Showing papers in "Reports on Progress in Physics in 1998"
TL;DR: Ferroelectric, dielectric, and piezoelectric properties of ferroelectric thin films and ceramics are reviewed with the aim of providing an insight into different processes which may affect the behaviour of Ferroelectric devices.
Abstract: Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin films and ceramics are reviewed with the aim of providing an insight into different processes which may affect the behaviour of ferroelectric devices, such as ferroelectric memories and micro-electro-mechanical systems. Taking into consideration recent advances in this field, topics such as polarization switching, polarization fatigue, effects of defects, depletion layers, and depolarization fields on hysteresis loop behaviour, and contributions of domain-wall displacement to dielectric and piezoelectric properties are discussed. An introduction into dielectric, pyroelectric, piezoelectric and elastic properties of ferroelectric materials, symmetry considerations, coupling of electro-mechanical and thermal properties, and definitions of relevant ferroelectric phenomena are provided.
1,835 citations
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TL;DR: The most suitable approach up to now for studying excited-state properties of extended systems is the Green function method as discussed by the authors, which has turned out to be a fruitful approximation to the self-energy.
Abstract: Calculations of ground-state and excited-state properties of materials have been one of the major goals of condensed matter physics. Ground-state properties of solids have been extensively investigated for several decades within the standard density functional theory. Excited-state properties, on the other hand, were relatively unexplored in ab initio calculations until a decade ago. The most suitable approach up to now for studying excited-state properties of extended systems is the Green function method. To calculate the Green function one requires the self-energy operator which is non-local and energy dependent. In this article we describe the GW approximation which has turned out to be a fruitful approximation to the self-energy. The Green function theory, numerical methods for carrying out the self-energy calculations, simplified schemes, and applications to various systems are described. Self-consistency issue and new developments beyond the GW approximation are also discussed as well as the success and shortcomings of the GW approximation.
1,458 citations
TL;DR: In this paper, the contribution of the technique of ferromagnetic resonance (FMR) has made to the understanding of the magnetic behavior of ultrathin single films is reviewed.
Abstract: The contribution that the technique of ferromagnetic resonance (FMR) has made to the understanding of the magnetic behaviour of ultrathin single films is reviewed. Experimental methods to measure FMR in situ in ultrahigh vacuum are presented. The temperature dependence of the magnetization, of the magnetic relaxation rate in the vicinity of the Curie temperature, and of the second- and fourth-order magnetic anisotropy energy (MAE) constants can be measured by FMR in situ for magnetic monolayers. Using the cases of Ni/Cu(001) and Gd/W(110) as examples, the role of the MAE for the quantitative description of temperature- and thickness-dependent reorientation transitions of the magnetization is discussed. Initial results for the anisotropy of the g-factor which is related to the anisotropy of the orbital moment (and the MAE) are presented.
780 citations
TL;DR: Models of neural networks are developed from a biological point of view and small networks are analysed using techniques from dynamical systems.
Abstract: Models of neural networks are developed from a biological point of view. Small networks are analysed using techniques from dynamical systems. The behaviour of spatially and temporally organized neural fields is then discussed from the point of view of pattern formation. Bifurcation methods, analytic solutions and perturbation methods are applied to these models.
668 citations
TL;DR: In this article, the authors provide a detailed review of the state-of-the-art properties of the group III nitrides (AlN, GaN and InN).
