Showing papers in "Reports on Progress in Physics in 1999"

The pseudogap in high-temperature superconductors: an experimental survey

Abstract: We present an experimental review of the nature of the pseudogap in the cuprate superconductors. Evidence from various experimental techniques points to a common phenomenology. The pseudogap is seen in all high-temperature superconductors and there is general agreement on the temperature and doping range where it exists. It is also becoming clear that the superconducting gap emerges from the normal state pseudogap. The d-wave nature of the order parameter holds for both the superconducting gap and the pseudogap. Although an extensive body of evidence is reviewed, a consensus on the origin of the pseudogap is as lacking as it is for the mechanism underlying high-temperature superconductivity.

Phase separation in confined systems

Abstract: We review the current state of knowledge of phase separation and phase equilibria in porous materials. Our emphasis is on fundamental studies of simple fluids (composed of small, neutral molecules) and well-characterized materials. While theoretical and molecular simulation studies are stressed, we also survey experimental investigations that are fundamental in nature. Following a brief survey of the most useful theoretical and simulation methods, we describe the nature of gas‐liquid (capillary condensation), layering, liquid‐liquid and freezing/melting transitions. In each case studies for simple pore geometries, and also more complex ones where available, are discussed. While a reasonably good understanding is available for phase equilibria of pure adsorbates in simple pore geometries, there is a need to extend the models to more complex pore geometries that include effects of chemical and geometrical heterogeneity and connectivity. In addition, with the exception of liquid‐liquid equilibria, little work has been done so far on phase separation for mixtures in porous media.

Self-organized criticality

Abstract: The concept of self-organized criticality was introduced to explain the behaviour of the sandpile model. In this model, particles are randomly dropped onto a square grid of boxes. When a box accumulates four particles they are redistributed to the four adjacent boxes or lost off the edge of the grid. Redistributions can lead to further instabilities with the possibility of more particles being lost from the grid, contributing to the size of each ‘avalanche’. These model ‘avalanches’ satisfied a power-law frequency‐area distribution with a slope near unity. Other cellular-automata models, including the slider-block and forest-fire models, are also said to exhibit self-organized critical behaviour. It has been argued that earthquakes, landslides, forest fires, and species extinctions are examples of self-organized criticality in nature. In addition, wars and stock market crashes have been associated with this behaviour. The forest-fire model is particularly interesting in terms of its relation to the critical-point behaviour of the sitepercolation model. In the basic forest-fire model, trees are randomly planted on a grid of points. Periodically in time, sparks are randomly dropped on the grid. If a spark drops on a tree, that tree and adjacent trees burn in a model fire. The fires are the ‘avalanches’ and they are found to satisfy power-law frequency‐area distributions with slopes near unity. This forest-fire model is closely related to the site-percolation model, that exhibits critical behaviour. In the forest-fire model there is an inverse cascade of trees from small clusters to large clusters, trees are lost primarily from model fires that destroy the largest clusters. This quasi steady-state cascade gives a power-law frequency‐area distribution for both clusters of trees and smaller fires. The site-percolation model is equivalent to the forest-fire model without fires. In this case there is a transient cascade of trees from small to large clusters and a power-law distribution is found only at a critical density of trees.

Irradiation effects in carbon nanostructures

Abstract: The paper reviews the principles of interaction of energetic particles with solid carbon and carbon nanostructures. The reader is first introduced to the basic mechanisms of radiation effects in solids with particular emphasis on atom displacements by knock-on collisions. The influence of various parameters on the displacement cross sections of carbon atoms is discussed. The types of irradiation-induced defects and their migration are described as well as ordering phenomena which are observable under the non-equilibrium conditions of irradiation. The main part of this review deals with alterations of carbon nanostructures by the electron beam in an electron microscope. This type of experiment is of paramount importance because it allows in situ observation of dynamic processes on an atomic scale. In the second part, radiation effects in the modifications of elemental carbon, in particular in graphite which forms the crystallographic basis of most carbon nanostructures, are treated in detail. It follows a review of the available experimental results on radiation defects in carbon nanostructures such as fullerenes, nanotubes and carbon onions. Finally, the phenomena of structure formation under irradiation, in particular the self-assembling of spherical carbon onions and the irradiation-induced transformation of graphitic nanoparticles into diamond, are presented and discussed qualitatively in the context of non-equilibrium structure formation.

