Showing papers in "Reports on Progress in Physics in 1975"

Co-operative Jahn-Teller effects

Abstract: The co-operative Jahn-Teller effect is a phase transition which is driven by the interaction between localized orbital electronic states and the crystal lattice. The possible origins, symmetries and properties of the electron-lattice interactions and how they lead to possible hamiltonians for the coupled system are discussed. The relation of these interactions with quadrupolar interactions and magnetostriction is included. The methods of solution of the hamiltonians lead to an understanding of the electronic states, the phonon spectrum and the mixed normal modes. The wide variety of experimental techniques used on this problem are reviewed in detail and the results are compared with theoretical expressions whenever possible. The application of external stress and magnetic field is of particular significance in the case of the rare earth compounds, because they can product effects which are larger than the low-temperature spontaneous effects.

Laser-induced breakdown of gases

Abstract: Powerful flashes of ultraviolet, visible and near-infrared laser light can cause breakdown and plasma formation in gases which are normally transparent at these wavelengths. The ionization processes involved are multiphoton absorption and inverse bremsstrahlung or cascade collisional ionization, in competition with deionization by diffusion, recombination and radiative energy losses. The relative importance of these mechanisms and their roles in leading to the initiation of breakdown and the earliest stages of plasma formation is examined. Theoretical treatments are summarized, and experimental results obtained with a variety of gases and metal vapours at pressures from millitorrs to hundreds of atmospheres and with flashes from picosecond duration to quasi-continuous illumination are reviewed.

Fluctuations near superconducting phase transitions

Abstract: The effects of thermodynamic fluctuations near superconducting phase transitions are reviewed. Above Tc, fluctuations towards the superconducting state lead to the appearance of excess conductivity, diamagnetism, specific heat, and tunnelling currents. Below Tc, fluctuations towards the normal state lead to the appearance of resistance in thin wires and the breakdown of fluxoid quantization in small rings. These effects are generally small, but they can be measured experimentally, particularly in superconducting samples of reduced dimensionality (on the scale of the Ginzburg-Landau coherence length) such as thin films, whisker crystals, and powders. These phenomena are explained using Ginzburg-Landau theory (the salient features of which are developed concurrently), and the current status of theoretical and experimental studies of these effects is surveyed.

The dynamic properties of monatomic liquids

Abstract: The computer simulation and inelastic neutron scattering studies of simple, monatomic liquids are reviewed, together with the theory appropriate for their interpretation. Computer simulation (molecular dynamics) studies influenced development of non-equilibrium statistical mechanics techniques for the calculation of correlation functions that enter the various measured susceptibilities. Studies of systems interacting via continuous potentials have developed to a high level of sophistication. Neutron scattering experiments are difficult to perform with high accuracy, yet offer a unique means of investigating the dynamics of simple liquids in the domain of wavevectors and frequencies larger than about 0.05 AA-1 and 5*1011 s-1 respectively. The problems faced in analysing neutron data to the level required are reviewed in detail. Theory has developed through the rigorous generalization of the Markovian theory of fluctuations set out by Landau and Lifshitz (1959). The development has taken the form of a generalized Langevin equation, which provides a framework within which neutron and molecular dynamics data may be interpreted.

The behaviour of surfaces under ion bombardment

Abstract: The processes involved when ions bombard surfaces are divided into two categories: those due to elastic collisions, i.e., sputtering and ion scattering; and those due to inelastic collisions, leading to excitation and ionization, resulting in the emission of X-rays, optical photons and electrons. The mechanisms involved in ion scattering and sputtering are reviewed in detail. The experimental energy and angular distributions of the secondary particles are considered and compared with current theories. The effect of sputtering on the erosion of surfaces and the effects of radiation blistering and radiation damage on surface topography are discussed. Considering inelastic effects due to incident ions, results for total yields, statistics of secondary electron emission, and their energy distribution are outlined. X-ray emission, including molecular X-rays and other solid state effects, are discussed. Finally optical photon emission due to de-excitation of excited atoms is treated.

Developments in atomic structure calculations

Abstract: The general availability of electronic computers has resulted in a variety of developments in calculations of the electronic structure of atoms. The starting point is generally the Hartree-Fock approximation, which inadequately accounts for the motion of the electrons. A treatment of electron correlation has been the aim of most of the recent work in this field, and the methods which have been proposed are reviewed. The accuracy to which energies (or at least energy differences) can be evaluated, and calculations of electron affinities and term ratios are considered. The evaluation of oscillator strengths (transition probabilities) and hyperfine structure are also covered. Available atomic structure programs based on the correlation methods are discussed.

Diffraction gratings (manufacture)

Abstract: Recent progress in the techniques of manufacture of spectroscopic diffraction gratings is described. Three main topics are discussed in detail; (i) the mechanical ruling of gratings, and the way in which the mechanical aspects of the ruling process affect the spectroscopic performance of the grating; (ii) the development of interference (or 'holographic') techniques for manufacturing gratings, and the relationship between ruled gratings and those made interferometrically; (iii) X-ray gratings for use at grazing incidence, and the different criteria that arise through the use of gratings in this way. The review was completed in April 1975.

