Showing papers in "Reports on Progress in Physics in 1984"

Nucleation and growth of thin films

Abstract: The purpose of this article is to describe the basic physical processes involved in the nucleation and growth of thin films of materials on solid surfaces. In this introduction the three modes of crystal growth which are thought to occur on surfaces in the absence of interdiffusion are described, and the relationships between the thermodynamics of adsorption and the kinetics of crystal growth are explored in general terms. This is followed by a brief review of atomistic nucleation theory, explaining the relations of such theories to experimental observables. In the next three sections, recent experimental examples of these three growth modes are given, which are interpreted where possible in terms of nucleation and growth theory. The last section discusses observations on the shapes of growing crystallites and the relation of such observations to nucleation and surface diffusion processes.

The inflationary Universe

Abstract: According to the inflationary Universe scenario the Universe in the very early stages of its evolution was exponentially expanding in the unstable vacuumlike state At the end of the exponential expansions (inflation) the energy of the unstable vacuum (of a classical scalar field) transforms into the energy of hot dense matter, and the subsequent evolution of the Universe is described by the usual hot Universe theory Recently it was realised that the exponential expansion during the very early stages of evolution of the Universe naturally occurs in a wide class of realistic theories of elementary particles The inflationary Universe scenario makes it possible to obtain a simple solution to many long-standing cosmological problems and leads to a crucial modification of the standard point of view of the large-scale structure of the Universe

Thermal conductivity of solids and liquids under pressure

Abstract: Thermal conductivity of solids and liquids under pressure is covered in this review. Experimental techniques are critically considered and compared, and an introduction to theory is provided. Results are presented and discussed for ionic crystals, normal molecular crystals, plastic crystal phases, clathrate hydrates, polymers and glass-formers, liquids, covalent and semiconducting crystals, rocks and metals. Special attention is given to isochoric conditions, change of crystal structure and molecular orientational disorder. Available reliable measurements at pressures up to a few GPa indicate the need for theoretical development, especially in connection with molecular crystals and ferromagnetic metals.

Magnetic amorphous alloys: physics and technological applications

Abstract: Magnetic amorphous alloys obtained by rapid quenching of the melt are excellent soft magnetic materials with a wide range of technological applications. They also represent a significant challenge to the scientific understanding of magnetic materials, since most of the existing theories of solids assume lattice periodicity. For these reasons, the magnetic and other properties of amorphous alloys have been very actively studied over the last decade. In recent years increasing attention has been directed to the fundamental understanding of the structural, thermal and magnetic properties of the amorphous alloys. It is not only scientifically but also technologically important to achieve such an understanding, since the amorphous alloys are, in many respects, so different from conventional crystalline magnetic materials. The author attempts to summarise recent progress in research on magnetic amorphous alloys and critically assesses the present level of understanding of this new class of magnetic materials, focusing mostly on the transition-metal-metalloid glasses. He starts with a review of early developments and a discussion on the nature of glasses and glass formation and proceeds to an extensive discussion of their atomic structure, both from the experimental and theoretical points of view.

Linear optical birefringence of magnetic crystals

Abstract: The study of changes in the refractive indices (with consequent changes of birefringence) of a transparent magnetic crystal which accompany changes in the magnetic order is becoming more popular. The authors review why this is. The first reason is that birefringence can be measured very accurately: the different experimental arrangements are reviewed. The second reason is because a birefringence measurement is an integrational spectroscopic technique and therefore it is studied both experimentally and theoretically as a branch of magneto-optics and hence gives information on the detailed energy level structure of the solid. The third reason is that in a number of interesting systems the birefringence is proportional to the magnetic energy over a wide temperature range and it is often a more convenient method of obtaining the magnetic specific heat than direct specific heat measurements; this is particularly true in magnetic crystals which show low dimensional ordering. The last reason is that in all magnetic crystals the birefringence change should vary like one of the thermodynamic critical exponents near to the phase transition. They review in detail the reasons why birefringence studies have become so successful for measuring critical exponents in pure and particularly mixed crystals.

Critical phenomena at interfaces

Abstract: Equilibrium aspects of the structure and thermodynamics of the interfacial region separating two coexisting phases near criticality are reviewed. Brief surveys of pertinent rigorous and exact results and experiments are included. Phenomenological approaches and renormalisation group results, along with implications on such topics as universality and interface stability, are discussed. Related topics such as roughening and wetting are also briefly discussed.

