Showing papers in "Reports on Progress in Physics in 1989"

The superposition model of crystal fields

Abstract: The superposition model was originally developed to separate the geometrical and physical information in crystal field parameters. Its success in the analysis of lanthanide spectra has been paralleled by the success of the related angular overlap model in the analysis of d-electron spectra. The basic ideas, method of application and reliability of the superposition model are discussed and its relationship with the angular overlap model is clarified. Developments described are the application of the superposition model to the ground (L=0) multiplet splittings of d5 and f7 ions, orbit-lattice interactions, transition intensities and correlation crystal fields. Special attention is paid to work which has been claimed to support or disprove the postulates of the model.

Electromigration in metals

Abstract: This paper provides an overview on the current understanding of electromigration in metals. The discussion is first focused on studies in bulk metals and alloys. This part includes a thermodynamic formulation of electromigration, a kinetic analysis of the atomic processes and a review of the theory. In addition, experimental results in interstitial and substitutional systems are summarised. The second part of the paper reviews the studies in metallic thin films. This emerged as an important area of electromigration studies since the late 1960s when electromigration damage was found to cause failure of conductor lines in integrated circuits. The discussion will review first the basic nature of electromigration in thin films with emphasis on the role of grain boundaries in mass transport and damage formation. Then the materials issues of electromigration will be explored according to the scaling trends in VLSI technology. Finally, the recent results of electromigration in fine lines and device contacts of dimensions in the micrometre range are discussed.

The relativistic mean-field description of nuclei and nuclear dynamics

Abstract: The relativistic mean-field model of the nucleus is reviewed. It describes the nucleus as a system of Dirac nucleons which interact in a relativistic covariant manner via meson fields. The meson fields are treated as mean fields, i.e. as non-quantal c-number fields. The effect of the Dirac sea of the nucleons is neglected. The model is interpreted as a phenomenological ansatz providing a self-consistent relativistic description of nuclei and nuclear dynamics. It is viewed, so to say, as the relativistic generalisation of the Skyrme-Hartree-Fock ansatz. The capability and the limitations of the model to describe nuclear properties are discussed. Recent applications to spherical and deformed nuclei and to nuclear dynamics are presented.

s-process nucleosynthesis-nuclear physics and the classical model

Abstract: Among the various processes responsible for the formation of the heavy elements in stars, the slow neutron capture process (s-process) is distinguished by the fact that it involves mostly stable isotopes. Therefore, the relevant nuclear physics data can be determined by experiments. With this rather reliable data basis, s-process nucleosynthesis offers an important testground of models for the late stages of stellar evolution, which are supposed to be the s-process site. The empirical counterpart for such models is the so-called classical s-process, a purely phenomenological picture, that is successfully used to derive the resulting abundances as well as information on the physical conditions during the s-process. The status of this classical approach is reviewed with emphasis on the implications for various stellar models of the s-process and in the light of results obtained by stellar spectroscopy. A brief account of the potential s-process chronometers is also presented.

Theory of electronic processes in atom scattering from surfaces

Abstract: The theory of electronically non-adiabatic processes in atom scattering on surfaces of solids, in particular metals, is reviewed. It is assumed that resonant tunnelling of electrons between an electronic orbital of the adatom and the conduction band of the substrate is the dominant charge exchange mechanism. The kinetic energy of the adatom is assumed to be in the intermediate range (eV to keV), which makes it possible to treat the adatom trajectory classically and to obtain a time-dependent electronic Hamiltonian. The aim of the theory is to find the deviations from adiabaticity, i.e. the fraction of the minority charge states of the atom after scattering and the spectrum of the electron-hole pairs created in the substrate. Under some simplifying approximations, these quantities can be calculated explicitly. Their dependence upon the adatom velocity and other parameters of the model is analysed, with particular attention paid to features which are characteristic of an extended substrate, in contrast to atom-atom or atom-molecule scattering. The present understanding of more complete models, for example inclusion of the intra-atomic Coulomb correlation, is also reviewed.

