Table of contents

Microscopic quantum chemical calculations and simulations based on atom-atom potentials have been undertaken for hole self-trapping in pure corundum ( alpha -Al₂O₃) crystals. A comparison of different modes of ionic relaxation during hole trapping has shown that the inward Jahn-Teller 40% displacement of two O ions accompanied by the 20% outward displacement of the two nearest Al ions is energetically the most favourable. Eighty per cent of the hole density is concentrated on these two O ions, thus confirming that a small-radius two-site polaron model similar to that for alkali halides (the V_K centre), is applicable here. The calculated absorption energy of the STH (2.9 eV) is close to that observed experimentally.

Via the quantum mechanical description of thermal neutron scattering at the hydrogen atom nucleus of the methane molecule, a new effect is discovered showing that a neutron with insufficient energy for creating one intravibrational phonon creates the phonon by simultaneous destruction of another intravibrational phonon of the methane molecule. The effect occurs not only with combined inelastic thermal neutron collisions on the intravibrational phonons of the methane molecule but also with combined inelastic scattering at the rotational and intravibrational phonons. Evidently this effect exists theoretically for all incident thermal neutron energies E₀ in (1 meV, 100 meV) investigated.

The electron spectrum and optical absorption of antiferromagnetic chromium are calculated within the relativistic Green function method, being an analogue of the KKR method. The influence of the spin-orbit interaction and the spin polarization on the electron spectrum structure is analysed. An additional optical absorption maximum in the infrared region that is absent in the non-relativistic calculation is shown to appear on the theoretical curve when the relativistic effects are taken into account. The energy positions of this and other maxima due to antiferromagnetic ordering at h omega <0.8 eV agree well with the experimental data.

The one-dimensional SU(N)-invariant t-J model consists of electrons with N spin components on a lattice with nearest-neighbour hopping and spin exchange J. The multiple occupancy of the lattice sites is excluded. The model is integrable at the supersymmetric point, t=J. The discrete Bethe ansatz equations are analysed and the solutions are classified according to the string hypothesis. The thermodynamic Bethe ansatz equations are derived for arbitrary band filling in terms of thermodynamic energy potentials for the classes of eigenstates of the Hamiltonian. These equations are solved in limiting cases, e.g., S=1/2, the ground state and the high-temperature limit. If the charge fluctuations are suppressed the Bethe ansatz equations map onto those of the SU(N)-invariant Heisenberg chain.

The distribution of eigenfunction nodes in a disordered electron system under a strong perpendicular magnetic field is studied. It is demonstrated that the distribution of eigenfunction nodes is closely related to the amplitude correlation of the eigenfunction in such a state. For the distribution of nodes the author obtained a formula fitting the numerical result, suggesting that the distribution is somewhere between the binomial distribution and the Poisson distribution.

Magnetic properties of the orbitally nondegenerate periodic Anderson model are investigated using Kotliar and Ruckenstein's slave-boson reformulation of the Gutzwiller variational method (1986). The Gutzwiller approximation is reproduced at the saddle-point level for T=0. This approach allows one to extend the analysis of Rice and Ueda (1985) to calculate the magnetization as a function of applied magnetic field. The authors find that a magnetic instability exists in the Kondo regime. The findings suggest that the Gutzwiller approximation is too biased towards the magnetic state.

The dependences of the specific saturation magnetization sigma (C), magnetic ordering temperature T_C(C), average hyperfine field H(C) and average isomer shift delta (C) on Si concentration are measured in crystalline Fe-Si alloys disordered by mechanical crushing. It is shown that topological disordering does not determine the magnetic properties. The peculiarities found are explained in terms of the local atomic structure parameters depending on the number of Si nearest neighbours of an Fe atom.

The authors report a comparative study of the L₃-M₄₅M₄₅ Auger spectra of Cu, Cu₂O and CuO. The large intensity of the uncorrelated two-hole band-like spectrum in the L₃-M₄₅M₄₅ Auger spectra of Cu₂O and CuO and the spectral shapes for these transitions indicate strong Cu 3d-O 2p hybridization in the oxides. The L₂-L₃M₄₅ CK rates obtained for these compounds indicate the stability of the Cu 3d level with increasing oxidation state of Cu. They also provide a quantitative estimate of the contributions of the different processes that lead to the formation of the L₃-M₄₅M₄₅ Auger satellite in Cu, Cu₂O and CuO.

X-ray and Raman measurements show that beta -BaZr₂F₁₀ becomes disordered at pressures greater than 3.7 GPa at ambient temperature. This transformation occurs via a largely displacive mechanism because the disordering is reversible. Comparisons of glassy BaZr₂F₁₀ and the high-pressure states of beta -BaZr₂F₁₀ provide structural insight into the medium-range order of ionic glasses. Specifically the authors believe that layers of tightly packed pentagonal bipyramidal units are present in the glass.