Table of contents

The tracer diffusion of a liquid in a porous media was studied using the neutron spin echo technique. The samples used consisted of porous Vycor glass saturated with a mixture of D₂O-H₂O (64/36) water to match out the coherent small-angle scattering component due to the neutron scattering density contrast between water and the Vycor matrix, in order to reduce the small-angle neutron scattering and the chemical diffusion contribution to the quasi-elastic scattering. A series of measurements has been made in the Q range 0.07-0.14 AA^-1 using the spin echo spectrometer IN11 at the ILL. This experiment in principle enables us to measure the tracer diffusion coefficient of the liquid within a pore and through the local pore network and hence to investigate the interconnectivity of these pores through a determination of <r²(t)< over length scales to about 100 AA.

A double-parabola model is used to calculate various properties of the critical droplet in homogeneous nucleation using the Cahn-Hilliard non-classical theory, and the results are compared with those predicted from the classical theory due to Becker and Doring. The calculation can be performed mostly analytically, and various quantities can be characterized by two parameters: the supersaturation which measures difference in chemical potential from two-phase coexistence, and the asymmetry of thermodynamic properties of the liquid and the vapour phase. The predictions of the non-classical theory do not differ much from those of the classical theory in liquid-vapour nucleation (liquid droplet formation), but the difference is significant in vapour-liquid nucleation (gas bubble formation). In particular, the non-classical nucleation rate of bubble formation is found to be significantly faster, typically 18 orders of magnitude, than the classical nucleation rate. The deviations of the non-classical results from classical data increase as the asymmetry becomes more pronounced.

The electrical resistivities rho of the liquid Sr-Bi and SrSb alloys have been determined for compositions covering the whole concentration range. Plotted as a function of composition the results resemble those obtained previously for Mg-Bi by other authors. There is a very sharp resistivity maximum at 60 at.% Sr, well up into the semiconducting range. The band structure of solid Mg₃Bi₂ has been calculated using the augmented spherical wave ASW) method. No gap has been found and it is concluded that solid Mg₃Bi₂ is metallic, in agreement with measurements found in the literature dating from the thirties and before. These measurements were checked and extended through the melting point. Finally, a band-structure calculation was carried out for Mg₃Sb₂, which is a semiconductor in the crystalline as well as in the liquid state.

Magnetic measurements on liquid Sb-Zn alloys around the composition of the intermediate phases SbZn, Sb₃Zn₄ and Zn₂Sb₃ are reported. The results give no clear evidence of an anomalous behaviour arising from the possible occurrence of association or chemical short-range order in the liquid state. The slightly increased temperature dependence of the magnetic susceptibility observed near the liquidus temperature seems to be that of a liquid semimetal. The deviations from the additive rule are very small and negative and tend to disappear with increasing temperature.

In this paper, the 120 keV Ar⁺-ion-irradiation-induced phase transformation from the Al₆₂Cu_25.5Fe_12.5 icosahedral quasicrystal (IQC) to B2(CsCl)-based phases has been studied by means of transmission electron microscopy. The dose dependence of the Ar⁺ ion irradiation effect and the subsequent in-situ heating and cooling behaviours after irradiation have been investigated. The experimental results show that the r⁺-irradiation-enhanced diffusion can accelerate the transformation from the Al₆₂Cu_25.5Fe_12.5 IQC to a B2-type or B2-based phase at room temperature and that this B2 or B2-based phase transforms reversely to the IQC when the Ar⁺ dose is equal to or lower than 10¹⁴ Ar⁺ cm^-2 and the temperature is increased to about 1000 K.

Motivated by a three-dimensional LDA band structure calculation for trans-polyacetylene (PA) we construct an effective two-dimensional model for a film of conjugated polymers. In real materials a non-perfect structural order is found. It has its origin in the finite conjugation length of single chain segments, impurities and crystalline defects. We model these effects through a fluctuating contribution to the interchain hopping amplitude. Using a supersymmetric path integral formalism we obtain the averaged one-particle Green function giving the electronic density of states (DOS) and furthermore the current correlation function determining the optical absorption coefficient and DC conductivity. As a consequence of the interchain coupling we find a broad shoulder at the band edge in the DOS and the optical absorption. The main effect of disorder is seen in a broadening of the interband transition peak and an increasing intraband absorption. The anisotropy in the absorption coefficient parallel and perpendicular to the chain direction allows us to fix the interchain coupling strength; the line shape contains information about the effective disorder strength.

By calculating the carrier density distribution and the effective one-particle potential self-consistently, we investigate the size dependence of the carrier ground state of small spherical semiconductor particles with the doping level fixed. The particles are assumed to be in an insulating medium or in the vacuum. The prominent peak just inside the carrier-deficient surface layer in the carrier density profile persists regardless of the particle size, while, with increasing size, the oscillatory density-profile feature inside the prominent peak becomes less and less conspicuous and reduces to nearly constant density to achieve charge neutrality. The remarkable variation of the potential bending with increase of the size depends upon where the newly occupied carrier states have their probability density concentrated. This variation of the potential bending often involves the energy intersection of two close energy levels with different angular momenta l.

