Table of contents

The ability to produce deliberately shaped or curved two-dimensional electron gases in semiconductors using recent developments in technology, for example regrowth of III-V semiconductors on patterned or etched substrates, opens the possibility of investigating not only the behaviour of electrons in a curved quasi-two-dimensional space, and the effects of varying that curvature, but also presents a novel way of investigating electron transport properties in a non-uniform transverse high magnetic field. It is shown that a semi-infinite two-dimensional electron gas subjected to a non-uniform magnetic field has, in addition to current-carrying edge states, one-dimensional states which lie within the interior of the gas, which also have a finite dispersion, an effect which may be used to create quantum wires or other structures. It is also shown that, in the absence of a magnetic field, curvature of the two-dimensional electron gas gives rise to a potential variation which is inversely proportional to the square of the radius of curvature, an effect which may also be used to confine the electronic motion to one dimension.

We present experimental observations of the dielectric and energy relaxations in the spin density wave state of TMTSF₂PF₆. These measurements reveal two distinct dynamical regimes. The dielectric relaxation shows a critical slowing down towards a 'static' glass transition around 2 K. Below this temperature very slow energy relaxation processes appear, which exhibit breaking of ergodicity associated with aging effects. These results are analysed within the framework of glass-forming systems.

The interactions between an atomic force microscope (AFM) tip and the perfect and defective (001) surfaces of LiF, NaCl and CaO have been studied by quantum-chemical and atomistic simulation techniques. The liquid which is usually present on surfaces in experimental conditions, is considered to be inert and not contributing to the imaging. However, its chemical interaction with the tip is taken into account via the specific microscopic structure of the very end of the tip, reflecting the possibility of its oxidation and protonation. Calculations were performed for three models representing the nano-asperity at the end of SiO₂ and MgO tips consisting of up to 66 atoms. The tip-surface interaction and related forces were calculated as a function of the chemical structure of the tip, its shape, and its distance from the surface. The associated tip and surface distortions caused by this interaction were investigated, We studied the atomic structure of Mg and O impurity defects near the (001) LiF surface, and OH^- molecular ion substituting for Cl on the (001) surface of NaCl, and calculated their stability adiabatic barriers for diffusion, and AFM images. It is demonstrated that the optimal tip-surface distance for 'atomic resolution' is about 3-5 AA, which corresponds to the presence of one or two liquid layers between the tip and the surface. The surface and defect distortion by the tip is small in this distance range and greatly increases at smaller distances, leading to creation of surface defects. The electrostatic contribution to the tip-surface interaction makes a basis for 'atomic resolution' at large distances, whereas much stronger 'chemical' interactions dominate at small distances. The results suggest that it should be possible to image charged impurities such as Mg or O ions substituting for the host ions in alkali halides by AFM.

The influence of the barrier shape on tunnelling of spin-polarized electrons through a vacuum gap between the tip of a scanning tunnelling microscope and a magnetic sample is analysed. It is shown that the spin-dependent transmission probability of the electrons depends strongly on the form of the barrier at the sample surface. This suggests that for a detailed interpretation of the images obtained by a spin-polarized scanning tunnelling microscope the barrier shape should be known.

It is shown that the fracture surface energy of heterogeneous brittle materials (polycrystals, composites, ceramics, rocks) depends on their microstructural characteristics, namely the fractal properties of the network of microcracks. A quantitative approach for the calculation of fracture energy from microcrack network fractal parameters is suggested. Very high experimental values of fracture energy for heterogeneous brittle materials can be easily explained in the framework of the fractal model.

Normal-incidence standing x-ray wavefield (NISXW) measurements have been made of the local adsorption site of Rb on Al(111) surfaces, particularly in an ordered ( square root 3* square root 3)R30 degrees phase, as a function of the sample temperature during adsorption or subsequent annealing. The results confirm the a top-site occupation for low-temperature (around 150 K) preparation, but show that room-temperature preparation leads to a structure having Rb atoms in surface substitutional sites. The overall structural situation is therefore essentially the same as that found previously by low-energy electron diffraction LEED for the Al(111)( square root 3* square root 3)R30 degrees -K phases. However, experiments involving annealing of the low-temperature prepared surface to room temperature indicate that only a small part of the surface easily transforms to the higher-temperature form, and indeed there is evidence that even in room-temperature preparations some fraction of the adsorbed atoms may remain in atop sites. The apparent conflict of this result with that from recent photoemission core-level shift and LEED data is discussed.

