Showing papers in "Reports on Progress in Physics in 1970"

Electrical phenomena in amorphous oxide films

Abstract: Thin layers of insulators, from 100 A to 20 000 A thick, have a number of interesting electrical properties. These properties include forming, which is a profound and essentially permanent change produced by an electrical field, and, after forming, differential negative resistance together with switching and memory phenomena. Electroluminescence and electron emission also occur. These features are shown by most amorphous or microcrystalline insulators of the appropriate thickness, and are particularly clearly shown by the oxides, such as Al2O3, Ta2O5 and SiO. In our review we survey the observed properties of metal-insulator-metal devices involving such insulating layers, and discuss the mechanisms which have been proposed for their operation. Further, since the unusual properties may be technologically important, we outline some of the possible uses of these devices and the relevance of the phenomena to the mechanism of corrosion of certain metals.

Optical phonons of small crystals

Abstract: The long wavelength optical phonons of ionic crystals give rise to dipole-dipole forces, whose long range nature causes the vibrations to depend on the size and shape of the crystal sample. This dependence affects significantly most spectral properties of all crystals whose dimensions are of the order of or shorter than the wavelength of reststrahlen (characteristically several tens of micrometres). In experimental work in which the infrared properties of ionic crystals were examined on small samples, in powder form or in colloidal suspension or in the shape of thin layers, the peculiarities due to size and shape were not always properly recognized. The infrared frequencies of the material which are derived with disregard to these peculiarities may be in error by some tens of wavenumbers. In the experimental section of the review we interpret the general characteristics of the experimental spectra and analyse in detail some representative infrared measurements in the light of the theory. The theory of optical vibrations which takes account of the finiteness of the specimen is formulated firstly in general terms and then by special reference to samples which have one, two or three dimensions short (slab, cylinder and sphere-like geometries). Synthetic spectra are drawn whose characteristic features are interpreted in terms of bulk and surface modes. In the theory retardation effects, i.e. the coupling between lattice and electromagnetic waves, are also included, and the quantized modes are combinations of these, i.e. polaritons. For tiny crystallites of such size that the characteristic infrared radiation wavelength is much larger than the sample size, retardation effects can be neglected. The theory becomes much simpler and the spectrum sharper. Some of the absorption peaks are due to optical surface modes, and their positions are simply related to the characteristic shapes of the crystallites. The theory is so presented as to provide a practical aid in the correlation of spectra with sample shape. The consequences on the spectra of other, less common variables of experiments, for example, the refractive index of the environments, are also calculated. The role of optical surface modes in Raman - and electron - scattering is then discussed. Simple geometrical arrangements of small sized crystals enable the spectra of surface modes to be scanned.

Infrared and microwave magnetoplasma effects in semiconductors

Abstract: The interaction of electromagnetic waves with free-carrier plasmas in semiconductors and semimetals is analysed, with particular emphasis on microwave and infrared effects arising in the presence of external magnetic fields. The general frequency- and field-dependent dielectric tensor is initially developed for a single isotropic band via the Boltzmann equation in the local approximation. Solution of the electromagnetic wave equation in the tensor medium yields two normal modes, whose complex propagation constants and polarizations are determined by the microscopic medium parameters as well as the angle θ between wave vector q and applied magnetic field B0. We first consider wave propagation in the lossless (collisionless) limit, and concentrate on the properties of normal modes in the Faraday (q parallel B0) and Voigt (q perpendicular B0) geometries. Essential features of the wave interaction with the medium are conveniently set forth in `contour maps' which display zeroes and infinities of the lossless dielectric constants, indicating resonances, dielectric anomalies and black-out regions for each mode of propagation over a wide range of frequencies and fields. Losses are then explicitly incorporated into the dielectric tensor, and their effect discussed in some detail. A number of experimental examples associated with the single band model are presented, including free-carrier absorption, cyclotron resonance, Faraday and Voigt effects, helicon waves, magnetoplasma reflection and small-particle effects. The presentation is subsequently generalized to more complicated systems. The dielectric tensor is developed for a multiple-carrier plasma and for anisotropic bands. A variety of new effects is seen to arise, including hybrid resonances, tilted-orbit resonances and Alfven wave propagation. `Contour maps' of appropriate lossless dielectric constants are again found convenient in discussing these new features. Effects of spatial dispersion (nonlocal phenomena) and of orbital quantization on the high-frequency response of a free-carrier system are then considered. The contribution of the polar lattice, which gives rise to a vast array of coupled magnetoplasma-phonon modes, is finally examined. A further elaboration of certain topics arising in this review is presented in an appendix, including a detailed discussion of electron dynamics, the `polariton' formalism, fundamentals of Kramers-Kronig analysis and magneto-optics of birefringent media.

