Table of contents

Molecular dynamics simulations are reported for supercooled liquid states of a realistic model of rubidium. Results for various quantities that monitor structural relaxation lend support to the basic notions in the dynamic theory of vitrification and the glass transition.

A fractal splitting rule for the one-dimensional Aubry model for any irrational number has been obtained and has been confirmed via a large-scale numerical calculation. The result clearly displays the spectrum splitting into three (at each stage) and the self-similarity property for incommensurate systems.

The crystal structure of stoichiometric BaVS₃ at room temperature, 100, 60 and 5K has been refined by the total profile analysis based on powder neutron diffraction data. The structural refinement at 100 K is consistent with the model that the driving mechanism for the 240 K transition is the freezing of the dynamical distortion and the static ordering of the V sublattice. The generalised R factor (Hamilton test) indicates that the space group for the phase stable between 240 and 80 K is Cmc2₁. The structural refinement of the 60 K data shows that the 80 K transition is not accompanied by an additional structural distortion. This means that the transition is not of Peierls type. The electron transfer model proposed by Massenet et al (1980) can explain all the physical properties of the phase stable between 80 and 31 K. The structural refinement of the 5 K data shows that the phase stable between 31 and 5 K has still the Cmc2₁ space symmetry. No reflections of magnetic origin have been observed at 5 K. This indicates that the magnetic ordering, observed for about half of the V sites at the 31 K transition by inelastic neutron scattering experiments, is not long range. The driving mechanism for the 31 K transition is not yet understood. The disproportionation of the V⁴⁺ cations into V³⁺ and V⁵⁺ would explain all the physical properties of the phase stable between 31 and 5 K; however, such a model is unlikely from the crystal chemical point of view. The NMR data of Nishihara and Takano (1981) indicating the presence at 1.3 K of two crystallographically independent V sites are reconciled with the neutron and X-ray diffraction data by assuming that either a fourth transition exists between 5 and 1.3 K or at the 31 K transition the space group changes from Cmc2₁ to Pmc2₁.

Ferromagnetic superconductors are superconducting at low temperatures, but below a certain temperature they become ferromagnetic and the superconductivity breaks down. In an intermediate state coexistence of superconductivity and ferromagnetism is observed. The authors use a generalised Ginzburg-Landau mean-field model to describe the superconductivity, the magnetisation and their mutual competition. Coexistence of superconductivity and magnetism is only possible through spatial varying structures which can be complicated because of the non-local character of the electromagnetic interaction. They consider structures with spatial variation order parameters. They consider easy-axis magnetic anisotropy. They discuss the structures known so far and develop a new alternating vortex lattice structure which combines both Bloch walls in the magnetisation and vortex lines in the superconductivity. This structure can have the lowest free energy of all structures over a certain temperature interval.

For pt.I see ibid, vol.19, no.33, p.6505 (1986). The authors use a generalised Ginzburg-Landau mean-field model to examine possible structures of the intermediate state of ferromagnetic superconductors which combine magnetisation and superconductivity. A new alternating vortex lattice structure is proposed for materials with an easy-plane magnetic anisotropy. One component of the magnetisation is periodically divided into large oppositely polarised domains. Each domain contains one row of vortex (anti-vortex) lines. The magnetisation does not vanish in the Bloch walls, instead it is perpendicular to the magnetisation in the domains and has an oscillating structure because of the interaction with superconductivity. It is sinusoidally oscillating and not a spiral wave independent of any anisotropy in the easy plane. The fraction of the magnetisation which oscillates is substantial. This new structure agrees better with the properties of the intermediate state of ErRh₄B₄ than previous structures.

A new real-space renormalisation group method is developed and tested for the case of the Anderson-Hubbard model. The development is concentrated on obtaining and subsequent analysis of one-dimensional recurrence relations which take into account the effects of interaction and temperature. The results are presented in a form that naturally permits the extension of the given treatment to the case of higher dimensions and off-diagonal bond percolation type of disorder. Although the method itself is not limited to low temperatures, actual computations were made for the case of low temperatures and weak-electron-electron coupling. For this case, the scaling expressions obtained for the ground-state energy and the energy gap are in satisfactory agreement with available exact results.

A model based on the band Jahn-Teller effect involving the flat electronic bands that evolve from the doubly degenerate Gamma ₁₂ state is applied to the study of the cubic-to-tetragonal and the tetragonal-to-tetragonal transitions in the A15 pseudobinary alloy Nb₃Sn_1-xSb/sub /x. The model is consistent with the experimental findings and the band-structure calculations. It has been predicted that C₃₃-C₁₃ should drop discontinuously at the tetragonal-to-tetragonal transition as the temperature is lowered.

The recursion method has been of use in many localised orbital calculations of the electronic structures of imperfect solids. However, most such calculations have been of local densities of states and there have been few attempts to extend or generalise the method. The authors show how the method can be used to calculate expectation values and response functions in general and so extend the possible range of applications considerably. The techniques described are efficient in that only one tridiagonalisation of the Hamiltonian is needed for each expectation value or response function.