Abstract: The group III nitrides (AlN, GaN and InN) represent an important trio of semiconductors because of their direct band gaps which span the range 1.95-6.2 eV, including the whole of the visible region and extending well out into the ultraviolet (UV) range. They form a complete series of ternary alloys which, in principle, makes available any band gap within this range and the fact that they also generate efficient luminescence has been the main driving force for their recent technological development. High brightness visible light-emitting diodes (LEDs) are now commercially available, a development which has transformed the market for LED-based full colour displays and which has opened the way to many other applications, such as in traffic lights and efficient low voltage, flat panel white light sources. Continuously operating UV laser diodes have also been demonstrated in the laboratory, exciting tremendous interest for high-density optical storage systems, UV lithography and projection displays. In a remarkably short space of time, the nitrides have therefore caught up with and, in some ways, surpassed the wide band gap II-VI compounds (ZnCdSSe) as materials for short wavelength optoelectronic devices. The purpose of this paper is to review these developments and to provide essential background material in the form of the structural, electronic and optical properties of the nitrides, relevant to these applications. We have been guided by the fact that the devices so far available are based on the binary compound GaN (which is relatively well developed at the present time), together with the ternary alloys AlGaN and InGaN, containing modest amounts of Al or In. We therefore concentrate, to a considerable extent, on the properties of GaN, then introduce those of the alloys as appropriate, emphasizing their use in the formation of the heterostructures employed in devices. The nitrides crystallize preferentially in the hexagonal wurtzite structure and devices have so far been based on this material so the majority of our paper is concerned with it, however, the cubic, zinc blende form is known for all three compounds, and cubic GaN has been the subject of sufficient work to merit a brief account in its own right. There is significant interest based on possible technological advantages, such as easier doping, easier cleaving (for laser facets) and easier contacting. It also appears, at present, that the cubic form gives higher electron and hole mobilities than the hexagonal form. The dominant hexagonal structure is similar to that found in a number of II-VI compounds such as CdS and they can therefore be taken as role models. In particular, the lower symmetry gives rise to three separate valence bands at the zone centre and exciton spectra associated with each of these have been reported by many workers for GaN. Interpretation is complicated by the presence of strain in many samples due to the fact that most material consists of epitaxial thin films grown on non-lattice-matched substrates (bulk GaN crystals not being widely available). However, much progress has been made in understanding the physics of these films and we discuss the current position with regard to band gaps, effective masses, exciton binding energies, phonon energies, dielectric constants, etc. Apart from a lack of knowledge of the anticipated valence band anisotropy, it can be said that GaN is now rather well documented. Less detail is available for AlN or InN and we make no attempt to provide similar data for them. The structure of the paper is based on a historical introduction, followed by a brief account of the various crystal growth methods used to produce bulk GaN and epitaxial films of GaN and the ternary alloys. This is then followed by an account of the structural properties of hexagonal GaN as measured by x-ray diffraction and electron microscopy, phonon properties from infrared and Raman spectroscopy, electrical properties, with emphasis on n- and p-type doping, and optical properties, measured mainly by photoluminescence. A brief comparative account of cubic GaN properties follows. Discussion of alloy properties in the context of their use in quantum well and superlattice structures forms an introduction to the device sections which close the paper. These include details of the technology necessary for etching, contacting and forming laser facets, as an introduction to recent results on LEDs and laser diodes. Having described the current position, we speculate briefly on likely future developments.
464 citations
TL;DR: In this article, a review of diffractive phenomena from surfaces, which up to now were investigated mainly with helium, is presented, and the theoretical background for diffraction calculations is outlined and representative examples of different applications.
Abstract: Atomic beam techniques are presently being used in many branches of surface physics such as studies of the particle-surface physisorption potential, surface structure, surface phonons, nucleation and growth on metal and insulator surfaces, surface diffusion and accommodation and sticking of molecules. This review concentrates on diffractive phenomena from surfaces, which up to now were investigated mainly with helium. The theoretical background for diffraction calculations is outlined and representative examples of different applications are given. The main subjects covered are: structural determinations of chemisorbed and physisorbed systems, investigations of disordered surfaces, selective adsorption resonances, diffusion and nucleation studies and investigations of growth and phase transitions on surfaces. Diffraction results obtained with Ne, Ar, and are also summarized.
394 citations
TL;DR: In this article, it was shown that the transfer of nucleons is an important process for the initiation of fusion, and it is likely that the production of isotopes of element 114 close to the island of spherical superheavy elements (SHEs) could be achieved by fusion reactions using targets.
Abstract: The search for new elements is part of the broader field of investigations of nuclei at the limits of stability. In two series of experiments at SHIP, six new elements were synthesized via fusion reactions using 1n-deexcitation channels and lead or bismuth targets. The isotopes were unambiguously identified by means of correlations. Alpha decay, not fission, is the dominant decay mode. The collected decay data establish a means of comparison with theoretical data. This aids in the selection of appropriate models that describe the properties of known nuclei. Predictions based on these models are useful in the preparation of the next generation of experiments. Cross sections decrease by two orders of magnitude from bohrium (Z = 107) to element 112, for which a cross section of 1 pb was measured. The development of intense beam currents and sensitive detection methods is essential for the production and identification of still heavier elements and new isotopes of already known elements, as well as the measurement of small -, - and fission-branching ratios. An equally sensitive set-up is needed for the measurement of excitation functions at low cross sections. Based on our results, it is likely that the production of isotopes of element 114 close to the island of spherical superheavy elements (SHEs) could be achieved by fusion reactions using targets. Systematic studies of the reaction cross sections indicate that the transfer of nucleons is an important process for the initiation of fusion. The data allow for the fixing of a narrow energy window for the production of SHEs using 1n-emission channels. The likelihood of broadening the energy window by investigation of radiative capture reactions, use of neutron deficient projectile isotopes and use of actinide targets is discussed.
