Showing papers in "Reports on Progress in Physics in 1985"

The surfaces of metal oxides

Abstract: The author reviews the present understanding of the electronic, geometric and chemisorption properties of metal oxide surfaces. It is restricted to experimental and theoretical studies of single-crystal oxide surfaces since only for those systems has it been possible to correlate surface properties with specific site geometry, ligand coordination, defect structure, etc. The geometric structures of the oxide surfaces that have been investigated to date are described in relation to bulk crystal structure and cation ligand coordination. The electronic structure of both perfect and defect surfaces is discussed for the various classes of metal oxides, and similarities and differences in their behaviour are correlated with surface geometry and cation electronic configuration. The chemisorption of several types of atoms and molecules on single-crystal oxide surfaces, both nearly perfect and containing point defects is reviewed and interpreted in terms of surface electronic and geometric structure. Some recent electron- and photon-stimulated desorption results on oxides are also reviewed, as the measurements of surface phonon and plasmon modes.

Compton scattering and electron momentum determination

Abstract: When radiation is Compton-scattered the emerging beam is Doppler broadened because of the motion of the target electrons. An analysis of this broadened lineshape the Crompton profile, provides detailed information about the electron momentum distribution in the scatter. The technique is particularly sensitive to the behaviour of the slower moving outer electrons involved in bonding in condensed matter and can be used to test their quantum-mechanical description. The review begins with a brief survey of the historical development of the subject to within a decade of the present. The behaviour of quantum systems from a momentum viewpoint, is explained and the conditions under which the scattering experiment can be interpreted directly in terms of electron momentum density are discussed. The experimental techniques with gamma -rays, X-rays and electron beams are compared. Finally, recent results on insulators and conductors are surveyed and the extent to which they challenge conventional assumptions of band theory is critically reviewed.

Microscopic theory of the nuclear collective model

Abstract: This articles reviews the development of a microscopic theory of nuclear collective structure as a submodel of the nuclear-shell model. It starts by showing how the so-called geometrical (Bohr-Mottelson-Frankfurt) collective model must be augmented by the addition of vortex spin degrees of freedom to make it compatible with the shell model. A unified symplectic model emerges that can be applied both with phenomenological and microscopic interactions. Examples are given of both kinds of calculation. It is shown how the full shell model space can be expressed in an Sp(3R) contains/implies U(3) basis in which form it naturally factors into collective and intrinsic subspaces. In this way, the collective content of a shell model state becomes immediately apparent. Thus a shell model interpretation is given of collective states, including the low-lying rotational bands, the so-called beta and gamma vibrations and the giant monopole and quadrupole resonances.

The magnetohydrodynamics of current sheets

Abstract: Examples of current sheets are summarized and their formation is described. A universal phenomenon in cosmic plasmas is the creation of sheets off intense current near X-type neutral points (where the magnetic field vanishes). These sheets are important as sites where the magnetic-field energy is converted efficiently into heat and bulk kinetic energy and where particles can be accelerated to high energies. Examples include disruptions in laboratory tokamaks, substorms in the earth's magnetosphere, and flares on the sun. The basic behavior of a one-dimensional sheet is presented, together with an account of the linear tearing-mode instability that can cause the field lines in such a sheet to reconnect. Such reconnection may develop in different ways: it may arise from a spontaneous instability or it may be driven, either from outside by motions or locally by a resistivity enhancement. Various processes are described that may occur during the nonlinear development of tearing, along with the many numerical and laboratory experiments that are aiding our understanding of this intriguing cosmical process.

Molecular beam epitaxy

Abstract: This article reviews the major physico-chemical aspects of molecular beam epitaxy (MBE), especially as applied to the deposition of thin epitaxial films of III-V compound and alloy semiconductors. The experimental requirements to achieve the necessary levels of control, purity and uniformity are described first to establish the basic features of the technique. This is followed by a rather detailed treatment of the growth process, including thermodynamic considerations, surface reaction kinetics and film growth dynamics. Because MBE provides a unique means of preparing clean surfaces having controlled stoichiometry and reconstruction, the evaluation of their crystallographic and electronic structure is dealt with at some length. The structure and composition of semiconductor-semiconductor interfaces, together with the formation of quantum wells and periodic superlattices, are topics of increasing importance and as such also receive considerable attention. Dopant incorporation is dealt with rather more briefly by reference to specific examples and finally some mention is made of materials other than III-V semiconductors which have been prepared by MBE.