Ultrathin films of vanadyl-naphthalocyanine (VONc) have been prepared on the (001) faces of RbI, KI, and KBr by molecular beam epitaxy. Molecular arrangements were determined by reflection high-energy electron diffraction. VONc molecules form square lattices ( square root 10* square root 10-R+or-18.4 degrees ) on these substrates. The lattices are commensurate to the substrates and the intermolecular distance varies from 1.64 nm on RbI to 1.48 nm on KBr, in keeping with the lattice constant of the substrates. The position of the molecules is primarily determined by electrostatic interaction between molecules and the substrates. VONc molecules stay on the surface via electrostatic interaction and steric hindrance between projecting naphthalene rings is avoided by minimizing the van der Waals interaction between molecules.

We study the induced potential of an interface electron interacting with bulk longitudinal-optical (BO) phonons as well as interface optical (IO) phonons using the Green-function method. The dependence of the induced potential on the magnetic field strength and on the distance of the electron from the interface is studied. The numerical results show that in weak magnetic fields both mod V_e-BO mod and mod V_e-IO mod are rapidly increasing functions of the magnetic field, but beyond a critical magnetic field B_c, mod V_c-BO mod and mod V_e-BO mod are slowly decreasing functions of the magnetic field. The numerical results also show that the electron-IO phonon interaction is dominant for weak magnetic fields and small electron departure from the interface. In the opposite limit, the electron-BO phonon interaction is dominant.

The ground-state energy and effective mass of a polaron in the quantum well (QW) of an AlAs/GaAs double heterostructure (DHS) are calculated as functions of the well width and the strength of the electric held applied along the growth direction. Every type of optical phonon mode that can exist in the DHS within the continuous model is considered separately. It is found that the contribution of the interface phonon to the polaron effect is much larger than that of the confined bulk phonon modes in QWS with width (d<50 AA and then falls quickly with increasing well width up to d approximately=200 AA. An electric field has hardly any influence on polaron effects in QWS with width d<50 AA, but when d>50 AA the influence becomes much stronger. The correction of the ground-state energy and effective mass originating from LO modes decreases and that from the interface modes increases with increasing strength of the electric held.

For the ( square root 3* square root 3)R30 degrees commensurate physisorption system Xe/Pt(111), the K/sub //-/, layer- and double-group-symmetry-resolved density of states (LDOS) as well as spin-resolved normal photoemission spectra for circularly and linearly polarized light have been calculated by means of a fully relativistic Green function formalism. Comparison with the LDOS obtained for an isolated Xe monolayer shows that the m_j splitting of the 5_ps2/-derived states of the Xe adsorbate is almost exclusively due to the lateral interaction within the Xe layer. In contrast. Pt derived features are strongly determined by the Xe overlayer via 'surface umklapp': firstly, electronic states localized in the two topmost Pt monolayers; and secondly, photoemission features originating from umklapped Pt bulk initial states. For circularly polarized light, we obtain very good agreement with experiment with regard to peak positions and sign of the spin polarization. Substantial discrepancies in absolute polarization values are ascribed to Auger-like electron-hole processes and call for an extension of our photoemission theory to incorporate these additional excitation channels. For linearly s-polarized light, the calculated spin polarization spectra depend very strongly on the lateral position of the Xe layer relative to the substrate.

In this paper we present a highly convergent renormalization scheme for the computation of Green's functions of interfaces in the case of tight-binding models. It allows the calculation of the Green's function of the whole infinite system, which is built up of a stack of a semi-infinite solid of material A, an interface region, and a semi-infinite solid of material B. As a first application we analyse the layer-resolved interface electronic structure of Sb/GaAs(110) and compare the results with calculations for (xML)Sb/GaAs(110), x=1,2 and 3. We find several interface states in the fundamental band gap of GaAs(110). Their energy dispersion and orbital composition are discussed in detail. Furthermore, we find that the interface electronic structure of (3ML)Sb/GaAs(110) still differs significantly from that of the system terminated by a semi-infinite Sb crystal.

In this work the effect of the ferromagnetic layer thickness on the interlayer exchange coupling in magnetic multilayers, which has rarely been considered so far, is investigated by employing the one-band tight-binding hole-confinement model. The numerical calculations for a simple cubic lattice show that although the oscillatory variation of coupling parameter J with the variation of non-magnetic layer thickness N has periods insensitive to the ferromagnetic layer thickness M, when M becomes smaller the amplitude and phase of such oscillations depend strongly on M, and J varies with M in an oscillatory-like fashion. Our theoretical results may be helpful in understanding the recent experimental data.

