Table of contents

Short-range order has been investigated in Ni(Al) solid solutions (with 6 and 10 at.% Al) by measurements of the residual electrical resistivity during thermal treatments. Local ordering results in an increase of resistivity, much stronger for the higher Al concentration. During isothermal annealing, the asymptotic residual resistivity, which characterises the equilibrium state of short-range order, was found to be a linear function of reciprocal temperature. In conditions of constant vacancy concentration, the approach to equilibrium could not be consistently represented by first-order kinetics, even with a distribution of relaxation times or by a sum of two exponentials. Indeed, for the 10 at.% Al alloy, the results clearly proved that the time constant was amplitude-dependent. Although a so-called cross-over was observed in that alloy, the quantitative evaluation of the relaxation of short-range order was carried out by application of a power law. The activation enthalpies of self-diffusion obtained lie in the same range as published diffusion data and they appear to increase when Al concentration goes from 6 to 10 at.%.

The changes in the optical properties of five samples of La_xPr_1-xP₅O₁₄ with different La contents were measured for spontaneous birefringence and rotation angle of the ellipsoid indicatrix axes induced by the ferroelastic phase transitions. Two types of domain and their movement and reproduction were observed in a temperature range from T_c to T_c-100 degrees C. The transition temperature is found to vary linearly with La content. The experimental data are explained and fitted to an approximation considering high-order terms relating to the order parameter.

In connection with the controversial question about the interband matrix elements of a stair-function potential in a crystal, the authors show analytically that they are non-vanishing, in general, for an arbitrary band model for both an infinite and a finite crystal. This is in accord with numerical studies of specialised models, but in contradiction to other analytical claims, which are analysed in some detail. The connection to the phenomena of Stark ladders and Zener tunnelling is discussed.

A technique of ab initio fully relativistic pseudopotential construction has been developed and used to construct pseudopotentials (PP) for all transition metals. Systematic trends in the Periodic table have been studied. It was shown that the correct evaluation of relativistic effects in the Dirac approximation could be of greater importance for transition metals than for simple metals due to specific features of the valence electron density distribution. Test calculations for a variety of atoms and atomic configurations indicated good transferability of the author's PPS. The influence of ionic radii and of the particular choice of exchange-correlation potential on the PP obtained was studied. LDA relativistic results were found to be qualitatively accurate in atomic spectra studies.

Effects of thermal relaxation on amorphous SiAu films are investigated by means of photoelectron spectroscopy and measurements of the electrical resistivity and specific heat. Annealing of the films in the amorphous state increases the resistivity and reduces the electronic density of states at the Fermi energy. These relaxation effects can be reversed by irradiation with 5 keV argon ions at low temperature. The results are discussed in the framework of structural relaxation, clustering and a scaling theory given by McMillan (1981).

Using a standard tight-binding model, the dependence of the localisation length xi on a perpendicular magnetic field in quasi-one-dimensional systems is investigated. A well known numerical method is used to calculate the localisation length as a function of the number of flux quanta per unit cell alpha and other system parameters. An attempt to explain the xi ( alpha ) curves perturbatively yields qualitative agreement and corrects the earlier results for xi as a function of energy and disorder W in the limit of large W obtained by similar techniques. Finally, the Lloyd model is re-examined with a magnetic field included. Previous claims of an exact solution for the Lloyd model for B=0 have been attacked but, the author believes not rigorously defeated. The author hopes to rehabilitate the Lloyd model by demonstrating its abilities in the magnetic field.

Novel surface resonances in doped semiconductor superlattices are shown to exist in association with depletion regions. Self-consistent calculations in a tight-binding formalism for the electronic states of a GaAs-AlGaAs structure exhibit resonance-like states in high-energy minibands. These resonances which extend throughout the superlattice, are strongly affected by applied electric fields, and are concurrent with significant changes in the local density of states. Calculated optical absorption curves show features related to surface resonances, making them accessible to experimental study.

The three-dimensional stability of static one-dimensional solutions of the Landau-Ginzburg equation is investigated. A general formula for the growth or decay rate is obtained for arbitrary local free energies, using a method developed earlier by Rowlands and Infeld (1979) for different equations. The results are then applied to quartic free energies. Among the real finite solutions obtained earlier, conoidal waves are shown to be stable, and solitary waves and kinks marginally stable. Two other types of non-linear periodic waves are shown to be unstable.

Electron and spin Green functions of a Hubbard ferromagnet are calculated, both starting from the Stoner ground state and for a ferromagnet with Hubbard subbands. The temperature dependences of the spin-wave stiffness and damping, the magnetisation, the local moment on a site and the thermodynamic properties are investigated. The role of non-quasiparticle contributions, described by branch cuts of electron Green functions, is discussed. The non-quasiparticle ('ferrospinon') correction to the linear term in the specific heat is obtained. Experimental data on 'half-metallic' ferromagnets (in particular, spin polarisation and longitudinal nuclear relaxation rate) are analysed.

