Table of contents

In situ Raman scattering spectroscopy is performed in order to probe the chemical dynamics and structure modifications at Ar and SF₆ low-pressure plasma-etched silicon surfaces. An Ar plasma induces relatively slight perturbation of the silicon surface, while a SF₆ plasma creates highly disordered overlayers induced by the fluorine atoms reacting at ion-induced defects. The overlayer structure is a mixing of an amorphous phase and 10 AA microcrystallites expected to be close to porous silicon structure.

High-resolution photoemission spectroscopy has been used to determine the surface core-level shift of the Rh(100) single-crystal surface. From the Rh 3d core level spectra the surface atoms were found to have 0.62+or-0.01 eV lower binding energy than the bulk atoms. The observed shift is in excellent agreement with recent theoretical calculations.

The kinetics of the A+A to 0 reaction on a correlated heterogeneous one-dimensional chain is studied. As a novel result it is found that the temporal behaviour of the density of A particles depends on the energetic topography of the surface when A particles are initially located preferentially on the more energetic sites, as happens in the case of thermodynamical equilibrium for the absorbate.

The authors have measured the work function changes and Na 2p core-level shifts for the systems Na/Cu(111), Na/Ni(111), and Na/Cu(110) as a function of alkali coverage. They find that the coverage dependence of the core-level shift in the first layer is well correlated with the work function change. They suggest that these quantities are related through the coverage dependence of the alkali dipole moment and the position of the centroid of the alkali valence charge density. In order to confirm this relationship, the authors use a simple fitting procedure which is able to reproduce the work function changes from the core-level shifts for all three systems.

The temperature and layer-thickness dependence of the magnetoresistance (MR) in magnetic multilayers is discussed with the use of the finite-temperature band theory in which the effect of spin fluctuations is taken into account by means of the static functional-integral method. It is shown that their model calculation explains well the observed MR data of Fe/Cr multilayers recently reported by Gijs and Okada.

The evolution of the electronic structure of ultrathin mercury layers on tungsten (110) with increasing thickness was studied by angle-resolved and resonant photoemission. There is evidence for the formation of new Hg electronic states arising from Hg adatom hybridization and quantum-well states. These results, combined with the resonant photoemission, suggest that the overlayer undergoes a gradual non-metal-to-metal transition with increasing coverage and are similar to the change of the metallic properties observed in free Hg clusters with increasing size. The Hg band structure exhibits little evidence for strong interactions or hybridization with the W(110) substrate, in spite of layer by layer growth of the Hg overlayer, and superlattice periodicity.

The electronic structure of a 2D gas subjected to a tilted magnetic field, with a strong component parallel to the GaAs/AlGaAs interface and a weak component oriented perpendicularly, is studied theoretically. It is shown that the parallel field component modifies the originally circular shape of a Fermi contour while the perpendicular component drives an electron by the Lorentz force along a Fermi line with a cyclotron frequency given by its shape. The corresponding cyclotron effective mass is calculated self-consistently for several concentrations of 2D carriers as a function of the in-plane magnetic field. The possibility of detecting its field-induced deviations from the zero-field value experimentally is discussed.

Point-contact junctions on normal-metal-superconductor bilayers show asymmetries of different magnitudes and signs in the differential resistance versus voltage curves for opposite-bias voltages. In the absence of Andreev reflection (i.e. for energies outside the energy gap) no asymmetry is found. The asymmetries are investigated with Ag, Cu, Pd, Pt, Ni, and Co as point-contact material on Ag-Pb bilayers. For all materials the authors provide the statistics of the asymmetries. They discuss effects that may be responsible for this new phenomenon.

Negative linear magnetoresistance of two-dimensional (2D) electrons have been found in a disordered array of antidots. The authors suggest that trajectories that roll along the array of antidots exist in a magnetic field. These trajectories have a mean free path larger than the average value for electrons with ordinary diffusion.

The resistivity, magnetoresistance and Hall effect of the rare earth silicides RSi_2-x, with x from 0 to 0.4, have been measured on several polycrystalline samples, single crystals and epitaxial thin films. All samples are metallic; the Hall effect measured for some phases shows that they are dealing with nearly compensated metals. The magnetoresistance displays peculiar features in antiferromagnetic phases, which are explained by the increase of spin disorder scattering at the metamagnetic transition.

The longitudinal optical vibrational modes have been studied in (GaAs)_n(AlAs)_m superlattices by means of FTIR spectroscopy. In the undoped samples, confined LO phonons have been observed; the dispersion of LO phonons obtained by FTIR spectroscopy was in good agreement with the Raman data. In the doped (GaAs)_n(AlAs)_m superlattices, coupling of confined vibrational LO modes with superlattice plasmons has been found. Analysis of the experimental results showed that the frequency of superlattice plasmons in the superlattices studied is mostly determined by the population of the miniband formed by the Gamma -like conduction band states. The fitting of the calculated reflection spectra to the experimental spectra allowed us to measure the population of the minibands, the effective mass and the vertical mobility of electrons.