The kinetics of migration of point defects to dislocations

Abstract: In this review we discuss the various ways in which dislocations interact with and provide sites for the segregation of point defects. Since there is, in general, a long-range interaction between a point defect and a dislocation, the kinetics of the migration of the point defects to the dislocations will depend on the form and magnitude of this interaction. The origin of the interaction and the various spatial forms it can take are therefore discussed in some detail and related to the resulting kinetics. The early stages of this segregation process can be explained and the kinetics satisfactorily correlated with experimental observations without a detailed discussion of the physical situation prevailing at the dislocation cores. However, in the later stages of the segregation process the observed kinetics very often show a strong departure from simple first order and, in this case, an explanation requires a discussion of the detailed point-defect behaviour within the dislocation core region. Various physically plausible dislocation core situations are discussed and related, where possible, to pertinent experimental observations.

Dislocations and stacking faults

Abstract: The properties of linear defects in crystals (dislocation lines) and of planar defects (stacking faults) are important in almost every branch of solid state physics. This article is intended to give a comprehensive introduction to dislocation theory for the physicist who is not a specialist in crystal plasticity. It begins with a survey of the established theory relating to the geometrical and topological properties of dislocations, the elastic theory of dislocations in a continuum, the atomistic or core properties of dislocations, and the dynamics of moving dislocations. General methods for finding the elastic field of an arbitrary dislocation loop are outlined, and the results of some recent calculations using anisotropic elasticity are summarized. These include the prediction, partially confirmed by experiment, that dislocations in certain ranges of orientation may have negative line tension. The current importance of atomistic calculations of core structure and related problems is emphasized, and the methods available for these calculations are discussed. More detailed descriptions of dislocation and stacking fault configurations are given for some of the common crystal structures, and recent work on complex defects resulting from vacancy aggregation in close-packed structures is included. The experimental and theoretical evidence for the recent conclusion that screw dislocations in body-centred cubic metals have an asymmetric core is also reviewed. The rather controversial theory of thermally activated dislocation motion is described in a separate section, and examples are given of the application of this theory to various models of the obstacles encountered by moving dislocations. The final section is concerned with the theory of dislocations in grain boundaries and interphase boundaries, and includes the concept of the surface dislocation tensor.

The metal-nonmetal transition

Abstract: An account is given of some of the mechanisms which can lead to a transition from a metallic to a nonmetallic state, when a parameter such as the interatomic distance or temperature is varied. The simplest of these is the band overlap or Wilson transition, which occurs when a conduction band overlaps a valence band; this is discussed in § 2 and for noncrystalline systems in § 15. These transitions can be described in the Hartree-Fock approximation. If the insulating property is due essentially to the repulsion between electrons (e2/r12), then the nonmetallic state is normally antiferromagnetic. The possibility of describing it by normal band theory with a spin-dependent potential is discussed in § 5. It is emphasized that antiferromagnetism can exist in the metallic state, and that the conditions for the appearance of conductivity and the disappearance of antiferromagnetism are not always the same. The nonmetallic behaviour, that is the existence of a Hubbard gap, normally persists above the Neel temperature (as in NiO), as does the gap in some metals, but not in chromium. Disordered systems, such as doped semiconductors, are discussed; here in the metallic state we suggest that the two Hubbard bands overlap, and that the metal-nonmetal transition can be described as an Anderson transition (§ 16). This model gives a simple explanation of the negative magnetoresistance. In some materials a transition occurs which does not involve magnetic moments or structural change, and for d bands, following Halperin and Rice, and Weger, we introduce the concept of an `orbital orientation wave' in degenerate d bands (§§ 5, 19.3, 19.4). A number of specific materials are discussed.

