Table of contents

The authors extend the treatment by Kaveh and Mott (1981) of the effect of weak localisation on the conductivity of disordered two-dimensional systems to include the effect of magnetic and electric fields on the localisation. It is shown that the change in conductivity due to the localization depends on only one parameter, the diffusion length. This result holds irrespective of the number of independent mechanisms which individually correspond to a separate diffusion length. Their experimental values of the conductivity for different temperatures, electric fields and different magnetic fields fall on the predicted universal curve. The derived formulae for the conductivity, when few mechanisms determine the length scale, are in agreement with our experimental data, which enables us to identify the individual diffusion lengths.

Variable-range hopping theory is modified to include a correlator between the distances and the energy differences of localised states. The DC conductivity for a Fermi glass at very low temperatures that he would predict if no correlation between energies and positions were included.

The authors have studied the three-dimensional ordering in a system of linear chains coupled with an interaction proportional to the derivatives of the field. By examining the behaviour of the Debye-Waller factor as well as the correlation function and length they have demonstrated that the system does not order at any finite temperature in both the incommensurate and commensurate regimes.

A two-dimensional jump type of band edge singularity is reported for a bona fide three-dimensional problem and is shown to result from non-intersecting twisted loci of vanishing group velocity (filaments of critical points). The evidence for this conclusion emerges from several numerical investigations and the singularity in the density of states is in accord with a qualitative expectation for such a topological feature. It now appears that earlier examples of linear critical lines have the stronger (logarithmic) singular behaviour due to their intersections and it is also suggested that other lower-dimensional singularities may be found in two- and three-dimensional systems. A brief account of the other singularities in the present two-magnon FCC problem is also given.

The authors study the phase diagrams of SrTiO₃ as function of temperature and of uniaxial stress along a general direction. In particular, new results are presented for stress along (110) and along (1+ delta , 1+ delta , 1-2 delta ). In the latter case, the trigonal pseudotetragonal phase transition is described by a three-state Potts model Hamiltonian with both quadratic and linear symmetry-breaking fields. Far away from the (zero-stress) multicritical point, the leading fluctuations are described by the underlying XY model. The first-order Potts model transition is turned second order by the symmetry-breaking fields via a tricritical point, at delta _t(<0), and at a critical point, delta _c(>0). The universal ratio delta _t/ delta _c is calculated to order epsilon =4-d. In the close vicinity of the multicritical point, the XY model underlying all of these results undergoes a crossover to a Gaussian model, and the Potts line approaches the temperature axis tangentially.

The Jahn-Teller phase transition of TmAsO₄ has been studied using elastic and inelastic neutron scattering techniques. The author has measured the order parameter ( beta =0.5) and also studied the acoustic phonons and the excitons above and below the transition. A theoretical calculation is made of the effects of a magnetic field on the phase transition in TmAsO₄ and further experiments are proposed.

The effect of tetragonal splitting of the electronic doublet on the properties of the cubic E(X) epsilon Jahn-Teller system is investigated taking account only of linear Jahn-Teller coupling. Ham reduction factors appropriate to the tetragonal symmetry are introduced, and their connection with the cubic reduction factors is established. A simple relation between the vibronic matrix elements of vibrational and electronic operators is derived. Numerical calculations are performed for weak to intermediate Jahn-Teller coupling where the electronic splitting competes with the Jahn-Teller interaction. The vibronic ground doublet is studied in detail. It is found that the reduction factor corresponding to q can drop below ¹/₂ considerably. Experimental consequences for electron paramagnetic resonance on a ²E state in a tetragonal field and for Mossbauer spectroscopy on a ⁵E state split by magnetic exchange are discussed. The theory predicts the spectra to be markedly different from what is expected either for a tetragonal purely electronic state or for a cubic Jahn-Teller coupled state.

Neutron scattering has been used to study both sublattice magnetisation and magnetic excitations in the Fe_1-xMn_xCl₂ mixed magnetic system for low Mn concentrations (x<or=0.25). First the temperature dependence of magnetic Bragg reflections exhibits an anomalous behaviour at low temperature which is imputed to the magnetisation of Mn²⁺ in the exchange field with Fe²⁺ ions. On the other hand, by inelastic neutron scattering the authors observe an overall decrease of the dispersion curves of the host excitations with Mn concentration. A comparison of the inelastic results with predictions of (i) a cluster model, and (ii) a mean crystal model suggests that, in addition to the effect of mixing on exchange, a strong concentration dependence of the trigonal anisotropy must be taken in account.

The ¹⁵¹Eu Mossbauer resonance has been used to study magnetic exchange interactions involving diamagnetic Eu³⁺ ions in antiferromagnetic SrEu₂Fe₂O₇. The odd number of neighbouring magnetic ions results in a large induced magnetic flux density at the europium nucleus, and a small quadrupole effect. The experimental data are combined with a detailed calculation of the exchange and crystal-field interactions to resolve an ambiguity in the magnetic lattice as determined from neutron diffraction data, and it is concluded that the Fe³⁺ spins are normal to the (110) mirror planes.

The deviations from the classical diffusive heat flow which occur at high frequencies are calculated in a more general way than was done hitherto. The effect of taking the Umklapp process quasi-conservation of wavenumber into consideration is also investigated. It is shown that this effect will considerably increase the magnitude of the first order deviation from the classical solution.

The temperature dependence of permittivity and remanent polarisation in monocrystalline and ceramic PbZrO₃ and the temperatures of the phase transitions between ferro-, antiferro- and paraelectric phases are influenced by applied field and hydrostatic pressure. A constant electric field broadens the ferroelectric phase range. Under the influence of hydrostatic pressure the ferroelectric phase occurs only at pressures less than 26 MPa for monocrystals and 40 MPa for ceramics. Results obtained confirm Samara's hypothesis of the existence of two soft modes in PbZrO₃, i.e. ferroelectric and antiferroelectric.

The authors measured the adiabatic dielectric permittivity epsilon _b of tris-sarcosine calcium chloride (TSCC) at 400 Hz near the ferroelectric transition for DC bias fields up to 5000 V cm^-1. They found epsilon _infinity =4.2+or-0.2, Curie-Weiss constant C⁺=38.3+or-0.3K, T₀=129.4+or-0.1K, and Landau coefficients A=(2.95+or-0.2)*10⁹ V m A^-1 s^-1 K^-1 and B=(3.5+or-0.1)*10¹⁵ V m⁵ A^-3 s^-3. They observed evidence of an internal bias field as reported previously by others.

Variations in the fluorescence line positions, linewidth, and lifetimes with hydrostatic pressure up to about 80 kbar are reported for samples of NdP₅O₁₄ and Nd_0.1Y_0.9P₅O₁₄. The results for the lightly doped sample are consistent with changes in the radiative decay rate while the very different results obtained on the heavily doped sample are interpreted in terms of increased radiationless quenching due to energy migration to trapping sites. Results are also reported on the variations in the fluorescence lifetimes of the R lines, and the positions of the N lines and vibronic peaks of ruby as a function of hydrostatic pressure.

The phenomenon of electromigration of atomic species during AES of glasses is discussed, and the induction time T_i, before any migration occurs, is defined theoretically for the two possible directions of migration. Experimental Auger peak shapes are shown to exhibit a growing asymmetry with irradiation time, in such a sense as to indicate the direction of the field present in the irradiated volume. The migration of sodium is hence found to be towards the surface, for ions originally with the analysed region.