Table of contents

From hyperfine interaction measurements (PAC) the diffusion of indium in zinc was identified as a two-stage mechanism. The heat of formation of a substitutional dilute Zn(In) alloy is Delta H_s=+0.37(3) eV below 500K and +0.24(3) eV or +0.18(3) eV above 500K, depending on the diffusion direction.

The susceptibility with respect to an arbitrary interaction lifting degeneracy in the SU(N) Kondo model is considered. The Wilson number that relates the small interaction response at low and high temperatures is derived and shown to be independent of the mechanism, e.g. magnetic field or crystal field. The result proves a conjecture on the Wilson number of Hewson and Rasul (1982).

It is shown that, despite the questions raised by G. Treglia et al. (1983), the previous conclusion on the surface reconstruction of Mo (001) and W (001) remains almost unchanged when one uses the self-consistent Hartree approximation (rigorous charge neutrality condition), and performs a more elaborate total energy minimisation.

Proton nuclear magnetic resonance was used to probe the environment of the intercalant molecule in low dimensional (aeolotropic) solids 3R_I-TaS₂(NH₃)_2/3 and 3R_II-TaS₂(N₂H₄)_4/3, and, specifically to look for phase transitions associated with changes in superlattice geometry. The proton resonance frequency for the intercalated molecules in the temperature interval 200 to 300K indicates that there are different magnetic environments in three temperature domains. (i) There is only one resonance field for molecules in isotropic magnetic environments. (ii) The high- and low-temperature domains (I and III) have molecules in only two different environments, whereas in the middle temperature domain (II) there are at least four different magnetic environments. (iii) The different magnetic environments measured by the ratio of anisotropic to isotropic resonance absorption intensity (r_ai) indicate that the ratios r_ai are constants of the sample in domains I and II. (iv) At low temperatures (domain III), r_ai depends on magnetic field polarisation effects and the temperature treatment of the sample. The authors conclude that the different magnetic environments observed for ammonia and hydrazine appear to be determined by the host, octahedrally coordinate TaS₂.

X-ray absorption near-edge study has revealed that Eu in EuRh₃B₂ is in a pure trivalent state while it exhibits a mixed-valence behaviour in EuFe₄Al₈. The average valency of Eu in EuFe₄Al₈ is found to be 2.9+or-0.05 at 300K.

The author discusses the problem of calculating the elastic dipole tensor G for point defects in ionic crystals. It is shown that the relation between G and the derivative with respect to bulk strain of the defect formation energy Delta E provides a natural and practical means of calculating G. This relation may be exploited either by computing Delta E for a series of values of strain and extracting the derivative numerically, or by making use of an explicit expression for G in terms of the relaxed ionic positions. The method of calculation is illustrated for some colour centres for which experimental values of G are available: the H, V_k and O₂^- centres in alkali halides and the V^- centre in MgO. He obtains satisfactory agreement with experiment for the H and O₂^- centres, but rather poor agreement of the V_k and V^- centres. He discusses what may be learnt from the comparison between theory and experiment for G and suggests that further compositions of this kind may contribute to the improvement of atomistic models for point defects in ionic crystals.

The thermal expansion of Mn- and V-doped Al₂O₃ has been measured in the temperature range 3-10K using a three-terminal capacitance dilatometer using 'pure' Al₂O₃ as a reference standard. The data for Mn-doped Al₂O₃ exhibit a sharp, positive peak at approximately=3.8K moving to approximately=4.5K after oxidising treatment, whereas the data for V-doped Al₂O₃ exhibit a small, broad, negative peak at approximately=3.8K. The Al₂O₃:V peak is attributed to the presence of V³⁺ and can be explained using a Schottky-like model with a Gruneisen coefficient of -6+or-2. However, the Al₂O₃:Mn peak, which is provisionally attributed to the presence of Mn³⁺, cannot be explained on the basis of a simple Schottky-like model using the published energy levels of Mn³⁺, predicted using a multimode Jahn-Teller model.

A local density dependence for the exchange-correlation potential is derived for a model insulating system. This derivation is contrasted with the standard methods based on the jellium model and the physical origin of the exchange-correlation potential in semiconductors and insulators is discussed. It is also shown that this new derivation can account for the failure of the local density approximation to produce the correct energy gap in bandstructure calculations.

Crystalline-electric-field parameters for the Tm³⁺ ion in TmAsO₄ have been determined. The parameter values give a good fit to the ¹⁶⁹Tm quadrupole interaction over the range 1.3-30K and to the Tm³⁺ electronic level splittings, in contrast to earlier work. Below the distortion temperature (T_D=6.13K) a large basal-plane contribution to the electric field gradient arises, due entirely to the cooperative Jahn-Teller effect, whilst the lattice contribution remains small. The origin of the Mossbauer effect lineshape asymmetry in this compound is also discussed.

The network generated from a Voronoi tessellation of space is a natural prototype for topologically random networks. To assess the effects of random coordination, the percolation and conduction properties of the Voronoi network and of its closest kin among regular networks, the triangular network, are calculated and compared by means of Monte Carlo simulations.

As Ising model with spins located homogeneously randomly and interacting via a purely antiferromagnetic exchange depending only on the spin separation is an extreme, clearly specified model exhibiting disorder and purely topological frustration. Two numerical studies of such a system, exact Gibbs evaluation and Monte Carlo simulation, have been performed and show no significant evidence for an ordered low-temperature phase in the thermodynamic limit.