Synchrotron X-radiation protein crystallography: instrumentation, methods and applications

Abstract: This review describes the utilisation of synchrotron X-radiation (SR) in protein crystallography. For the general reader a brief discussion is given of the basics of protein structure analysis including an outline of the problems of sample radiation damage, the inherent weakness of individual reflections and the large amounts of data that need to be collected as well as the 'phase problem' of crystallography. The properties of synchrotron radiation emitted by relativistic electrons in simple circular orbits in bending magnets and by more sophisticated electron motions induced in wiggler or undulator magnets are discussed. The impact of newer high brightness synchrotron radiation sources in protein crystallography is assessed. The instrumentation required to monochromatise and focus the radiation onto typically small protein crystal samples and to detect the diffraction pattern is described. Both step scanning and energy dispersive techniques to optimise anomalous dispersion are covered. A gazetteer is given of the instruments providing data collection facilities for protein crystallography which are available on SR sources around the world. The modifications that are needed to standard data processing techniques are dealt with, taking account, for instance, of the position, angle and wavelength correlatable properties of photons incident to a sample from an SR source; properties which affect reflection prediction in both camera or diffractometer work.

A comparative study of methods for thin-film and surface analysis

Abstract: A number of features characteristic of different thin-film analytical methods are reviewed and evaluated. The principles, approach for quantification and prominent problems of the most commonly used methods (ESCA, AES, SIMS, LEIS, RBS and NRA) are discussed. Evaluation of the special advantages and disadvantages of the different methods points out the need for a synergetic multi-method approach in thin-film analysis.

Spectroscopy and impurity behaviour in fusion plasmas

Abstract: The authors review spectroscopic work and impurity behaviour in laboratory plasmas devoted to controlled nuclear fusion research. A necessary prerequisite for this is a discussion of the atomic physics (rate coefficients, ionisation models and spectral emission) involved in the interpretation of plasma spectroscopic data. The largest part of this review is concerned with spectroscopic work in magnetically confined Tokamak plasmas and laser-produced, inertially confined, plasmas, since these two approaches have received the greatest attention. In Tokamak plasmas, impurity radiation constitutes an important loss process; it is therefore necessary to find ways to avoid impurity contamination. On the other hand, in inertial confinement, 'impurities' are necessary to absorb the beam energy and are, moreover, an essential diagnostic tool, particularly for the compressed material. Spectroscopic work of fusion interest in other laboratory plasma devices is also briefly reviewed.

Coherence in inelastic low-energy electron scattering

Abstract: The results of electron-photon coincidence experiments have given a new insight into the excitation of atoms by electron impact. This article reviews the way in which this new experimental technique has provided an exacting and fundamental test for the theory of the excitation process. The basic experimental method is first described, and a comprehensive treatment of excitation of the n1P states of helium is given. The extension of the technique to the study of excitation of the n=2 states of atomic hydrogen is then presented. Coincidence measurements also provide information on the role of spin in the collision, and this subject is treated in detail for excitation of heavy atoms by both unpolarised and spin-polarised beams of electrons. Finally, the electron-photon coincidence method permits observation of the time evolution of excited atomic states, and this review describes the attempts made to observe quantum beats in the radiative decay.

Effective operators in the atomic hyperfine interaction

Abstract: The many-body perturbation theory (MBPT) is reviewed and applied to the atomic hyperfine interaction. Graphical methods are introduced by without mathematical details. The results are interpreted in terms of effective operators. For systems with a single valence electron-such as the alkali atoms-this operator has the same form as the ordinary hyperfine operator and is identical to the operator commonly used in the analysis of experimental hyperfine data. The origin of different contributions to this operator is discussed. Numerical results are given for the 22S and 22P states of the lithium atom, where accurate MBPT calculations have recently been performed. For systems with several valence electrons additional parameters are needed or, alternatively, the parameters of the one-body effective operator are allowed to be term-dependent. Recent experiments and corresponding theoretical investigations on alkaline-earth elements, with two valence electrons, are reviewed and, in particular, MBPT calculations on the calcium atom are discussed.