301 citations
TL;DR: In this paper, a brief survey of the technique of sputter depth profiling is given, along with some introductory remarks about aims and scope, historical background and present state of the art, and a summary of optimized experimental conditions for high-resolution depth profiles highlights the use of sample rotation and of low energy primary ions.
Abstract: Beginning with some introductory remarks about aims and scope, historical background and present state of the art, a brief survey of the technique of sputter depth profiling is given. The fundamental principles of ion/solid interactions during sputtering and the resulting changes in surface composition and surface topography as well as their influence on the conversion of sputtering time to sputtered depth are illustrated by some examples. Together with the specific analysis method employed, such as secondary ion mass spectrometry or Auger electron spectroscopy, these effects determine the shape of the depth profile of interfaces and of thin layers. Recent developments in evaluation and quantification of depth profiles are reviewed with emphasis on the role of the depth resolution function in profile reconstruction, including experimental determination and theoretical modelling of the depth resolution function by the three fundamental parameters: atomic mixing, surface roughness and information depth (MRI model). A summary of optimized experimental conditions for high-resolution depth profiles highlights the use of sample rotation and of low-energy primary ions. Practical application of depth profiling at interfaces is further elucidated by typical examples in the fields of surface and grain boundary segregation, corrosion, oxidation and interdiffusion controlled reactions at thin-film interfaces in electronic materials. An outlook on future trends focuses on theoretical profile simulation methods and on instrumental developments.
206 citations
TL;DR: In this article, the basic properties of 2:14:1 hard magnetic phases as well as technological procedures used for manufacturing R-Fe-B and R- Fe-C permanent magnets are given.
Abstract: A review is given on the basic properties of 2:14:1 hard magnetic phases as well as technological procedures used for manufacturing R-Fe-B and R-Fe-C permanent magnets. First, we analyse the phase diagrams, crystal structure of hard magnetic phases and composition ranges in which these phases form solid solutions, as a result of various types of substitution. Then, the magnetic properties of and -based compounds are described. The routes used for manufacturing R-Fe-B and R-Fe-C permanent magnets are detailed. The magnets' microstructure as well as their coercivities are described in correlation with composition and manufacturing routes. The properties of nanostructure magnets are then presented. Finally, the stability of Nd-Fe-B and Nd-Fe-C magnets in various working conditions is analysed.
187 citations
TL;DR: In this article, the design of analogue electronic experiments to investigate phenomena in nonlinear dynamics, especially stochastic phenomena, is described in practical terms and the advantages and disadvantages of this approach, in comparison to more conventional digital methods, are discussed.
Abstract: The design of analogue electronic experiments to investigate phenomena in nonlinear dynamics, especially stochastic phenomena, is described in practical terms. The advantages and disadvantages of this approach, in comparison to more conventional digital methods, are discussed. It is pointed out that analogue simulation provides a simple, inexpensive, technique that is easily applied in any laboratory to facilitate the design and implementation of complicated and expensive experimental projects; and that there are some important problems for which analogue methods have so far provided the only experimental approach. Applications to several topical problems are reviewed. Large rare fluctuations are studied through measurements of the prehistory probability distribution, thereby testing for the first time some fundamental tenets of fluctuation theory. It has thus been shown for example that, whereas the fluctuations of equilibrium systems obey time-reversal symmetry, those under non-equilibrium conditions are temporally asymmetric. Stochastic resonance, in which the signal-to-noise ratio for a weak periodic signal in a nonlinear system can be enhanced by added noise, has been widely studied by analogue methods, and the main results are reviewed; the closely related phenomena of noise-enhanced heterodyning and noise-induced linearization are also described. Selected examples of the use of analogue methods for the study of transient phenomena in time-evolving systems are reviewed. Analogue experiments with quasimonochromatic noise, whose power spectral density is peaked at some characteristic frequency, have led to the discovery of a range of interesting and often counter-intuitive effects. These are reviewed and related to large fluctuation phenomena. Analogue studies of two examples of deterministic nonlinear effects, modulation-induced negative differential resistance (MINDR) and zero-dispersion nonlinear resonance (ZDNR) are described. Finally, some speculative remarks about possible future directions and applications of analogue experiments are discussed.
