Table of contents

Layered Pb/Ag thin films have been produced by vacuum evaporation on cooled glass substrates. Measured X-ray diffraction spectra showed the peaks characteristic of a superlattice. A new model of a non-ideal superlattice is proposed in which thin layered films are composed of a large number of texturised grains. Calculated X-ray diffraction spectra are compared with experimental data. The agreement between the experimental data and the results of the calculates supports the model of layered growth of silver and lead within the grain. The influence of room-temperature annealing was observed.

A general formula for the partial structure factors of an m-component mixture is derived. The compressibility of the mixture obtained by Kirkwood and Buff (1951) can be written in terms of the partial structure factors in the long-wavelength limit. For the m-component hardsphere system, the explicit expressions for the partial structure factors are obtained based on Hiroike's (1969) exact solution of the Percus-Yevick equation. The formula are applied to the liquid Li-Pb alloys, which is considered as a ternary mixture of Li, Pb and Li₄Pb. It is shown that the theoretical total structure factor is in good agreement with the observed one due to Ruppersberg and Reiter (1982) by the neutron diffraction experiment.

The authors present the results of calculations of the partial structure factors and radial distribution functions of liquid Li₄Pb at different temperatures. Assuming that this alloy is partially ionic, they have modelled the interionic forces by pairwise potentials that are strongly repulsive at small separations r and electronically screened Coulombic at large r. This model was motivated by the fact that an r-dependent ordering potential, which exhibits approximately screened Coulombic decay for larger r, can be extracted from the neutron diffraction data for S_CC(q) in Li₄Pb. The calculations, which are based on the mean-spherical and the hyper-netted-chain approximations, show that the wavenumber dependence of the measured concentration structure factor S_CC(q) can be reasonably well accounted for by the model with effective electron charges of about 0.5 and 4-2.0 at the Li and Pb sites, respectively, and an inverse screening length of 1.1 AA^-1. These parameters are consistent with values obtained from the ordering potential extracted from experiment. In order to explain the observed temperature dependence of S_CC(A) it is necessary to assume that the charge transfer between species decreases with increasing temperature.

The effect of depinning on the position and shape of the dislocation damping peak associated with point-defect drag is considered. Although this peak exhibits a standard Debye form as a function of frequency, a significant narrowing of the damping versus temperature maximum, as well as a shift towards lower temperatures, results from the interference of depinning. The effect depends on the heating rate. A similar behaviour of the differentiated modulus defect is predicted. However, in both cases the peak narrowing remains hidden in the apparent activation energy, conventionally derived from the peak temperature displacement upon frequency variation, is used in evaluating the peak width instead of the true activation energy for the underlying relaxation process. The analytical results obtained suggest a procedure for revealing the true peak narrowing by determining the true activation energy for point-defect diffusion along with the dislocation. A method for experimentally 'decoupling' this activation energy from the activation energy for superimposed depinning is proposed. This method is based on measuring the peak temperature displacement when varying both the frequency and the heating rate.

The attenuation of longitudinal acoustic waves propagating along the (110) axis in single-crystal PdH_0.64 has been investigated over the temperature range 80-300K and the frequency range 10-270 MHz. At a given frequency the attenuation exhibits a maximum as a function of temperature. The attenuation peaks are attributed to the stress-induced ordering of hydrogen atoms on the interstitial sites. These peaks occur when omega tau _R=1, where omega /2 pi is the frequency of the ultrasonic wave and tau _R is the relaxation time for stress-induced ordering. An Arrhenius plot of tau _R^-1 against inverse temperature gives an activation energy of 0.23+or-0.02 eV over the temperature range 190-240K, approximately the same as that deduced by other techniques for the hopping of hydrogen between interstitial sites. Surprisingly, the experimental value of tau _R at any particular temperature is about an order of magnitude shorter than the mean time of stay of hydrogen on an interstitial site as determined by other experimental techniques. Possible reasons for this discrepancy are discussed.

The authors first propose an approach for treating a correction from the higher-order perturbative terms to the usual low-order pseudopotential perturbation calculation for the one-valence-electron states in a disordered metal. Then, the previously used energy-independent non-local model pseudopotential (EINMP) with this correction is applied along with the original EINMP to the variational thermodynamic calculation in the framework of a hard-sphere reference system for the alkalis in the liquid phase and for the K-Rb, Na-K and Na-Cs liquid alloys. The results obtained for (i) the hard-sphere diameter, sigma , (ii) the isothermal compressibility, chi _T, and (iii) the excess entropy, S^exc, are carefully examined and discussed. It appears that in going from the pure alkalis to their binary alloys, (i) the change in sigma follows closely the concept of valence-electron charge transfer, introduced previously to interpret the corresponding concentration dependence of the electrical resistivities and (ii) the deviations of chi _T and S^exc from ideality for the Na-K and Na-Cs alloys are largely due to the valence-electron charge transfer, which affects sigma in S^exc and also affects the contribution from the volume dependence of the screening to chi _T.

