Table of contents

We demonstrate that the giant magnetoresistance (GMR) effect in magnetic multilayers can be explained quantitatively in terms of the scattering of electrons from a spin-independent random potential that arises from the grown-in defects within the multilayer. We have calculated the GMR ratio for Co/Cu and Fe/Cr multilayered systems within the Kubo - Greenwood formalism assuming that the on-site atomic energies are disordered randomly within a realistic spd tight-binding model. Our predictions are in good agreement with experiment and demonstrate how the GMR ratio depends on the features of the electronic band structure. In particular, we obtain the enhancement of GMR in Co/Cu multilayers at electron energies up to about 1 eV above the Fermi level that has recently been observed by Monsma et al [1995 Phys. Rev. Lett. 74 5260]. We predict no such enhancement for Fe/Cr multilayers.

A modified geometrically based free-energy functional for hard spheres is proposed which gives reliable results even for situations of extreme confinements that reduce the effective dimensionality D. It is accurate for hard spheres between narrow plates (D = 2), inside narrow cylindrical pores (D = 1), and is exact in the 0D limit (a cavity that cannot hold more than one particle). This functional also predicts the hard-sphere fluid - solid transition in excellent agreement with the simulations.

We apply a recent adaptation of White's density matrix renormalization group (DMRG) method to a simple quantum spin model, the dimerized XY chain, in order to assess the applicability of the DMRG to quantum systems at non-zero temperature. We find that very reasonable results can be obtained for the thermodynamic functions down to low temperatures using a very small basis set. Low-temperature results are found to be most accurate in the case when there is a substantial energy gap.

Porous silica dispersed with silver nano-particles (about 3 nm) within its pores (cages) has been prepared by a new method. The microstructures, particle size distribution and optical absorption have been examined. We have found that in the process of alternating exposure to the ambient air and annealing, this new material assumes a optical switching and memory effect in the visible wave band or a reversible transition between transparency and opacity.

Adaptive and transferable electron densities applicable as input in Harris density functionals and first-principles inter-atomic potentials are constructed from calculations for the atom in question embedded in a homogeneous electron gas. The density profile of each atom depends explicitly on the average density in which the atom is located, which forms the basis for the adaptivity. Harris functional results for Al and Ni atoms in numerous configurations are compared to the corresponding self-consistent results. The new density construction is shown to give systematically better results than superimposed fixed atom-like densities.

In the low-density regime bound states between negative (repulsive) test charges are obtained when many-body effects (exchange and correlation) are incorporated in the screening function of the three-dimensional electron gas via the local-field correction. The Schrödinger equation is solved in the momentum space by diagonalizing the corresponding matrix. We also perform variational calculations and find good agreement between the two methods. For high electron density ( is the density parameter) no bound states are found. Below a critical density the number and the energy of bound states increase with decreasing electron density. For large the binding energy for the ground state saturates near . We discuss the wave functions of the ground state and of the lowest excited states. We also present results for the effects of exchange and correlation for a positive (attractive) test charge and we discuss results for the ground state and excited states.

As in the Landauer - Büttiker approach to transport, a transmitter is, at fixed energy, characterized by its reflection and transmission coefficients. Generalizing a prior approach, we establish a variational principle to determine, without magnetic field, the distribution of the current over the different channels if the transmitter is coupled incoherently to its surroundings. For transmitters coupled with each other, a general demand of additivity defines the form of the variational functional. Contacts are treated as special transmitters, and, as a test, the results of the standard model are reproduced.

A typical serial resistance is defined as the resistance within a long incoherently coupled chain, with no regard to contacts and reservoirs. This resistance is strictly additive. It is shown that well within the chain a relaxed current and density distribution is established that is independent of the conditions at the ends of the chain. This distribution coincides with the optimal distribution that minimizes the resistance of each of the single transmitters that are the building blocks of the chain. For a sufficiently long chain, the serial resistance is determined by the linear dependence of the inverse total transmission on the number of single transmitters.

The relaxational behaviour of a long chain implies corresponding features of the reflection and transmission matrices in the asymptotic regime, especially a factorization of the transmission matrix, expressing memory loss with respect to the ingoing and outgoing channels.