The Jordan-Wigner transformation is used to obtain a fermion representation for the uniform spin-¹/₂ magnetic chain in the presence of second-neighbour interactions. The authors show explicitly how to compensate for the phase reversals due to intermediate sites by introducing correction factors to bilinear XY terms that introduce quartic interactions for second neighbours. Their study extends Neimeijer's work (1971) on the ground-state properties of the second-neighbour uniform Heisenberg chain to finite temperatures, for all ratios of the two interactions, and accommodates an arbitrary ratio of XY and Ising effects as well as XY anisotropy and an external magnetic field. A careful analysis of the ground-state properties precedes a numerical search for energetically stable solutions in all parts of the phase diagram, for the Heisenberg case in the absence of an external field. The emphasis in this work is placed on the antiferromagnetic solutions.

The paramagnetic susceptibilities of all the RCo₂ compounds for which measurements are available are found to obey a Curie-Weiss law consistent with the modified indirect exchange model. The rare-earth ions are in a well defined tripositive valence state. Paramagnetic moments and paramagnetic Curie temperatures are obtained for these materials for the first time. The paramagnetic moments of these materials are changed from their free-ion values by the effects of itinerant electron polarisation, an effect which is particularly large for SmCo₂ whose paramagnetic moment is almost three times greater than the free-ion value. The modified indirect exchange model is found to be able to give a full description of the magnitudes of the rare-earth, cobalt and diffuse moments in polarised neutron experiments. The band structures of the light RCo₂ compounds are found to be distinctly different to those of the heavy RCo₂ compounds.

The crystal and magnetic structures of polycrystalline CeD_x(2.2<or=x<or=3.0) were determined in the temperature range from 1.3 to 380K by means of conventional and high-resolution time-of-flight neutron diffraction measurements. At high temperatures the structures correspond to FCC (Fm3m) symmetry, with a statistical distribution of deuterium on tetrahedral and octahedral sites (displaced along the directions (111) from the ideal position ¹/₂, ¹/₂, ¹/₂). Whereas CeD_x for x>or=2.90 remains approximately cubic down to 1.5K, second-order phase transitions to ordered structures with tetragonal symmetry (basic translations a_t approximately a_c, c_t approximately 2a_c, space group I4; in contrast to Titcomb and co-workers (1974)) occur at temperatures T_c=290(3) and 330(5)K for x=2.18 and 2.29 respectively. Between x=2.29 and x=2.96 the magnetic ordering changes from ferromagnetism (T_c=3.7(2)K, ordered magnetic moment mu (Ce³⁺)=1.1(1) mu _B at 1.3K) to FCC II antiferromagnetism (T_N=4.2(2)K, mu (Ce³⁺)=0.6(1) mu _B at 1.3K, mu //(111)).

The Mossbauer-effect spectra of a polycrystalline sample of natural pyrite (FeS₂) have been measured in the temperature range 290-430K. On the basis of the centre shift and f' as a function of temperature the plus and minus second moments of frequency of the thermal vibrations of Fe atoms have been established: omega (2)=(5.68+or-0.18)*10¹³ and omega (-2)=(4.35+or-0.18)*10¹³ s^-1. The ratio of these moments (1.3+or-0.1) corresponds to the Debye model with theta _D=568+or-20K. The anisotropy of the Fe atom thermal vibrations at room temperature is represented by the mean-square amplitudes (z²)=(0.14+or-0.08)*10^-22 m², where the z axis is identified with the (111) crystallographic direction and (x²)=(y²)=(0.30+or-0.05)*10^-22 m². The detection of this anisotropy by the Mossbauer methods is difficult owing to the small value of the asymmetry of the lines.

A new method of determining surface electronic structure is described in which the surface atomic layers are embedded on to a semi-infinite substrate. An embedding potential, derived from the bulk Green function, is added to the Hamiltonian for the surface region, which can then be resolved by conventional surface band-structure techniques such as the LAPW method used here. Results are presented for Al (100) and Ni (001), illustrating the way that the embedding potential produces the true surface density of states rather than the discrete states of the usual slab calculations.

The lattice parameter and the characteristic temperature theta _p of the fine crystalline particles of copper, silver, gold and lead in a range from 60 to 2000 AA in diameter were determined by the X-ray diffraction method. The lattice parameters did not change appreciably from the bulk value in contrast to the observations by electron diffraction by Vermaak and Kuhlmann-Wilsdorf (1968) and by Solliard and Buffat (1977). The characteristic temperature theta _p of copper, silver and gold decreases with decreasing particle size. This is due to the softening of the thermal vibration of surface atoms in fine metal particles. The surface characteristic temperature theta _s was estimated from the particle size dependence of theta _p on the assumption that the effective thickness of the surface is of the order of one atomic layer. The results were in good agreement with LEED experiments. However, theta _p of lead particles was not found to change with the decrease of particle size.

The sub-band structure of hole space-charge layers in Si is investigated both theoretically and experimentally. The sub-band energies and the k/sub /// dispersion are calculated self-consistently in the Hartree approximation. The results are compared with measured sub-band excitations as obtained by resonant inelastic light scattering techniques. Various transitions are observed in a wide range of carrier concentrations on (100) as well as (110) and (111) Si surfaces. From the lineshapes and linewidths information on the k/sub /// dispersion of the individual sub-bands is obtained. By analysing the polarisation of the scattered light the authors were able to separate collective and single-particle excitations.