Table of contents

The general task of the present work is to test the correlation between experimental resistivity and atomic ordering in alloys. The authors used the Rossiter model to describe the SRO contribution to the total electrical resistivity. This model allows them to calculate the SRO resistivity at a given temperature as long as the Cowley-Warren parameters are known. The theory of Clapp and Moss (1966) has been applied to obtain the Cowley-Warren parameters as functions of temperature and interaction energies of atoms at different distances. Hence, the SRO resistivity could, like the Cowley-Warren parameters, be represented as a function of interaction energies and temperature. To test this model the authors used experimental resistivity data for Ni₃Mn, and from the total resistivity they obtained the SRO contribution, to which they fitted the model function. From the above calculations the authors got the values of interaction energies, from which they could obtain the temperature dependencies of the Cowley-Warren parameters.

Sound velocity measurements carried out between 10 and 300 K in a Pd₈₅Pt₁₅ single crystal at 0 and 0.29 H contents for five different propagation modes, have shown that C', C₄₄ and C_T are strongly decreased by H loading (C_T=¹/₃(C₁₁-C₁₂+C₄₄)). Smaller relative changes are observed for longitudinal modes. The bulk modulus as well as the anisotropy factor turn out to be increased by H impurities. The Debye temperature as absolute zero is found to be 271+or-5 K for n=0 and 264+or-5 K for n=0.29.

The Raman spectra of GeO_2-SiO₂ glasses have been studied using samples of different GeO₂ contents, which are found to have the general features of the spectrum of vitreous SiO₂(v-SiO₂). It is demonstrated that substitution of Ge for Si in the v-SiO₂ network prevents the formation of ring structures. A new vibrational band located at 570-700 cm^-1 was found in the spectra of high-GeO₂-content samples (above 5.0 at.%GeO₂). It is also shown that the W₃ vibration of the v-SiO₂ network is reduced in highly GeO₂ doped GeO₂-SiO₂ glasses.

Penetration profiles of Zn in Si were recorded with the aid of the spreading-resistance technique after in-diffusion from the vapour phase at 1262 K. The conversion of resistance data into concentrations of substitutional Zn is based on an acceptor level at E_V+0.24 eV detected by Hall effect measurements on the authors' Zn-diffused samples. The diffusion behaviour is found to be affected by surface conditions. In dislocation-free Si crystals with damaged surfaces the profile shape as well as the incorporation rate of Zn reveal a self-interstitial-limited diffusivity which depends on C^-2. Together with the much larger diffusion constant in highly dislocated samples this provides strong evidence that Zn in Si migrates by the kick-out mechanism.

Band-structure calculations of ferromagnetic Fe_1/3TaS₂ and Mn_1/3TaS₂ using the augmented spherical-wave method are reported. The calculations give a spin magnetic moment of 4.35 mu _B per Fe for Fe_1/3TaS₂ and 4.18 mu _B per Mn for Mn_1/3TaS₂. The local moments at Fe and Mn induce a magnetic polarisation of the Ta 5d(z²) band, which is parallel to the local moment for Fe_1/3TaS₂ and antiparallel for Mn_1/3TaS₂. Measurements of the transport properties of Fe_0.28TaS₂ show a large spin-disorder contribution to the resistivity and a large anomalous Hall effect. The anomalous Hall effect is analysed in terms of side jump and skew scattering contributions. The large magnetisation anisotropy indicates the presence of Fe²⁺, in agreement with the band-structure calculations.

The effective-medium approximation formulation of electron transport in disordered metals due to Roth and Singh (see Phys. Rev. B, vol.25, p.2522, 1982) is reconsidered using the muffin-tin potential model. The theory is reduced into a tractable form by using the non-overlapping condition of muffin-tin spheres. The major part of the off-shell correction is shown to be absorbed in the on-shell expression by adding simple correction factors.

The configurationally averaged zero-temperature DC conductance of weakly disordered systems of finite width is calculated using the coherent-potential approximation. There is no smooth transition from the quantised quasi-ballistic regime at low disorder to the structureless conductance for larger disorder. Instead the conductance oscillates as a function of the Fermi energy in the intermediate region. The perturbation theory agrees well with the numerical results obtained previously.

A comparative study of the dependence on T of the resistivity in non-magnetic Ni₃₀Pt₇₀ and paramagnetic Co₃₀Pt₇₀ alloys in their ordered (L1₂-type structure) and disordered (A₁) phase is presented. In order to make measurements of as far as possible real thermodynamic equilibrium states, the heating rates were chosen taking into account previous (Co₃₀Pt₇₀) or present (Ni₃₀Pt₇₀) order kinetics data. The authors find that in Ni₃₀Pt₇₀ the resistivity can be analysed in terms of two main contributions: (i) a term of 4 K residual resistivity associated with the chemical disorder, and its variation with T, which is separately measured using a specific device; and (ii) a phonon-like term, proportional to T. In Co₃₀Pt₇₀, a further term associated with the spin-disorder scattering at present. In the paramagnetic range, this term decreases with T as the chemical disorder increases, the decrease being especially large at the order-disorder (L1₂-A₁) transition. Such behaviour is attributed to the existence of short-range magnetic order in the paramagnetic state of Co₃₀Pt₇₀, which would be enhanced by the increase of the chemical disorder, i.e. by the increase of the Co-Co coordination, thus reducing the spin-disorder contribution to the resistivity. All the data are in very good agreement with a previous comparative study of equi-atomic NiPt and CoPt phases, which led to a similar conclusion.