Electronic properties of semiconductor alloy systems

Abstract: Semiconductor alloys provide a natural means of tuning the magnitude of the forbidden gap and other material parameters so as to optimise and widen the application of semiconductor devices. With the advent of small-structure systems, such as quantum wells and superlattices, the effects of alloy composition, size, device geometry, doping and controlled lattice strain can be combined to achieve maximum tunability. The concepts and ideas peculiar to the description and modelling of semiconductor alloy systems are reviewed with a view to providing a link between electronic structure and optical and transport properties.

Experimental studies of positrons scattering in gases

Abstract: The current status of certain aspects of positron scattering in gases is reviewed. A brief resume of the experimental techniques used in this field is also given. Results for total scattering cross sections in a number of gases are presented along with a detailed discussion of potential systematic errors which can affect such measurements. Important features of the cross sections are pointed out and comparisons are made with electron scattering data. Results from experiments which go beyond total cross section determinations are discussed. Emphasis is placed upon recent measurements of positronium formation cross sections. Recent positron lifetime studies in dense gases are reviewed. The subjects covered include positron annihilation in clusters, ortho-positronium annihilation in bubbles and positronium formation in spurs.

Angular momentum transfer and charge cloud alignment in atomic collisions: intuitive concepts, experimental observations and semiclassical models

Abstract: The authors discuss intuitive concepts to describe alignment and orientation effects in collision processes with, or leading to, an atomic np state. For direct excitation one can understand the atomic angular momentum transferred in terms of a rolling ball, and for excitation (de-excitation) in a molecular picture one can visualise the alignment angle of the atomic p charge cloud in terms of a transition from a body-fixed molecular picture (small internuclear distances R) to a space-fixed picture (large R). These concepts are illustrated by experimental results for e+Na* and Na++Na* collisions. Semiclassical theory is discussed for both the direct and the molecular inelastic processes, giving a theoretical foundation for these models. Detailed results are reported for the time development of the charge cloud in Na++Na* collisions as a model case, illustrating the concept of body-fixed versus space-fixed electron motion and its limitations. Further examples are the molecular process N2+Na* and the atomic process Xe+Ba* at thermal energies. In all cases long-range rotational ( Sigma - Pi ) coupling determines the charge cloud motion.

X-ray telescopes

Abstract: This review summarises the present knowledge about X-ray reflection from real mirrors, with particular emphasis on X-ray scattering. The mirror configurations for a telescope are presented and their imaging capabilities are discussed. The fabrication techniques and the supporting metrology methods used for the manufacture of highly aspherical telescope mirrors are outlined, including direct grinding and polishing as well as replication methods. An account is given of the achievements obtained so far for the telescopes built for solar and non-solar X-ray astronomy applications.

Matter under extreme conditions of temperature and pressure

Abstract: Research on matter at extreme conditions has provided new insights into material behaviour at atmospheric pressure. The application of high pressure is the simplest way to change the lattice spacing and has led to the discovery of new phenomena, new phases and new forms of electronic and ionic order. The application of high temperatures leads to melting, atomic and electronic excitation, and eventually to ionisation and the formation of a plasma state. The combination of high pressures and temperatures continues to present a severe challenge to experimental and theoretical advances. However, current experimental methods permit material studies at pressures of several megabars and temperatures of tens of thousands of degrees Kelvin. The rapid surge in computer technology has, in turn, permitted the solution of many previously intractable theoretical problems. High-pressure research supplies information about the properties of geologic materials which, when coupled with data from seismic waves and space probes, provides information about the structure of planetary interiors.

Advances in studies of nuclei far from stability

Abstract: Studies of the structure and decay modes of nuclei far from stability have yielded many new insights which were not seen in nuclei near the line of beta stability. The results of recent experimental and theoretical investigations are presented and the new insights being gained are discussed. Some of the highlights described include ground-state proton radioactivity, beta -delayed 2n, 3n and 2p decays, the coexistence of states built on quite different nuclear shapes in the same nucleus, new regions of strongly deformed nuclei, and the importance of the reinforcement of proton and neutron shell gaps on the shape of a nucleus.

The interacting boson model of nuclear structure

Abstract: Most of this review is devoted to a description of the interacting boson model in its different forms and to a discussion of its applications to a range of nuclei. Possible relations between the interacting boson model and the collective model are described and some attempts to derive the interacting boson model from the shell model are reviewed. To help the non-specialist reader the article begins with a survey of the more firmly established nuclear models.

Transient annealing of semiconductors by laser, electron beam and radiant heating techniques

Abstract: The annealing of semiconductors is of critical importance for successful electronic device fabrication. The present review surveys the new field of transient annealing and covers all timescales below those available with the conventional furnace. The work outlined includes the use of techniques which rely upon transient energy deposition in semiconductors from laser, electron beam, ion beam and other radiant sources. The many advances which have been achieved using these transient annealing methods in both fundamental and applied areas of physics are described.