160 citations
TL;DR: A review of the current understanding of this problem is reviewed in the light of many recent developments from theory, experiment and observation as mentioned in this paper, with further details being available in many recent reviews.
Abstract: Considerable publicity has accompanied Song and Richards' recent measurement of the rotation of the Earth's inner core and the observation of a reversal of the magnetic field in Glatzmaier and Roberts' computer model of the geodynamo. Additionally, the Galileo spacecraft has returned data suggesting the existence of magnetic fields in Io, Ganymede and perhaps Europa. These have given further impetus to the growing interest and activity in the problem of planetary magnetic-field generation and the core flows responsible for it. Here, the current understanding of this problem is reviewed in the light of many recent developments from theory, experiment and observation. The essential aspects of geodynamo theory are included, with further details being available in many recent reviews. The fundamental results of hydromagnetic flow in rapidly rotating systems are followed by a discussion of the latest thinking on the power source for the geodynamo. This replenishes the energy of some basic state but it may be instabilities of this basic state that result in the complex motions responsible for field generation. We discuss instabilities deriving their energy from buoyancy, the magnetic field and shear in the core flow. Our computational models can usually be run using an arbitrary choice of parameters. From this we learn of potential pitfalls when planetary values are used. In particular, the problems associated with the low values of the Ekman number E and Roberts number q are highlighted. Approaches to overcoming these are discussed and the resulting progress in producing full numerical dynamo models is reviewed. Finally, we discuss how the ideas and models developed primarily with the Earth in mind can be adapted and applied to the other planets and satellites known to have magnetic fields.
TL;DR: In this article, a review of NMR-NQR spectra and relaxation rates in two-dimensional quantum antiferromagnets driven towards the superconducting state by charge doping is presented.
Abstract: After a mention of the structural, magnetic and electronic properties of high-temperature superconductors (HTSC), the basic principles of NMR-NQR experiments in these compounds are presented, emphasizing the marked differences and the novel aspects of the latter systems in comparison with metals and conventional superconductors. It follows a review of NMR-NQR spectra and relaxation rates in two-dimensional quantum antiferromagnets (particularly ) driven towards the superconducting state by charge doping. The main results obtained in the normal state of HTSC are summarized, while the problems of the spin-gap and of the superconducting fluctuations are discussed to a certain extent, by including the most recent contributions. An overview is given on the main conclusions derived from NMR-NQR experiments in the superconducting state. A section is devoted to the insights into the vortex lattice and the flux lines motion that have been obtained from NMR line narrowing, and echo dephasing. This review deals mostly with three systems, , and .
TL;DR: In this article, a review of magnetic resonance imaging (MRI) in solid materials is presented, in terms of its merits relative to other approaches to solids MRI and the subtleties of its implementation.
Abstract: Magnetic resonance imaging (MRI) is well known in a clinical context as a technique capable of delivering highly detailed anatomical images, particularly of soft tissue. The MRI method is completely non-invasive and allows spatial resolution down to a few micrometres in three dimensions. Image contrast is governed by one of several nuclear magnetic resonance parameters and might reflect water mobility, chemical potential, self-diffusion coefficient, coherent flow or temperature, depending upon the exact form of the MRI measurement. Less widely realized is the enormous potential for the use of MRI in materials science. The flexibility that makes MRI such a valuable clinical tool is equally applicable in a non-medical scenario, but the greater technical difficulties associated with MRI in solid materials have hitherto limited the development of the technique in this area. This review describes in detail one approach to MRI in solid materials which is currently benefiting from rapidly increasing application: stray field (magnetic resonance) imaging (STRAFI). An introduction to the phenomenon of nuclear magnetic resonance and particularly its detection in solids is followed by a description of the steps necessary for its use as an imaging modality. The limits of MRI spatial resolution in liquids and solids are briefly discussed. STRAFI is placed in context throughout this introduction. The STRAFI technique is then described in detail, in terms of its merits relative to other approaches to solids MRI and the subtleties of its implementation. The principal areas of current STRAFI application are reviewed and developments with which STRAFI advancement is closely linked, are also described. In conclusion, some consideration is given to the promising future of stray field MRI as a widely accepted research tool in materials science and to the development of the technique itself.
