Showing papers in "Reports on Progress in Physics in 1955"

Paramagnetic Resonance II

Abstract: In this report, which is complementary to that of Bleaney and Stevens in Reports on Progress in Physics, Volume XVI, paramagnetic resonance data are collected together for crystalline solids containing ions of the transition groups, and those parts of the theory necessary for an understanding of the results are presented in a fairly simple way. After a brief introduction, the magnetic properties of various types of compounds are considered, and a simple discussion is given of the spin Hamiltonian which is used to describe the lowest energy levels. Some details are also given of the structures of the ten crystal types which are most commonly used, and of the relation between the structure and the resonance spectrum of a single crystal. Finally, the resonance data which are available for ions of the 3d, 4d, 5d, 4f and 5f transition groups are tabulated. These data are expressed as values of the parameters in the appropriate spin Hamiltonian, and, where applicable, formulae are given for (1) energy levels, (2) susceptibility, (3) specific heat tail CT2/R, in terms of these parameters.

Magnetostriction and Magnetomechanical Effects

Abstract: The energy of a ferromagnetic substance comprises three parts, the exchange, anisotropy and magnetostatic (demagnetizing) energies. These all vary with the state of strain of the substance and so it will deform spontaneously if the deformation reduces the total energy (equal to the sum of the magnetic energy and the elastic energy). This phenomenon is known as magnetostriction and the three magnetic energies respectively give rise to the volume magnetostriction, the linear magnetostriction and the form effect. The greater part of this report is taken up with the linear magnetostriction since this is the effect most easily and often observed. The formal theory as developed by Becker and others is given in § 2. § 3 reviews the various attempts to calculate the magnetostriction constants in terms of interatomic forces. The fact that the linear magnetostriction can interact with stress to give rise to an additional anisotropy is discussed in § 6 together with some consequences of this fact. § 7 deals with the volume magnetostriction and related topics which arise from the variation of the exchange energy with interatomic distance. The form effect which arises incidentally in the analysis of volume magnetostriction is treated more fully in § 8. The only secondary effect of any importance, the ΔE effect, is discussed in § 9. The final section describes such topics as magnetic strain analysis and the magnetostriction of ferrites and antiferromagnetics.

Theory of Radiation

Abstract: Recent developments in the theory of radiation have brought this subject to a very complete state. There is a nearly closed domain, in which the interaction between charged particles and the electromagnetic field can be considered as isolated from other phenomena such as mesonic interactions. Within this domain it is now possible to calculate any process with a precision only limited by the labour of the actual calculations. This progress requires for its achievement that quantum field theory should be developed in a relativistically covariant form. The early sections of this review are devoted to a systematic description of the methods of quantization as applied to various matter fields - the scalar meson field considered first for its simplicity, the electromagnetic field and the electron-positron field. Coupled field equations are then discussed, and their solution in the interaction representation is described. The Feynman-Dyson procedure is established for the evaluation of S-matrix elements and applied to typical cases. A general description is given of different methods of renormalization of the S-matrix, and it is shown how this is equivalent to an elimination of unobservable parameters, and a description of phenomena in terms of the (formally infinite) electron mass and charge. A series of individual problems, exhibiting radiative corrections, including the Lamb shift and the anomalous magnetic moment of the electron are considered in more or less detail. Some brief concluding remarks are appended on the development of approximations likely to be useful in meson theory.

Methods of Measuring Strong Magnetic Fields

Abstract: Descriptions which are briefly theoretical and essentially practical are given of a number of the techniques of magnetic field measurement, used or developed in the period of about twenty years since the last review of the subject. Particular attention is given to the application of the phenomenon of magnetic resonance while other subjects discussed are magneto-resistance, Hall effect, peaking strips, the generator principle and various fluxmeters. The uses which are made of the force exerted on a current-carrying conductor in a magnetic field are given though some of these are not, strictly speaking, field measurements. The measurement of the position of the magnetic median plane of a magnet is discussed. The last paragraphs are devoted to a description of a few methods which do not find very wide application. A list of about 150 references is appended. These are classified in groups under the heading of each particular method.

Ionization by Relativistic Particles

Abstract: A summary is given of the theories of the relativistic increase of energy-loss by ionization and of the density effect. The experimental evidence is then discussed and compared with theoretical predictions in a number of sections, each dealing with a particular experimental technique: proportional counters, cloud chambers, low pressure Geiger counters, photographic plates, crystal counters and scintillators. A brief description is also given of experiments in which the energy-loss of fast electrons in thin foils is measured.

The Ionization and Dissociation of Complex Molecules by Electron Impact

Abstract: The report deals with certain experimental and theoretical aspects of collision processes in polyatomic molecular gases Experimental techniques, dating from about 1932, are reviewed, with more attention given to recent methods Measurements of critical potentials, cross sections, and initial kinetic energy of product fragments are described A survey of the theoretical implications of these data is then given, with reference to the significance of electron-impact ionization potential measurements, for molecules and free radicals, bond dissociation energies for a wide range of molecular types, and certain collision processes involving negative ions

A Survey of Field Theory

Abstract: These six articles attempt to summarize the concepts and results of the field theory of elementary particles in terms which, it is hoped, will be understood by experimentalists or theoretical physicists working in other fields. Part I is a general, partly historical, introduction. The concept of virtual states and the physical ideas underlying the renormalization of mass and charge are explained. The outline of the theory starts in Part II, which deals with the description of a quantized field - the quantized electromagnetic field for example - and its relation to an assembly of particles. Part III describes the interactions of such particles. It is shown how solutions to problems in electrodynamics (the interaction of electrons and positrons with photons) may be obtained as a power series in the fine structure constant e2/4πhc(=1/137). A number of possible meson-nucleon interactions are then developed by analogy with electrodynamics. To decide which of these actually occurs in nature, it is necessary to determine from experiment the spin and intrinsic parity of the π-meson. Part IV is devoted to this problem. The charge independence of nuclear forces expressed in terms of isotopic spin is then used to determine, as uniquely as possible, the π-meson-nucleon interaction in qualitative agreement with experimental data. Quantitative agreement has not yet been obtained owing mainly to the largeness of the coupling constant. In Part V the experimental evidence on π-mesons is reviewed. It is emphasized that a variety of effects can be explained on the assumption that there is a short-lived excited state of the nucleon. Part VI starts with a discussion of the classical limit of the electromagnetic and Dirac fields and concludes with some considerations of the properties of the recently discovered heavy mesons and some general regularities of the couplings between them.