We studied the H vibrations in H-doped YBa₂Cu₃O_xH_y, (y=0.6) by neutron spectroscopy. Neutron spectra were taken at 10 and 14 K from both orthorhombic (x approximately=6.9) and tetragonal (x approximately=6.3) samples. The spectra exhibit large H-induced intensities in the energy range between 40 and 130 meV. In comparison to orthorhombic samples, the H vibrations in tetragonal samples are shifted to lower energies by approximately 11%. This shift is opposite to the spectral shift of the lattice vibrations between H-free orthorhombic and tetragonal YBa₂Cu₃O_x, frequently attributed to differences in the electron-phonon coupling. Our results show that any such differences in the electron-phonon coupling are not apparent in the H vibrations. The results also support previous suggestions that the H occupies sites in the close neighbourhood of the Cu(l) atomic planes.

The low-frequency (< 9 THz) phonon dispersion curves in the main symmetry directions of the fluoroperovskite crystal KMgF₃ have been measured at room temperature by means of coherent inelastic neutron scattering. These dispersion curves are well described by a rigid-ion model. The mean square displacements (MSDS) of the ions and the phonon density of states are deduced.

We have measured the stress dependence of the change in sound velocity for Sb-doped Ge over a concentration N range 1.1*10¹⁶-1.3*10¹⁸ cm^-3 and a temperature range 2.3-4.2 K. The revised density of states for N>10¹⁷ cm^-3 has been derived from fitting to the temperature dependence of the change in sound velocity under zero stress. Qualitative agreement is obtained between the calculation of the stress dependence using the density of states derived and the experiment.

We consider an electron-hole system illuminated by a laser and show that the system shows bistable and multistable behaviours. The mechanisms responsible for these behaviours are discussed in terms of excitonic and phonon interactions. It is found that both the excitonic and the phonon effects contribute to the multistability in the system. With an increase in the strength of the phonon interactions, the system passes from a phase of multistability to a phase of bistability; the reverse happens as the strength of the electron-hole interaction increases.

We report an experimental study of the propagation of low-frequency (1-30 MHz) edge excitations in a two-dimensional electron gas confined electrostatically by means of gate induced depletion. This technique allows control of edge channel structure without change of the bulk properties of the electron gas. We have observed a strong dependence of the velocity of the edge excitations on gate voltage and demonstrate that such spectroscopy is a sensitive probe of the two-dimensional edge electronic structure.

The object of this work is to determine the atomistic and electronic configuration of the (F₂⁺)* centre in NaF. This centre combines an F₂⁺ centre with an impurity dipole consisting of an Mg²⁺ substitutional ion and an Na vacancy. There are many conceivable configurations for such a combination, and our calculations lead us to propose a particular one. Our result agrees with an earlier experimental conclusion that, in its ground state, this centre has the excess electron localized in a single vacancy, more like an F centre than an F₂⁺ centre. Our method uses effective potentials derived for interaction of the excess electron with host and impurity ions. We also present a detailed analysis of the F- and F₂⁺-centre ground states, including isotropic hyperfine constants. The F₂⁺-centre results support the picture of substantial localization about this defect's: centre.

The free-ion parameters for Eu³⁺ in LaOF, LaOBr, GdOCl, and YOCl crystals and in LaOCl crystals at high pressure (up to 13 GPa) are calculated and fitted to the experimental "free-ion" energy levels. The Slater parameters F_kappa and the spin-orbit coupling parameter zeta decrease with increasing pressure and with decreasing Eu³⁺-ligand distances along the two host series LaOCl-GdOCl-YOCl and LaOBr-LaOCl-LaOF. The shift of the free-ion parameters along the latter series exhibits an "anti-nephelauxetic series" and therefore has a positive correlation with the overlap integrals. On the basis of the free-ion calculations and the calculations of the overlap integrals between Eu³⁺ and the ligands, a combined cloud-expanding model is proposed to explain the behaviour of the free-ion parameters.

Non-conventional spectroscopy techniques, in which the electron paramagnetic resonance (EPR) and optical absorption (OA) spectra are thermally detected (TD) via a carbon thermometer at liquid-⁴helium temperatures, are used to investigate the GaP:V system in n-type samples. New TD-EPR results are described. The presence of most of the TD-EPR lines appears to correlate with that of the V²⁺ TD-OA signal. A comparison of the photo-induced effects on the TD-OA V³⁺ and V²⁺ spectra and on the TD-EPR lines is made in order to help in the identification of the latter. A set of resonances in TD-EPR is attributed to a V²⁺(II) centre, with a ⁴T_mod ground state, similar to that seen in GaAs:V. A trigonal model where V²⁺(II) is part of a complex in which one of the neighbouring p ions is replaced by a defect is used to fit the experimental data. Further TD-EPR lines can be due to another V²⁺ ion having also a ⁴T_mod ground state but which appears to be different from V²⁺(II).

