Showing papers in "Advances in Physics in 1985"

A dynamic scaling assumption for phase separation

Abstract: A review of the dynamics of phase separation is presented, which focuses on the scaling assumption of the problem. Conventional linear and nonlinear theories are briefly reviewed. The problems with these conventional theories, and how the scaling idea can be used to overcome them are discussed. The growth rates of the droplets, domains, or grains are discussed in terms of the scaling assumption, and experimental tests of the scaling assumption for growth laws and scattering intensities are reviewed. To explain singular properties of the structure function and growth rate, which are observed experimentally, a strong correlation among droplets or grains on phase separations is postulated. A nonconventional form of the scattering function and a nonconventional droplet growth rate are explained qualitatively.

Neutron diffraction methods for the study of residual stress fields

Abstract: A technique is described for the measurement of the residual stress tensor averaged over a specified volume within a component. The method involves measurement of small changes in lattice spacing using high resolution neutron diffraction. The stress is inferred from these measurements of the strain, and the theory of the relationship between the two quantities is described, including the effects of crystalline anisotropy. The various types of high resolution neutron diffractometer suitable for the work are described. Experimental results validating the method are given for a simple bent bar of mild steel of known strain, a plastically strained mild steel bar, and a mild steel tube of known torsional strain. Examples of the method in practical use are given by a cracked fatigue test specimen, a double-V test weld and a weld joining a tube to a plate. A more detailed example is the anisotropic response of a polycrystalline sample under elastic and plastic strain; this is illustrated by measurements...

An applied science perspective of Langmuir-Blodgett films

Abstract: Langmuir-Blodgett (LB) films are prepared by transferring floating organic monolayers onto solid substrates. A combination of innovative chemistry and a carefully engineered instrument (a Langmuir trough) can result in high quality monomolecular assemblies displaying a high degree of structural order. The technique was first reported about fifty years ago and since then, LB films have been widely used as model systems in fundamental research. However, it is only during the past decade that the extensive applied potential has been recognised. Their precise thickness, coupled with the degree of control over the molecular architecture, has now firmly established a role for these layers in thin film technology. Following a historical introduction, this review describes the preparation and characterization of LB films and summarizes their importance in basic science. Particular emphasis is then placed on their potential applications, especially in fields related to electronics. The article concludes w...

Self-organization processes in continuous media

Abstract: This review considers the dynamical behaviour of dissipative continuous media which can be described by nonlinear partial differential equations having more than two non-trivial conserved quantities in the absence of dissipation. Such a medium often exhibits a spectral condensation of a certain physical quantity to a specific wavenumber region resulting in the formation of an ordered structure, even when starting from an initially turbulent state or one driven by a stochastic field. Examples discussed here include two-dimensional incompressible fluids, two-and three-dimensional magnetohydrodynamic fluids, the atmospheres of rotating planets, the electrostatic potential in inhomogeneous magnetized plasmas, and the solition gas as described by the Korteweg-de Vries equation. The relationship between the onset of chaos and self-organization in a soliton system, as well as in some localized vortex solutions, is also discussed.

Diffusion of hydrogen in metals

Abstract: The present status of our understanding of the diffusion of hydrogen in metals, both experimental and theoretical, is reviewed. Discussions are focused on the mechanism of diffusion of hydrogen isotopes H, D and T in f.c.c. and b.c.c. metals; the positive muon (μ +) is referred to where appropriate. An up-to-date compilation of diffusion data as a function of temperature and isotope mass has been made, and a clear distinction in general diffusion behaviour in f.c.c. and b.c.c. metals is noted. Subsequently, the results obtained from the Gorsky effect, nuclear magnetic resonance and quasi-elastic neutron scattering that provide information on elementary jump processes are discussed. A conceptual framework of the quantum diffusion of light interstitials in metals is given, including the recent Kondo theory that emphasizes the crucial importance of particle-conduction electron interactions in the diffusion process, especially at low temperatures. It is shown with the help of recent estimates of the ...

Coexistence of superconductivity and magnetism theoretical predictions and experimental results

Abstract: Superconductivity and ferromagnetic ordering are two antagonistic types of ordering, and their mutual influence leads to many interesting phenomena which have been studied recently in ternary compounds. Theoretical analysis of ferromagnetic materials which are type II superconductors near the superconducting transition point T cl shows that they become type I near the magnetic transition point T M. The proposed theory constructed for the case T M « T cl predicts the formation of a transverse domain-like (DS phase) magnetic structure below T M. The electronic spectrum appears to be gapless in the DS phase of clean compounds with a re-entrant transition. The change from type II to type I behaviour as the sample is cooled to T M has been observed in ErRh4B4. Experimental data for HoMo6S8, HoMo6Se8 and ErRh4B4 give evidence for the coexistence of super-conductivity and non-uniform magnetic ordering below T M. Mutual influence of superconducting and magnetic orderings is also studied.

