Showing papers in "Advances in Physics in 1975"

Charge-density waves and superlattices in the metallic layered transition metal dichalcogenides

Abstract: The d1 layer metals TaS2, TaSe2, NbSe2, in all their various polytypic modifications, acquire, below some appropriate temperature, phase conditions that their electromagnetic properties have previously revealed as ‘anomalous’. Our present electron-microscope studies indicate that this anomalous behaviour usually includes the adoption, at some stage, of a superlattice. The size of superlattice adopted often is forecast in the pattern of satellite spotting and strong diffuse scattering found above the transition. Our conclusions are that charge-density waves and their concomitant periodic structural distortions occur in all these 4d1/5d1 dichalcogenides. We have related the observed periodicities of these CDW states to the theoretical form of the parent Fermi surfaces. Particularly for the 1T octahedrally coordinated polytypes the Fermi surface is very simple and markedly two-dimensional in character, with large near-parallel walls. Such a situation is known theoretically to favour the formation of...

Structural and electrical properties of granular metal films

Abstract: Granular metal films (50–200,000 A thick) were prepared by co-sputtering metals (Ni, Pt, Au) and insulators (SiO2, Al2O3), where the volume fraction of metal, x, was varied from x = 1 to x = 0.05. The materials were characterized by electron micrography, electron and X-ray diffraction, and measurements of composition, density and electrical resistivity at electric fields e up to 106 V/cm and temperatures T in the range of 1.3 to 291 K. In the metallic regime (isolated insulator particles in a metal continuum) and in the transition regime (metal and insulator particles in a metal continuum) and in the transition regime (metal and insulator labyrinth structure) the conduction is due to percolation with a percolation threshold at x⋍0.5. Tunnelling measurements on superconductor-insulator-granular metal junctions reveals that the transition from the metallic regime to the dielectric regime (10–50 A size isolated metal particles in an insulator continuum) is associated with the breaking up of a metal ...

Interpretation of S-state ion E.P.R. spectra

Abstract: Although a considerable body of data exists on the parametrization of the ground-state splittings of S-state ions in crystals, relatively little progress has been made in obtaining a quantitative understanding of the mechanisms which determine these parameters. In the course of summarizing our present understanding, we emphasize the need for making planned experiments explicitly aimed at testing theoretical models, such as those proposed in this article. The variable frequency E.P.R. technique is described in some detail, as it has proved to be particularly useful in this respect.

Sloping convection in a rotating fluid

Abstract: Laboratory experiments on thermal convection in a fluid which rotates about a vertical axis and is subject to a horizontal temperature gradient show that when the rotation rate Ω exceeds a certain critical value ΩR (which depends on the acceleration of gravity, the shape and dimensions of the apparatus, the physical properties of the fluid and the distribution and intensity of the applied differential heating) Coriolis forces inhibit overturning motion in meridian planes and promote a completely different type of flow which has been termed ‘sloping convection’ or ‘baroclinic waves’. The motion is then non-axisymmetric and largely confined to meandering ‘jet streams’, with trajectories of individual fluid elements inclined at only very small (though essentially non-zero) angles to the horizontal. The kinetic energy of the waves derives from the interaction of slight vertical motions with the potential energy field maintained by differential heating, and it is dissipated by friction arising largely...

Phonon-assisted transition rates I. Optical-phonon-assisted hopping in solids

Abstract: An extensive calculation of the optical-phonon-assisted transition rates for non-adiabatic electronic hopping motion in a solid is presented. Holstein's Molecular Crystal Model is used as a basis for study and the computation involves no restrictions on either the magnitude of the electron-lattice coupling strength, the temperature, or the difference between the electronic energies of the initial and final sites. In the strong-coupling small-polaron regime, the jump rates, associated d.c. conductivity, a.c. conductivity, and electric-field dependence of the d.c. conductivity, for a crystal are all calculated. These transport properties manifest qualitatively distinct behaviours corresponding to whether the temperature is above or well below the optical-phonon temperature. In the low-temperature regime the energy-conserving processes which involve the absorption of the minimum amount of vibrational energy provide the dominant contribution to the thermally activated jump rates. At sufficiently high...

Charged dislocations in ionic crystals

Abstract: Experiments and theories concerning charged dislocations in alkali halide crystals are reviewed in detail, with particular attention to the way in which the experiments should be interpreted and to the range of applicability of the sweep-up and various forms of thermal-equilibrium models. Possible effects on mechanical properties and internal friction are analysed. The transient and steady-state effects of plastic deformation on ionic conductivity are described and new interpretations involving charged dislocations are proposed. A survey of results on the scattering of light by alkali halide single crystals leads to the conclusion that charged dislocations do not play an important role. Evidence about charges on surfaces and on dislocations in AgCl and AgBr is reviewed and compared with that for alkali halides. Comparisons are also made with MgO, the CsCl and CaF2 structures, semiconductors and ice.

Magnetic behaviour of cobalt, iron and manganese dissolved in palladium

Abstract: This paper is meant to be a report on the experimental work on dilute Pd-based alloys with Co, Fe and Mn. These alloys exhibit the phenomenon of giant moments. The importance of measurements on paramagnetic alloys is emphasized. From these measurements the conclusion can be drawn that Co and Fe dissolved in Pd does not behave like a normal paramagnet, i.e. according to a Brillouin function. This result makes it possible to explain the existing discrepancy in the interpretations of magnetic measurements on one hand and of specific-heat experiments on the other. The main conclusions of this paper are: The giant moment should be accounted for by ‘normal’ values of the magnetic quantum number (3/2 for Co, 2 for Fe and 5/2 for Mn) and a large value of geff. Paramagnetic alloys of Mn in Pd behave according to Brillouin functions, but alloys of Co or Fe in Pd do not. Hence, a number of interpretations of magnetic measurements should be considered as incorrect. The localized model for ferromagnetism can ...

