Showing papers in "Reports on Progress in Physics in 1997"

Atomic physics with super-high intensity lasers

Abstract: We review the phenonomena which occur in multiphoton physics when the electric field of the applied laser radiation becomes comparable with the Coulomb field strength seen by an electron in the ground state of atomic hydrogen. This field is reached at an irradiance of approximately . The normal perturbative photon-by-photon based picture of the interaction of individual electrons with the field is replaced by a tunnelling picture in which, in a time of the order of, or less than one optical cycle, atomic wavepackets are generated which escape the confining Coulomb potential. These wavepackets are strongly influenced by the laser, `quiver' and may be accelerated back to the parent ion in a recollision process. Phase-coherent effects locked to the laser field become important: high harmonics are generated from these recollisions. We discuss the theory of such effects, and review progress in understanding how this quiver motion can be coherently controlled. We discuss ionization dynamics and review mechanisms by which atoms may be stabilized in very strong fields. Finally, we discuss relativistic effects which occur at very high-intensities.

Applications of Monte Carlo methods to statistical physics

Abstract: An introductory review of the Monte Carlo method for the statistical mechanics of condensed matter systems is given. Basic principles (random number generation, simple sampling versus importance sampling, Markov chains and master equations, etc) are explained and some classical applications (self-avoiding walks, percolation, the Ising model) are sketched. The finite-size scaling analysis of both second- and first-order phase transitions is described in detail, and also the study of surface and interfacial phenomena as well as the choice of appropriate boundary conditions is discussed. Only brief comments are given on topics such as applications to dynamic phenomena, quantum problems, and recent algorithmic developments (new sampling schemes based on reweighting techniques, nonlocal updating, parallelization, etc). The techniques described are exemplified with many illustrative applications.

Nanometre-size tubes of carbon

Abstract: We review the present state of understanding of the structure, growth and properties of nanometre-size tubes of carbon. Two different types of carbon nanotubes, namely single-shell nanotubes made of single layers of graphene cylinders and multishell nanotubes made of concentric cylinders of graphene layers have now become available. The subtle structure parameters such as helicity in the carbon network and the nanometre diameters give the nanotubes a rich variety in physical properties. Recent experimental progress on the measurements of properties using electron-energy loss spectroscopy, Raman spectroscopy, electron-spin resonance, electrical conductance, mechanical stiffness and theoretical predictions on electronic and mechanical properties of nanotubes will be discussed. In addition to synthesis techniques, methods to purify and make aligned arrays of nanotubes will be described. Different approaches for fabricating composite structures using nanotubes as moulds and templates and their future implications in materials science will be evaluated. Finally, promising areas of future applications, for example as tiny field-emitting devices, micro-electrodes, nanoprobes and hydrogen storage material will be outlined.

Tight-binding modelling of materials

Abstract: The tight-binding method of modelling materials lies between the very accurate, very expensive, ab initio methods and the fast but limited empirical methods. When compared with ab initio methods, tight-binding is typically two to three orders of magnitude faster, but suffers from a reduction in transferability due to the approximations made; when compared with empirical methods, tight-binding is two to three orders of magnitude slower, but the quantum mechanical nature of bonding is retained, ensuring that the angular nature of bonding is correctly described far from equilibrium structures. Tight-binding is therefore useful for the large number of situations in which quantum mechanical effects are significant, but the system size makes ab initio calculations impractical. In this paper we review the theoretical basis of the tight-binding method, and the range of approaches used to exactly or approximately solve the tight-binding equations. We then consider a representative selection of the huge number of systems which have been studied using tight-binding, identifying the physical characteristics that favour a particular tight-binding method, with examples drawn from metallic, semiconducting and ionic systems. Looking beyond standard tight-binding methods we then review the work which has been done to improve the accuracy and transferability of tight-binding, and moving in the opposite direction we consider the relationship between tight-binding and empirical models.