Defects in liquid crystals

Abstract: Defects are local breakings of symmetry in an ordered medium. The physics of defects has long been reduced to the study of dislocations in solids, and to the main physical phenomena they are responsible for, like plastic deformation. Dislocations break translational symmetries. Disclinations break rotational symmetries and are the basic defects of media with continuous symmetries, like liquid crystals. In this review, it is stressed how their study has contributed to a renewal of the physics of defects. Static and dynamic properties in the nematic, cholesteric, blue, smectic and columnar phases of liquid crystals are described in detail, up to the most recent results, for small-molecule thermotropic liquid crystals as well as for lyotropic and polymeric liquid crystals. The authors also discuss the homotopy classification of defects, including point defects, and compare it with the Volterra classification; finally they present the curved-crystal description of frustration in the light of the relevant situation in liquid crystals. The usefulness of the concepts introduced for liquid crystals for the study of other systems (structural problems in biology, dissipative structures, magnetic domains, etc.) is also emphasised.

Plastic Ball experiments

Abstract: The Plastic Ball spectrometer is a 4 pi detector with particle identification for charged particles. It was first used at the Berkeley Bevalac to study nucleus-nucleus collisions at beam energies of hundreds of MeV per nucleon. Recently it has been moved to the CERN SPS for studies of target fragmentation at energies of hundreds of GeV per nucleon. The authors present a complete review of the spectrometer and the physics obtained at the Bevalac, with some recent additions from the work at CERN.

Study of baryon-baryon interactions and nuclear properties in the quark cluster model

Abstract: The author discusses the application of the quark cluster model to the baryon-baryon interaction. The short-range repulsion and spin-orbit force in the nucleon-nucleon potential are shown to be well described by the quark-quark interaction and quark exchange between the two nucleons. An extension of the study to systems with strangeness is made. There the author studies more details about the effect coming from the composite structure of the baryon and the quark-quark interaction. The dihyperon, which is a flavour SU(3) singlet state, is also discussed in terms of the quark cluster model. Finally the effects of quark degrees of freedom in nuclei, especially on the electromagnetic properties of the deuteron, are studied and compared with the results of conventional nuclear theory.

The supernova 1987A in the Large Magellanic Cloud

Abstract: Two years after the discovery of supernova 1987A in the Large Magellanic Cloud the huge amount of new and exciting information is still being analysed by astronomers and astrophysicists all over the world. The authors try to summarise the present situation concerning both the observational facts and their theoretical interpretation. In particular, they focus their attention on the information contained in the light curve and the spectra, and in the detection of neutrinos. It is shown that many previously open questions have now been answered, but also that many new problems await a solution. These include the unexpected blue appearance of the progenitor star, its presupernova evolution, the explosion mechanism, apparent deviations from spherical symmetry and their origin, properties of matter at high densities and temperatures, etc. All these questions are far from being fully understood. Nevertheless, an attempt is made to give at least some hints on how solutions may be obtained.

Antiprotonic-hydrogen atoms

Abstract: Experimental studies of antiprotonic-hydrogen atoms have recently made great progress following the commissioning of the low-energy antiproton facility LEAR at CERN in 1983. At the same time the understanding of the atomic cascade has increased considerably through measurements of the X-ray spectra. The life history of the p-p atom is considered in some detail, from the initial capture of the antiproton when stopping in hydrogen, through the atomic cascade with the emission of X-rays, to the final antiproton annihilation and production of mesons. The experiments carried out at LEAR are described and the results compared with atomic cascade calculations and predictions of strong interaction effects.

Compact maser sources

Abstract: The past decade has seen tremendous growth in the study of cosmic maser sources. Radio interferometers have fully resolved the structure of the sources on all angular scales, and have enabled large numbers of sources to be studied in detail for the first time. Astronomers now have a far clearer picture of the disposition of hydroxyl, water and silicon monoxide masers in star-forming regions and in circumstellar envelopes than was possible ten years ago. There have also been a major new developments, including the discovery of extremely powerful megamasers in the nuclei of distant galaxies, the discovery of strong 12 GHz methanol masers in star-forming regions and the discovery of widespread 89 GHz hydrogen cyanide masers in circumstellar envelopes. Large numbers of new maser sources have been fond using the all-sky infrared survey by the IRAS satellite to provide candidates. Maser sources of different types can be identified by their characteristic infrared emission. IRAS follow-up surveys have also revealed new types of circumstellar maser associated with stars in unusual stages of their evolution. On the theoretical front there have been major advances in our understanding of saturation and competitive gain. New mechanisms for producing polarisation in saturated masers have been found, and new pump mechanisms have been proposed for several maser lines, including the first satisfactory pump for strong water masers. This review summarises these and other important developments, concentrating mainly on the new observations and the new types of astrophysical investigation which these have made possible. The use of masers for astronomical distance measurements is also discussed.

