Table of contents

Temperature dependence of the lineshape parameter (H-parameter) of Doppler-broadened 511 keV two-photon annihilation line has been measured for a Cu-28at.%Al-4at.%Ni single crystal quenched from 1000 degrees C into ice water, and the behaviour of quenched vacancies before and after martensitic transformation has been investigated. The present experiment has revealed for the first time the fact that vacancies are distorted just above the transition temperature of the martensitic transformation. According to this result, the authors propose that vacancies are generally distorted just above the transition temperature of the displacive structural phase transformation. The electrical resistance measurement has been also made, and the results are compared with the temperature dependence of the H-parameter.

An expression for the integrated density of states of a liquid metal is given in Roth's effective-medium approximation. The expression can be regarded as a generalisation of the Friedel formula for the single-impurity problem to the situation where an infinite number of scattering potentials are distributed irregularly with finite density throughout all space. It is applicable to both the tight-binding- and the muffin-tin-potential models of liquid metals and is useful for accurate determination of the Fermi levels for these model liquid metals. It is also useful for showing that the EMA preserves several sum rules for the density of states.

The temperature-dependent electrical resistivity of pure bulk aluminium is investigated in response to recent reports of large conductivity enhancements resulting from superconducting fluctuations. No evidence for superconducting fluctuations were observed. Instead, the resistivity was found to increase as T² in samples having a residual resistance ratio greater than 4000, and to increase more rapidly in less-pure samples.

The influence of dislocations on the linewidth of the Mossbauer spectrum is studied for annealed and cold-rolled samples of armco-iron. The observed difference of 0.017 mm s^-1 in the line width for the two samples can be successfully explained using the theory given by Kvashnina and Krivoglaz (1967).

Contributions to the zero-temperature total energy of sodium are calculated as a function of structure at constant volume. The results emphasis the attractive features of continuous deformation models for the martensitic phase transition over explanations based on stacking fault considerations.

The authors investigate the Blandin-Heine-Stroud-Ashcroft suggestion that the Hume-Rothery rules for the phase stability of alpha and beta brasses arise as a consequence of the rapid variation of the free electron density response function in the neighbourhood of 2K_F. Using pseudopotential theory the authors show that the most structure-sensitive contribution to the free energy of a random binary alloy is the one-electron contribution arising from the virtual crystal bandstructure of neutral pseudo-atoms. The calculated difference in the one-electron contribution to the free energy between different crystal structures shows rapid variations at concentrations which are characterised by particular electron per atom ratios Z for which 2K_F is equal to the first or second reciprocal lattice vector of the alloy.

Isochronal and time-linear annealing of pure nickel and dilute NiSb-alloys (0.03 and 0.1 at.%Sb) cold rolled to different thickness reductions have been studied by positron lifetime, angular correlation and Doppler-broadening measurements. The results show that free vacancy migration occurs at the stage III around 80 degrees C. A small amount of Sb impurities induces three-dimensional vacancy clustering during the stage III. These microvoids grow continuously with further heating. After an annealing at 335 degrees C they become visible in the transmission electron microscope. The average microvoid size as a function of annealing temperature is estimated. The microvoids anneal out at 450 degrees C during the recrystallisation.

Self-consistent jellium calculations of the electron structure of hydrogen in Al and Mg have been supplemented with a first-order calculation of the energy contribution of the pseudopotential lattice. It is found that the most stable interstitial position of hydrogen in both Al and Mg is the tetrahedral one, with heats of solution of 1.3 eV and -0.1 eV respectively. Within vacancies, the hydrogen is predicted to prefer off-centre positions with vacancy trapping energies of about 1 eV for Al and 0.1 eV for Mg. The results are compared with available experimental data and good agreement is found.