Ring-laser tests of fundamental physics and geophysics

Abstract: HeNe ring-laser gyros are standard sensors in inertial guidance; mirror reflectances now reach 99.9999%. Present research instruments have an area of , a passive quality factor of , and a resolution of the frequency difference of counter-rotating optical beams approaching microhertz. In the Sagnac effect, this difference is proportional to the angular velocity. Present resolution is limited by thermal drifts in frequency pulling, itself reflecting mirror backscatter. The capability of ring lasers for measurements of geodesic interest, including seismometry and earth tides, and for detection of other sources of non-reciprocal refractive indices, including axions and CP violation, are discussed. In standard polarization geometries the observable is necessarily time-reversal odd. Scaling rules for dimensions, finesse etc summarizing past progress and suggesting future potential are given.

Biosensors: recent advances

Abstract: This review discusses recent advances in biosensor technology which draw on the disciplines of physics, chemistry, biochemistry and electronics. We first show that a biosensor consists of three components, a biological detection system, a transducer and an output system. Biological receptors are briefly reviewed, followed by a detailed discussion of immobilization procedures for the efficacious attachment of receptor molecules to a transducer surface. Two classes of transducers, optical and piezoelectric, which involve interesting physics and show particular promise for commercial biosensors, are discussed thoroughly. We comment briefly on practical factors affecting the commercialization of various biosensors.

Time-resolved fluorescence and photon migration studies in biomedical and model random media

Abstract: This review highlights time-resolved fluorescence kinetics and photon transport in tissues and other biomedical media with a special emphasis on ultrafast measurements of key optical parameters. Measurements of fluorescence decay lifetimes from human breast and atherosclerotic artery tissues in the uv and visible region are described after a brief description of fundamentals of fluorescence kinetics. A time-dependent diffusion model for photon migration and various ultrafast methods for time-resolved light scattering measurements to obtain key optical parameters of tissues and other model turbid media are presented. The usefulness of optical parameters as markers in optical diagnostics and imaging is considered. Time-gated measurements of ballistic and snake photons to obtain shadowgrams and an inverse numerical reconstruction of the interior map of a turbid medium from time-resolved data in the context of optical tomography are presented.

The Newtonian gravitational constant: recent measurements and related studies

Abstract: Improvements in our knowledge of the absolute value of the Newtonian gravitational constant, G, have come very slowly over the years. Most other constants of nature are known (and some even predictable) to parts per billion, or parts per million at worst. However, G stands mysteriously alone, its history being that of a quantity which is extremely difficult to measure and which remains virtually isolated from the theoretical structure of the rest of physics. Several attempts aimed at changing this situation are now underway, but the most recent experimental results have once again produced conflicting values of G and, in spite of some progress and much interest, there remains to date no universally accepted way of predicting its absolute value. The review will assess the role of G in physics, examine the status of attempts to derive its value and provide an overview of the experimental efforts that are directed at increasing the accuracy of its determination. Regarding the latter, emphasis will be placed on describing the instrumentational aspects of the experimental work. Related topics that are also discussed include the search for temporal variation of G and recent investigations of possible anomalous gravitational effects that lie outside of presently accepted theories.

Parity violation in atoms

Abstract: Optical experiments have demonstrated cases in which mirror symmetry in stable atoms is broken during the absorption or emission of light. Such results, which are in conflict with quantum electrodynamics, support the theory of unification of the electromagnetic and weak interactions. The interpretation of the experimental results is based on exchanges of weak neutral bosons between the electrons and the nucleus of the atom. A concise review of these phenomena in atomic physics is presented. The role of precise caesium parity-violation experiments, as a source of valuable information about electroweak physics, is illustrated by examples pertaining to experimental conditions which, in some cases, are not accessible to accelerator experiments. We give the basic principles of experiments, some under way and others completed, where a quantitative determination of the nuclear weak charge, , which plays for the exchange the same role as the electric charge for the Coulomb interaction is to be, or has been achieved. In the most recent and most precise experiment the accuracy on is limited to 1 % by the uncertainty due to atomic physics calculations. Such a result challenges specialists in atomic theory and nuclear structure, since a more accurate determination of would mean more stringent constraints upon possible extensions of the standard model. Moreover, clear evidence has recently been obtained for the existence of the nuclear anapole moment, which describes the valence electron interaction with a chiral nuclear-magnetization component induced by the parity-violating nuclear forces. In writing this review, our hope was to make clear that any improvement in atomic parity-violation measurements will allow the exploration of new areas of electroweak physics.