Neutron capture process in astrophysics

Abstract: The development of the field of study of neutron capture processes in astrophysics is reviewed as a gradual evolution of improving microscopic input nuclear physics coupled with more sophisticated stellar models and computational technology. Particular emphasis is placed on discussion of contemporary scenarios for the stellar environments responsible for the nucleosynthesis of heavy nuclei. The division into slow (s-process) and rapid (r-process) nucleosynthesis regimes is described and the evolution toward scenarios which involve neutron fluxes intermediate between these two extremes is discussed. Uncertainties in the theories and input physics are highlighted.

The physics of quantum well structures

Abstract: Recently developed methods for growing epitaxial layers of semiconductors have included the ability to exercise independent and spatial control over both the semiconductor band gap and the level of doping. A whole new range of tailored test structures is available for probing physical phenomena on the very short length and time scales appropriate to semiconductors. Many of the effects rely on the quasi-two-dimensionality of the carriers, but under suitable conditions the transition to higher and lower dimensions can be achieved. Some of the key concepts, principal results and current lines of research in this rapidly developing field are summarised here.

Complex atomic spectra

Abstract: The long history of the theory of complex atomic spectra, as distinct from series spectra, is reviewed from the period of the 1930s, when quantum mechanics was rapidly applied to solve a variety of problems, to the present day, when, at a single stroke, elaborate computer programs are used to fit many hundreds of atomic energy levels to theoretical models. Emphasis is placed on the use of of annihilation and creation operators. With their help, the role that Lie groups play in atomic spectra can be described in analogy to SO(3), the special orthogonal group corresponding to rotations in ordinary three-dimensional space. Configuration interaction is represented by effective operators that act within the states of the unperturbed configuration under study. These effective operators are also usefully constructed from annihilation and creation operators. A table is given in which the least-squares fits to the levels of atomic configurations comprising at least three electrons (or electron holes and electrons) are listed.

Method for counting noble gas atoms with isotopic selectivity

Abstract: A method has been developed for direct counting of noble gas atoms and has been demonstrated for selected isotopes of krypton. In principle, a few atoms of the noble gases argon, krypton, xenon and radon can now be counted with isotopic selectivity whether stable or radioactive. A concept was originated in which a laser method would be used to count noble gas atoms of a particular isotope that are moving freely in an enclosure. As the concept developed, a parallel with Maxwell's sorting demon became quite obvious since the plan was to sort out only atoms of a given type (Z selection), e.g. krypton atoms, from any other atom in the enclosure and then to sort the atom by isotope (A selection) before removing the atoms from the gas compartment. The plan was to count each atom as it was stored in a target until all atoms were counted.

The management of radioactive wastes

Abstract: In order to manage radioactive wastes, whether they are gases, liquids or solids one has to take decisions on several key points. For example, whether to disperse them in the atmosphere or the sea or to isolate them from Man; whether to process a sludge by dewatering; whether to remove activity from a solid waste, thus producing a secondary waste. These and many other decisions about treatment, packaging, storage and disposal have to be taken on the basis of sound technical information and taking account of national and international laws and regulations. But it is not sufficient just to manage waste efficiently, it is also essential to respond to the natural concern of the general public and to satisfy them and their political representatives that wastes are and will be managed safely. This article describes the quantities of wastes, their origin and classification, the legislative background and how wastes are managed at present. This is an international subject but it is described here largely from a United Kingdom point of view.

Experimental aspects of two-body ion-ion collisions

Abstract: The review is primarily concerned with experimental aspects of two-body ion-ion collisions that have been studied with intersecting beams. After an account of experimental methods and techniques there are discussions of interactions between positive and negative ions leading to mutual neutralisation, detachment and association. This is followed by a review of collisions between positive ions which result in ionisation or charge transfer. The relevance of these processes to aeronomy, fusion, astrophysics and other topics is indicated and, wherever possible, comparisons are made between the results of experiment and theory.

Microcomputer calculations in physics

Abstract: The use of microcomputers to carry out computations in an interactive manner allows the judgement of the operator to be allied with the calculating power of the machine in a synthesis which speeds up the creation and testing of mathematical techniques for physical problems. This advantage is accompanied by a disadvantage, in that microcomputers are limited in capacity and power, and special analysis is needed to compensate for this. These two features together mean that there is a fairly recognisable body of methods which are particularly appropriate for interactive microcomputing. This article surveys a wide range of mathematical methods used in physics, indicating how they can be applied using microcomputers and giving several original calculations which illustrate the value of the microcomputer in stimulating the exploration of new methods. Particular emphasis is given to methods which use iteration, recurrence relation or extrapolation procedures which are well adapted to the capabilities of modern microcomputers.