Precision measurements of the dispersion relations for some low-energy phonons in Cd have been performed at 77K by inelastic neutron scattering. Phonon group velocities have been extracted from the data with an accuracy of 5-10%. Several abrupt changes of group velocity as a function of wave vector have been identified as Kohn anomalies. The wave vectors and strengths of the anomalies are compared with calculations based on a model electron-ion potential and a spherical Fermi surface. Discrepancies between experiment and theory are qualitatively explained in terms of deformations of the Fermi surface from sphericity. These deformations are consistent with those determined by Stark and Falicov (1967). Extension of the calculations to include third-order terms in the model potential leads to the prediction of further phonon anomalies, none of which is observed.

With the aid of the theory of irreversible processes the author has succeeded in deriving relationships between the intrinsic diffusion coefficient, the tracer diffusion coefficient, and the correlation factor for a jump of single vacancies in closely packed alloys of the substitution type. The derived relations include those obtained by Manning (1967). For very dilute or infinitely dilute solutions the thermodynamic factor, the ratio of the intrinsic diffusion coefficients, the jump frequency relationships and the correlation factor of the impurity can be calculated from experimentally determined diffusion data.

The d electronic state in the realistic structurally relaxed models of amorphous and liquid transition metals is investigated by using the recursion method. The local density of states is discussed extensively with relation to the local atomic environment. It is found that the density of states for the amorphous case has a double-peaked structure, whereas that for the liquid case is single-peaked. The electronic transport is discussed in a model after Mott (1972) involving s-d transitions and with different mean free paths for s and d electrons. The numerical results of the resistivity and thermoelectric power show quite excellent agreement with experimental ones.

The authors present a simple calculation of the bandstructure contribution to the dipolar tensor of dilute transition metal alloys. Their numerical results exhibit the correct order of magnitude and predict systematic variations of the dipolar tensor with the impurity charge.

Optimised model potential theory, including orthogonalisation of the conduction states to the core states, has been used to calculate the temperature dependence and magnitude of the electric field gradient in the metals Be, Zn, Cd and In. Good agreement with measured temperature dependences is obtained, with improved values for magnitudes compared to previous model potential theory calculations. Anisotropy in the thermal vibration of the atoms is found to affect noticeably the calculated temperature dependences, and correlation and exchange interactions among conduction electrons are found to affect the magnitudes substantially. Problems in obtaining a simple explanation for the empirical ³/₂ power law for the temperature dependence are discussed.

Measurements of the electrical resistivity, rho , and thermopower S, of high-purity liquid Ca and Sr are reported. It is found that, at the melting temperature, rho =23.0, S=15.2, and (1/ rho )(d rho /dT)=+6.1 for Ca, and rho =87.0, S=0.3, and (1/ rho )(d rho /dT)=-0.46 for Sr, where the units are mu Omega cm, mu V degrees C^-1, and degrees C^-1, and degrees C^-1, respectively. These values are compared with recent calculations, special attention being given to the unusually large and positive value of (1/ rho )(d rho /dT) for Ca.

The electrical resistivity of four dilute PtCr alloys containing between 0.05 and 0.3 at.% Cr has been measured between 1.4 and 300K. The incremental resistivity Delta rho (T) for these alloys is assumed to contain two dominant contributions: the first, which decreases monotonically with increasing temperature, is due to magnetic scattering while the second arises from conventional deviations from Matthiessen's rule. With plausible assumptions about the latter, the parameters characterising the magnetic scattering have been estimated from data over a wide temperature range.

The effect of planar defects, particularly antiphase boundaries, upon the electronic structure and magnetic ordering of the ground states of certain binary and Heusler alloys is investigated theoretically. Because the magnetisation in question arises from bands of itinerant d electrons, the systems are taken to be described by a variant of Hubbard's Hamiltonian for a narrow s band. The planar defects are represented as a pair of adjacent planes between which anomalous values exist for the nearest-neighbour transfer matrix elements which constitute the kinetic part of the Hamiltonian. A tractable if crude model is generated by decoupling the equations of motion of the corresponding Green functions in the Hartree-Fock approximation, and self-consistent solutions are then obtained which manifest the site density of states and the distribution of magnetic moments in the vicinity of the defect. Comparison of the total energies of the pertinent ground states shows that, when the anomalous transfer matrix integrals are sufficiently larger than those characteristic of the bulk, the defect may support a narrow domain wall. Such a phenomenon has been observed to be associated with the periodically recurring stacking faults in Pd₃Mn and the random antiphase boundaries found in the CuMnAl Heusler alloys and in MnAl.

