Table of contents

The dispersion equation for acoustic plasmons in a high-density degenerate two-component Fermi gas is given and analysed. The random phase approximation (RPA) dielectric function is used, with exact analytic continuation from the upper to the lower half of the complex frequency plane. For small wavevector the acoustic plasmon frequency and damping depend on only two parameters, the ratio of Fermi velocities and the ratio of Thomas-Fermi wavevectors. A large mass ratio is not necessary for weak damping.

Nitrogen-implanted steels of different composition (mainly a variable chromium content) and structure have been studied by nuclear reaction analysis (NRA) and conversion electron Mossbauer scattering (CEMS). For every given composition and structure, the relationship between a parameter measuring the amount of iron nitride produced and the nitrogen dose retained in the near-surface region is found to be almost linear with a nearly constant positive slope. The dose threshold for iron nitride formation increases with increasing chromium content and defect concentration in the matrix.

The authors present an extension of the blip-function expansion of Weeks, Chandler and Andersen (WCA) (1971) to liquid binary alloys. It turns out that the fact that realistic interionic pair potentials tend to be non-additive for most alloys poses a serious problem. They propose to resolve this problem by modifying the separation of the pair potentials in repulsive short-range and long-range terms in such a way that the repulsive parts are always additive, the non-additivity being entirely absorbed in the long-range terms. They show that this results in a first important improvement of the structure of liquid alloys over a simple hard-sphere picture. However, we emphasise that this should only be considered as a first step towards a systematic application of thermodynamic perturbation theory to liquid alloys.

X-ray powder diffraction results for various hydrides Tm₆Mn₂₃H_x and Er₆Fe₂₃H_x are reported and included in an extensive plot of hydride-cell volume V_hydride against host-cell volume. All tetragonal hydride phases identified to data are observed to satisfy x>9, V_hydride<1.883(5) nm³ and c₀>a₀. It is demonstrated that this is compatible with hydrogen-induced distortion of the j₁ interstitial sites which are otherwise too small to accommodate hydrogen atoms.

A detailed analysis of positron lifetime spectra measured for In between 77 and 420K with high time resolution clearly shows the validity of the simple two-state trapping model. With the rate sigma _1V of positron trapping per unit vacancy concentration assumed to be independent of temperature the vacancy formation enthalpy H_IV^F=0.54+or-0.03 eV is determined from the parameters deduced from the lifetime spectra. The influence of a temperature dependence of sigma _IV suggested by theoretical considerations is discussed. A comparison with earlier self-diffusion data yields the vacancy migration enthalpy H_IV^M=0.27+or-0.04 eV, which is discussed together with results of non-equilibrium experiments.

A hierarchy of the various decoupling schemes produced by the periodic Falicov-Kimball Hamiltonian is established. The higher-order schemes are related to the alloy analogy and the methods used in that case: ATA and CPA. The physical picture assumed in each treatment and its influence on the valence transition are discussed.

Bloch spectral functions and component-projected Bloch spectral functions are defined within the framework of the KKR CPA method. The methods proposed are applied to a discussion of the band structure and the Fermi surface of TiN_x.

The valence electron momentum density (EMD) and the momentum density of positron annihilation pairs (MDAP) are calculated ab initio for alkali metals from Li to Cs. It is shown that the proportion of valence electrons having their momenta within the central Fermi surface ranges from 75% (Cs) to 93% (Na); the momenta of the remaining valence electrons lie in the Umklapp Fermi surfaces centered at the surrounding reciprocal lattice points. In the calculation of the MDAP, various enhancement factors describing the effect of the many-body electron-positron interaction are examined; it seems that the recent model of enhancement of Umklapp terms presented by Sormann et al. (1982) is not fully adequate. A relation between the EMD and MDAP is briefly discussed and the connection between the occupation of the central Fermi surface and other parameters of the electronic structure is pointed out.

Electronic energy bands and optical absorption spectra have been calculated by the KKR method for the two BCC transition metals Cr and Fe. The method of Segall and Yang (1980) was used for efficient calculation of the structure constants and the muffin-tin form of the crystal potential has been used to calculate the logarithmic derivatives. From the band structure so obtained, the density of states and epsilon ₂ spectra were calculated for both metals for photon energies up to 6 eV. The various structures found in the spectra were compared with the latest experimental results and good agreement was observed between the calculated and the experimental values. The structures in the absorption spectra have been successfully interpreted in terms of various band-to-band transitions inside the Brillouin zone.

Variations of the liquid alloy resistivity with local chemical order are studied within the framework of Faber-Ziman nearly free electron theory. Electron-ion interactions are presented by Ashcroft's empty-core pseudopotentials and structural changes arising from chemical ordering are evaluated with the help of geometrical models. These calculations show that chemical ordering cannot produce resistivity variations larger than about 20% whatever the valency of the alloy components may be. It is therefore concluded that discrepancies between measured and calculated resistivities larger than 20% should be attributed to electronic rearrangements producing changes in the pseudopotentials, or to departures from the nearly free electron approximation.

An exact expression is obtained for the off-diagonal part of the conductivity tensor of a man-body electron system up to the first order in an applied magnetic field. The formalism includes all types of electron scattering mechanism and is independent of the choice of the gauge describing the magnetic field. The resulting formula is expressed in terms of the three-body correlation function and includes the Lorentz force in an explicit operator form.