Simple transport properties of simple metals: classical theories and modern experiments

Abstract: The low-temperature DC electrical and thermal resistance of nearly-free-electron metals, such as the alkali metals and aluminium, are ideal for testing one's understanding of the basic scattering processes of electrons and phonons in a metallic environment. The greatly improved experimental capabilities of the past decade facilitated measurements, the results of which made clear that the traditional models were at least incomplete. Significant progress has hence been made in understanding the role of electron-electron and electron-phonon scattering, both normal and Umklapp; their sample dependence; other deviations from Matthiessen's rule; the influence of the lattice thermal conductivity on the transport tensor elements; the linear magnetoresistivity, and related fields. Yet, crucial questions like the existence of a charge density wave ground state in the alkali metals remain open, and new ones show up.

Electron lithography for the fabrication of microelectronic devices

Abstract: Electron lithography is a technique which is in widespread use for making masks and reticles for the manufacture of integrated circuits. It is also commonly used in research laboratories for exposing micron or sub-micron patterns directly on wafers. In addition, direct exposure is used, to a limited extent, in manufacturing plants, although generally at coarser resolutions. The reasons for using electron lithography in these applications are explained, and a history of the technique's evolution is given. The two major types of instrument-the scanning instrument and the projection instrument-are described and their limitations are pointed out. Interactions between electrons with energies of tens of keV (the energies typically used in electron lithography), the substrate (wafer or mask plate) and the resist material (in which a latent image is formed) are discussed. It is shown how these interactions affect the quality of the resist image before and after development.

High resolution laser spectroscopy of atomic systems

Abstract: Laser experiments now dominate the field of high resolution optical spectroscopy of atoms This review attempts to show why lasers have had such an impact on the subject and to describe the techniques in use today It starts by looking at characteristics of present day laser sources and at detection methods and techniques for frequency measurement Then the various ways of performing Doppler-free high resolution laser spectroscopy are dealt with in turn and examples are given from different areas of atomic spectroscopy to show the wide application of lasers The main methods dealt with include beam techniques (thermal atomic beams and fast ion beams), multiphoton spectroscopy, saturation methods and laser cooling of atoms and ions The review concludes by highlighting a number of areas where further development can be expected in the near future

New unorthodox semiconductor devices

Abstract: A distinctive feature in the development of solid-state electronic devices has been the long time interval that has usually elapsed between the initial conception of the device, and its experimental realisation. The technology bottleneck which gives rise to this phenomenon has recently been significantly widened by the emergence of several new technologies which are enabling the realisation of device structures which, although not entirely new in concept, are receiving a revived interest because they are now technologically feasible. The author discusses a range of new device structures, many of which fall into the above category, and the ways in which new advances in technology have made them realistic contenders for many device applications. In addition a number of structures which rely upon entirely new phenomena are presented, with experimental demonstration considerably enhanced, and in some cases only made possible by the emergence of ion-implantation and molecular beam epitaxy.

High-performance waveform-sampling analysis ― current status and trends

Abstract: The most recent developments in the area of high-speed and low-level signal detection are described Here, some interesting advances have taken place due to the possibility of using computer-aided signal processing and cryoelectronic Josephson sampling procedures While the operational principle of room-temperature signal averaging concentrates more on the priority of extremely high-voltage sensitivity (optimally 1 nV), the low-temperature Josephson sampler rather shifts the emphasis to extremely high time resolution (optimally 6 ps) Both novel schemes of high-performance waveform-sampling analysis may yield an overall efficiency limited at the characteristic rise time sensitivity product of about 10-17 V s An outline of the present uses and future prospects indicates that these methods represent an important tool in such advanced fields as low-temperature solid-state physics and cryoelectronics and deserve more attention both from a fundamental point of view and for many technological applications

Microwave semiconductors devices

Abstract: The field of microwave semiconductor devices has expanded greatly in recent years, both in the functions that may be performed and the range of devices available for any function. The upper frequency limits of transistor operation have also marched steadily upwards, with millimeter-wave (above 30 GHz) operation being achieved in many laboratories before 1980. The aim of this review is to provide an introduction for those new to microwave semiconductors and an update for those who have not been closely involved for some time. Following the introduction is a section on carrier transport, which is a topic of great importance in the understanding of device operation. The individual families of devices are discussed in the succeeding sections, starting with varistor and varactor diodes in which the variation of resistance or reactance is the important feature. The next section introduces transit-time phenomena and the devices designed to take advantage of them.