A new scheme is proposed to describe the scattering of electrons by deformations due to lattice defects. It is based on the assumption that these deformations are quite homogeneous on an atomic scale with the consequence that the potential seen by the electrons is quite similar to that obtained from self-consistent band-structure calculations for homogeneously deformed lattices. As a first example the scattering by the dilatation field of an edge dislocation is treated. The electrical resistivity is evaluated by solving the Boltzmann equation. taking into account the shape of the Fermi surface and the anisotropy of the scattering probability. Experimental and theoretical values of the electrical resistivity due to edge dislocations in Al are found to be in qualitative agreement showing that the scattering by the dilatation is the dominant mechanism in the case of edge dislocations. As was stated by Watts we find the anisotropy relation rho perpendicular to / rho //<1.5. That means the edge dislocations have a non-zero component of electrical resistivity rho // along the dislocation line.

We have studied relativistic effects on the tunnelling of electrons through a multi-barrier system (MBS) consisting of rectangular barrier potentials, by deriving, for the purpose, relativistic formulae for the transmission coefficient and associated relativistic conditions for resonant tunnelling. Among other things, we have discussed critically the quantitative extents of relativistic impacts on tunnelling through MBSS, especially in the context of their measurability.

A sharp transition from reversible behaviour to irreversible behaviour in the superconducting mixed state of single-crystal samples of CeRu₂ is described. Magnetization, AC susceptibility, resistivity and specific heat measurements under an applied field are reported. The results are discussed in relation to conventional models and those of more novel superconductivity, such as the Fulde-Ferrel state.

We develop a linearized theory for the classical Heisenberg model, which allows us to approximately solve for the local distortion of spins around an impurity in a non-collinear antiferromagnet. Provided that the ratio of disturbed to undisturbed bonds is small, the theory should be applicable. The theory is particularly useful when alloying lifts the degeneracy often found in non-collinear magnets. We look at the cases of gamma -Mn alloys in general, and Mn₃Pt in particular. We can successfully predict the experimentally observed phase transitions caused by alloying in these materials, providing further evidence for the exotic "hedgehog" phase.

Two mechanisms of diffusive kink motion in a quasi-one-dimensional rhombic antiferromagnet, placed in an external magnetic field, are considered, together with their effect on the soliton central peak in the dynamic structure factor (DSF). The temperature and field dependence of the diffusion coefficients is calculated. It is shown that non-dissipative diffusion gives zero contribution to the DSF, while ordinary Einstein diffusion changes its form considerably in the region of small wavevectors.

Mossbauer absorption spectra have been recorded for powder and single-crystal samples of the antiferromagnetic compound K₂FeCl₅.H₂O over the temperature range 1.3-293 K and also at 4.2 K in applied magnetic fields up to 10 T. Details of the electrostatic hyperfine interaction have been deduced from the paramagnetic spectra. In the antiferromagnetic phase the spectra show the Fe spins to be aligned along the crystal a axis. An external field of 3.2 T reoriented the spins to the c axis. The observed spin reorientation phase transition was found to be very sharp, characteristic of a pure antiferromagnet. Comparisons are made with a recent study of (NH₄)₂FeCl₅.H₂O.

An investigation of the relatively low-frequency (28 meV=7 THz) motion of protons in KH₂PO₄ (KDP) has been carried out using time-of-flight (TOF) neutron spectroscopy. A study of the relevant coherent response from single-crystal KD₂PO₄ (DKDP) has also been undertaken. Regarding the incoherent inelastic peak in KDP, the temperature dependences of the excitation energy, the energy width and the intensity were measured. The same measurements were carried out on a 90% deuterated sample. Strong damping of the peak in the paraelectric phase was observed for both samples, while the peak position was nearly temperature independent. Moreover, the temperature variation of the thermal parameter for the proton exhibited a marked discontinuity at T_c. It has been revealed by a coherent-scattering measurement around the (303) and (503) zones that this mode has a collective nature with energy dispersion in DKDP, and that it also shows strong damping in the paraelectric phase. The results suggest that the mode couples to the fluctuation of dipole moments, and that the fluctuation can possibly be described in terms of the order-disorder model.

An algorithm for computing spectral-angular and angular distributions of parametric X-ray radiation (PXR) produced by ultrarelativistic particles in crystals under multiple Bragg diffraction is developed. The algorithm is based on the methods applied in the dynamical theory of X-ray multiple diffraction. The angular distribution of three-wave PXR angular distribution into the simulated forbidden reflex (222) in the Ge crystal is computed. It is shown that using multiple PXR generation it is possible to generate X-ray beams with double angular collimation approximately=1 angular s² and spectral width approximately=10^-3.