Table of contents

High-resolution electron microscopic images of porous silicon films have been obtained and their microstructures have been analysed by means of digital image processing.

The cross-sectional structure of (Ge_mSi_n) superlattices grown on Si(100) at 603 K is investigated via bright-field (BF) images of the diffraction-contrasted transmission electron microscope (CTEM), selected-area diffraction (SAD) and high-resolution electron microscopy (HREM). The superlattices have 3D island structure from the fifth period upward, while the initial few periods have two-dimensional (2D) growth. The boundaries of the 3D islands are approximately parallel to the (011) plane. Analysis of the experimental results suggests that the 3D island structure in (Ge₁₂Si₁₂)₅₀ are formed by the relaxation of the misfit strain.

The authors report structural studies on a new C₆₀ clathrate formed with ether as a guest molecule. Single-crystal X-ray diffraction shows that the unit cell is based on the same square-planar layers of C₆₀ molecules as found in the C₆₀-n-pentane clathrate. However, the stacking sequence of the layers is different, and reciprocal space maps reveal a significant stacking disorder. Model calculations suggest a simple rule for the sequence of layers.

The ability to control the electrical resistivity of semiconducting tetrahedral amorphous carbon (ta-C) films is reported. Phosphorus is incorporated during film growth from a plasma formed with a cathodic vacuum arc by using a phosphorus-containing carbon cathode. By changing the P content in the cathode from 0-1% mass the room temperature resistivity of the films can be changed from 10⁷ Omega cm to 5 Omega cm. Electron energy-loss spectroscopy shows that incorporation of P does not change the amorphous tetrahedral nature of the carbon films deposited by the vacuum arc method.

The authors calculate the AC and DC current through a quantum double-barrier heterostructure taking into account Coulomb interactions in the well. The AC conductance has a periodic dependence on the external field frequency corresponding to the single-electron-level spacing, and as a function of the leads' chemical potential with a period determined by the interaction. These two different periods enable probing of the two characteristic energy scales of the quantum-dot structure.

The irreversible magnetization and the associated critical current density of the high-temperature superconductor La_1.85Sr_0.15CuO₄ are investigated as a function of the orientation ( theta , phi ) of the applied field (H) with respect to the crystallographic axes. For small angles (v= pi /2- theta ) between vortex lines and a-b planes, the authors show for the first time that the decrease of the pinning force component parallel to these planes occurs in a narrow range v approximately=1 degrees where it is of first order in mod v mod . This behaviour seems to be related to the existence of a 'staircase' (kinked) type of vortex lattice. Neither the usual anisotropic lattice (valid for a homogeneous superconductor), nor the more recently proposed combined lattice scheme, is compatible with the authors' results in this range of field orientation.

FeCo-Ag thin films with a grain structure, in which the FeCo alloy and the Ag metal are mutually insoluble, were prepared by a vacuum deposition method. The maximum magnetoresistance (MR) ratio observed in these films was 13.0% at room temperature in the magnetic field region between -1 and 1 T. Here the features of the MR curve corresponded well with those of the magnetization curve. This indicates the appearance of a giant MR effect in such grain-type alloy thin films. Though the magnetic anisotropy field (Hk) associated with the MR characteristics that were obtained is extremely large (>1 T), proper annealing was found to be effective for decreasing the Hk value without causing a drop in the MR ratio.

The authors have derived interatomic pair potentials for liquid transition metals using the tight-binding cluster Bethe lattice method (CBLM) in which the important role of s electrons (changing the number d electrons, screening d charge transfer and band mixing effect via s-d hybridization) can be treated self-consistently. Coupled with molecular dynamics simulations they have calculated the static structure factors S(q) of the 3d liquid transition metals, which are in good agreement with the experimental ones. The results for the electronic density of states of these metals as well as their cohesive energy are also presented. The authors have also discussed their results in the light of those obtained in the Wills-Harrison and Pettifor models.

Monte Carlo simulations of the liquid-vapour interface for a site-site intermolecular potential of methylchloride are reported. The introduction of non-uniform samplings for computer modelling of non-uniform systems has been considered and it is suggested that they may be of interest when the transition region is an important portion of the whole system. The role played by dipolar coulombic interactions in the interfacial properties has been discussed and the results of the simulations for a system without charges (LJ system) are reported. The interfacial transition region has been studied from both the structural and the energetical point of view. The orientational order in the transition region reflects the competition between anisotropy of the site-site potential and the orientational ordering induced by dipolar interactions. The surface tensions ( gamma s) are not significantly dependent on the size of the system but are very sensitive to the long-range corrections.

The problem of the formation of the d resonances in alloys of Mg and Al with a number of d metals (Ni, Cu and Zn) has been investigated both experimentally and theoretically. High-resolution ultraviolet photoelectron spectroscopy (UPS) spectra have been taken for these alloys and the electronic structure has been calculated by the linear muffin-tin orbital (LMTO) method, within the atomic sphere approximation. The theoretical results allow for the interpretation of the shapes of experimental spectra, while it turns out that the widths of the d bands can be described by the Friedel-Anderson model only qualitatively.

Using a two-conduction-band periodic Anderson model, the influence of a magnetic field on the thermodynamic properties of heavy-fermion systems is studied by means of the slave-boson mean-field theory. A depression of the heavy-fermion state by the magnetic field is found. A theoretical explanation for the anomalous volume magnetostriction of heavy-fermion systems is also given.