The correlation between mechanical stress and magnetic anisotropy in ultrathin films

Abstract: The impact of stress-driven structural transitions and of film strain on the magnetic properties of nm ferromagnetic films is discussed. The stress-induced bending of film-substrate composites is analysed to derive information on film stress due to lattice mismatch or due to surface-stress effects. The magneto-elastic coupling in epitaxial films is determined directly from the magnetostrictive bending of the substrate. The combination of stress measurements with magnetic investigations by the magneto-optical Kerr effect (MOKE) reveals the modification of the magnetic anisotropy by film stress. Stress-strain relations are derived for various epitaxial orientations to facilitate the analysis of the substrate curvature. Biaxial film stress and magneto-elastic coupling coefficients are measured in epitaxial Fe films in situ on W single-crystal substrates. Tremendous film stress of more than 10 GPa is measured in pseudomorphic Fe layers, and the important role of film stress as a driving force for the formation of misfit distortions and for inducing changes of the growth mode in monolayer thin films is presented. The direct measurement of the magneto-elastic coupling in epitaxial films proves that the magnitude and sign of the magneto-elastic coupling deviate from the respective bulk value. Even a small film strain of order 0.1% is found to induce a significant change of the effective magneto-elastic coupling coefficient. This peculiar behaviour is ascribed to a second-order strain dependence of the magneto-elastic energy density, in contrast to the linear strain dependence that is valid for bulk samples.

Experiments on ladders reveal a complex interplay between a spin-gapped normal state and superconductivity

Abstract: In recent years, the study of ladder materials has developed into a well-established area of research within the general context of strongly correlated electrons. This effort has been triggered by an unusual cross-fertilization between theory and experiments. In this paper, the main experimental results obtained in the context of ladders are reviewed from the perspective of a theorist. Emphasis is given to the many similarities between the two-dimensional high-Tc cuprates and the two-leg ladder compounds, including Sr14-xCaxCu24O41 ([14-24-41]) which has a superconducting phase at high pressure and a small hole density. Examples of these similarities include regimes of linear resistivity versus temperature in metallic ladders and a normal state with spin gap or pseudogap characteristics. It is remarked that the ladder [14-24-41] is the first superconducting Cu oxide material with a non-square-lattice layered arrangement, and certainly much can be learned from a careful analysis of this compound. A short summary of the main theoretical developments in this field is also included, as well as a brief description of the properties of non-Cu-oxide ladders. Suggestions by the author on possible experiments are described in the text. Overall, it is concluded that the enormous experimental effort carried out on ladders has already unveiled quite challenging and interesting physics that adds to the rich behaviour of electrons in transition metal-oxides, and in addition contributes to the understanding of the two-dimensional cuprates. However, considerable work still needs to be carried out to fully understand the interplay between charge and spin degrees of freedom in these materials.