Optical applications of dielectric thin films

Abstract: This review is divided into seven sections entitled "Introduction", "Basic theoretical framework", "Readily available materials", "Applications", "Performance and limitations of narrow band interference filters", "Problems in the preparation of optical coatings" and "Probable future developments". In the introductory section the scope of the review is presented and a survey is given of already published reviews which are relevant to the subject. A brief history of the subject is also presented and the reasons for continuing interest in it are discussed. The introduction is concluded by mention of problems which are currently engaging the attention of workers in the field. In the next section the matrix theory relating the optical properties of single or multilayer dielectric thin films to the basic film parameters (refractive index and thickness) is developed with particular reference to the assumptions that are made in establishing this theoretical framework. It is emphasized that the conditions implied in these assumptions are not always satisfied in practice, and that differences between observed and predicted optical performance are therefore to be expected. In addition, the basic matrix formulation is used to derive other useful expressions for the optical properties of multilayers, e.g. the reiterative amplitude reflectance formula usually associated with multiple-beam calculations, the generalized Airy formula associated with the `two effective interfaces' approach, and the method of equivalent layers for symmetrical systems. The calculation of Poynting flux and the concept of `potential transmittance' are also discussed. The section on readily available materials consists basically of a table listing the relevant physical properties of commercially available materials suitable for the vacuum deposition of dielectric thin films. A brief description of such practical detail is necessary as a prelude to the following section. In the section on applications, details are given of the design and application of filters consisting of single or multilayer dielectric thin films. The term `filters' is used here in its widest sense and the term includes antireflection coatings, mirrors, edge filters, beam splitters and narrow band filters. Magneto-optical applications of dielectric films are also described briefly. The fifth section is devoted to a more detailed examination of the properties of narrow band interference filters, specifically their performance and limitations. This choice of emphasis is made for two reasons. Firstly, the advantage of narrow band interference filters over other means of spectral selection such as prism or grating monochromators is marked and can be discussed on a quantitative basis. Secondly, the structural `defects' of layers which afflict all thin film devices produce the most easily measurable effects in narrow band filters. These effects are discussed under the subheading "Limitations". The section on problems in the preparation of optical coatings is concerned with the precise control of the optical thickness of individual films, since this is usually crucial to the performance of the complete filter. This subject is divided into two separate parts, one concerned with the uniformity of thickness of the film over a relatively large substrate area, and the other with the control of optical thickness during the deposition of the film, commonly referred to as `thickness monitoring'. In both cases the limitations of the available techniques are emphasized. The review is concluded with comments on probable future developments of the subject.

Quantization of general relativity

Abstract: Several theories of quantum general relativity have been formulated. Each of these theories has elements of arbitrariness and ambiguity as well as technical difficulty which make it less than satisfactory. However, the construction of these theories has revealed much about the structure of general relativity as a dynamical system and has spurred the development of new approaches to quantum theory. Canonical, sum-over-histories and source theory approaches to the quantization of the gravitational field in the absence of matter are reviewed in terms of a unified notation. Discussions of quantum theory and general relativity are provided to make the review self-contained for readers with a general physics background. The quantization of open space-time geometries (graviton scattering) is treated in sufficient detail to reveal the basic mathematical structure of the formalism. The quantization of closed universes (quantum cosmology) is discussed with particular attention to the superspace concept and the construction of finite-dimensional model quantum theories. Superspace, the domain manifold of the quantum state functional in general relativity, is also discussed separately.