TL;DR: In this paper, the historical development of CVD diamond deposition with the main focus on the most important techniques, hot-filament and microwave assisted CVD, will be resumed, and the control of structural and morphological properties during the deposition which is a prerequisite of oriented growth and the doping of diamond which is needed for semiconductor and sensor applications.
Abstract: Diamond is a unique material with several outstanding physical and chemical properties. It has the highest thermal conductivity at room temperature and it is transparent from the UV to the far IR. Furthermore it has the highest hardness, the highest Young's modulus and it is chemically inert and radiation hard. These and other properties of diamond are of great interest for various commercial applications. Much progress has been made in the last decade to produce diamond with chemical vapour deposition (CVD) techniques. Today, CVD diamond plates of more than 10 cm in diameter and more than 1 mm in thickness are commercially available whose properties, especially the optical and thermal ones, are comparable to the best single-crystal diamonds. In this overview, the historical development of CVD diamond deposition with the main focus on the most important techniques, hot-filament and microwave assisted CVD, will be resumed. We describe the control of structural and morphological properties during the deposition which is a prerequisite of oriented growth and the doping of diamond which is needed for semiconductor and sensor applications. The second part of this overview will discuss optical, thermal, thermomechanical and electronic properties of single-crystal diamond and CVD diamond. Finally, we give a description of several applications such as IR windows, heatspreaders, temperature sensors, piezoresistive sensors, diodes and transistors.
TL;DR: An irreversible thermodynamic treatment of gel swelling shows the intrinsic instability of polyelectrolyte gels and suggests new ways of approaching a microscopic model for osmosis.
Abstract: Biological polyelectrolyte gels consist of insoluble aggregates of molecules which collectively form structural fibrils and these fibrils, or their chemically bound side chains, have a net electrical charge. These gels may be visualized as negatively charged fibrils immersed in aqueous solutions which include free diffusible ions (mainly sodium, potassium and chloride). All living cells and most of the extracellular spaces of the body are polyelectrolyte gels and they strive to swell by the absorption of additional fluid because of the Donnan potentials generated by their fixed charge. We review Donnan swelling using the cornea of the eye as prime material. Donnan swelling requires knowledge of only one parameter such as: (a) the electrical potential within the gel or (b) the distribution of any mobile ion inside and outside the gel or (c) measurement of the gel pressure or (d) the fixed charge density on the fibrils, in order to calculate all the other relevant factors. We describe the conditions (which usually exist in biological tissue) when the microscopic distribution of the fixed charge density within the gel is not important to the Donnan phenomena. Fixed charge density is generated by two sources: permanent negative charges in the structural fibrils and transient mobile ion binding to the fibrils. Ion binding to large molecules is reviewed. In the case of the cornea, transient mobile ion binding is the predominant factor in generating fixed charge density under physiological conditions. An irreversible thermodynamic treatment of gel swelling shows the intrinsic instability of polyelectrolyte gels and suggests new ways of approaching a microscopic model for osmosis. In order to stabilize the two forces (osmotic potential and chemical potential) which generate the polyelectrolyte gel instability we review the types of third forces which must be present in order to stabilize biological gels. These third forces include van der Waal's force, metabolically driven ion pumps or fibrillar cross-linking. In the case of the cornea, it is shown that the gel pressure is exploited in order to help make the tissue transparent to light.
TL;DR: The development of unstructured grid-based, finite-element methods for the simulation of fluid flows is reviewed, employing methods based upon a Galerkin discretization in space together with an appropriate finite-difference representation in time.
Abstract: The development of unstructured grid-based, finite-element methods for the simulation of fluid flows is reviewed. The review concentrates on solution techniques for the compressible Euler and Navier-Stokes equations, employing methods which are based upon a Galerkin discretization in space together with an appropriate finite-difference representation in time. It is assumed that unstructured assemblies of triangles are used to achieve the spatial discretization in two dimensions, with unstructured assemblies of tetrahedra employed in the three-dimensional case. Adaptive grid procedures are discussed and methods for accelerating the iterative solution convergence are considered. The areas of incompressible flow modelling and optimization are also included.
TL;DR: The LIGA (LIthographie, Galvanoformung, and Abformung) process as mentioned in this paper is based on a combination of lithography, electroforming and replication processes.