Isotopes in the interstellar medium and circumstellar envelopes

Abstract: Measurements of the present-day abundances of elements and isotopes, combined with model calculations, allow us to trace the history of nucleosynthesis in the universe. Throughout this review, emphasis will be placed on descriptions of the measurement processes and the interpretations needed to obtain actual isotope and element abundances from measurements. Comparisons of the abundances of isotopomers of a given element are less affected by systematic effects than are comparisons of the abundances of different elements. Thus ratios of isotopomers should be given a greater weight when data and models are compared. As is generally accepted, the universe began with an explosive event, the Big Bang. The nucleosynthesis associated with this event produced `primordial' abundances of the `light elements', deuterium, , , and . Subsequent stellar processing of the light elements has altered the relative abundances, and also produced heavier elements such as carbon, nitrogen and oxygen. Stellar nucleosynthesis products from solar and larger mass stars are expelled into the interstellar medium (ISM). The goal of studies of the abundances of the light elements is to estimate the primordial abundances, that is, the abundances produced in the Big Bang. It is believed that D is always net destroyed in stars; and may be net produced, is certainly net produced. In the Solar System itself, results are obtained from in situ measurements with space probes to Jupiter, measurements of solar wind constituents, the analysis of the content of meteorites, and spectral line measurements of the solar photosphere. For sources outside the Solar System, these data are based on spectral line measurements of gas-phase species. The ratio of gas-phase abundances of elements, such as carbon to lithium may be affected by differing amounts of condensation onto dust grains; however such a process will not affect the ratio of isotopes such as . The most reliable measurements of D to H ratios are based on spectroscopic measurements of Lyman series ultraviolet absorption lines from foreground interstellar gas. Measurements of clouds in our galaxy have been obtained with satellites such as the International Ultraviolet Observatory, Copernicus, and the Hubble Space Telescope. The most interesting new development is the measurement of distant clouds with large redshifted velocities. Such data can be taken with Earth-bound optical telescopes. In the near future the Far Ultra Violet Explorer will refine and extend measurements of D/H ratios in relatively nearby regions. Abundances of in the ISM have been measured using the hyperfine transition of , in galactic H II regions which are ionized by high-mass stars. is the most abundant of the light elements. The primordial abundance must be very accurately determined if one wishes to use this quantity to estimate the baryon density in the early universe. Recently /H ratios have been measured in a number of metal-poor compact blue galaxies. These sources seem to have had little stellar evolution, so the ratio should be close to the primordial value. Estimates of the primordial abundance of are made for a population of old stars found far from the plane of our galaxy. A refinement of Li abundance estimates requires a more detailed understanding of the Li destruction processes in stars.

Emulsions: basic principles

Abstract: Emulsions are metastable colloids made out of two immiscible fluids, one being dispersed in the other, in the presence of surface active agents. Emulsion droplets exhibit all the classical behaviours of metastable colloids: Brownian motion, reversible phase transitions as a result of droplet interactions that may be strongly modified, and irreversible transitions that generally involve their destruction. They are obtained by shearing two immiscible fluids leading to the fragmentation of one phase into the other. Because the lifetime of emulsions may become significant (more than a year), they become good candidates for various commercial applications. All these applications have already led to an important empirical control of these materials, from their formation to their destruction. Besides this empirical background, which is widespread among the various specific applications, the basics of emulsions are certainly progressing and we aim in this paper to give an overview of the most recent advances. We will particularly focus on interdroplet forces, reversible phase transitions and colloidal structures, monolayer adhesion and emulsion gels, coarsening and destruction and finally some stability criteria for double emulsions.

X-ray holography

Abstract: A review of holographic methods using hard x-rays is presented. The main emphasis is put on those techniques which aim to produce atomic resolution. For this reason `the inside source concept' and its experimental realizations are discussed in detail. Apart from the general description of holographic imaging with atomic resolution, specific features such as the effect of multiple scattering, translation periodicity and angular dependence of the atomic scattering factor are also given.

Extended irreversible thermodynamics revisited (1988-98)

Abstract: We review the progress made in extended irreversible thermodynamics during the ten years that have elapsed since the publication of our first review on the same subject (Rep. Frog. Phys. 1988 51 1105-72). During this decade much effort has been devoted to achieving a better understanding of the fundamentals and a broadening of the domain of applications. The macroscopic formulation of extended irreversible thermodynamics is reviewed and compared with other non-equilibrium thermodynamic theories. The foundations of EIT are discussed on the bases of information theory, kinetic theory, stochastic phenomena and computer simulations. Several significant applications are presented, some of them of considerable practical interest (non-classical heat transport, polymer solutions, non-Fickian diffusion, microelectronic devices, dielectric relaxation), and some others of special theoretical appeal (superfluids, nuclear collisions, cosmology). We also outline some basic problems which are not yet completely solved, such as the definitions of entropy and temperature out of equilibrium, the selection of the relevant variables, and the status to be reserved to the H-theorem and its relation to the second law. In writing this review, we had four objectives in mind: to show (i) that extended irreversible thermodynamics stands at the frontiers of modern thermodynamics; (ii) that it opens the way to new and useful applications; (iii) that much progress has been achieved during the last decade, and (iv) that the subject is far from being exhausted.

Rheo-nmr: nuclear magnetic resonance and the rheology of complex fluids

Abstract: The application of nuclear magnetic resonance methods to the study of complex fluids under shearing and extensional flows is reviewed. Both NMR velocimetry and spectroscopy approaches are discussed while specific systems studied include polymer melts, rigid rod and random coil polymers in solution, lyotropic and thermotropic liquid crystals and liquid crystalline polymers, and wormlike micelles. Reference is made to food systems.