Gorsky-effect measurements of the diffusion coefficient of tritium (T) in V, Nb and Ta are reported. The measurements were performed at small T concentrations (less than about 1.4 at.%), and in the temperature range from -140 degrees C to 100 degrees C. Comparison with results obtained for H and D shows an increase of the activation energy with increasing isotopic mass. For Nb and Ta, the pre-exponential factors are identical for all the isotopes, whereas for V this factor is appreciably larger for T and for H and D. The reduction of the relaxation strength in the presence of precipitation allows a rough determination of the solubility limit, which is found to be isotope independent for Nb and Ta. In V, the solubility limit for T is shifted slightly to lower temperatures. The H, D or T induced lattice expansion calculated from the relaxation strength shows no dependence on isotopic mass.

The impurity cluster formalism has been extended to describe the scattering of Bloch electrons by a cluster centred around an interstitial impurity. An important feature of this extension is that the host wavefunction coefficients and Green function at the interstitial site can be obtained numerically from their values at the next-neighbouring atoms. The theory has been applied to Cu(H). Calculations of Dingle temperatures x^* and cross-sectional area changes delta A reveal that x^* is mainly determined by the hydrogen potential, whereas delta A is additionally sensitive to the environment of the hydrogen. The experimental x^* values can be nicely reproduced simulating screening effects by means of a potential shift. Charge neutrality is achieved satisfactorily using a seven-atom cluster which means that the influence of H is not restricted to its own muffin-tin sphere. Because of the sensitivity of delta A to charge transfer the merits of the potentials available can be judged to their full extent only if this quantity is measured as well. The effect of lattice distortion around H appears to be very small. Comparison of calculated and measured anisotropy of x^* confirms that the hydrogen occupies the octahedral and not the tetrahedral interstice in Cu.

Isomer shifts and Debye temperatures for substitutional ¹¹⁹Sn impurity atoms in the FCC metals Al, Ag, Au, Cu, Pb, Pd, Pt and Rb were determined by Mossbauer emission spectroscopy on the 24 keV gamma radiation of ¹¹⁹Sn. The radiation sources were prepared by ion implantations of radioactive ¹¹⁹In and ¹¹⁹Sb isotopes into single crystals and high-purity foils of the respective metals. Correlations are discussed between measured isomer shifts and electronic properties of the host materials (the Fermi-level free-electron density n_F^1/3 and the cell-boundary electron-density parameter n_WS^1/3 from the Miedema theory of alloy formation). The Debye temperatures for ¹¹⁹Sn are interpreted in terms of the Einstein-Debye and the Mannheim models for impurity lattice vibrations.

The linear combination of atomic orbitals method is applied to clusters of 365 atoms, which are constructed to model the pair distribution functions of liquid and amorphous Fe. From these models the authors compute densities of states, spectral functions and electrical conductivities. Densities of states are quite similar in both models. Some evidence of residual E-k dispersion relations is found in the peaks of the spectral functions, but no evidence is found for any negatively sloped portion of the E-k curve. Although s states are found to be more mobile than d states, the conductivity is dominated by d states because of their much greater density.

An attempt is made to obtain a good fit to the Fermi surface of platinum using the interpolation scheme. The starting point is a set of parameters which give a good fit to the optical data. The authors find that it is possible to obtain a good fit to the Fermi surface as well as to the optical data.

Resistivity calculations based on the nearly free electron model are presented for many noble and transition liquid metals. The triple-ion correlation is included in the resistivity formula according to the self-consistent quasi-crystalline approximation (SCQCA). Two different methods for describing the conduction band are used. The problem of applicability of the nearly free electron model to different metals is discussed.

The electronic properties have been studied of four Ca_1-xAl_x metallic glasses (x=0.25, 0.30, 0.35 and 0.40) prepared by liquid quenching. The measurements involved are the electrical resistivity in the range 2-700K, the Hall coefficient at 77-300K, the low-temperature specific heat at 1.5-6K and the magnetic susceptibility at 77-600K. All experimental data indicate a nonfree-electron behaviour. The effective electron concentration and the Fermi momentum are deduced from the Hall coefficient and found to be far below the free-electron values. Both the electronic specific heat coefficient and the magnetic susceptibility decrease with increasing Al content. The relatively large paramagnetism suggests the presence of d-character states at the Fermi level. The electrical resistivity is empirically found to obey the relation rho = rho ₀-BT^1.2(B>0) below 120K and to be proportional to temperature with a negative TCR in the range 120-400K.

