such as lifetimes, Lande factors, chemical shifts, collision cross section, etc. The so-called 'optical pumping' techniques are also described. The last two sections are devoted to examples of applications of these techniques. A special emphasis is laid on energy levels close to a limit of dissociation and to molecular predissociation. It is shown that optical pumping makes it possible to study very weak predissociation effects which are otherwise completely unseen and are probably extremely frequent in simple excited molecules.