A thorough study of the series has been carried out. All of the samples have the same orthorhombic crystallographic structure. However, the magnetic, magnetoelastic and electrical properties show drastic changes with the Tb content. The decrease in the intensity of the ferromagnetic double-exchange interaction has been correlated with the increase in the Tb content and the decrease in the Mn - O - Mn bond angle, which leads to a decrease in the electron transfer between Mn 3d and O 2p orbitals. Two magnetic phases have been detected at low temperatures: an insulator spin-glass phase in samples with high Tb contents and a metallic ferromagnetic phase in samples with low Tb contents.

We calculate variationally the binding energies of a hydrogenic impurity and a Wannier exciton in an arbitrary corner of well material surrounded by the barrier material. The results show that the binding energy of ground impurity states in the corner can be comparable with that of highly excited impurity states in the bulk when the corner structure becomes small. However, the behaviour of the exciton in the corner is not the same as that of impurity; the binding energy of the exciton varies with the corner structure insensitively. The dependences of the impurity and exciton binding energies on the dielectric mismatch between the well material and barrier material are also discussed.

Tunnelling spectroscopy measurements were carried out on the mixed crystal to investigate the change in the density of states in the CDW state. The temperature dependences of the CDW energy gap widths were obtained. Experiments showed that the flattening of the peak in the curve with increasing x corresponds to the decrease in the CDW energy gap. A model was proposed in which the anomalous characteristics in this system were explained in terms of the competition of the two conduction mechanisms associated with the metallic carriers and the `excitonic' carriers.

The single-vortex dynamics in ladders of overdamped Josephson junctions is investigated by means of numerical simulations. We derive the velocity (v), the coefficient of viscosity and the height of the dynamical barrier for the cell-to-cell vortex motion as functions of the bias current , the magnetic field penetration depth and the vortex position (x). The vortex dynamics can be satisfactorily described in terms of the motion of a particle subjected to a potential , the form of which is analysed.

The magnetic structure in the ordered phase of the nearly one-dimensional Heisenberg antiferromagnet has been measured using elastic neutron scattering. crystallizes in the orthorhombic Pnma space group with spin chains running along the crystallographic b-direction. Below the ordering temperature the magnetic structure is incommensurate along the chain direction with a temperature-independent ordering wavevector q = (0, 0.472, 0) (rlu}). The occurrence of an incommensurate structure is shown to be the consequence of frustration on the spins induced by the exchange interaction between chains. Group theory is used to determine the possible magnetic structures compatible with the symmetry of the crystal. The results show that at T = 0.3 K the spin ordering is cycloidal with spins rotating in a plane that contains the propagation direction b. A mean-field calculation of the magnetic ground-state energy including exchange anisotropy effects is used to study the stability of the observed structure. Values for the interchain exchange constants that are consistent with the features of the magnetic structure are proposed.

We present an experimental study of slow domains in semi-insulating GaAs using time-resolved, two-dimensional electro-optic voltage probing. A detailed analysis of the shape of the bulk domains proves that they are formed by symmetric charge dipoles made up from ionized traps. The study of the dependence of the domain parameters on the applied voltage allows us for the first time to reconstruct experimentally the bulk j(E)-characteristic of SI-GaAs and to show that prevailing theoretical models fail to explain the observations. The experimentally determined field dependence of the free-carrier concentration reveals a strongly field-enhanced trapping with no critical field. The drift velocity of domains is experimentally found to be exclusively determined by the free-carrier concentration at the centre of the domain.

ZnSe microcrystals have been embedded into Pyrex glass matrix for the first time. The optical extinction and photoluminescence spectra of these ZnSe-doped (-embedded) glasses are measured at various temperatures. The specimens exhibit a clear shift of the optical extinction (absorption) edge due to the spatial confinement of carriers and excitons in three dimensions. Also, the photoluminescence spectra show a weak edge emission band at their absorption edges and an intense broad emission band at longer wavelength. The broad emission band shifts to the longer-wavelength side with increasing time elapsed after excitation. Taking into account some imperfections in the crystallite and impurities in the matrix glass, the results obtained are discussed and are compared with the properties of ZnSe bulk and film crystals.

Positron annihilation spectroscopy is used to obtain information about structures and concentrations of - and -GP zones in - Ag alloys. Measurements of positron lifetimes and Doppler broadening are reported. For the interpretation we present a method to estimate numerically the lifetimes of positrons trapped in spherical precipitates and give results for a series of possible concentrations and internal structures of the precipitated particles. From a comparison with the experimental data we conclude that the silver atoms are distributed homogeneously inside the -GP zones and that the silver concentration of both types of zone is .