The electronic properties of Cu_1-xTi_x metallic glasses (x=0.4, 0.5 and 0.6) have been studied through measurements of the low-temperature specific heat, the magnetic susceptibility and the electrical resistivity. The electronic specific heat coefficient increases with increasing Ti content. This behaviour is consistent with the UPS results and indicates the dominant contribution of the Ti 3d states at E_F. The magnetic susceptibility of the amorphous phase is temperature independent and is of the order of 10^-4 EMU mol.^-1. It also increases with increasing Ti content. The presence of a large orbital paramagnetism is deduced and attributed to the unfilled d states at E_F. The electrical resistivity increases with decreasing temperature for all alloys studied. The Ziman theory cannot be applied for such highly resistive alloys. The low-temperature resistivity below 100K does not fit to the relation rho = rho ₀-AT² (A>0), suggesting that an extra scattering mechanism is present and supersedes the effect due to the temperature dependence of the Debye-Waller factor.

The Hall resistivity of amorphous Fe_xNi_80-xB₁₈Si₂ alloys (x=0, 5, 8, 10, 12 and 15) has been measured in the paramagnetic region. Except for x=0 these alloys are ferromagnetic with Curie temperatures T_C<or=316K. For the separation of the normal and anomalous Hall coefficients the authors also measured the field and temperature dependence of the magnetisation in the corresponding temperature interval. The normal Hall coefficient R₀=-(1+or-0.1)*10^-10 m³ A^-1 s^-1 was roughly independent of x. The anomalous Hall coefficient R_s was positive, a factor of one hundred bigger, and it increased with x. The results for R₀ and R_s are compared with those for similar systems and discussed in terms of the recent theoretical models.

The resistance and magnetoresistance of the intermetallic magnetic superconductor Y₄Co₃ were measured in the temperature range 1.74-4.2K and in fields of up to 1.1 T. The authors extracted the slope of the upper critical field near the superconducting transition, and the specific heat coefficient gamma . The main conclusions are that the superconducting state is fully quenched just above the transition temperature, and that the authors are dealing with a dirty superconductor. Moreover, the specific heat coefficient gamma calculated from theory is in good agreement with the experimental value.

The metals in the middle of the transition metal series have a tendency towards antiferromagnetism, ordered AB alloy structures and lattice distortions based on a twofold superlattice. By contrast, at the beginning and end of the series the corresponding tendency is towards ferromagnetism, chemical segregation rather than alloy formation, and stable simple structures. The authors show how all these results can be understood in a unified way in terms of a general theorem published earlier. The argument applies quite generally to any set of tight-binding bands such as the d bands, independent of the specific structure. In a general sense the superlattice formation for approximately a half-filled band is like a Peierls transition, but it results from the electronic structure as a whole with no special role attached to the Fermi surface. The case of threefold superlattices is also discussed with examples.

The application of cold-working causes a remarkable reduction in the magnetisation of the intermetallic Ni₃Al compound. The cold-working effect is explained in terms of the dislocation theory with consideration of the ferromagnetic and paramagnetic interactions of Ni atoms. The magnetisation at 0K decreases in proportion to the dislocation density in the early stage of plastic deformation and to the square root of it in the advanced stage.

The magnetic properties of Eu²⁺ (4f⁷ electronic configuration) based amorphous alloys Eu_1-xX_x, where X=Mg, Zn, Cd and Al and x approximately=0.3, have been investigated by studying local moments from ¹⁵¹Eu Mossbauer spectroscopy and bulk magnetisation in low AC and high DC fields. The divalent 4f⁷ state of Eu atoms implies vanishing interactions with crystal fields. The magnetic properties are then assumed to be depicted by localised 4f⁷ moments interacting via RKKY interactions, in view of the pure s (Mg, Zn, Cd) or sp (Al) character of the valence electron shells of the components. Amorphous Eu_1-xAl_x (x=0.225, 0.30 and 0.40) are concentrated spin glasses (mictomagnets). Amorphous Eu_0.70Zn_0.30 (T_C=133K) and Eu_0.75Cd_0.25 (T_C=134K) reveal all the properties of soft ferromagnets in the presence of some distribution of the exchange interaction, resulting from the disorder. Amorphous Eu_0.70Mg_0.30 exhibits a double magnetic transition, i.e. from paramagnetic to ferromagnetic, at T_C=131K and from ferromagnetic to spin glass at T_SG approximately=52K. The latter re-entrant spin-glass transition is consistently characterised both from AC susceptibility and from the anomalous temperature dependence of the hyperfine field distribution. Amorphous Eu_0.70Mg_0.30 is concluded as representing an example of a cluster spin glass (with intercluster frustration) whereas the amorphous Eu_1-xAl_x alloys would present short-range frustration.