Recent advances in the study of wind waves

Abstract: The field of wind-generated ocean surface gravity waves is reviewed for the period covering the last fifteen years. Theories and observations relevant to understanding the physics of wind waves are discussed, as well as techniques for measuring and forecasting waves.

Principles and problems of phase-shift analysis

Abstract: Phase-shift analysis is the extraction of the scattering amplitude from the scattering cross section and other experimentally observable quantities such as polarizations When only elastic scattering is energetically allowed, unitarity determines the unobservable angle-dependent complex phase of the scattering amplitude with, at most, only a few discrete alternative solutions Above the inelastic threshold the unitarity constraint on a scattering amplitude is only an inequality and a continuum of different amplitudes will correspond to exactly the same observables In practical cases these differences can be important Extra theoretical input of a dynamical nature can, in principle, remove the continuum ambiguity but, because numerical analytic continuation is always involved, data of absurd accuracy are required Thus unique answers can, in practice, only be found by introducing further model-dependent assumptions; it is important to recognize this and ensure that these assumptions are as dynamically plausible as possible Recent results using the structure of the amplitude in both kinematic variables suggest that fixed-t dispersion relations might form a sound basis for an inelastic phase-shift analysis

Outline of axiomatic relativistic quantum field theory

Abstract: Hilbert space and the C*-algebra approach are outlined. There follows sections devoted to Wightman's axiomatic treatment with recent generalisations. The review surveys the work covered in the period 1954-74.

The establishment of thermal equilibrium in paramagnetic crystals

Abstract: An account is given of the various processes of electron spin relaxation concerned in establishing thermal equilibrium in dielectric crystals containing paramagnetic ions of the transition elements. The discussion is illustrated by reference to the experimental results, and includes some consideration of the possible role of impurities in those cases where the observed relaxation has had no ready explanation in terms of accepted mechanisms.

Absorber theory of radiation

Abstract: Developments in the Wheeler-Feynman absorber theory are surveyed. The action on the emitter of radiation by complete, incomplete, time-symmetric and time-asymmetric absorbers is described in physical terms. Some discussion is given of the prevailing divergent views of the inter-relationship between absorber theory and the arrow of time. The application of absorber theory to general fields is reviewed briefly. Some experiments, actual and proposed, to detect the advanced effects predicted by absorber theory for an imperfect universe are discussed.

The foundations of thermometry

Abstract: The whole field of fundamental thermometry in the range 0.5 to about 3000K is reviewed. A historical outline of the development of the theory and methods of thermometry is given. It is shown how the need arose for practical temperature scales and eventually for an international organization responsible for drafting and promulgating an International Practical Temperature Scale. The areas that combine to produce practical scales of temperature are described. These are thermodynamic temperature measurement, the realization of fixed points, precision interpolating instruments, and radiation thermometry. Detailed discussions are included of many of the experimental and theoretical problems at present encountered in precision work. All of this is brought together in a discussion of the present IPTS-68, including an assessment of its merits and defects and a look towards future developments.

Anisotropic and nonlinear viscoelastic behaviour in solid polymers

Abstract: Two aspects of the mechanical behaviour of solid polymers are reviewed: (i) anisotropic mechanical behaviour of oriented polymers, and (ii) nonlinear viscoelastic behaviour. In the case of anisotropic behaviour, the discussion includes a comprehensive account of progress to date in obtaining experimental data, together with recent attempts at understanding the behaviour in structural terms. The discussion of nonlinear viscoelastic behaviour, on the other hand, is almost entirely at a phenomenological level. Here it is attempted to bring together the many different approaches and to show their common relationships, particularly between the engineering formulations and those based on continuum mechanics.

Nucleon removal and rearrangement energies

Abstract: The process of nucleon removal from a nucleus is analysed together with the associated nuclear rearrangement. Several possible definitions of the rearrangement energy are reviewed, and their connection with experimentally observable quantities is investigated.

Inelastic laser light scattering from synthetic and biological polymers

Abstract: The development of laser sources and the improvement of other spectroscopic equipment have led to increased application of inelastic light scattering techniques. A general theory of inelastic scattering from fluctuations in dielectric constant is presented. The experimental techniques are discussed under the headings of Rayleigh, Fabry-Perot and Raman spectroscopy in the frequency ranges

Pion condensation and other abnormal states of matter

Abstract: In this review the author considers abnormal states of matter; that is, the phase transitions induced in matter placed in an abnormal environment. He considers matter in huge magnetic fields such as might be associated with pulsars, the effects of supercritical electric fields such as might be produced by the concentration of electric charge during a collision between extremely heavy ions, and briefly comments on the nature of the phase transition induced by supercritical gravitational fields (black holes). However, the bulk of this article reviews the behaviour of matter at extremely high densities, i.e. densities in excess of 2.8*1014 g cm-3, corresponding to supercritical strong interaction fields associated with an abnormal concentration of baryon number. Such an environment may be produced in the cores of neutron stars or in nuclear shock waves induced in nuclei as a result of extremely energetic heavy-ion collisions.