The spectrum of surface acoustic phonons in films of CaF₂ on GaAs(111) has been investigated by means of Brillouin light scattering. We report the first experimental observation of the disappearance of the Rayleigh phonon with increasing film thickness in these structures of the fast-on-slow or accelerating type. For propagation along <121> at the disappearance of the Rayleigh surface phonon, a quasi-resonance in the continuum, whose spectral position tends to that of the Rayleigh phonon of CaF₂(111) for larger value of film thickness, is detected in the spectrum. Because of this behaviour we identify this quasi-resonance as the pre-Rayleigh mode of the film. When propagation is along (110) the role of the pre-Rayleigh mode of the film is played by the pseudosurface acoustic mode. The theoretical discussion is performed by comparing the Brillouin spectra with the surface projected phonon density of states for shear vertical phonons numerically calculated for CaF₂/GaAs(111) heterostructures.

We have investigated the vacuum ultra-violet photodissociation of molecular O₂ physisorbed on graphite using synchrotron radiation with photon energies in the range 13-35 eV. The yield of desorbed O⁺ ions shows a threshold at approximately 19.5 eV and resonances at 24.5 eV and 28.5 eV, contrasting strongly with the gas phase photodissociation cross section. The principal mechanism of O⁺ production appears to be dipolar dissociation driven by photoelectrons generated in the substrate. An enhancement in the detection efficiency of low-energy ions, compared with previous measurements, has identified an additional dissociation mechanism leading to the desorption of O^- ions. Specifically, the results support a channel for low-energy O^- desorption, attributed to the dissociative attachment of photoelectrons, again generated in the graphite substrate, to the physisorbed O₂ molecules.

The phase transitions in KTa_1-xNb_xO₃ (KTN) with x=0.076 have been investigated by various methods. The modifications of the light diffraction patterns recorded as a function of the temperature reveal three successive changes in symmetry. These results are consistent with the anomalies detected for the same crystal in the temperature dependence of both the dielectric permittivity and the light transmission and confirm the sequence of cubic-tetragonal-orthorhombic-rhombohedral structural phase transitions. A careful analysis of the temperature dependence of the integrated intensity for each transverse optical phonon line below the cubic-tetragonal transition shows similar anomalies at each transition. These transitions are preceded in the cubic phase by the formation of precursor clusters as in very dilute KTN crystals. These microregions are evidenced from the occurrence of intense quasi-elastic scattering well above the first transition and thereby of the activated phonon density of states bands.

The method of vapour-solid couples was applied in order to investigate interdiffusion and intrinsic diffusion in dilute Al-Zn alloys. Although an aluminium oxide layer could not be avoided in our experiments, typical concentration-penetration curves were measured by electron probe microanalysis. The Boltzmann-Matano evaluation of the profiles yielded interdiffusion coefficients in agreement with previously published results. Contrary to the expected behaviour, bulges were observed by metallographic inspection at those regions of surfaces where zinc diffused into the samples. From results of interdiffusion and from measurements with only 1h diffusion time, it is deduced that the penetration of Zn atoms is nearly unaffected by the surface oxide layer and shows no local differences. Therefore, the reason for the growth of the bulges is the diffusion of the host component Al, which is possible only at preferred sites of the oxide layer. The ratio D_Zn/D_AI was determined at those sites where the oxide layer was penetrated by both Zn and Al atoms. From the value measured for an infinitely dilute solution, (D_Zn/_Dl))₀=1.70+or-0.25, the vacancy flow factor (L_AlZn/_ZnZn)₀=-0.19+or-0.13 was derived. Vacancy jump frequency ratios according to the five-frequency model were calculated which differ from unity only slightly and are similar to those obtained for the systems Ag-Zn and Cu-Zn. A weak binding between vacancies and Zn impurity atoms was estimated by means of two different methods.