The Higgs boson

Abstract: A comprehensive review of the phenomenology of the conventional Higgs boson is presented. The fundamentals of the standard model of electroweak interactions are reviewed. Experimental limits on light Higgs bosons are assessed and prospects for further experimental searches are evaluated.

The spectroscopy of highly ionised atoms

Abstract: Spectroscopic investigations of the structure of highly ionised atoms have undergone significant developments in recent years. The experimental progress is largely due to the introduction of several powerful light sources, such as laser-produced plasmas. Tokamaks and beams of fast, excited ions. The results obtained with these laboratory devices complement the data from astrophysical observations of the solar corona and solar flares. The experimental techniques are described in some detail. The results are discussed with respect to energy level studies along isoelectronic sequences, inner-shell excited levels, forbidden transitions, lifetimes and transition probabilities as well as the results of spectroscopic studies of quantum electrodynamics, QED. The experimental results are compared with theoretical predictions.

Neutral beam heating of fusion plasmas

Abstract: The author deals with the neutral beam heating of toroidal fusion plasmas. A definition of toroidal fusion plasmas and neutral beam heating is attempted in the introduction by describing fusion processes, the hierarchy of confinement devices and heating methods. This simultaneously provides the reader, assumed to be a non-plasma physicist, with the necessary background information.

Synergetics: an overview

Abstract: As is witnessed by the program of the Italian Society for Systemics, as well as by the program of this congress, systemics deals with complex systems that are practically ubiquitous. Just to mention a few examples: schools, universities, and more generally education, hospitals, and more generally health care, factories, companies, cities, economy. In a more general frame, we may consider society. Globalization has been furthered considerably by the internet and the increasing traffic. A different field dealing with complex systems is legislation and still more pressing issues, such as ethics. Biological systems are outstanding examples of complex systems. Of particular interest is the brain. Its action is increasingly mimicked by computers as well as by computer nets. Robotics as well as multi-agent systems provide us with still more examples. There are, however, much simpler model systems in physics and chemistry, such as lasers, fluids and chemical reactions that again and again may play an important role in developing new ideas on how to cope with complex systems. A field of particular interest is quantum physics that will not be covered, however, in the subsequent talk.

Electronic transport in low-dimensional structures

Abstract: Reviews the transport properties of electrons in semiconductor heterojunction structures, in which the degrees of freedom for motion of the charge carriers are reduced by confining potentials, thereby producing low- (i.e. two-, one- or even zero-) dimensional electronic structures. A theoretical treatment of quantisation effects due to two-dimensional (2D) and one-dimensional (1D) confinement, with and without an applied magnetic field, will be given, and used to interpret a range of experimental results obtained from GaAs/AlxGa1-xAs 2D and 1D heterojunction structures. For 2D samples, these results cover low-field mobility measurements, which give information on the scattering mechanisms present, and high-field magnetotransport studies, including the quantum Hall effect and parallel conduction processes. In 1D structures, quantum and ballistic transport processes will be discussed.

Electron scattering off nuclei

Abstract: Electrons and photons are the ideal probes to study the hadronic structure of nuclei and elementary particles. At the interface of nuclear and particle physics, electro- and photonuclear experiments have provided us with clear evidence for subnuclear effects in nuclei. Such investigations become even more interesting in view of recent puzzling results about the free nucleon, e.g. its strange spin structure in deep inelastic scattering and the low threshold amplitude for photoproduction of neutral pions. The advent of a new generation of high-energy, high-intensity electron accelerators operating with a continuous beam will soon open a new frontier, the direct investigation of correlations by coincidence techniques. Systematic experiments with complete kinematics and separation of the independent structure functions will provide new information on the internal degrees of freedom of hadrons and on their interactions in the nuclear medium.

Energy measurement of elementary particles

Abstract: The authors review the techniques used to measure the energy of elementary particles. The methods discussed span more than 20 orders of magnitude in energy, ranging from a small fraction of an electronvolt, where relic remainders of the Big Bang are being looked for, to the 1020 eV domain, where the highest-energy cosmic rays can be found. The emphasis is, however, on techniques employed in particle physics for studying collision processes at accelerators in the GeV-TeV range.