Slow-neutron multiple scattering

Abstract: Slow-neutron scattering is perhaps the most powerful method presently available for investigating the structure and dynamics of materials on the atomic level. With relatively few exceptions it can be used to advantage for any material whether it be in the solid, liquid or gaseous state. In the past few years the importance of applying multiple-scattering corrections has come to be recognized, particularly in the case of liquids and gases for which one is interested in mapping the complete scattering function. The present article presents a review and extension of the theoretical basis for such corrections from the point of view of the Vineyard theory of multiple scattering. The theory is formulated for a sample of arbitrary size and shape and a general expression is obtained for the effective scattering function as an expansion in terms of orders of scattering (single, double, etc.). The general properties of this expansion are investigated with particular emphasis on the question of convergence ...

Physical cluster mechanics: Statistical thermodynamics and nucleation theory for monatomic systems

Abstract: We extend previous computations of mechanical stability of atomic microclusters to the realm of statistical thermodynamics, obtaining thermodynamic functions for small, solid-like Van der Waals clusters of less than some 100 atoms possessing non-crystalline structures of ‘polytetrahedral’ type. These are shown to be almost invariably at a thermodynamic advantage over alternative lattice structures of the same number of atoms, at least for the Lennard-Jones potential in the harmonic-oscillator/rigid-rotor approximation. The dependence of thermodynamic functions upon cluster size appears to be essentially monotonic in the number of internal degrees of freedom; although there are certain exceptional structures, particularly with icosahedral symmetry, there proves to be little evidence for the occurrence of ‘magic numbers’ for stability at any temperature and within the size-range considered. Particular attention is given to the heat capacity of model systems in relation to their vibrational spectra....

Electron spin resonance of magnetic ions in metals an experimental review

Abstract: The contribution that the technique of electron spin resonance has made to our understanding of the behaviour of localized moments in metals is reviewed. The main areas of application of the technique are discussed at a simple theoretical level and its limitations are pointed out. A comprehensive review of the experimental results obtained from electron spin resonance is presented and, in appendices to the article, a comprehensive list of experimental results is given indexed to author, impurity and host and to areas of application. In a final section of the review an attempt is made to suggest those areas in which electron spin resonance still has an important part to play in adding to our understanding of magnetism in metals.

The optical and electrical effects of high concentrations of defects in irradiated crystalline gallium arsenide

Abstract: The damage produced by fast neutron irradiation of gallium arsenide has been studied by a number of techniques. The electrical resistivity, which increases with dose at low doses to semi-insulating values, shows a remarkable, specimen-independent decrease for doses greater than 1017 n cm-2 from values of ca. 109 Ω cm to 3 Ω cm for the highest dose of 1.5 × 1020 n cm-2. In this high dose region the temperature dependence of the resistivity at low temperatures is given by exp [b/T 1/4] and it is suggested that in this highly disordered state conduction occurs by tunnel-assisted hopping between defect levels in the band gap. The presence of such levels is indicated by the strong optical absorption tail which is produced from 0.1 eV to the crystalline edge at 1.5 eV. Although at the highest doses the samples contain a high degree of disorder, X-ray diffraction shows that they are basically crystalline. Lattice parameter determinations show that it increases with dose, linearly at first, then tending ...

Exact formulae for the electrical resistivity

Abstract: The theory of exact formulae for the electrical resistivity is reviewed. The derivation of an exact formula for the resistivity and the proof of the equivalence of this formula with the linear response formula for the electrical conductivity are presented. One form of the resistivity formula is evaluated in the limit of weak scattering. The model for this calculation is a system of non-interacting free electrons scattered by random fixed impurities. It is shown that the evidence which suggested that the resistivity formula is simpler to evaluate than the conductivity formula is misleading.

Transport properties of electrons in energy band tails

Abstract: Transport properties of electrons in energy band tails of disordered semiconductors are studied experimentally using a material system in which (i) the width and shape of the band-tail are approximately known and (ii) the Fermi energy is controllable. The material is heavily-doped, closely-compensated, crystalline n-GaAs whose compensation ratio can be made arbitrarily close to unity by the use of two techniques that are described in detail. This control of the Fermi level through compensation permits the measurement of the transport properties of electrons at various energies in the band-tail. Using band tails having a width of ∼50 meV, measurements have been made of the temperature dependence of the d.c. conductivity and Hall coefficient, the frequency dependence of the a.c. conductivity and the electric field dependence of the d.c. conductivity (the last two at low temperatures). The evidence demonstrates the progressively greater localization of states deeper in the tails. No sign is found of...

Optical homodyne detection

Abstract: This paper contains a theoretical treatment of optical homodyne detection applied both to radiometry and to scattering in the visible and near infra-red, together with some experimental comparisons. A signal estimator, corresponding to the difference in the detector output a.c. power when a given optical field is present or absent, is introduced. The accuracy with which the mean signal intensity can be measured is derived in terms of the inverse root relative variance (IRRV) of this estimator. It is shown that most conventional microwave-based analyses of this problem need revision since they ignore, or treat in an ad hoc manner, both signal fluctuations and photodetection quantum statistics. A rigorous analysis is given including both these effects. The predictions of this analysis compare favourably with existing experimental data. The results are then used to compare signal-intensity measurement by direct or homodyne detection under various operating conditions. Finally, going beyond the measu...