Segregation and immiscibility in liquid binary alloys

Abstract: An overview of the thermophysical properties of segregating and demixing liquid binary alloys is presented encompassing the simple approaches that are commonly used by physicists, chemists and metallurgists in general. The various experimental and theoretical information available for such systems are put together to establish a respectable understanding between the experimental results, theoretical approaches and the empirical models. The key to understanding is the deviation that the properties exhibit from Raoult's law and the marked change in the liquid phase as a function of composition, temperature and pressure. The characteristic behaviour is ascribed to an outcome of the interplay of the energetic and structural re-adjustment of the constituent elements on mixing. After summarizing the experimental technique and some results, a comprehensive microscopic approach, based on statistical, electronic and hard sphere-like theory, is undertaken to further the understanding of the origin of the intriguing processes that are associated with the immiscible and phase-separating liquid alloys. We conclude by providing a brief account of the kinetic aspects of phase separation with some intended industrial applications.

Electron - ion recombination processes - an overview

Abstract: Extensive theoretical and experimental studies have been carried out for the past 20 years on electron - ion recombination processes, as they are applied to the analysis of astrophysical and laboratory plasmas. We review the basic understanding gained through these efforts, with emphasis on some of the more recent progress made in recombination theory as the recombining system is affected by time-dependent electric fields and plasma particles at low temperature. Together with collisional ionization and excitation processes, recombination is important in determining ionization balance and excited-state population in non-equilibrium plasmas. The radiation emitted by plasmas is usually the principal medium with which to study the plasma condition, as it is produced mainly during the recombination and decay of excited states of ions inside the plasma. This is especially true when the plasma under study is not readily accessible by direct probes, as in astrophysical plasmas. Moreover, external probes may sometimes cause undesirable disturbances of the plasma. Electron-ion recombination proceeds in several different modes. The direct modes include three-body recombination (TBR) and one-step radiative recombination (RR), all to the ground- and singly-excited states of the target ions. By contrast, the indirect resonant mode is a two-step dielectronic recombination (DR), which proceeds first with the formation of doubly-excited states by radiationless excitation/capture. The resonant states thus formed may relax by autoionization and/or radiative cascades. For more exotic modes of recombination, we consider off-shell dielectronic recombination (radiative DR = RDR), in which an electron capture is accompanied by simultaneous radiative emission and excitation of the target ion. Some discussion on attachment of electrons to neutral atoms, resulting in the formation of negative ions, is also given. When resonance states involve one or more electrons in high Rydberg states, presence of an external or intrinsic electric field in the vicinity of the target ions can seriously affect the ionic states involved and the resulting reaction rates. Such perturbative fields can be intrinsic, as in the case of the plasma ion field, or externally imposed. A proper theoretical treatment of this difficult problem is crucial in understanding the recombination process which takes place in a field contaminated environment. The simple off-shell dressing procedure of high Rydberg states by a time-dependent field is reviewed, and the possibility of an anomalously large enhancement in the rates, due to the momentum coherence effect (MCE), is discussed. The presently available data on recombination rates are summarized, and several important deficiencies and future directions for further research are pointed out. Based on the detailed calculations for a number of cases, several empirical rate formulae for RR and DR processes have been generated to summarize the data for ready applications. As the collection of atoms is cooled to very low temperatures, , and the bound electrons are ionized by laser irradiation to states of very precisely controlled energies, the prospect for accurate experimental measurements of very-low-energy recombination rates is considered, where the electron temperature can be very low. Therefore, it is of interest to reconsider theoretically some new phenomena which may occur at such cold environments, in which the electron de Broglie wavelength can be very large, and both the density and coherent effects, as well as possible field effects, must be properly taken into account. Finally, a broader understanding of the various recombination processes may be achieved by studying their relationships to other reactions initiated by electron, ion and photon impact.

Theory of semiconductor surface reconstruction

Abstract: We present a review of semiconductor surface reconstruction. Experimental and theoretical results on atomic geometry, electronic states, phonon modes, and bonding are presented for clean cleaved, clean epitaxially grown, overlayer covered, surfactant mediated epitaxially grown, and defect induced reconstructed semiconductor surfaces. Energetic aspects of reconstructions are discussed using empirical as well as first-principles theoretical approaches.