Electron-positron annihilation at high energies

Abstract: The experimental physics programme of PEP and PETRA is presented. The data from 11 independent experiments are collated according to subject matter and compared with theory. The author starts with a brief history of e+e- interactions and then goes on to give brief technical details of the two accelerators and the 11 experimental detectors. The physics of hadron production is considered and the production yield per event of all hadrons detected is given in convenient tabular form. The confrontation of the measurements of hadron production with QCD theory is considered, together with the status of the understanding of fragmentation. A section is devoted to the electroweak interaction and results are presented from an analysis combining the data from the various experiments on lepton and quark pair production.

Nucleon-alpha reactions in nuclei

Abstract: The experimental data on nucleon-alpha reactions to discrete and continuum states are reviewed, together with the theories that have been proposed to account for them. At low energies the compound-nucleus mechanism predominates, and as the energy increases the pre-equilibrium and direct processes become more likely. These theories are described and the results of typical calculations compared with the experimental data. Particular attention is devoted to J-dependent and spectator effects, and to the competition between the triton pick-up and alpha knock-out mechanisms.

Physics of B mesons

Abstract: In this report I discuss the physics of B mesons which were first observed directly in 1986 through their full reconstruction. Emphasis is put on the experimental results on exclusive and inclusive B decays obtained since then by the e"e- experiments ARGUS at DESY and CLEO at Cornell. The results are discussed in the framework of the standard model with three families, on which in particular the BoBo mixing observed in 1987 by ARGUS puts strong constraints.

Measurement of short-lived particles in high-energy physics

Abstract: The authors discuss the techniques of measuring lifetimes in the picosecond range; the relevant particles are the tau lepton, and charm and beauty hadrons. The different types of vertex detectors used to recognise decays and to measure decay distances are described. Lifetimes are generally determined from decay distances; problems caused by experimental cuts, resolution, backgrounds, and unknown momenta are discussed. The measurement of the b lifetime is presented as an example. They conclude by describing decay vertex tagging at present and future colliders.

Modelling of VLSI semiconductor manufacturing processes

Abstract: The manufacture of complex integrated circuits demanded by present-day system designers requires the assembly of a large number of interacting process steps. Many of these process steps cannot be chosen without considering the effect of the other steps involved in the manufacturing process. A great deal of understanding of the basic physical principles must be employed before a successful manufacturing process can be defined. The optimisation of the steps and their timely introduction to enable the device to enter the marketplace dictates that an efficient means of predicting the impact of a particular recipe must be employed. This review surveys the various physical principles that underly the processing steps, highlights some of the anomalous behaviour that occurs and describes the tools currently available to help the process physicist design diffusion, oxidation, ion implantation, deposition and photo-lithographic routines. An example of the use of these tools, and an indication of forthcoming developments, will be given.

Non-perturbative calculations in the Standard Model

Abstract: The Standard Model is a remarkably successful theory of elementary particle interactions on energy scales less than a few hundred Gev. It excludes gravity, so cannot be a complete theory. However, no experiment disagrees sufficiently with the predictions obtained using perturbative approximations to call into question its validity for the other interactions. Its full solution is a major challenge which must be met if we are to have confidence in extending the theoretical framework to much higher energies. As yet unimagined non perturbative mechanisms may be exposed. Computer simulation offers the only known means to this end. This paper explains how the Standard Model may be formulated outside perturbation theory and describes some of the numerical results that have been obtained so far.

Nuclear physics in the United Kingdom 1911-1986

Abstract: After a brief introduction on work in classical radioactivity this article attempts to present the main contributions made in the United Kingdom, up to 1986, to the concept and understanding of the atomic nucleus. Particular phases of the development of nuclear physics in the country have been chosen and in each (if relevant) attention is given to the interaction between the Universities, Government Laboratories and Government and other funding agencies. From the work thus supported some examples of experimental and theoretical advances have been selected.

The great age of British magnetism

Abstract: The author gives his personal recollections of the developments in the science of the magnetisation curve and of magnetic materials in the United Kingdom during the 'great age' of 1940-70. He does not give full details of the scientific arguments but tries to show a broad canvas of how he recollects the main problems presented by the subject and gives a personal view of the men and their contributions to the solutions of these problems.