The afterglow and energy transfer mechanisms of active nitrogen

Abstract: The complex spectrum of active nitrogen is reviewed from the vacuum ultraviolet to the infrared. Its variations with change of pressure, temperature and additive are described. Many of the mechanisms which have been individually proposed to account for various spectral features of the afterglow are also reviewed. Some of these mechanisms, suitably modified, together with some new suggestions, are shown to provide a comprehensive and self-consistent model that accounts for all of the complex spectroscopic phenomena of active nitrogen. Collision-induced radiationless transitions are shown to be very significant in the afterglow, particularly when relevant selection rules are taken into account. The B 3Πg (ν = 12), B prime 3Σu− (ν = 8), a 1Πg (ν = 5) and a prime 1Σu− (ν = 7) levels are populated by collision-induced radiationless transitions from the shallow 5Σg+ state. The population distributions among the B 3Πg (ν = 12−9), B prime 3Σu− (ν = 8−4), a 1Πg (ν = 5−0) and a prime 1Σu− (ν = 7−0) levels are then each determined by the competing decay processes of spontaneous radiation, and collision-induced vibrational relaxation and electronic quenching. The B 3Πg (ν = 6) level is populated by collision-induced radiationless transition from the A 3Σu+ (ν = 16) level. The B 3Πg (ν = 4−0) levels are populated by transitions from B prime 3Σu− (ν = 8−4) and 3Δu (ν = 7−0). In the cooled afterglow the C 3Πu (ν = 4) level is populated by an inverse predissociation of N(4S) and N(2D) atoms; the C 3Πu (ν = 0) level is populated by collision-induced radiationless transitions from 5Πu (ν = 0). The 5Πu (ν = 0) level lies a little below, not above, the C 3Πu (ν = 1) level. The 5Σg+ (ν = 0) level lies 950 ± 50 cm−1 below the N(4S) + N(4S) dissociation limit.

Vibration analysis by holography

Abstract: This article covers aspects of holography useful to practical workers in the field of vibrational analysis. After an introduction, it is divided into four main sections, the first of which (§ 2) deals with the basic theory and practice of holography. Several ideas which may help to solve particular problems, such as the illumination of long objects, are described, holographic recording media are briefly covered, and the section is concluded with a description of some techniques for the maximum utilization of the available coherent light. The second main section (§ 3) deals exclusively with the technique of stroboscopic holography - its theory and practice - and variations in the basic technique are described with the advantages of `live' and `frozen fringe' techniques being discussed in detail. Section 4 covers time-averaged holography in similar detail, with particular attention being given to some of the many modifications to the basic technique which have been devised to overcome some of its disadvantages. The section concludes with a typical utilization of the method of time-averaged holography applied to the study of plate vibrations. The final section (§ 5) covers one or two techniques for altering the fringe sensitivity of the stroboscopic and time-averaged methods and also describes several related methods of vibrational analysis which may have advantages over the holographic system in some experimental situations.

The three-body problem in nuclear physics

Abstract: In this article the author has attempted to state and explain the nature of the three-body problem in nuclear physics. Its solution is fundamental to a knowledge of the precise form of the nucleon-nucleon interaction. It is shown that final state interaction and nucleon-nucleon bremsstrahlung experiments can yield invaluable information on the two-body system, and can enable us to distinguish between possible models for the two-body potential. This kind of two-body information is available only from three-body experiments. Great advances in the theoretical understanding of, and in our ability to handle, the three-body system have been made since the pioneer work of Faddeev and these are discussed in the text. The experimental state of the art is reviewed in some detail, and attention is drawn to those experiments which would appear to be the most rewarding at the present time. It is not yet possible to say the extent to which specifically three-body forces are relevant to the solution of the three-body problem.

Group theory and topology in solid state physics

Abstract: Firstly, various formal concepts of finite group theory are discussed with particular reference to their use in recent work on solid state theory. The pictorial viewpoint is stressed wherever possible throughout the discussion. Groups of operators are treated, including the anti-unitary operators involved in the magnetic groups. Next, the use of group theory in conjunction with matrix methods is discussed, and this leads to the treatment of equivalent operators. An account is given of several methods used in the many-electron problem, including a treatment of second quantization from a group theoretical viewpoint. The class sum operator approach is outlined as a means of linking the group theoretical approach and the traditional Dirac approach to quantum mechanics. Some topics from crystal field theory and electronic band theory are treated as illustrations of the general principles, and some recent work in these two fields is reviewed. The basic terminology of graph theory is then given, and several applications to solid state theory are treated. Some topics which involve the fixed points and indices of mappings are discussed, as are the similarities and differences between the theory of one-, two- and three-dimensional lattices. An account is given of the effect of periodic boundary conditions in reciprocal space on the dispersion curves for lattice vibrations, with particular reference to the theory of critical points in the density of states function. Finally, a survey is given of experimental methods which employ strong magnetic fields to investigate the topology of the Fermi surface in metals.