A theory of two complementing luminescence lines observed in some A_IIB_VI semiconductors doped with transition metal impurities is proposed. The first line is known to be attributable to an intracentre transition; the second line appears as a result of a second-order process of interference between radiative and non-radiative transitions with participation of the d shells of the impurity atoms. A detailed calculation of the intensity of such a process is presented. It is shown that the specific resonance can make this second-order process quite strong if the impurity-phonon coupling is strong enough. The possibility of observing pairs of complementing lines in various A_IIB_VI and A_IIIB_V systems is discussed. Some unresolved problems associated with the effect are also touched upon.

The t-J model has been proposed to describe the physics of copper oxide planes in high-temperature superconductors. A real-space quantum renormalization-group method is developed in order to study the t-J Hamiltonian in a square lattice. The zero-temperature phase diagram is analysed for electron concentrations near the half-filled band and ratio J/t. A critical line is found that separates two distinct regions: one of a gas of holes in an antiferromagnetic background, and the other consisting of a phase in which holes segregate into a hole-rich and a pure antiferromagnetic region, as demonstrated by chemical-potential eigenvalue calculations near the phase attractor. The results are close to the experimental observations.

The rare-earth gallium oxide perovskites RGaO₃ (R=La,Pr,Nd) are promising substrates for epitaxy of high-T_c superconductors. In this paper the crystal structures of orthorhombic and rhombohedral RGaO₃ have been studied by neutron diffraction, which allows precise localization of light atoms such as oxygen, which are of importance in phase transitions. The lattice parameters, atom positions and tilt angles of the oxygen octahedra were determined in the temperature range 12-1773 K. Lanthanum gallate LaGaO₃ shows a phase transition from an orthorhombic phase (Pbnm) to a rhombohedral phase (Rc) at 425 K. The results are discussed in the classification schemes developed for distorted perovskites.

The dynamics of the ZnCl₄ anion in two compounds, one exhibiting a structural transition to an incommensurate phase (Rb₂ZnCl₄) and another not having any structural transition at all (Cs₂ZnCl₄), are studied through ³⁵Cl NQR frequency ( nu ) and relaxation time (T_1Q) measurements as a function of temperature T in their high-temperature (paraelectric) phases. Estimates of the potential barriers hindering the reorientational motions of ZnCl₄ anions along axes parallel and perpendicular to the a axis of the crystal structure are obtained from the T_1Q data. These results are analysed in conjunction with a qualitative comparison of relative mean square amplitudes of such motions obtainable from frequency data, in order to investigate the differences in the dynamics and structure of the ZnCl₄ anion. There seems to be correlation between the dynamics of the anion in this phase and the possibility that the compound may exhibit a structural instability at low temperatures in this family.

The self-diffusion of H in the substoichiometric rare-earth hydride halide YBrH_0.78 was investigated by incoherent quasielastic neutron scattering. YBrH_0.78 exhibits a layered structure in which the H atoms occupy tetrahedral interstitial sites located in Y bilayers, which are sandwiched by Br bilayers. As this arrangement suppresses H transfer between different Y bilayers, the present study investigates an essentially two-dimensional H diffusion within a given Y bilayer. The self-diffusion coefficient of the H was determined to be between 750 and 900 K. At 900 K, its value is D=(1.2+or-0.4)*10^-6 cm² s^-1. The temperature dependence can be described by an Arrhenius relation with an activation energy E=(0.48+or-0.12) eV. The momentum-transfer dependence of the measured spectra suggests the presence of noticeable correlation effects in the H diffusion.

The contribution of the induction energy to the stability of layered structures in MX₂ (halide) systems is investigated. A simple potential, based upon the ionic model but including many-body induced dipole effects in a self-consistent way, reproduces the experimental structure map for a wide range of materials. The predicted stability of the layered crystals is confirmed in molecular dynamics simulations at high temperatures. The potential role of other induction terms is briefly discussed.

In the case of electromagnetic waves it is necessary to distinguish between inward and outward on-shell integral equations. Both kinds of equation are derived. A correct implementation of the photonic KKR method then requires the inward equations and it follows directly from them. A derivation of the KKR method from variational principles is also outlined. Rather surprisingly, the variational KKR method cannot be entirely written in terms of surface integrals unless permeabilities are piecewise constant. Both kinds of photonic KKR method use the standard structure constants of the electronic KKR method and hence allow for a direct numerical application. As a by-product, matching rules are obtained for derivatives of fields on different sides of the discontinuity of permeabilities.

Electronic and ground-state properties of indium iodide in the orthorhombic TlI-type structure (space group D_2h¹⁷) are reported. These properties are derived from the one-particle Kohn-Sham equations in the local-density approximation. The ab initio norm-conserving nonlocal pseudopotential of Bachelet, Hamann and Schluter is used. The corresponding calculational technique is described briefly. Analytical expressions for the matrix elements are obtained. Results are presented for the valence and conduction band energies, densities of states, effective masses and charge densities. In terms of these, a consistent explanation of various experimental measurements of InI single crystals has been obtained. Furthermore, by examining the orbital character and the calculated charge densities of various states, the origin of the bonding in indium iodide has been determined.