Ultrasonic imaging of the human body

Abstract: Ultrasonic imaging is a mature medical technology. It accounts for one in four imaging studies and this proportion is increasing. Wave propagation, beam formation, the Doppler effect and the properties of tissues that affect imaging are discussed. The transducer materials and construction of the probes used in imaging are described, as well as the methods of measuring the ultrasonic field. The history of ultrasonic imaging is briefly reviewed. The pulse-echo technique is used for real-time grey-scale imaging and the factors that limit the spatial and temporal resolutions are considered. The construction and performance of transducer arrays are discussed, together with the associated beam steering and signal processing systems. Speckle and scattering by blood are introduced, particularly in the context of the observation of blood flow by means of the Doppler effect and by time-domain signal processing. Colour flow imaging, and the colour coding schemes used for velocity and power imaging, are explained. The acquisition and display of three-dimensional images are discussed, with particular reference to speed and segmentation. Specialized imaging methods, including endoluminal scanning, synthetic aperture imaging, computed tomography, elasticity imaging, microscanning, contrast agents, and tissue harmonic imaging, are reviewed. There is a discussion of issues relating to safety. Conclusions are drawn and future prospects are considered.

Ligand-receptor interactions

Abstract: The formation and dissociation of specific noncovalent interactions between a variety of macromolecules play a crucial role in the function of biological systems. During the last few years, three main lines of research led to a dramatic improvement of our understanding of these important phenomena. First, combination of genetic engineering and X ray cristallography made available a simultaneous knowledg of the precise structure and affinity of series or related ligand-receptor systems differing by a few well-defined atoms. Second, improvement of computer power and simulation techniques allowed extended exploration of the interaction of realistic macromolecules. Third, simultaneous development of a variety of techniques based on atomic force microscopy, hydrodynamic flow, biomembrane probes, optical tweezers, magnetic fields or flexible transducers yielded direct experimental information of the behavior of single ligand receptor bonds. At the same time, investigation of well defined cellular models raised the interest of biologists to the kinetic and mechanical properties of cell membrane receptors. The aim of this review is to give a description of these advances that benefitted from a largely multidisciplinar approach.

Accelerator mass spectrometry and its applications

Abstract: The state of the art in accelerator mass spectrometry (AMS) is reviewed. The review is divided in two parts. The first covers the general methodology, followed by its specific elaborations for the commonly measured long-lived isotopes such as 10Be, 14C, 26Al, 36Cl and 129I, as well as other isotopes with emerging applications. The second part considers the very wide spectrum of applications that now employ AMS, and groups these according to research area rather than isotope. The period up until late 1998 is covered, with an emphasis on post-1990 developments and literature.

Electron diffraction and microscopy of nanotubes

Abstract: Carbon nanotubes were discovered by electron microscopy in the carbon soot produced in an electric arc between graphite electrodes, as used in the production of fullerenes. Details of this microstructure have been studied mainly by the combined use of electron microscopic imaging and electron diffraction. Due to the small size of the tubes, diffraction patterns of single tubes, which are the most informative ones, can only be obtained by electron diffraction. For a complete interpretation of the observed diffraction effects a detailed theory is required. Successively more refined approximations of the theory allow us to understand the origin of the different features of the diffraction patterns. The most complete kinematical theory for the diffraction by single shell chiral straight tubes is obtained by the direct summation of the complex amplitudes of the waves scattered by the carbon atoms arranged on a helically wound graphene network. The closed form analytical expressions deduced in this way make it possible to compute the geometry and the intensity distribution of diffraction space. Diffraction patterns are computed as planar sections of this diffraction space. High-resolution electron microscopic images reveal the geometry of individual graphene sheets and their defects in multishell tubes. As well as the characteristic features of straight nanotubes those of helix shaped tubes are also discussed. It is shown how the combined use of electron diffraction and electron microscopy makes it possible to completely characterize the geometry of carbon nanotubes.