The nature of bonds in paper and the behaviour of paper under mechanical strain

Abstract: Work done during the past twenty years on the physics of fibre bonds in paper and the behaviour of paper under mechanical strain is reviewed. The first major section of the article illustrates the chemical nature of the cohesion in paper and reports the results of direct observations on fibre-to-fibre bonds. A statistical theory of the geometry of fibrous networks is shown to be a basis for the characterization of fibre bonding in paper. The second major section is concerned with the mechanical behaviour of paper. This is shown to be a complex subject because paper exhibits almost every known rheological property. Direct, conventional rheological methods, molecular theories and the concept of elastic networks are discussed in this context. Further complications arise from any changes in the water content of the environment. Such effects are conveniently thought of as a competitive process wherein cellulose and water compete for hydrogen bonds for their hydroxyl groups. Not least among the rheological phenomena that paper exhibits is mechanical fracture. This is a discontinuous process that is found to be intimately connected with the mechanical work done on the specimen and is principally governed by its geometrical distortion.

The medical applications of ultrasonics

Abstract: Ultrasonic techniques are complementary to other physical methods used in surgery, therapy and diagnosis. Most medical applications of ultrasound are based on the properties of longitudinal waves in the frequency range 1-15 MHz. The relevant plane-wave equations are given, and the methods of production, the form and the measurement of the ultrasonic field are discussed. Ultrasonic waves travel at similar velocities (about 1500 m s−1) in most biological tissues, and are absorbed at a rate of about 1 dB cm−1 MHz−1. Absorption occurs chiefly due to relaxation processes. It leads to thermal effects in biological systems. Mechanical effects, such as streaming and cavitation, are also important in certain situations, particularly at low frequencies. Highly focused ultrasound is used in neurosurgery; it is the only method for producing trackless damage deep in the brain. In vestibular surgery, ultrasonic irradiation is used routinely for the treatment of Meniere's disease; it can alleviate the symptoms without damage to the hearing. Therapeutic applications include the treatment in physiotherapy of various soft-tissue ailments, and the production of aerosols for inhalation. The examination of soft tissues is possible both by pulse-echo and continuous-wave techniques. Ultrasonic diagnosis provides information about the position and extent of characteristic impedance discontinuities; this information cannot be obtained directly by any other method. One- and two-dimensional displays, and time-position wave forms, are produced by pulse-echo techniques. Continuous-wave techniques include those based on the Doppler frequency shift of ultrasound reflected by moving structures.

Flux compression theories

Abstract: The simplest nontrivial theories of flux compression are worked out in complete detail. The approach is phenomenological and sufficiently general to cover both explosive and electromagnetic flux compressions. Extensions to gravitational flux compression (stellar collapse) are also discussed. The theories are based on exact solutions of the free boundary (Stefan) problem which connects the implosion dynamics with the flux losses. Parameter-independent comparisons of the solutions with experiment support the adequacy of the models. Comprehensive methods for data analysis have been developed. With these it is possible to evaluate the phenomenological parameters which govern the implosions solely in terms of information derived from experimental records. In cases where liner thickening is essential the analytical flux compression theories must be supplemented by computer assisted solutions. Generalizations of the analytical data reversion methods can be applied to these numerical solutions. Comparison of the experimental records and computer simulations lead to direct determinations of the implosion parameters.

Molecular beam masers

Abstract: The purpose of this article is to review some of the developments and achievements of molecular beam masers from the time of their conception to the present and to delineate areas of particular interest and activity. A discussion of the early history of molecular beam masers is followed by an outline of the general principles and methods of preparation and use of emissive molecular beams. In particular, electrostatic methods of state selection of molecules and maser cavity resonators are discussed in some detail. An outline of the theory of molecular beam masers is then presented with reference to a number of specific problem areas. The technique of beam maser spectroscopy is discussed with emphasis on spectral resolution and problems of sensitivity. Molecules studied by beam maser spectroscopy are tabulated and discussed. The noise, gain-bandwidth and saturation characteristics of beam maser amplifiers are reviewed. The properties of beam maser oscillators are outlined, which include oscillation conditions, noise, amplitude and frequency characteristics and transient behaviour. The paper concludes with a brief discussion of future trends.