Metal-insulator transitions in non-crystalline systems

Abstract: This review gives an account of the changes in our understanding of the metal-insulator transition in non-crystalline systems since the application to it of scaling theory and experiments on doped semiconductors at millikelvin temperatures. The first four sections give an account of previous work. §5 discusses briefly the scaling theory, and §6 deduces from the Kubo-Greenwood formula that when the Fermi energy of a ‘metal’ lies at a small energy ΔE above a mobility edge, the conductivity is of the form 0.03 e 2 ħL, where L is the size of the specimen, the inelastic diffusion length, or, in a magnetic field, LH = (cħ/eH)½, or the localization length at an energy ΔE below the transition, whichever is the smaller. Mott's ‘σmin’ is observed in doped semiconductors in a magnetic field such that LH < a, a being the distance between donors, and in some liquid systems. The solved and unsolved problems of long-range Coulomb interaction are described, both for metallic conduction and for hopping. Applicati...

Magnetic excitations in crystal-field split 4f systems

Abstract: A survey is given of magnetic excitations in metallic 4f systems when the crystal-field splitting is comparable to or larger than the magnetic interaction between different sites. Particular emphasis is put on the interaction of crystal-field split rare-earth ions with conduction electrons. A number of physical effects is discussed which result from that interaction. The modifications of the magnetic excitations are described which result when the coupling of phonons to the crystal-field levels is so strong that a bound state between the two types of excitations is formed. For a study of these phenomena the method of neutron scattering is an indispensable tool. For that reason it plays a central part in this review.

Resistivity anomalies due to valence fluctuations

Abstract: A survey is given of the resistivity anomalies of metals with Ce, Eu and Yb in intermetallic compounds and in concentrated and dilute alloys. For their interpretation a dynamic alloy model is proposed. A formalism is given by which the anomalies can be reproduced semiquantitatively.

Quantum 1/f noise associated with ionized impurity scattering and electron-phonon scattering in condensed matter

Abstract: Scattering of charged particles is accompanied by the emission of soft photons. Handel's theory of 1/f noise, based on the infrared quasi-divergent coupling of the system to the electromagnetic field, indicates that the current associated with a beam of scattered particles will exhibit 1/f noise. His derivation is valid in a vacuum. Here we extend his results and obtain the fluctuation spectrum for the fluctuations in cross-section and for the scattering rates w kk′ in k-space, using the Born approximation. Next we consider mobility fluctuations due to these scattering rates, employing the relaxation time solutions of the Boltzmann transport equation, valid in non-degenerate semiconductors. Explicit results are obtained for the mobility-fluctuation noise caused by ionized impurity scattering, acoustic phonon scattering, optical phonon scattering, polar optical phonon scattering, and intervalley scattering. We derive Hooge's law, and the Hooge parameters for the above-mentioned processes are obtai...

Impurity states and electron transport in gapless semiconductors

Abstract: Specific features of transport phenomena in gapless semiconductors which are due to both peculiarities of the electron spectrum of an ideal crystal and those of the impurity states in these semiconductors, are discussed. A detailed critical analysis is made of a large number of experimental papers dealing with studies of the conductivity, Hall effect, and magnetoresistance in HgCdTe crystals as a function of temperature, pressure, and magnetic field. On the strength of this analysis it is concluded that the perturbing action of the potential of acceptor-type impurities leads to effective overlap of the valence band with the conduction band. As a result, when the impurity concentration is sufficiently high, and exceeds that at which the Mott-Anderson transition occurs, the density of states of a p-type zero-gap semiconductor differs substantially from that of an ideal crystal, and is similar to that of a semimetal.

Polarization dependence of natural and field-induced one-photon and multiphoton interactions

Abstract: Natural and induced linear and nonlinear processes are discussed from a unified viewpoint. In each case the dependences of a nonlinear optical process on the geometry of the experimental arrangement, including the polarization of the light beams, on the one hand, and on the physical couplings on the other are separated in an optimally efficient and convenient manner. The method is based on a generalization of the quantum theory of angular momentum, in diagram notation, to all molecular and crystal symmetry groups. Recent applications of such methods, including Raman and Compton scattering, magnetic circular dichroism, Raman optical activity and the hyper-Raman effect, are reviewed and extended.