The nonlinear physics of musical instruments

Abstract: Musical instruments are often thought of as linear harmonic systems, and a first-order description of their operation can indeed be given on this basis, once we recognise a few inharmonic exceptions such as drums and bells. A closer examination, however, shows that the reality is very different from this. Sustained-tone instruments, such as violins, flutes and trumpets, have resonators that are only approximately harmonic, and their operation and harmonic sound spectrum both rely upon the extreme nonlinearity of their driving mechanisms. Such instruments might be described as `essentially nonlinear'. In impulsively excited instruments, such as pianos, guitars, gongs and cymbals, however, the nonlinearity is `incidental', although it may produce striking aural results, including transitions to chaotic behaviour. This paper reviews the basic physics of a wide variety of musical instruments and investigates the role of nonlinearity in their operation.

Merged-beams experiments in atomic and molecular physics

Abstract: The merged-beams technique is powerful for the experimental study of certain classes of atomic and molecular processes that cannot be as readily or accurately addressed by other methods. The principal advantages of the technique are the ability to make quantitative studies of collisional interactions with high resolution at low relative energies, to collect products that have undergone appreciable angular scattering, and to investigate processes involving short-lived or chemically-reactive species. Despite continuing advances in ion-source and particle-beam technologies, merged-beams experiments remain a challenge, constituting a relatively small but growing fraction of the worldwide effort in atomic and molecular collisions research. This review outlines the fundamental principles of the merged-beams method, reviews techniques and progress, and focuses on three active programs to highlight the advantages of the method for addressing fundamental questions in atomic and molecular physics.

Transport properties of dilute alloys

Abstract: The review introduces the reader to a theoretical method describing the transport properties of dilute alloys on an ab initio basis. The calculations start from density or spin density functional theory using a Green function formalism to investigate the underlying electronic structure of the ideal and perturbed system on equal footing. The residual resistivity is calculated solving the quasiclassical Boltzmann equation. The theory is outlined and various methods and approximations developed to solve the transport equation are reviewed and compared with respect to accuracy and validity. It will be demonstrated that the theory is able to quantitatively account. The success and limitations of these calculations are discussed for a large variety of systems, non-magnetic and ferromagnetic, in comparison with experimental results. It will be shown that these calculations confirm empirical rules and concepts, elucidate the microscopic processes behind the trends and can be used to make a theoretical material design.

Ultra-low energy antihydrogen

Abstract: The study of CPT invariance with the highest achievable precision in all particle sectors is of fundamental importance for physics. Equally important is the question of the gravitational acceleration of antimatter. In recent years, impressive progress has been achieved at the Low-Energy Antiproton Ring (LEAR) at CERN in capturing antiprotons in specially designed Penning traps, in cooling them to energies of a few milli-electron volts, and storing them for hours in a small volume of space. Positrons have been accumulated in large numbers in similar traps, and low-energy positron or positronium beams have been generated. Finally, steady progress has been made in trapping and cooling neutral atoms. Thus the ingredients to form antihydrogen at rest are at hand. This report will describe the techniques available to produce, decelerate, and accumulate antiprotons at low energy, how to generate high-density plasmas of low-energy positrons, and how to combine these two species into antihydrogen. Once antihydrogen atoms have been formed, they can be captured in magnetic gradient traps and standard spectroscopic methods applied to interrogate their atomic structure with extremely high precision for comparison with the hydrogen atom. In particular, the 1S-2S transition, with a lifetime of the excited state of 122 ms and thereby a natural linewidth of five parts in , offers in principle the possibility to directly compare matter and antimatter properties at a level of one part in . Other quantities of interest, such as the hyperfine structure splitting of the ground state, will also be discussed. Finally, we will give a brief outlook into the future and comment on some of the possible antiproton facilities which could be used to continue this field of research well into the next century.