Physics applied to archaeology I. Dating

Abstract: An account is given of the three main chronological applications of physics in archaeology: radiocarbon dating, thermoluminescence dating and archaeomagnetism In addition the following techniques are outlined: potassium-argon dating, uranium series dating (including ionium dating), fission track dating and dating by chemical change (including hydration of obsidian and weathering layers on glass) Some mention of dendrochronology and varve chronology is included in the section on radiocarbon Radiocarbon dating, applicable to wood and some other organic remains, is predominant in this field and forms the basis of most prehistoric chronologies Its techniques of measurement are now well established and the main research emphasis is concerned with the fluctuations in the radiocarbon content of the atmosphere that have been inferred from small systematic errors shown up in the dating of known-age samples Possible causes for these fluctuations are past variations in climate, in sunspot activity and in geomagnetic field intensity There is evidence from archaeomagnetic studies that the latter has been significantly different from its present day value, possibly varying sinusoidally with a maximum about 2000 years ago and a minimum about 6000 years ago Archaeomagnetism, which is based on the magnetization `fossilized' in clay when it is fired, also provides data on the past secular variation of the geomagnetic direction and to a limited extent this can be used for dating; the geophysical interpretation of the data is also of interest Thermoluminescence is a fairly recently developed method and the main research emphasis is still concerned with various complications of technique It is important because it is directly applicable to pottery and pottery styles are the `grammar' of most archaeological chronologies Potassium-argon dating is a well developed geological technique and is notable in the present context for its dating of volcanic material associated with early hominid remains Uranium series dating has the possibility of being applicable to bone and shell of the paleolithic period, this being too early to be reached by radiocarbon Fission track dating is primarily a geological technique but it holds promise for archaeology, particularly in early periods Obsidian dating and glass layer counting are based on the hydration caused by weathering The former needs to be calibrated against radiocarbon but in a limited number of cases the latter appears to be absolute although the difficulty of understanding the physicochemical mechanism involved casts some doubt on its validity

The physics and dynamics of the stratosphere and mesosphere

Abstract: The physics and dynamics of the stratosphere and mesosphere (the region from about 10 km to 80 km in the atmosphere) are discussed with reference to recent observational and theoretical investigations. Following a short review of the principal techniques of measurement the main findings as regards the composition, radiation fields, temperature and winds are summarized and related to meteorological theory. A short account is also given of wave phenomena in this region. A selected first list of references useful for further reading is included.

The nuclear surface

Abstract: The nuclear surface cannot be sharply defined since the nuclear density is a continuous function of radius. By convention, one defines a surface thickness parameter as the width of the region in which the nuclear density falls from 90% to 10% of the central density. This parameter is known to be approximately 2?5 fm for the whole range of spherical nuclei, at least for those nuclei for which a central region of roughly constant density has meaning, that is above, say, A = 40. Experimentally it is an ill-defined parameter and appears to be independent of mass number - again this statement applies only to spherical nuclei. It is a matter of some speculation whether or not the constitution of this surface region is the same as that of the interior. Theoretically a preponderance of neutrons (i.e. a neutron/proton ratio greater than N/Z) is desirable in the surface and the experimental measurements are consistent with a `neutron skin' of perhaps 0?2 to 0?3 fm. Though the size of the nuclear charge distribution has been determined with great precision, the nuclear matter distribution is known with much less certainty and one still cannot rule out the possibility that protons and neutrons have a common distribution. One must also consider the possibility that nucleons in the nucleus are clustered into sub-units such as ?-particles. Experiments designed to test this by knocking out these clusters look specifically at the nuclear surface since the mean free path of these sub-units in the central region must be too small to allow them to escape even if struck by a deeply penetrating probe. The results of such experiments seem to indicate that the nuclear matter in the surface region is largely condensed into ?-particles. Another class of experiments involving meson-probes indicates a degree of spatial correlation of nucleons in the nuclear surface which is consistent with ?-particle clustering. Recently doubt has been thrown on the interpretation of these experiments, so clustering into ?-particles cannot be considered to be established.

Statistical theory of transport processes in a classical dense fluid

Abstract: This paper is concerned with the statistical theory of classical dense fluids which are in a condition of nonequilibrium. Various theories based on the Liouville equation are considered as a means of relating the phenomenological transport coefficients of fluid mechanics, such as viscosity and thermal conductivity, to molecular properties. The arguments are restricted to simple fluids composed of a single chemical species composed of essentially spherical molecules and internal molecular degrees of freedom are neglected. Some numerical results of certain aspects of the theory are considered.