Spin-polarized photoemission

Abstract: Spin-polarized photoemission has developed into a versatile tool for the study of surface and thin film magnetism. In this review, we examine the methodology of the technique and its application to a number of different problems, including both valence band and core level studies. After a detailed review of spin-polarization measurement techniques and the related experimental requirements we consider in detail studies of the bulk properties both above and below the Curie temperature. This section also includes a discussion of observations relating to unique metastable phases obtained via epitaxial growth. The application of the technique to the study of surfaces, both clean and adsorbate covered, is reviewed. The report then examines, in detail, studies of the spin-polarized electronic structure of thin films and the related interfacial magnetism. Finally, observations of spin-polarized quantum well states in non-magnetic thin films are discussed with particular reference to their mediation of the oscillatory exchange coupling in related magnetic multilayers.

The realization of atomic resolution with the electron microscope

Abstract: The high-resolution electron microscope has evolved into a sophisticated instrument that is capable of routinely providing quantitative structural information on the atomic scale. Applications of atomic-resolution imaging can now be found in many scientific fields, and its impact on the knowledge and understanding of atomistic processes has been profound. Better control over instrumental parameters, enhanced reliability of signal recording, and novel methods for imaging and data processing are areas of highly active, ongoing research. Agreement over reliability factors, serious discrepancies in absolute contrast levels, and inversion of crystal scattering are identified as unresolved issues requiring further attention.

Open questions in the magnetic behaviour of high-temperature superconductors

Abstract: A principally experimental review of vortex behaviour in high-temperature superconductors is presented. The reader is first introduced to the basic concepts needed to understand the magnetic properties of type II superconductors. The concepts of vortex melting, the vortex glass, vortex creep, etc are also discussed briefly. The bulk part of the review relates the theoretical predictions proposed for the vortex system in high temperature superconductors to experimental findings. The review ends with an attempt to direct the reader to those areas which still require further clarification.

Tunnelling and relaxation in semiconductor double quantum wells

Abstract: Double quantum wells are among the simplest semiconductor heterostructures exhibiting tunnel coupling. The existence of a quantum confinement effect for the energy levels of a narrow single quantum well has been largely studied. In double quantum wells, in addition to these confinement effects which characterize the levels of the isolated wells, one faces the problem of describing the eigenstates of systems interacting weakly through a potential barrier. In addition, the actual structures differ from the ideal systems studied in the quantum mechanics textbooks in many aspects. The presence of defects leads, for instance, to an irreversible time evolution for a population of photocreated carriers. This irreversible transfer is now clearly established experimentally. The resonant behaviour of the transfer has also been evidenced, from the study of biased structures. If the existence of an interwell transfer is now clearly established from the experimental point of view, its theoretical description, however, is not fully satisfactory. This review focuses on the theoretical description of the energy levels and of the interwell assisted transfer in double quantum wells. We shall firstly outline the problem of tunnel coupling in semiconductor heterostructures and then discuss the single particle and exciton eigenstates in double quantum wells. In the remaining part of the review we shall present and critically review a few theoretical models used to describe the assisted interwell transfer in these structures.

Trapped-ion and trapped-atom microwave frequency standards

Abstract: The stability and accuracy of atomic microwave frequency standards (atomic clocks) have been improving at a rate exceeding one order of magnitude per decade for the past 30 years. This sustained improvement has been driven mainly by the many and diverse technological applications requiring highly stable and accurate time and frequency standards. There is no reason to suspect that this rate of progress is slowing. Over the last decade, research and development in atomic frequency standards has tended increasingly towards the use of atom and ion trapping technologies to approach the supposedly ideal unperturbed atomic frequency reference consisting of a single atom or ion either motionless or in free-fall in a perfect, field-free vacuum. This work has been facilitated by the relatively recent development of techniques whereby laser light is used to cool, confine and manipulate atoms or ions. This paper reviews the current status of atomic microwave frequency standards based on trapped atoms and trapped ions, and attempts to explain the motivation for their development and the principles of their operation, with particular emphasis on the physics of factors which limit their performance.