The dynamics of the Earth's inner and outer cores

Abstract: A brief historical introduction to the study of the Earth's core is followed by a review of recent advances in the dynamics of the liquid outer core and the rotation of the solid inner body. In particular, the scaling of the fluid motion equations is reviewed and a detailed derivation of the 'subseismic equation' which governs small-amplitude, low-frequency oscillations is given following methods first outlined by Rochester. Attention is given to the compressibility of the outer core and its role in dynamo theory as well as its effect on Proudman-Taylor flow, where it produces a fluid motion with helicity. The theorem of J.B. Taylor is also generalised to real core compressibility. Rotational motions of the inner core are examined under the strong gravity torque exerted on it by the rest of the Earth. Both the motion ignoring back reaction on the mantle's rotation and the full coupled problem are solved.

Applications of low temperature scanning electron microscopy

Abstract: Scanning electron microscopy performed on specimens cooled to the temperature of liquid helium yields interesting information on the spatial structures of various sample properties. In many applications of low-temperature scanning electron microscopy the perturbation of the sample due to the electron beam can simply be treated as a local heating effect. In this case the spatial resolution is limited by the thermal healing length of the particular geometric configuration. The spatial resolution can be improved considerably by means of high-frequency beam modulation and utilising the modulated signal. Irradiation with the electron beam results in two important sample responses. The electron-beam-induced local heating causes an electric resistance change yielding a two-dimensional voltage image. On the other hand, the region heated locally by the beam represents a phonon source. This provides the mechanism for obtaining a two dimensional phonon image. Recently, these principles have been applied in the following areas: hotspots in superconducting microbridges, spatial structures in superconducting tunnel junctions and phonon focusing in single crystals.

The neutron optical potential

Abstract: The nucleon optical model is described, with particular reference to the analysis of neutron scattering and reaction cross sections. The central, spin-orbit and isospin terms in the potential are defined and the results of phenomenological determinations of their parameters reviewed. The methods used to determine the asymmetry and isospin terms are described in detail. The optical potential as negative energies may be determined from the properties of the single-particle states and thus enables its behaviour at both positive and negative energies to be studied. Some of the methods used to calculate the optical potential from the nucleon-nucleon interaction are described and the results compared with the phenomenological potential. The coupled-channels formalism enables both elastic and inelastic scattering from collective nuclei to be analysed simultaneously. Some analyses made in this way are reviewed, together with similar analyses of charge-exchange isobaric analogue state reactions.

Metals, nonmetals and metal-nonmetal transitions: some recollections

Abstract: The author, in a historical review, recollects places, occasions, colleagues and circumstances associated with the study of metals, nonmetals and metal-nonmetal transitions.

T Tauri stars: an overview

Abstract: The review presents the main aspects of the body of knowledge on T Tauri variable stars, which are presumed to be young, gravitationally contracting low-mass stars. The author first reviews their often exotic properties in the different wavelength ranges accessible to modern astronomy and discuss the data's immediate interpretation. He then describes some theoretical results of protostellar evolution computations with the intention of defining a framework for discussing the quantitative models which have been proposed for T Tauri stars. Although a unified picture of the T Tauri phenomenon has yet to emerge, major advances have occurred in the last few years in the study of physical processes potentially relevant to these stars (e.g. the role of magnetic fields in driving stellar winds and stellar activity). He reviews these current models and identifies those properties of the stars which now seem well understood. Lastly, at the risk of being quickly outdated, he goes on to draw some tentative conclusions about the nature of the T Tauri phenomenon.

Ancient climates: investigation with climate models

Abstract: Mathematical models of the climate system have recently become an important aspect of the study of ancient climates. Three geologic climate problems have received particular attention: (i) the faint early Sun paradox; (ii) the warm apparently ice-free Cretaceous period and (iii) the glacial-interglacial cycles of the last 700000 yr. There are three basic limitations in investigating geologic climate problems: (i) limitations in reconstructing the climate state; (ii) limitations in specifying the forcing factors which influenced the climate and (iii) limitations in determining the climate sensitivity to specific forcing factors. The three climate problems which are described here are widely different in terms of the degree to which the climatic record can be reconstructed, the importance of specific factors and in the types of models which have been applied to investigate climate sensitivity. Each time period is sufficiently different in terms of the climatic record and in terms of the evidence for a particular forcing factor to be a well-posed climate modelling problem. For the most part a simulation or replica of these climates is not yet possible, but based on sensitivity experiments there has been a convergence toward understanding the climatic response forcing factors and describing the causes of climatic change during Earth history.