The relation between two well known charge-density-wave (CDW) transport phenomena, namely the apparent increase in the threshold field E_T for continuous Frohlich conduction in short specimens, and the memory effect seen when the direction of current flow is reversed, is investigated in experiments on NbSe₃ at temperatures T between 59 and 114 K. The experiments show that the processes of phase-slip that occur near the current terminals during continuous Frohlich conduction are initiated thermally, probably by the creation of a vortex, and are responsible also for relaxing the metastable distortion of the CDW that remains after current ceases.

A new M-type hexagonal ferrite with composition LaZnFe₁₁O₁₉ has been synthesised and its cation distribution and magnetic properties have been investigated. The location of Zn ions among the five sublattices of the M structure has been deduced from temperature dependent Mossbauer spectra. It is found that less than 10% of Zn ions may occupy the pseudo-tetrahedral 4e sublattice, the remaining Zn ions being located at the tetrahedral 4f_IV sublattice. From both, the low value of the saturation magnetisation, M₀=19.0 mu _BFU^-1, and the observation of a high field differential susceptibility it is concluded that LaZnFe₁₁O₁₉ has a non-collinear magnetic structure. Based on the hyperfine interactions and cation distribution obtained from the Mossbauer spectra the authors propose the existence of a random spin canting in the octahedral 12k and 2a sublattices. A theoretical analysis in the scope of the random spin canting model is performed in order to justify the observed noncollinear disordered spin structure.

The T=0K phase diagram of the Heisenberg model for finite S on the square lattice with exchange interaction up to third neighbours is studied. The nearest-neighbour interaction is assumed to be ferromagnetic. In the classical (S to infinity ) approximation the model has, in addition to the ferromagnetic phase, an antiferromagnetic and two non-equivalent helical phases depending on the amount of competition between the interactions. To lowest order in 1/S these phases are still present but quantum fluctuations bring a significant modification to the phase diagram: with the exception of the ferro-helix transition line, all the phase boundaries are shifted with a considerable enhancement of the region with antiferromagnetic order. In addition, between the two non-equivalent helices a new phase sets in which is characterised by an infinite degeneracy of the ground state with absence of long-range order even at T=0K. In the S to infinity limit this phase collapses into line in the parameter space.

Many experimental results have indicated the existence of a freezing temperature of about 2.5 K and an overall behaviour equivalent to that of a spin glass state. The analysis of field cooled DC magnetisation measurements however shows that no critical behaviour appears in the first coefficients of the non-linear susceptibility. It is suggested that the very high degree of frustration prevents the development of a true spin glass transition. A simple model with uncompensated antiferromagnetic clusters consistently accounts for the observed properties.

The authors present the results of a detailed quantitative analysis of mode intensities in inelastic electron tunnelling spectra of the formate ion. They find that the theoretical approach of Phillips and Adkins (1985) can account successfully for the intensities of those spectral components that can be evaluated unambiguously from experimental spectra. They conclude that useful information can be obtained from quantitative aspects of inelastic electron tunnelling spectra, even in cases where the complexity is such that any attempt at full analysis would not be feasible.

The optical spectra of Nd³⁺ and Pr³⁺ doped in PbWO₄ crystals obtained at liquid N₂ temperature have been analysed. The experiment results have been interpreted within the framework of the crystal-field theory. The isolated configuration interactions, configuration mixing interactions and crystal-field interaction were considered for this analysis. The free-ion parameters resulting from the least-squares fit to 15 free-ion levels of Nd³⁺ give an RMS deviation of 41.4cm^-1. The crystal-field calculations for Nd³⁺ have been performed using the approximate site symmetry D_2d and the exact symmetry S₄. The crystal-field parameters appropriate to S₄ symmetry obtained from least-squares analysis, covering 30 Stark levels yield an RMS deviation of 12.3cm^-1. Similar analysis for Pr³⁺ in PbWO₄ is being reported.

The methyl tunnelling in alpha -crystallised toluenes C₆H₅CH₃ and C₆D₅CH₃ has been studied by high-resolution inelastic neutron scattering. Two tunnel peaks are measured, corresponding to the two inequivalent methyl groups of this crystalline structure. Their frequencies change when deuterating the benzene ring. They are found to exhibit different temperature and pressure (up to 4.6 kbar) effects. All the collected data lead to the determination of two potential models for the methyl group rotation.

The L_III X-ray absorption spectra of some binary and ternary compounds of dysprosium have been recorded using a 0.4m diameter Cauchois-type bent-crystal spectrograph. It is observed that the L_III absorption discontinuity in the compounds shifts to the high-energy side with respect to that in the dysprosium metal. The chemical shifts are found to be governed by the effective charges on the absorbing ions, which are calculated on the basis of Suchet's theory. The plot of the chemical shift, Delta E, against the effective ionic charge, q, has been used to determine the charges in the ternary intermetallic compounds in which they cannot be calculated theoretically. The extended X-ray absorption fine structure (EXAFS) is interpreted with the help of the graphical method given by Lytle and co-workers.