Superlattices of high-temperature superconductors: synthetically modulated structures, critical temperatures and vortex dynamics

Abstract: We present in this paper an overview of the work that has been performed on high-temperature superconductor superlattices, with a special emphasis on the superconducting properties of multilayers. The critical temperature and superconductivity in ultra-thin films, the vortex dynamics, and the field-temperature vortex phase diagram in multilayers are studied and compared to bulk behaviour. In particular, the roles of the individual layer thicknesses and the conductivity of the spacer material are studied, as well as the coupling between superconducting layers and the effect of finite multilayer thickness. Comparisons with other systems are made, and an interpretation is proposed for the understanding of superconductivity in ultra-thin oxide films. The field-temperature vortex phase diagram is determined in coupled multilayers and compared to single crystal results. Finally, we discuss the development of novel oxide systems and heterostructures, and the possibilities brought out by new material combinations.

Movement and fluctuations of the vacuum

Abstract: Quantum fields possess zero-point or vacuum fluctuations which induce mechanical effects, namely generalized Casimir forces, on any scatterer. Symmetries of vacuum therefore raise fundamental questions when confronted with the principle of relativity of motion in vacuum. The specific case of uniformly accelerated motion is particularly interesting, in connection with the much debated question of the appearance of vacuum in accelerated frames. The choice of Rindler representation, commonly used in general relativity, transforms vacuum fluctuations into thermal fluctuations, raising difficulties of interpretation. In contrast, the conformal representation of uniformly accelerated frames fits the symmetry properties of field propagation and quantum vacuum and thus leads us to extend the principle of relativity of motion to uniform accelerations. Mirrors moving in vacuum with a non-uniform acceleration are known to radiate. The associated radiation reaction force is directly connected to fluctuating forces felt by motionless mirrors through fluctuation - dissipation relations. Scatterers in vacuum undergo a quantum Brownian motion which describes irreducible quantum fluctuations. Vacuum fluctuations impose ultimate limitations on measurements of position in space - time, and thus challenge the very concept of space - time localization within a quantum framework. For test masses greater than Planck mass, the ultimate limit in localization is determined by gravitational vacuum fluctuations. Not only positions in space - time, but also geodesic distances, behave as quantum variables, reflecting the necessary quantum nature of an underlying geometry.

Time-resolved diffraction studies of muscle using synchrotron radiation

Abstract: Muscle contraction is one of those biological phenomena that we can all appreciate in our everyday lives. Sometimes it is when we are resting quietly and are aware of our heartbeat. At other times it may be when we are exerting ourselves and become short of breath, or when we exercise for a long period and our muscles start to ache. The way in which muscles produce force has exercised the minds of philosophers and scientists at least since the days of Erasistratus in the third century BC. Nowadays, of course, we know a very great deal about muscle structure, physiology and biochemistry, but we still do not know exactly what the molecular process is that produces movement. An ideal way of probing this process would be to be able to obtain signals from the relevant molecules as they actually go through their normal force-generating routine in an active muscle. The spatial dimensions involved are in the region of 1 - 50 nm, thus precluding the use of light microscopy, and the time regime is microseconds to milliseconds. Techniques with the appropriate spatial resolution might be electron microscopy and x-ray diffraction, but electron microscopy cannot yet be carried out on living tissue. X-ray diffraction methods can clearly have the right sort of spatial resolution, but what about recording diffraction patterns in the very short times involved (say 1 ms)? It is here that the high flux from synchrotron storage rings comes into its own. Using synchrotron radiation from, say, the SRS at the CCLRC Daresbury Laboratory it is possible to record x-ray diffraction patterns from living muscles in the millisecond time regime and to follow how these diffraction patterns change as the muscles go through typical contraction cycles. Unfortunately, x-ray diffraction is not a direct imaging method; the observed distribution of diffracted intensity needs to be interpreted in some way to give useful information on the spatial relationships of the force-generating molecules. This review details the practical methods involved in recording time-resolved x-ray diffraction patterns from active muscles and the theoretical approaches that are being used to interpret the diffraction patterns that are obtained. The ultimate aim is to produce a series of time-sliced images of the changing molecular arrangements and shapes in the muscle as force is produced; together these images will form `Muscle - The Movie'.