The effect of the electron-phonon renormalisation on the thermodynamics of weak coupling superconductors is discussed, starting from the equations appropriate for a strong-coupling system derived by Bardeen and Stephen (1964). For isotropic superconductors, the expected factor of (1+ lambda ), where lambda is the mass renormalisation parameter, is obtained. It cancels out of the various ratios of experimental quantities, giving just the BCS results. In order to re-derive the usual results for the anisotropic superconductor, with the effect of the mass renormalisation explicitly included, it is necessary to modify the definition of the Fermi surface average to include the quasiparticle velocity. Some of the difficulties of trying to obtain similar results, starting from the formulae derived by Wadal (1964), are also discussed. A simple way to include corrections due to deviations of 2 delta ₀/k_BT_c from its BCS value, is given.

The tunnelling density of states measured on the normal side of a proximity-effect double-layer is experimentally investigated. An empirical model for the calculation of the density of states is suggested and is shown to account for most of the observed features. The importance of taking into consideration, in such studies, the influence of the normal metal upon the superconductor is pointed out.

Crystal field parameters and pressure dependent susceptibility and Knight shift data for tri-valent rare-earth pnictide and chalcogenide compounds are analyzed. The three rare-earth valence electrons are assumed to distribute themselves amongst the anion p bands and rare-earth 5d bands. It is argued that under pressure, the charge transfer from the rare-earth to the anion p band is modified and that this process dominates in producing the observed pressure dependence of the crystalline electric field parameters. The rate of transfer of p electrons back to the rare-earth 5d band under pressure is found to be approximately one order of magnitude less for the chalcogenides than for the pnictides and it is suggested that this is due to smaller d-p overlap arising from the position of the chalcogenide p band with respect to the Fermi level and the d band level.

The magnetisation curve of a Bi_95.6Mn_4.4 alloy was measured at 118K. The sample was tested both in the unstressed state and exposed to ultrasonic stress pulses. These stresses changed the magnetisation of the sample if ultrasonic waves were applied additionally to the magnetic field in those sections of the magnetisation curve where magnetisation changes are irreversible. The effect can be explained qualitatively on the basis of the Stoner-Wohlfarth theory.

A detailed study has been made of the magnetisation and magnetostriction of a single crystal of Pr in pulsed fields up to 18 T and in the temperature range 3-78K. Measurements were taken for all combinations of strain and applied field directions along the three symmetry axes to determine the modes of distortion. These results are analysed using a molecular field Hamiltonian, and the single-ion magnetoelastic coefficients are derived. The dominant strain-dependent interaction is shown to arise from the crystal field.

A comparison is made between the experimental and calculated magnitudes of the microwave Faraday rotation and ellipticity related to conduction electron spin resonance (CESR) of a lithium dispersion in paraffin wax. Experimental magnitudes are taken from a previous paper by Theobald et al. (1972), whereas calculated magnitudes are obtained from equations which have been transported from the classical paramagnetic case to the special case of CESR. In this trans position, the static susceptibility is in fact replaced by the conduction electron spin susceptibility of the effective lithium sample. The effective volume of lithium was only a part of the total volume, because the skin depth was smaller than the size of particles at the frequency and temperature of the experiment. Finally, a good agreement is found between the experimental and calculated magnitudes of the CESR Faraday effect of lithium.

The authors report EXAFS (extended X-ray absorption fine structure) measurements performed in the 1000-2500 eV range using synchrotron radiation delivered by the ACO storage ring (LURE). The K edge EXAFS spectra of Al in elemental aluminium and alpha alumina have been analysed; it is shown that accurate measurements of the first-neighbour distances are possible even for low-Z atoms. In the case of the metal, the authors emphasise the importance of the screening of the core hole potential in the first 150 eV part of the spectrum.