An experimental study of helicon resonance in metals

Abstract: This review is devoted to helicons-electromagnetic waves propagating in the electron-hole plasma in metals at low temperatures. The review is divided into two parts. In the first part the author presents the theory of, and the experimental data on, helicons in the absence of effects caused by Landau quantisation of conduction electron energy in a strong magnetic field. The author emphasises the peculiar features of helicon propagation which are due to the anisotropy of the electron spectrum of metals, effects not observable in a gaseous plasma. An analysis of data on the anisotropy of the collision damping, the Landau damping and on dopplerons-a new branch of electromagnetic excitations in the vicinity of the Doppler-shifted cyclotron resonance-is given. He also discusses the results of studies of the acoustic satellites of helicon resonance and the peculiarities of helicon propagation in the intermediate state of type-I superconductors. In the second part he presents the results of observations of new effects caused by Landau quantisation, namely oscillations of the phase velocity of helicons, non-linear helicon resonance and the anomalous attenuation of helicons in metals with diamagnetic domains. Examples are discussed in which the helicon resonances are employed to investigate transport relaxation in high magnetic fields, the production of defects by plastic deformation, open orbits and the absolute amplitude of the de Haas-van Alphen effect.

Neutron scattering-modern techniques and their scientific impact

Abstract: The sustained interest in the neutron and its use as a probe of the structure and dynamics of condensed matter is examined against the background of neutron availability. An analysis is made of developments in neutron source brightness, instrument physics and experimental methodology which have been or are likely to be of outstanding value in physics, chemistry, biology and materials technology studies. The role of pulsed sources as the next step ahead in neutron source brightness, their need for extensive instrument development to realise this potential and their complementarity with steady-state reactors is analysed using newly available experimental results.

Identification techniques for nuclear particles in space plasma research and selected experimental results

Abstract: The author presents an account of the rapid development in instrumental techniques proposed for satellite applications since 1970. Brief historical remarks on the evolution of thoughts on space plasma composition are followed by an introduction to space missions with a varying degree of emphasis on ion composition measurements. The characteristic technical constraints, which distinguish satellite instruments from laboratory equipment, are emphasised. Secondly, individual instruments are described with the emphasis placed on spectrometer systems used to determine particle identity. In spite of the fact that a satellite instrument is generally a self-contained unit with an electronics package, which is equally important for the overall performance, aspects of the electronics are addressed only to the extent it is required to convey insight into the functional principle of a particular solution to the particle identification problem. Finally, the physics of nuclear particles as important constituents in space plasmas is accentuated by discussing selected experimental results. The composition of the solar wind and related principles, the role of heavy ions as tracer particles and as a significant plasma component in the Earth's magnetosphere are delineated.

Cosmic rays of very high energy

Abstract: A detailed examination is made of the evidence about the mass composition as a function of energy and it is concluded that protons predominate over most of the energy range; this conclusion is not universally accepted, however, and many authors prefer an enrichment of heavy nuclei in the range 1014-1016 eV. There is less argument about the form of the energy spectrum and it is generally agreed that a power law representation applies over wide ranges of energy, the spectrum having a constant exponent to about 3*1015 eV, followed by an increased exponent to 1019 eV, after which the spectrum flattens again. Interactions in the atmosphere are dealt with in detail. It is argued that there is a breakdown in the scaling behaviour apparent at accelerator energies ( approximately 1012 eV). Recent pp intersecting ring experiments seem to bear out this view. The interesting question of the small anisotropies in arrival directions is considered at length and it is demonstrated that a consistent pattern emerges, the amplitude of the first harmonic rising steadily with energy. The phase of the harmonic varies with energy and there is the intriguing possibility that ultra-high energy gamma -rays, present to the extent of one in a thousand at 1015 eV, may be responsible. The final question to be addressed concerns the likely sources of cosmic rays and their mode of propagation. The preferred view is put forward, viz that the sources in the Galaxy (pulsars, supernova remnants, etc) provide the bulk of the particles with energy below about 1019 eV.

Lattice field theories

Abstract: The use of lattice approximations to study relativistic quantum field theories is surveyed. The models for scalar, gauge and fermionic degrees of freedom are defined. The standard methods for analytic study of these models using strong coupling, duality and mean field methods are outlined. The important area of numerical Monte Carlo simulations is also studied. These methods are used in the study of quantum chromodynamics in detail, including its spectrum. The arguments leading to the 'proof' that phi 4 is trivial theory are presented.

Metals, non-metals and metal-non-metal transitions: some recollections

Abstract: Cet article retrace l'histoire de la physique du solide depuis cinquante ans avec un regard particulier sur les transitions metal-isolant