High- - a negative-U interpretation of cuprate superconductivity, incorporating questions of crossover, micro-inhomogeneity and a compound order parameter

Abstract: The two-subsystem, negative-U interpretation of high-temperature superconducting (HTSC) behaviour in the mixed-valent, square-planar cuprate metals is pursued further. The question of the symmetry of the order parameter is central to understanding the HTSC phenomenon. Many have by now examined the potentialities of a mixed order parameter to cope with the wealth of data of all types available from these materials. It is emphasized here that it is actually a mistake for the present mixed-valent, highly tight-binding systems to seek a single, spatially homogeneous, compound order parameter. The systems are demonstrably micro-inhomogeneous due to charge segregation as in many substituted systems very close to Mott localization. In these circumstances there is a fascinating interplay between magnetism, charge and structural aspects to the problem at the unit cell level. Questions of local moment seeding, of local trapping of carriers structurally and magnetically, of on-site Jahn - Teller and bonding distortions, on the one hand, are highly intertwined, on the other, with somewhat more delocalized aspects to the problem, like SDWs, CDWs, the resonant valence band state, etc. All these matters have to be treated simultaneously, in a way appropriate to the precise level of carrier doping secured. Control over the outcome is dependent, moreover, from system to system upon the detailed degree of covalency introduced into the problem by the particular counter-ions employed. The present review makes an attempt to hold together all the published information on a coherent basis. That basis is seen as being provided by a negative-U sequencing of state energies in the mixed-valent cuprates, once it is accepted that the systems are not homogeneous at the micro-level. Results on the LSCO system, for example, find interpretation in terms of moderately well organized discommensurations in charge and spin distribution. The fact that this approach has been followed from the beginnings of high-temperature superconduction, now nearly ten years ago, without any sustained discouragement from the accumulating data, indicates that it provides a means of interpretation worth more direct attention than it has received to date. In particular it provides very naturally a means to understand what it is that confines high-temperature superconduction to its narrow range - the square-planar, layered, mixed-valent cuprates. There is in essence only one high-temperature superconductor and this paper attempts again to say why.

Material-specific gap function in the t - J model of high-temperature superconductors

Abstract: We present theoretical arguments and experimental support for the idea that high- superconductivity can occur with s-wave, d-wave, or mixed-wave pairing in the context of a magnetic mechanism. The size and shape of the gap is different for different materials. The theoretical arguments are based on the t - J model as derived from the Hubbard model so that it necessarily includes three-site terms. We argue that this should be the basic minimal model for high- systems. We analyse this model starting with the dilute limit which can be solved exactly, passing then to the Cooper problem which is numerically tractable, then ending with a mean-field approach. It is found that the relative stability of s-wave and d-wave depends on the size and the shape of the Fermi surface. We identify three striking trends. First, materials with large next-nearest-neighbour hopping (such as ) are nearly pure d-wave, whereas nearest-neighbour materials (such as ) tend to be more s-wave like. Second, low hole-doping materials tend to be pure d-wave, but high hole-doping leads to s-wave. Finally, the optimum hole-doping level increases as the next-nearest-neighbour hopping increases. We examine the experimental evidence and find support for this idea that the gap function in the high-temperature superconductors is material specific.

Nonlinear systems: between a law and a definition

Abstract: A discussion relevant to the logical foundation of nonlinear dynamic systems theory and statistical theory is presented. The initial point of view is associated with the question of whether or not an equation can represent a physical law. It is argued that the answer depends on which of the associated concepts/processes - that of direct physical measurement or that of counting - is more important in the theory of the relevant system. Without the inclusion of the concept of counting in the set of the most basic physical concepts, consideration of the nonlinear dynamic (or statistical) systems which may have an `iterative mathematical nature' is seen to be logically unsatisfactory. The ergodic hypothesis is considered, from the points of view of direct physical measurement and counting. It is shown that for the foundation of statistical theory we must consider `measurement in principle' even when we cannot, by reason of the complexity of the system, actually perform this measurement. The discussion inevitably involves many concepts, which reflects the essence of the very complicated situation under discussion. This gives the work, which is intended, first of all, for a reader with a physics background, a logical-critical inclination. The latter is justified also by the opinion here that the solution to the fundamental problems of `nonlinear physics' cannot come from the existing physics. It is argued that the topic of intuitionistic logic has to be considered for axiomization of the nonlinear dynamic theory. Though the work is not a review of the results of the existing dynamic or statistical theories, the discussion of the logical problems in the foundation of these theories is relatively much more complete than it usually is in such reviews. The classical results of analytical mechanics are finally considered, including some nontrivial applications.