Two-photon momentum densities and the two-dimensional angular correlation of positron annihilation radiation (2D ACPAR) in paramagnetic Cr are calculated by using the band-structure method due to Hubbard and Mijnarends (1972). The 2D ACPAR surfaces, N(p_y, p_z), are calculated for four sets of momentum directions with p_x (the axis of integration) along the (110), (111), (100) and (112) directions. The results are presented and are discussed in terms of the contributions from different bands and the topology of the Fermi surface. The results indicate that in the first Brillouin zone the N-centred ellipsoidal hole pockets of paramagnetic Cr give rise to significant structure in the N(p_y,p_z) surfaces.

The Doppler-broadened positron annihilation spectrum from a single crystal of zinc has been studied as a function of temperature and of the direction of photon emission. An anisotropy in the behaviour of the spectrum line height parameter was observed in the prevacancy temperature region with linear slopes 0.60*10^-4K^-1 in the (0001) direction and 1.02*10^-4K^-1 for (1010). Optimum fittings with the two-state trapping model gave differing values for the monovacancy formation enthalpy, implying an inadequacy in the present understanding of the prevacancy rise.

It is shown that the temperature dependence of the total structure factor of the liquid Li_0.8Pb_0.2 alloy obtained experimentally by Ruppersberg and Reiter (1982) can be qualitatively well understood within the compound-forming model by taking into account the temperature dependence of the total packing fraction and the fraction of the 'molecule'.

An SXS experimental determination of valence density of states has been performed for Pd in PdH_x (x approximately 0.6). From their results, the authors ascertain that in PdH_x a structure at about 7.4 eV from the maximum of the 4d-5s band is observed in agreement with a previous theoretical calculation.

A variational theorem is used to obtain an expression for the spin polarisation (or exchange) energy of a free atom directly, rather than as the difference between two large numbers, and a numerical demonstration is provided for a nickel atom. The spin polarisation energy of several open shells, and the coupling between them, is then expressed as a quadratic form in the spins of the occupied valence orbitals, with local exchange integral matrix elements as coefficients. The authors have evaluated these matrix elements for all d transition metal and light actinide atoms. The spin polarisation energies of transition metal atoms are added to the measured cohesive energies to obtain valence bond energies.

In a recent letter Mahan and Hansch (1983) derive a new transport equation for the linear response of many-body systems to DC perturbations. In this letter the author shows how time-dependent external fields can be included in the analysis; as an application, Holstein's (1964) electron-phonon transport equations, originally obtained from the Kubo formula, are derived.

A simple new empirical relation for the pressure dependence of the superconducting transition temperature T_c is proposed. The validity of the relation is checked using the T_c results obtained from high-pressure band-structure calculations for FCC La and Pr.

It is shown that the triple-Q spin density wave state with components Q_x, Q_y and Q_z can be stabilised in the first-kind FCC antiferromagnet by the fourth-order term with respect to the transfer integral. Using the interacting virtual bound state approach, this state is found to be more stable than the simple-Q state in the nearly half-filled band region.

The magnetic properties of melt-spun iron-zirconium alloys are drastically altered by the addition of hydrogen. The transformation of the magnetic order in Fe₈₉Zr₁₁ from collinear ferromagnetism to asperomagnetism below 60 K is suppressed in Fe₈₉Zr₁₁H₂₀, which remains ferromagnetic at all temperatures. Hydrogenation increases both the Curie temperature and the moment per iron atom, from 252K and 1.6 mu _B, respectively, to 382K and 2.0 mu _B.

Core electron energy loss spectra (CEELS) of 4d to 4f excitations are presented for Ce intermetallic compounds. The spectral fine structures of 4d⁹4fⁿ⁺¹ final states show features which can be correlated to the f-count as derived from other core level spectroscopies. Comparison with 4d CEELS of gamma Ce and CeO₂ reveals, however, that the detailed spectral shapes of 4d excitations in Ce intermetallics differ from those expected from a simple superposition of f¹ and f² final state configurations.