For pt.II see J. Phys. C, vol.21, p.4097 (1988). Dispersion relations for bulk and surface magnetic polaritons in semi-infinite layered structures composed of two different magnetic materials are analysed theoretically. The considerations are restricted to the case of in-plane magnetisation and to wave propagation in the Voigt geometry. Some exemplary wave spectra are presented for structures composed of magnetic and non-magnetic materials as well as for structures where both constituents are magnetic.

The lattice constant, magnetic susceptibility and the bottleneck parameter of the Gd(Al_1-xBi_x)₂ intermetallic compounds were measured in the paramagnetic temperature range. Fundamental disagreement was found between the concentration dependences of the density of states at the Fermi level estimated on the basis of magnetic measurements on the one hand and ESR measurements on the other hand.

The linewidth of the ferromagnetic resonance absorption in nickel, iron and cobalt was measured in pulsed magnetic fields up to 35 T at frequencies between 400 and 900 GHz. With this configuration the authors were able to determine the intrinsic relaxation in the temperature range between 10 and 300 K. Their results show that there is a universal behaviour of the relaxation for the 3d ferromagnets.

Nuclear magnetic resonance measurements of ¹H, ²D and ⁵¹V spin-lattice relaxation rates T₁^-1 in TaV₂H_x(D_x) (0.22<or=x<or=1.54) have been performed over the temperature range 10-440 K. For all the studied samples the double-peak temperature dependence of T₁^-1 is observed. The high-temperature relaxation rate maximum is associated with H(D) diffusion, and the low-temperature one is attributed to localised motion of H(D) atoms. The experimental results are analysed to obtain the parameters of the two types of motion as functions of temperature and H(D) content. At 300 K the diffusion rate is found to increase with increasing H(D) concentration. The characteristic period of the localised motion, tau _L, exhibits unusual temperature dependence, tau _L approximately exp(-T/T₀) with T₀ approximately=50 K.

Results of ⁵⁷Fe Mossbauer investigation in a layered cyanide Mg(Mg((CH₂)₆N₄)₂Fe(CN)₆)₂.nH₂O for different stages of hydration are presented. Mossbauer spectra in the partially hydrated stages show an asymmetry in the intensity of the two absorption peaks of the quadrupolar doublet. The powder pattern has been analysed in terms of superposition of two asymmetric doublets arising from two inequivalent sites. An explanation for the asymmetry of the intensity in the doublet spectrum has been suggested in terms of the Goldanskii-Karyagin effect.

The author reports the observation of the photoluminescence in antiferromagnetic MnS. A strong emission band centred at 1.43 eV is observed at 78 K in MnS powder. The Stokes shift from the lowest 3d crystal-field absorption peak is at least 0.6 eV. The temperature dependences of the emission peak energy, the integrated intensity and the decay curve of the photoluminescence are measured at various temperatures from 78 to 200 K. The results are interpreted qualitatively by a model in which the emission originates from the impurity- and defect-perturbed Mn²⁺ excitons by taking into account the spin ordering of Mn²⁺ ions.

LiF-TLD 100 exposed to ionising radiations and then suitably annealed shows an enhanced thermoluminescence response for the second exposure. Several models have been proposed to explain this phenomenon which has been termed 'radiation-induced sensitisation'. In this paper it is shown that the sensitisation is observed only in LiF and KCl doped with impurities having limited solid solubilities. It is proposed that the sensitisation occurs owing to the redistribution of the impurities during the thermal annealing of the exposed samples.

Recently Kang and co-workers (see ibid., vol.2, p.1665, 1990) reported that the electrical resistance of an organic superconductor kappa -(BEDT-TTF)₂Cu(NCS)₂ noticeably decreases as a function of hydrostatic pressure; especially up to 1 kbar at 77.4 K where the resistance rapidly decreases by a factor of almost 10. These strain-sensitive resistance behaviours are consistently explained by the extended Hubbard model for a quarter-filling: an inter-site Coulomb repulsion is assumed to open an energy gap (300-500 K at room temperature) at the Fermi level and delicately interplay with the strain-sensitive transfer integrals of quasi-two-dimensional itinerant pi -electrons.

Experimental evidence is presented for the appearance of new plateaux in the differential conductance of a ballistic 1D channel in the high DC bias regime. In zero magnetic field, the authors observe steps in the differential conduction of e²/h, as opposed to the conventional value of 2e²/h for a spin-degenerate 1D subband. The new plateaux are found to be at quantised values to an accuracy of a few per cent and are described well by a model that assumes the bias is dropped symmetrically in the 1D channel. In addition, all the plateaux disappear with increasing DC voltage bias which can be explained in terms of an enhancement of the tunnelling probability. The same effects persist in a transverse magnetic field and the authors are able to calculate subband spacings as a function of the applied magnetic field.