Spin-polarized - liquids

Abstract: Dilute liquid - liquids offer a model system for testing theories of weakly interacting fermions. The ability to spin polarize the nuclei adds an extra dimension to the field. In this review, the extraordinary way in which the polarization of the nuclear spins can modify the macroscopic behaviour of the liquid is explained. Frequent reference to recent experiments and theoretical developments are made and the various techniques for producing high polarizations are described.

Our cometary environment

Abstract: The encounter of a small armada of spacecraft with Halley's Comet in 1986, the disintegration and multiple impact of Comet Shoemaker - Levy 9 on Jupiter in 1994, and the application of new technologies to the detection of distant solar system bodies, have led to great revisions in the understanding of comets. Further, rapid improvements in computing power and numerical techniques have permitted the dynamical evolution of comets and asteroids to be followed far into the future and past, and the relationships between families of small interplanetary bodies to be explored. The small body environment is now generally recognized as strongly interacting with the terrestrial one, and may be hazardous on timescales of human as well as geological interest. We review our current understanding of the cometary environment, with particular regard to the hazard it presents. It appears that many comets are handed down from the Oort - Opik cloud, which is dynamically sensitive to the galactic environment, through the planetary system into Earth-crossing orbits. Thus, the terrestrial environment is subject to stresses which vary cyclically on a number of timescales from planetary to galactic.

Heavy-quark effective field theory

Abstract: The heavy-quark expansion in QCD has become a widely used tool in heavy-quark physics The theoretical aspects of the expansion ( being the mass of the heavy quark) and some techniques that appear in practical calculations are reviewed Some applications to processes involving heavy quark decays are discussed

Temperature-dependent electronic structure: from heavy fermion behaviour to phase stability

Abstract: The temperature (T)-dependent density functional approach and some methods for calculating bands in structurally disordered systems are described. The band results are used for determining thermodynamic properties. The methods are applied to determine T-dependent properties in systems which are sensitive to temperature as the result of special density-of-state features at the Fermi energy. Such systems include heavy fermion Ce-compounds, FeSi, and fcc Ce, and the properties are specific heat enhancements, large magnetic susceptibility, and the - transition, respectively. The phase stability of transition metals at high T are studied with the purpose of separating the electronic and vibrational effects. T-dependent effects in copper oxides leading to nonlinear electron - phonon coupling are discussed. Finally, calculations of momentum densities are shown to give a sensitive indication of thermal disorder in alkali metals. It is found that the effects of disorder are often similar to the effects of strong correlation.

Composite fermions in the quantum Hall effect

Abstract: The quantum Hall effect and associated quantum transport phenomena in low-dimensional systems have been the focus of much attention for more than a decade. Recent theoretical development of interesting quasiparticles - `composite fermions' - has led to significant advances in understanding and predicting the behaviour of two-dimensional electron systems under high transverse magnetic fields. Composite fermions may be viewed as fermions carrying attached (fictitious) magnetic flux. Here we review models of the integer and fractional quantum Hall effects, including the development of a unified picture of the integer and fractional effects based upon composite fermions. The composite fermion picture predicts remarkable new physics: the formation of a Fermi surface at high magnetic fields, and anomalous ballistic transport, thermopower, and surface acoustic wave behaviour. The specific theoretical predictions of the model, as well as the body of experimental evidence for these phenomena are reviewed. We also review recent edge-state models for magnetotransport in low-dimensional devices based on the composite fermion picture. These models explain the fractional quantum Hall effect and transport phenomena in nanoscale devices in a unified framework that also includes edge state models of the integer quantum Hall effect. The features of the composite fermion edge-state model are compared and contrasted with those of other recent edge-state models of the fractional quantum Hall effect.

The trans-Neptunian region

Abstract: The search for additional planets in the Solar System beyond the known planets has fascinated the human race since time immemorial. During the second half of this century it was realized that the source of the new comets that we observe today must also be located beyond the planet Neptune and so as time progressed, searches of the trans-Neptunian region were conducted more in the hope of finding proto-comets rather than a new planet. In 1992, a body was found orbiting the Sun in the region beyond the Planets and, since that date the known population has grown to a total of nearly forty bodies. This review is concerned with the search for these bodies and a summary of our knowledge to date of the known members of the population.