Nuclear structure in cold rearrangement processes in fission and fusion

Abstract: In fission and fusion of heavy nuclei large numbers of nucleons are rearranged at a scale of excitation energy very small compared with the binding energy of the nuclei. The energies involved are less than 40 MeV at nuclear temperatures below 1.5 MeV. The shapes of the configurations in the rearrangement of a binary system into a monosystem in fusion, or vice versa in fission, change their elongations by as much as 8 fm, the radius of the monosystem. The dynamics of the reactions macroscopically described by a potential energy surface, inertia parameters, dissipation, and a collision energy is strongly modified by the nuclear structure of the participating nuclei. Experiments showing nuclear structure effects in fusion and fission of the heaviest nuclei are reviewed. The reaction kinematics and the multitude of isotopes involved are investigated by detector techniques and by recoil spectrometers. The advancement of the latter allows one to find very small reaction branches in the range -. The experiments reveal nuclear structure effects in all stages of the rearrangement processes. These are discussed, pointing to analogies in fusion and fission on the microscopic scale, notwithstanding that both processes macroscopically are irreversible. Heavy clusters, as , , nuclei with closed-shell configurations N = 82, 126, Z = 50, 82 survive in large parts of the nuclear rearrangement. They determine the asymmetry in the mass distribution of low-energy fission, and they allow the synthesis of superheavy elements, currently up to element 112. Experiments on the cold rearrangement in fission and fusion are presented. Here, in the range of excitation energies below 12 MeV, the phenomena are observed most convincingly.

Mechanism of vortex motion in high-temperature superconductors

Abstract: Different models for the description of vortex motion in real type-II superconducting systems are introduced and applied to high- and low- superconductors, ranging from pin breaking in homogeneous systems to different vortex shear models, which describe the plastic flow of vortices within percolation paths across the superconductor taking into account a distribution of superconducting properties. It is demonstrated, that collective pinning (for instance caused by oxygen vacancies) is sufficiently strong to provide the necessary intrinsic volume pinning force in high- material if a single vortex approach is applied. Thus, inhomogeneities in the sample have to be considered, which lead to a flux shear dominated mechanism of vortex motion in real systems. This mechanism automatically explains a number of interesting phenomena and properties of high- material: the Arrhenius-like phase transition, the `s'-shape of the voltage-current characteristics in the double logarithmic plot, the existence of a reversible regime close to , and the correct temperature scaling, field scaling and magnitude of the volume pinning force. Finally, the inhomogeneity of the superconducting material is derived from experimental data via inversion schemes obtained for different models. The resulting distribution functions for local superconducting properties are comparable for all schemes. Possible explanations for the shape of the distributions are given.

Interface states at semiconductor junctions

Abstract: The experimental and theoretical progress in understanding the electronic structure and the related parameters of Schottky interfaces and heterojunctions is reviewed. Particular emphasis is devoted to the solution of several historical controversial points, to the impact of novel ab initio theoretical approaches, to new experimental techniques based on synchrotron light and free electron lasers, to the efforts towards controlled modifications of interface parameters and to the foreseeable future developments of this vigorously progressing and technologically crucial field.

Archaeological dating using physical phenomena

Abstract: A review is given of the science-based techniques that have been used to establish archaeological chronologies from the million-year range down to the historical period. In addition to the discussion of nuclear, atomic and chemical methods indication is given of the way in which the Earth's magnetic field and perturbations of the Earth's orbital motions are useful in this.

Application of finite element methods to the simulation of semiconductor devices

Abstract: In this paper a survey is presented of the use of finite element methods for the simulation of the behaviour of semiconductor devices. Both ordinary and mixed finite element methods are considered. We indicate how the various mathematical models of semiconductor device behaviour can be obtained from the Boltzmann transport equation and the appropriate closing relations. The drift-diffusion and hydrodynamic models are discussed in more detail. Some mathematical properties of the resulting nonlinear systems of partial differential equations are identified, and general considerations regarding their numerical approximations are discussed. Ordinary finite element methods of standard and non-standard type are introduced by means of one-dimensional illustrative examples. Both types of finite element method are then extended to two-dimensional problems and some practical issues regarding the corresponding discrete linear systems are discussed. The possibility of using special non-uniform fitted meshes is noted. Mixed finite element methods of standard and non-standard type are described for both one- and two-dimensional problems. The coefficient matrices of the linear systems corresponding to some methods of non-standard type are monotone. Ordinary and mixed finite element methods of both types are applied to the equations of the stationary drift-diffusion model in two dimensions. Some promising directions for future research are described.