The unified method for molecular dynamics and density functional theory (MD/DF) introduced by Car and Parrinello is based on zero-temperature density functional theory. We have incorporated the finite-temperature extension of density functional theory proposed by Mermin into a consistent fictitious Lagrangian framework. Such an extension of the original MD/DF method is desirable for two rather different reasons. First this framework provides a general method to treat electronic states at finite temperature or in non-equilibrium excited states. Second it can alleviate certain practical problems that arise when Kohn-Sham DF methods in general and MD/DF in particular are applied to metallic and near-metallic systems. Our approach involves dynamically varying occupation numbers which is important for states near the Fermi energy. We show that the added degrees of freedom of these states can be used to accelerate the convergence to the electronic ground state. In MD simulations this improved response of the electrons also leads to an increase in the rate of energy transfer from the ionic to the electronic degrees of freedom. Our method is illustrated by calculations on crystalline metallic carbon and simulations of liquid silicon.

We have shown in an analytical way supported by intuitive arguments that all the eigenstates of an infinite quasiperiodic chain can be of extended nature in an unusual way. The well known copper mean chain provides such an example. Earlier works, based on calculations with systems having finite sizes, showed the co-existence of localized and extended states for this particular system. Within the framework of the real space renormalization group scheme we transform a perfectly ordered chain and a copper mean chain into a period doubling sequence for which an area-preserving dynamical map is already in existence. The recursion relations for the Hamiltonian parameters of the effective period doubling sequences generated from an ordered chain and a copper mean chain are then compared to extract information regarding the true nature of the eigenstates of the latter.

We investigate the role of electron correlations on the plasmon dispersion in alkali metals using a sum rule approach. The single pair contribution to the low-energy part of the excitation spectrum is calculated in the framework of the Landau theory of Fermi liquids and used to estimate the plasmon contribution to the compressibility sum rule. The plasmon contribution to the f-sum rule is calculated employing a non-local effective interaction (g-matrix) recently proposed in the literature. The analysis accounts for the strong density dependence of the dispersion coefficient alpha exhibited by experimental data. The average energy of multipair excitations is also estimated.

Crystal-field parameters (B_k,m) were obtained for LaOBr doped with Tm³⁺ from the analysis of luminescence spectra. The crystal-field Hamiltonian H_CEF= Sigma _kmB_km⁺ Sigma i C_km(r_i) for the C_4v symmetry comprises five non-zero B_k,m parameters. The results of model crystal-field calculations in C_4v symmetry for Tm³⁺ in LaOBr are compared with experimental data, for the lowest ten (S, L)J states 4^f12 configuration. The set of crystal-field parameters (CFPs) calculated on the basis set of 54 sublevels reproduced the experimental level scheme within an RMS deviation of 11 cm^-1.

Within the McClure model a dispersion equation for the energy spectrum of the Bi-Bi_1-xSb_x superlattices in the envelope function approximation is derived. The effective mass of minibands, the Fermi energy and the concentration of free carriers in a superlattice with X=0.1 and the period d=10 nm are determined. The dependence of the miniband energy upon the ratio r=d^I/d^II is obtained (d^I is the thickness of the Bi layers and d^II is the thickness of the alloy layers). It is shown that there is a transition from the semiconducting state (r<0.31) to the semimetallic state (r>0.31) in the superlattice with X=0.1 and d=10 nm due to an energy overlap between the minibands at the L and T points of the Brillouin zone. For d^I=4 nm and d^II=6 nm the Fermi surface of electrons is closed whereas the Fermi surface of holes is open in the direction of the superlattice wavevector q. In the superlattice with X=0.12 and d^I=d^II=60 nm the transverse components of the effective-mass tensor change signs at a certain value of q=q_o. As a result, at q>q₀ the dependence of the miniband energy on the wavevector k in the plane of layers has a shape like a 'camel's back'.

Using a technique entailing localized photo-generation of excess carriers on the (100) face of a PDATS crystal, an asymmetry between the distances holes drift and those of the electrons is found. By modelling the electric fields between surface electrodes, an average distance for the hole drift is estimated, and is deduced to be about 60 mu m with lower and upper bounds of 40 and 80 mu m respectively. Further experiments suggest that the holes do not undergo any form of recombination but could remain in the crystal for long times. The effect of this in this and other photo-conduction experiments is considered.

A new configuration of electron waveguide, namely the y-branching electron waveguide, is proposed. Mode-splitting behaviour is revealed to be the result of symmetry of the structure and transverse mode of the waveguides. The transport properties of the structure are theoretically studied by applying the boundary-matching method. The effects of multireflection and resonance in the structure are demonstrated. Prospective applications are also suggested.