Abstract: The realization of precision deep microstructures requires high-energy, intense parallel beams of x-rays from synchrotron radiation sources and novel process technology. Deep x-ray lithography with synchrotron radiation is basically a shadow printing process in which a two-dimensional pattern is accurately transferred from a mask into a resist material by chemical changes induced by the radiation. Subsequent electroforming and moulding processes are used to manufacture microstructures from metals, plastics and ceramics. This process, known as LIGA (LIthographie, Galvanoformung, and Abformung), first developed in Germany, is based on a combination of lithography, electroforming and replication processes. The development of the LIGA process for the fabrication of a wide range of precision microstructures has been stimulated by the increasing use of synchrotron radiation sources for lithography. Applications for microstructures exist in many sectors of industry. These include chemical and process engineering, biomedical instrumentation, automotive and aerospace technology, environmental monitoring and information technology. Emphasis is placed on three main areas, micromechanics, micro-optics and microfluidics, which are emerging with the widest range of industrial applications. This paper reviews the progress being made in microfabrication technology using x-ray beam lithography and the LIGA process. It includes a description of synchrotron radiation, storage ring sources, the fabrication processes, applications and potential markets. Reference is also made to European networks and R&D activity worldwide.
TL;DR: An overview of accelerators with superconducting cavities is given in this paper, where the performance of the best single-cell cavities comes close to the intrinsic limitation of the material.
Abstract: Superconducting cavities have been in operation in accelerators for 25 years. In the last decade many installations in storage rings and linacs have been completed. Meanwhile, nearly 1 km of active cavity length is in operation in accelerators. Large-scale applications of superconducting radiofrequency systems are planned for future linear colliders and proton linacs. Superconducting cavities have been proved to operate at higher gradient, lower AC power demand and more favourable beam dynamics conditions than comparable normal conducting resonators. The performance of the best single-cell cavities comes close to the intrinsic limitation of the superconducting material. Complete multicell structures with all auxiliaries (couplers, tuner, etc) lag behind in performance because of their complexity. In this paper, an overview of accelerators with superconducting cavities is given. Limitations of superconducting performance are described and research and development efforts towards understanding and curing these effects are discussed in detail. Fundamentals of superconductivity and radiofrequency cavity design are briefly explained.
TL;DR: In this paper, a brief introduction to the interaction between light and the solid-gas interfaces is given, and the mechanisms of photo-induced processes are discussed and related to the experimentally determined cross sections.
Abstract: This article reviews aspects of photochemistry on solid surfaces. In order to understand the photo-induced processes a brief introduction is given to the interaction between light and the solid-gas interfaces. The adsorption of molecules on solid surfaces, and the negative ion resonances (NIR) by inelastic electron scattering are briefly discussed. There are three photoinduced processes which occur on surfaces: photoinduced desorption (PID), photoinduced dissociation and photoinduced reactions. The mechanisms of the photoinduced processes are discussed and related to the experimentally determined cross sections. Photoinduced processes are driven: (i) by direct electronic excitation of the adsorbate, (ii) by substrate excitation, (iii) by both adsorbate and substrate excitation, or (iv) by charge transfer dissociation. The usual experimental methods and the light sources are presented. Most of the experimental examples deal with photo-stimulated experiments in the IR-visible and UV region. In some examples photon stimulated desorption of ions of physisorbed molecules is studied by using VUV synchrotron radiation in the energy range between 13-40 eV. In addition to desorption and dissociation, photo-excited molecules can react with each other and form new chemical bonds leading to new species on the surface. Photoinduced polymerizations of formaldehyde on Ag(111) occur after irradiation with light at .
TL;DR: In this article, the authors discuss some of the relevant computational advances which permit the simulation of large-scale problems involving nonlinear solids within realistic time frames and computational resources, particularly in relation to the computational treatment of finite-strain elasto-plastic (viscoplastic) deformation, the modelling of frictional contact conditions and element technology capable of dealing with material incompressibility.