The aggregation of F centres and OH^- molecules to F_H(OH^-) centres in KBr leads to a very interesting optical and thermal bistability. The structures of the bistable F_H(OH^-) centres were investigated by using optical and conventional detection of electron nuclear double resonance (ENDOR). The analysis of the ENDOR spectra of both F_H(OH^-) configurations is presented. It is shown that in both bistable configurations the OH^- molecule resides on a next-nearest anion site (fourth shell). The bistable configurations differ in the orientation of the OH^- dipole with respect to the F centre. The thermal bistability is tentatively explained as being due to entropy changes with temperature.

The authors have performed quantum-mechanical calculations of tunnelling current drained through a narrow probe weakly attached to quantum wires. The amplitude of the current is approximately proportional to the local density of states in the wires. They demonstrate that the spatial variation of the probability distribution of an electron due to quantum interference effects is obtained by scanning the probe along the wires.

The title materials display anomalous electrical, magnetic and optical properties. They have the characteristics of Mott insulators showing high electrical resistance and ligand-field optical spectra, but at the same time they adopt a magnetic ground state which is neither of Curie-Weiss type nor indeed of some standard antiferromagnetically ordered condition. The observed state is one of lowish, isotropic, and almost temperature-independent paramagnetism. The authors have studied the magnetic properties of Sc-doped TiOX, seeking possible indications of the existence of the resonant valence bond or RVB state-the state currently suggested by some to be responsible for high-temperature superconductivity in the mixed-valence cuprates. It is necessary to distinguish between RVB and some extreme case of magnetic frustration in these S=1/2, 2D oxyhalide systems. The authors' tentative conclusion is that TiOX presents an RVB ground state which Sc doping renders glassy and static.

The authors have successfully prepared the carbides R₂Fe₁₇C_x (R identical to Y, Gd, Tb, Dy, Ho and Er) with a high carbon content x=1.5 and 2.0 by the melt-spinning method. The carbides have a single rhombohedral Th₂Zn₁₇ structure phase and are highly stable to at least 1100 degrees C. The Curie temperatures T_C are raised enormously and the saturation magnetizations change little upon carbonation. The average enhancements of T_C are about 220 K for R₂Fe₁₇C_1.5 and 265 K for R₂Fe₁₇C_2.0. The volume dependence of the exchange interaction has been discussed. The Curie temperatures are analysed in terms of a mean-field model. The spin reorientation is observed to be 133 K for Er₂Fe₁₇C_1.5 and 141 K for Er₂Fe₁₇C_2.0.

Specific-heat measurements have been performed on polycrystalline R₂Zn₁₇ compounds (R identical to Pr, Nd, Tb, Dy, Ho, Er and Y) in the temperature range from 0.3 to 150 K. For all the compounds, except for R identical to Y, magnetic anomalies in the specific heat have been observed, indicating the existence of magnetic order at low temperatures.

The zero-field splitting (ZFS) parameters D, E and F for an Mn²⁺ centre in a BiVO₄ single crystal are calculated using the point-charge electrostatic model and the superposition model. The calculated ZFS parameters at the Bi and V sites are compared with the experimental values for Mn²⁺. The calculated second-order axial and rhombic ZFS parameters at the Bi site turn out to be similar to those from experiment. The superposition model and the point-charge model give similar results. This supports the notion that the Mn²⁺ impurity substitutes for the Bi³⁺ ion in BiVO₄.

A nuclear magnetic double-resonance experiment, performed on the ²³Na spin system in a single crystal of NaNO₂ at 130 K, has shown that the spin-lattice relaxation is magnetic, as opposed to quadrupolar, at that temperature. The approach to equilibrium of the high-frequency satellite of this spin system, after excitation by a selective 90 degrees pulse, was found to be quite non-exponential at 130 K and roughly in agreement with a simple theory. The approaches to equilibrium of both the high-frequency satellite and the central resonance after excitation of the latter by a selective 90 degrees pulse were found to be exponential, in marked contrast to the theory. The reason for the discrepancy is not known.

An expression for the static polarizability of a system of electrons moving in an external potential is obtained in terms of occupied wavefunctions of a single-particle Hamiltonian. The formalism is exact within the density functional theory. For a single electron in an external potential the polarizability is expressed in terms of the ground-state charge density.

The electronic structures of Cr³⁺ ions on the Nb site and the Li site of LiNbO₃ were calculated under the actual site symmetry C₃ with the spin-polarized self-consistent-charge discrete variational X_alpha (SCC-DV-X_alpha ) method. Special emphasis is placed on a discussion of the potential of LiNbO₃:Cr³⁺ as a tunable laser crystal.

Photoluminescence investigations have been performed on solid C₆₀, C₆₀/C₇₀ and C₇₀ samples. The photoluminescence spectra of C₆₀ exhibit a main peak at 1.69 eV and well resolved peaks at 1.65 eV, 1.60 eV and 1.5 eV which the authors assign to different phonon replicas. The temperature dependence of the photoluminescence spectrum is also investigated. They find that the 1.77 eV emission peak, which appears at low temperature, clearly originates from C₆₀. The enhancement of this peak seems to be closely related to the amount of C₇₀ existing in C₆₀ samples. In this case C₇₀ acts as an impurity leading to a local distortion of the pure C₆₀ structure.