The analysis of electronic structures has been carried out for the transition-metal compounds showing the corundum-type crystal symmetry using the suggested tight-binding method for interacting bands. With the self-consistent field approximation, the branches of the electronic spectra and energy gaps have been analytically calculated. The role of the electron correlations was found to be decisive for the dielectrization of spectra for which no additional assumptions, e.g. the existence of spin- or charge-density waves, was necessary. The data obtained provide an explanation for the appearance of the insulator state in such compounds as Ti₂O₃, V₂O₃, Cr₂O₃, alpha -Mn₂O₃ and alpha -Fe₂O₃. The calculated values of band gaps agree reasonably with the experimental data available. The Peierls problem is solved for the corundum-structure d compounds.

The values of the isotropic and anisotropic Dy-Fe exchange interactions and the CEF parameters for Dy(Fe₁₁Ti) are evaluated from the magnetization curves along the crystal axes measured on the single crystal in the range of temperatures between 4.2 and 300 K. The anisotropic exchange interaction is two orders smaller in magnitude than the isotropic exchange interaction but is comparable with the CEF interaction and apparently affects the magnetization processes.

The initial oxidation of two groups of tin structures with different chemical bindings, designated as alpha ₂-Sn type and beta -Sn type, simultaneously grown by different electrochemical reductions is investigated by Mossbauer spectroscopy, and partly by X-ray photoelectron spectroscopy. It is shown that during the growth process the tin types become oxidized either to a mixture of tin(II) and tin(IV) oxides, or to tin(IV) oxides, but that the later oxidation in air at room temperature, monitored by periodic Mossbauer studies, is always to tin(IV) oxides. The kinetic results show that the mechanisms of oxidation of the two tin types are almost identical and that, if there is some difference, as suggested by parallel electron diffraction studies, it is beyond the sensitivity of the method. The oxidation of the tin types is a two-stage process, with a very large initial rate of oxidation, which decreases after aging for 1 or 2 months. Numerical equations describing parts of the oxidation curves are derived. The influence of impurities on the oxidation process is discussed.

The second-harmonic generation (SHG) of nominally pure lead phosphate Pb₃(PO₄)₂, was reinvestigated. SHG is confirmed at room temperature albeit with a large spatial variation within each sample. The signal did not disappear when the crystal was heated well above the ferroelastic transition temperature of 453 K. Chemical analysis using X-ray microprobe techniques showed a weak oxygen deficit for all samples. It is suggested that SHG is not necessarily indicative of an intrinsic symmetry breaking process in Pb₃(PO₄)₂ but may be related to structural imperfections.

Absorption spectra of KCl:Bi³⁺ crystals were measured at various temperatures. Six absorption bands were observed at 335, 245, 212, 207, 201 and 196 nm. The first five bands are assigned to the A, B, C₁, C₂ and C₃ bands and the last to the charge-transfer D' band. The emission from KCl:Bi³⁺ excited in the A absorption band was measured as a function of exciting photon energy and temperature. The A-band excitation produced two emission bands peaking at 388 and 430 nm, the excitation spectra of which are not the same. The temperature dependence of the decay times is described for the two emission bands. The polarized emission spectrum and the angular dependence of polarization ratios of the A-band emission were also investigated to determine the symmetry axes of the Bi³⁺-vacancies complex. The results are interpreted in terms of a model in which the reduction effect of the strong spin-orbit interaction on the Jahn-Teller effect coupling to the E_g mode is taken into account An additional perturbation caused by two cation vacancies is required to present the definitive assignment of the A-band emission from KCl:Bi³⁺.

The room temperature reflectivity of the misfit layer compounds (MS)_nTS₂ (T=Nb, Ta) is investigated in the spectral range 1000-20000 cm ^-1. The reflectivity for the E-vector parallel to the (001) planes and the incident light along (001) is dominated by the charge carrier response. This part of the spectra can be described using the Drude formula with the plasma frequency given by omega p=(Ne²/ epsilon ₀ epsilon _infinity m*)^0.5 (N=charge carrier concentration, m*=effective mass, epsilon _infinity =high-frequency dielectric constant). For compounds with M=Sn, Pb the number of charge carriers is equal to that of the parent compound TS₂. The observed anisotropy in the (001) plane is rather small and can be related to small differences in the effective masses (m₍₀₁₀₎^*/m₍₁₀₀₎^* approximately=0.98). This effect is attributed to the orthorhombic distortion of the (TS₂) sublattice. For compounds with M=La, Sm, Tb, electron donation is indicated and the plasma frequency is determined by about 0.5, 0.2, 0.25 holes per T ion, respectively, in a single (TS₂)-related conduction band.