The impurity resistivity is calculated for various solutes dissolved in copper using a 'dilute' Korringa-Kohn-Rostoker description of the host and impurity phase shifts deduced from experiment. It is shown how a concise solution of the Boltzmann equation can be obtained by exploiting the cubic symmetry. The transport relaxation time is calculated and compared with the total scattering time and with various other approximate relaxation times. It is found that the spherical-band approximation is frequently quite inaccurate while the Ziman approximation reproduces the resistivity accurately but exaggerates the anisotropy of the relaxation time.

The thermoelectric power of copper at high temperatures is investigated using APW calculations of the band structure and the matrix elements of the electron-phonon interaction. The matrix elements depend upon the electron energy and the scattering rate of conduction electrons increases with increasing energy. If the results are interpreted in pseudopotential language, the positive sign of the thermoelectric power is mainly ascribed to the non-local nature of the pseudopotential.

Specific heat and thermal expansion experiments have been performed on Gd_xY_1-xCu₂ compounds between 1.2K and 50K and from 1.4K up to 77K, respectively. The specific heat data are analysed in an electronic, lattice and magnetic part resulting in a gradual variation of the Debye temperature with composition and in a magnetic entropy proportional to the number of Gd atoms. From the thermal expansion data below T_N a magnetic contribution to the volume at zero temperature of negative sign is deduced. The magnetic volume is roughly proportional to x² and is attributed to the loss of short-range polarisation at the antiferromagnetic spin configuration. By comparing the specific heat and thermal expansion data, the electronic, lattice and magnetic Gruneisen parameters have been determined.

The magnetovolumes of Cr and a Cr_99.5 V_0.5 alloy relative to a paramagnetic Cr₉₅V₅ alloy are deduced from the thermal expansion measurement of Roberts et al. (1983). The data indicate a relatively small ground-state magnetovolume of about 0.15% in Cr, and a collapse of the self-consistent moments above the Neel temperature, where the magneto-volume due to the remaining critical fluctuations decays approximately exponentially with temperature.

Results of magnetisation and ⁵⁷Fe Mossbauer measurements show that the strong ferromagnetic coupling in Pd₃Fe is markedly altered in the hydride Pd₃FeH_0.7. The magnetic moment of (4.59+or-0.03) mu _B is reduced by the presence of hydrogen to (0.83+or-0.03) mu _B. This reduction is accompanied by a decrease of approximately 46% in the magnetic transition temperature of the hydride compared with that of Pd₃Fe (499K). In addition Mossbauer spectra at 4.2K in external magnetic fields indicate that anisotropic antiferromagnetic coupling dominates in the hydride. Results are compared with those for Pd₂AuFe where similar effects are observed.

The magneto-volume effect of Fe-based amorphous alloys has been studied by making a systematic study of the thermal expansion and magnetic properties of (Fe_xM_1-x)₈₅N₁₅ where M=Cr, Mn, Co and Ni. It has been found that the magneto-volume effect, omega _m, is a simple function of the effective 3d electron number N_eff with the calculation based on the charge transfer model with a maximum at N_eff=8.2, where the spontaneous magnetisation also reaches a maximum. It has also been found that the ratio of the spin-wave stiffness constant obtained by neutron scattering, D_s, to that estimated from the temperature dependence of magnetisation, D_m, exhibits the same N_eff dependence as omega _m, thus providing clear experimental evidence that the deviation of D_s from D_m in Fe-based amorphous alloys is due to the Invar effect.

The local atomic and magnetic structures of the ferromagnetic Fe_xB_1-x amorphous alloys, where x=0.14, 0.17 and 0.25, have been studied by neutron scattering using pulsed spallation neutrons. The atomic and magnetic structures have been separately determined by applying a magnetic field (5 kOe) parallel and perpendicular to the scattering vector Q. The Fe-Fe, Fe-B and B-Fe coordination numbers could be determined from high-Q measurements (Q_max approximately=25 AA^-1). The results show that there is no significant structural change in Fe_1-xB_x amorphous alloys with 0.14<or=x<or=0.25. The magnetic moments reside mainly on Fe atoms. These results give experimental support to the idea that the Invar effect of these alloys can be interpreted in terms of the electronic structure of the alloys. A comment is given for the method of determining the magnetic structure.

¹¹⁹Sn Mossbauer spectra have been recorded below the Neel temperature (T_N=378K) in the antiferromagnetic compound FeSn₂. A second magnetic transition occurs at T_t=93K. Above T_t, the ¹¹⁹Sn transferred hyperfine field is totally anisotropic in agreement with the magnetic structure deduced from a previous neutron diffraction study. The spin direction is close to (100) at room temperature and it deviates slightly when the temperature decreases down to T_t. Below T_t, large changes are observed in the Mossbauer spectra but a detailed interpretation of these spectra has not been obtained. The transferred fields result from a covalent bonding between the transition metal and tin, as is also observed in MnSn₂. This bonding also accounts for the variation in ¹¹⁹Sn quadrupolar splitting from MnSn₂ to CoSn₂.