Theory of Z boson decays

Abstract: The precision data on Z boson decays from LEP-I and SLC colliders are compared with the predictions based on the minimal standard theory. The Born approximation of the theory is based on three most accurately known observables: Gµ - the four fermion coupling constant of muon decay, mZ - the mass of the Z boson, and (mZ) - the value of the running fine structure constant at the scale of mZ. The electroweak loop corrections are expressed, in addition, in terms of the masses of higgs, mH, of the top and bottom quarks, mt and mb, and of the strong interaction constant s(mZ). The main emphasis of the review is focused on the one-electroweak-loop approximation. Two electroweak loops have been calculated in the literature only partly. Possible manifestations of new physics are briefly discussed.

Electron-ion scattering

Abstract: Recent advances in the elucidation of electron-ion scattering phenomena is reviewed, with particular emphasis on the new generation of experiments where scattered electrons are analysed and detected. The sensitivity of measurements as a probe of collision dynamics, application to plasma studies, and future directions are considered.

Dynamical symmetries in the structure of nuclei

Abstract: The use of spectrum generating algebras in the description of the nuclear many-body system is reviewed. General notions of symmetry and dynamical symmetry in quantum mechanics are introduced with the help of simple examples. It is then indicated how techniques based on symmetry considerations can be used to find the analytical solutions for the problem of an aggregate of interacting particles (bosons and/or fermions). Some older ideas due to Wigner, Racah and Elliott are succinctly summarized to put more recent advances in a proper perspective. It is then shown that similar techniques are used in a model of the nucleus in terms of interacting bosons due to Arima and Iachello. Subsequent extensions of this model to odd-mass nuclei lead to the consideration of mixed systems of bosons and fermions and, most notably, to supersymmetry.

Exploring solid state physics properties with radioactive isotopes

Abstract: Radioactive atoms have been used in solid state physics for many years. Established nuclear techniques such as M??bauer spectroscopy, perturbed angular correlation, -NMR and emission channelling have now been joined by new and successful tracer techniques like radioactive deep level transient spectroscopy, capacitance voltage measurements, Hall-effect measurements or photoluminescence spectroscopy. Numerous radioactive species, ranging from to , are employed to attack problems involved with defects or impurities in metals, semiconductors and superconductors. This paper aims to give an idea of the potential of modern `radioactive solid state physics' by briefly explaining the techniques used and describing some typical experiments.

El Niño and La Niña predictable climate fluctuations

Abstract: The appearance of unusually warm sea surface temperatures in the eastern equatorial Pacific, the signature of El Nino, alters the location and intensity of regions of deep convection in the tropics, and thus affects the global atmospheric circulation. The increase in the temperature of the surface waters is part of the oceanic response to the altered atmospheric conditions, especially the changes in the trade winds over the Pacific. This circular argument - the warm surface waters are both the cause and consequence of the changes in atmospheric conditions - implies that the interactions between the tropical Pacific Ocean and the atmosphere amount to a positive feedback and can result in natural modes of oscillation with timescales of the order of a few years. The Southern Oscillation, between complementary El Nino and La Nina states, corresponds to such a mode. Considerable progress has been made towards a capability to predict El Nino, La Nina, and climate fluctuations in general: an array of instruments now monitors the tropical Pacific; coupled ocean-atmosphere models capable of simulating and forecasting El Nino are growing rapidly in realism and skill. The predictability of El Nino appears to vary with time, probably because the Southern Oscillation is self-sustaining and hence highly predictable during some decades, damped and hence difficult to predict in other decades. During the latter periods, bursts of westerly winds that sporadically persist over the western equatorial Pacific for a week or two, have a spatial structure that enables them to excite the Southern Oscillation. Attention is now turning to the exchanges between the tropical and extra-tropical oceans that influence the decadal modulation of El Nino.

Symmetries in the structure of nuclei

Abstract: The use of spectrum generating algebras in the description of the nuclear many-body system is reviewed. General notions of symmetry and dynamical symmetry in quantum mechanics are introduced with the help of simple examples. It is then indicated how techniques based on symmetry considerations can be used to find the analytical solutions for the problem of an aggregate of interacting particles (bosons and/or fermions). Some older ideas due to Wigner, Racah and Elliott are succinctly· summarised to put more recent advances in a proper perspective. It is then shown that similar techniques are used in a model of the nucleus in terms of interacting bosons due to Arima and Iachello. Subsequent extensions of this model to odd-mass nuclei lead to the consideration of mixed systems of bosons and fermions and, most notably, to supersymmetry.