The authors investigate the binding energy separation between the many-body ground-state singlet and higher-lying magnetic states for the f¹-f² Anderson model to next-leading order in the 1/N expansion, where N denotes the orbital degeneracy of the f states. They formulate integral equations for the magnetic-state amplitudes that appear at next-leading order in 1/N and solve these, together with the integral equations that describe the singlet, numerically. They find that the total separation between magnetic and singlet states is small, even in the valence-fluctuation regime, and hence demonstrate the robustness of the leading-order result. They find that the maximum binding-energy separation is shifted towards higher valence, in contradiction to results of the minimal-degeneracy model. They also present results for the valence and charge susceptibility.

In this paper the current-voltage characteristics and dynamic conductance are computed for a tunnel junction with an embedded magnetic impurity which is subjected to a steady magnetic field. Using a theory that does not employ the transfer Hamiltonian technique, the approach is based on a dynamical method described by Cini, and employs the Keldysh formalism.

The tweed microstructure in the nonstoichiometric crystal of YBa₂Cu₃O_{7- delta} is studied by computer simulation. A two-dimensional model of 99*99 unit cells represents a layer of this crystal with an oxygen deficit and exhibits a ferroelastic tetragonal-orthorhombic phase transition. The simulation allows one to determine the temperature-oxygen-concentration phase diagram. Below the transition temperature the strain fluctuations form a grid of needle-shaped domains, the density of which increases with increasing degree of nonstoichiometry. On quenching, the tweed texture orders through the intermediate stripe phase to a single orthorhombic domain. Oxygen vacancies, appearing due to nonstoichiometric oxygen concentration, gather along the domain walls, and hence slow down the kinetics of the annealing process.

A low-field magnetization study of polycrystalline URu₂Si₂ reveals a magnetic structure with substantial irreversibility around T=30 K, with applied fields H<or=50 mT. Possible origins of such phenomena are discussed.

The first-order Neel transition in chromium and a change in behaviour between the paramagnetic and the antiferromagnetic phase are observed in its electrochemical properties. The explanation must relate the kinetics of anodic oxide film growth to changes in the electronic structure at the magnetic phase transition.

The authors report low-temperature magnetic relaxation measurements of nanometre-scale antiferromagnetic horse-spleen ferritin particles. It is argued that the magnetic relaxation signal is due to the excess uncompensated spins. Their key observation is the non-thermal character of the relaxation below 2 K. The observed features are in agreement with theoretical suggestions on quantum tunnelling of the Neel vector between easy directions.

The site of Ti in LiNbO₃ was studied by means of gamma - gamma perturbed angular correlation and the PIXE-channelling technique. It is found that, after in-diffusion, Ti is situated in the oxygen octahedron normally occupied by a Li ion. However, evidence for association with a defect or slight displacement from the ideal site is observed.

An investigation of low-frequency inelastic scattering (LOFIS) and pre-resonant Raman spectroscopy has revealed the role of the modifying cations, Ba²⁺, Zn²⁺ and Pb²⁺ on the bonding properties of the Eu-O bond in Ba(PO₃)₂, Zn(PO₃)₂, and Pb(PO₃)₂ glasses. Analysis of the results has shown that the electronegative character of the Pb²⁺ ions and the ionic radii of the Ba²⁺ and Zn²⁺ ions play a major role in the short-range order of the metaphosphate glasses. Furthermore, it was shown, using LOFIS, the influence of the modifying cations on the size distribution of the structural units found in the metaphosphate glasses.

The magneto-optical (MO) Kerr effect of FeX compounds with X=Co, Ni, Pd, Pt is studied theoretically using self-consistent band structure calculations. The MO spectra so determined agree with experimental Kerr spectra in overall shape but not in magnitude, the latter being slightly underestimated for FeCo and overestimated for FePd and FePt. The trend in the spectra for FeNi, FePd and FePt is explored and by investigating the effect of different crystal structures for FeCo and FePd the authors demonstrate that the Kerr spectrum depends sensitively on the local atomic environment of the Fe and Pd or Co atoms. Their calculations suggest that the computed spectra are those of highly ideal samples, which only approximately possess the atomic characteristics of the surfaces and bulk of real polycrystalline samples.

Measurements of the time-resolved intensity and decay kinetics of the 408 nm emission band for KI:Pb²⁺ in the 4.2-100 K temperature range are reported. Two fast components and one slow component are observed. The temperature dependences of three decay times are interpreted in the framework of a three-excited-state-level model including multiphonon processes between two radiative and underlying metastable levels. Non-radiative quenching is also included to fit the decrease in overall intensity in the temperature range investigated.

Three different kinds of (Br₂^-+e) emission band in KCl:KBr mixed crystals have been investigated based on cathodo- and thermoluminescence measurements. the green emission band is considered to be the normal (Br₂^-+e) pi emission; the violet and blue bands observed below 120 K are suggested to originate from the (Br₂^-+e) emission perturbed by a Cl^- ion at two different sites. The relationships between the intensities of these three bands and the temperature are also reported.