Abstract: The paper discusses some of the relevant computational advances which permit the simulation of large-scale problems involving nonlinear solids within realistic time frames and computational resources. The need for rigorous consideration of both theoretical and algorithmic issues is emphasized, particularly in relation to the computational treatment of finite-strain elasto-plastic (viscoplastic) deformation, the modelling of frictional contact conditions and element technology capable of dealing with material incompressibility. Practically important aspects such as adaptive mesh refinement procedures are discussed and attention is given to choice of appropriate error estimators for elasto-plastic materials and the transfer of solution parameters between successive meshes. The role of explicit solution techniques in the simulation of large-scale nonlinear problems is also discussed. The concept of discrete elements is briefly described and their applications to a wide range of solid mechanics problems illustrated. Some advances in the field of iterative equation solution methods are reviewed and their potential advantages in the simulation of large-scale nonlinear solid mechanics problems are demonstrated.
TL;DR: In this article, a systematic analysis is presented of isotopic and disorder effects observed in crystals of various isotopic composition via low-temperature large-radius exciton spectroscopy.
Abstract: Most of the physical properties of a solid depend to a greater or lesser degree on its isotopic composition. Scientific interest, technological promise and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled semiconductor and insulator crystals. A systematic analysis is for the first time presented of isotopic and disorder effects observed in crystals of various isotopic composition via low-temperature large-radius exciton spectroscopy. Substituting a light isotope with a heavy one increases the interband transition energy and the binding energy of the Wannier-Mott exciton as well as the magnitude of the longitudinal-transverse splitting. The nonlinear variation of these quantities with the isotope concentration is due to the isotopic disordering of the crystal lattice and is consistent with the concentration dependence of line half-widths in exciton reflection and luminescence spectra. The common nature of the isotopic and disorder effects in the crystals of C, LiH, ZnO, ZnSe, CuCl, CdS, , GaAs, Si and Ge is emphasized. The review closes with an outlook on the exciting future possibilities offered through isotope control of a wide range of semiconductor and insulator crystals.
TL;DR: In this article, the particle-hole level and state densities required to calculate the cross sections of pre-equilibrium nuclear reactions are reviewed using the equidistant spacing model, explicit expressions for the total density of states and for the density of final accessible states.
Abstract: The particle-hole level and state densities required to calculate the cross sections of pre-equilibrium nuclear reactions are reviewed. Using the equidistant spacing model, explicit expressions are found for the total density of states and for the density of final accessible states. These are modified to take account of the restrictions due to the Pauli principle and the finite depth of the nuclear potential. The dependencies of the densities on spin, isospin, linear momentum and pairing are described. The effects of departures from the equidistant spacing model, particularly those due to shell structure, are also discussed. Some comparisons are made with realistic densities obtained by full combinatorial calculations Some recommendations are made concerning the best choices to be made for pre-equilibrium calculations, combining accuracy and convenience.
TL;DR: In this paper, the results of different analyses are combined and interpreted within the framework of the standard model (SM) of electroweak interactions, severely limiting the room for modifications of these quantities from new physics.
Abstract: The measurements of the partial decay widths and forward-backward asymmetries for and test the Z-couplings to the initial state -pair and the heavy quarks in the final state. The four LEP detectors have registered about four million hadronic Z-decays each and SLD at SLC has recorded 300 000 Z-decays with highly polarized electron beams. The high statistics as well as the good tracking, vertexing and particle identification capabilities of the detectors allow high-precision measurements of these quantities. The measurements of the electroweak observables with heavy quarks are reviewed. The results of the different analyses are combined and interpreted within the framework of the standard model (SM) of electroweak interactions. In all cases good agreement with the SM predictions is found, severely limiting the room for modifications of these quantities from new physics.
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TL;DR: In this article, the main objectives of astrometry are presented and the most important features or phenomena that intervene in the measurement of positions of celestial objects are shortly described, and two classical astrometric techniques that are still very much used, especially since the invention of CCDs, transit instrument and astrophotography, are described.
Abstract: First, the main objectives of astrometry are presented and the most important features or phenomena that intervene in the measurement of positions of celestial objects are shortly described. Then, the two classical astrometric techniques that are still very much used, especially since the invention of CCDs, transit instrument and astrophotography, are described. The third section is devoted to the application of interferometric techniques to astrometry, in optical and in radio wavelengths. In the fourth section, it is shown how much precise time measurements are important in modern astrometry, in particular for ranging to the Moon or planets, and in studying pulsars. Then, astrometry from satellites is presented describing the Hipparcos satellite and its results, and the applications of the Hubble Space Telescope. Finally, after presenting the new needs of astrophysics for more accurate astrometry, a description of two major projects, GAIA and SIM, and of a few other smaller satellites that may be launched during the next decade is given.