The authors have calculated the electronic band structure of Nb-Ta (001) superlattices by means of the surface-Green-function-matching method. An empirical tight-binding Hamiltonian including d orbitals and up to second-neighbour interactions is employed. The influences of the interactions at the interfaces and the relative thicknesses of the constituent materials on the superlattice spectrum are studied and discussed.

A model calculation of the spin-canting effect for Li-Zn ferrite is presented here to improve the existing calculations and to determine an accurate relationship between the magnetic moment at 0 K and the Zn substituent level. It is assumed that, although the B sublattice can be subdivided into two commensurate sublattices B' and B" regarding the spin orientations because of the spin-canting effect, the spin orientation is actually random at local sites. In contrast with previous calculations, the exchange integral ratio J_AB/J_BB in this paper is expressed in terms of correlated molecular-field coefficients. By self-consistent iterative computation, excellent agreement between the theory and the experimental results is realised, which the authors consider to be the best of those published.

Evidence is presented for believing that Sn atoms dissolved in Ni₃Fe lie mainly on Fe sites. The author shows that the hyperfine field at an Sn nucleus is determined by the moments in its first- and second-neighbour shells, and its magnitude is consonant with that found in other magnetic materials.

The electro-optic coefficients r_ij and birefringence components of lithium thallium tartrate (LTT) single crystals have been measured in the 5-300 K temperature range, as also have the refractive indices at room temperature. The coefficients r₄₁, r_a and r_b behave in an anomalous way in the vicinity of T_c approximately=12 K where LTT undergoes a phase transition; below T_c, r₄₁ reaches a high value of 500 pm V^-1. Their behaviour can be explained by considering the transition to be of a proper ferroelectric-proper ferroelastic nature, triggered by a balance between the values of clamped susceptibility, elastic stiffness and piezoelectric coefficient, as suggested earlier. A model of built-in strain is employed to interpret the existence of the symmetry-breaking coefficients r_a and r_b in the symmetric phase. The measured optical properties are used to re-examine the possibility of distinguishing domains in polarised light.

The effect of thermal and optical excitation on the optical absorption band of reduced LiNbO₃ has been investigated for crystals with different Li/Nb ratios. Fe- and Cu-doped samples were also studied. Although the main effect of the excitation is to enhance the infrared part of the spectra at the cost of the ultraviolet-visible region, a number of new features appear depending on stoichiometry and doping. In all cases, the effect of thermal excitation is reversible. The spectra for all undoped samples, before as well as after the various treatments were fitted with four gaussian bands peaking at 1.6, 2.3, 3.1 and 3.9 eV. Additional bands have to be used to fit the spectra for doped crystals (2.5 eV for Fe-doping and 2.8 eV and 3.3 eV for Cu-doping). The possible origin of the bands is discussed in relation to other available information, but a definite assignment cannot be offered yet.

An analysis of the energy moments of X-ray absorption spectra at the copper K edge has been used to assign characteristic energies for a series of copper compounds with formal valences of +1, +2, and +3. The results indicate that there is a good correlation between the characteristic energies defined in this way for near-edge X-ray absorption spectra and the charge distribution in these copper compounds.

A fully relativistic one-step theory of photoemission has been applied to calculate spin-resolved normal-emission spectra induced by circularly polarised photons between 6 and 25 eV from the unreconstructed (111) surfaces of Pd, Ir and Pt. Consistent with the simultaneously calculated bulk band structures, individual spectral features are identified as bulk interband transitions and one-dimensional density of states transitions between initial and final states of specific double-group symmetry types. Theoretical peaks have experimental counterparts, with some deviations (up to 0.3 eV) in energy, but the observed spectra exhibit in addition 'ghost peaks' reflecting dominant peaks in the opposite-spin spectra, which-beyond experimental artifact-indicates some spin-flip process. Relative peak heights are in reasonable agreement except for an observed enhancement for photon energies such that the final states are associated with a flat f-like band.

The authors develop a theory of energy-, angle- and spin-resolved photoemission spectroscopy of non-magnetic crystalline metals and their alloys on the basis of relativistic quantum mechanics. In particular, the theory is applicable to cases of more than one atom per unit cell and random solid solutions. They illustrate it by explicit first-principles calculations for pure Cu, Ag and Au, ordered Cu₃Au and disordered Cu_0.75Au_0.25.

A new expression is given for the long time diffusion coefficient D_L(k) of charged interacting colloidal spheres in suspension, as a function of the wavenumber k, near k=k_m, where the static structure factor has a maximum. The expression is based on a physical analogy between a mode description of the behaviour of atomic fluids (as observed in neutron scattering) and of colloids (as observed in light scattering). Use of this expression in conjunction with a hard-sphere model yields good agreement with extant data on colloids.

Discrete multilevel spontaneous resistance fluctuations have been observed in a microstructure fabricated on a delta -doped GaAs layer base. The authors suggest that this behaviour of the resistance is caused by random discrete changes of states of individual impurity atoms. They have been able to observe the influence of the change of the state of a single scatterer on the universal conductance fluctuations. Comparison with theory is made.