Table of contents

We have extended the single-impurity cluster model to include the effects of charge transfer to the conduction band for the interpretation of the core level photoemission spectra of highly correlated, small-gap semiconducting and metallic transition metal compounds. The model includes the charge fluctuations between the local transition metal site and the conduction band and has been successfully applied to reinterpret the spectra of some 3d transition metal chalcogenides, reproducing well the large asymmetry of the main peaks, as well as the other satellite structures.

The chemical and magnetic structures of Ho/Pr superlattices have been studied by a combination of X-ray and neutron scattering techniques. While the Ho blocks are HCP, the Pr blocks retain their bulk-like DHCP structure, but with a stacking of the hexagonal atomic planes that is not coherent between successive blocks. Below a Neel temperature of approximately 110 K, the Ho magnetic moments order into a basal plane spiral with a magnetic coherence length that is limited to the width of a single Ho block, even for a sample comprising of only six atomic planes of Pr. These observations are discussed in terms of the differing band structures of the two elements.

The spatially inhomogeneous order parameter considered in the Landau description of incommensurate phases is analysed from a structural viewpoint using the superspace approach. All modulation functions, including that of the order parameter, are defined along the internal space. Taking a displacive case for concreteness and within the constant-amplitude approximation a general differential equation for the order parameter phase, theta , which generalizes the well known sine-Gordon equation, is derived. No reference is made to any particular expression for the free-energy expansion, only consistency arguments between the Landau description and the structural properties of an incommensurate phase are used. It is shown that, under certain quite common conditions, this general equation can be approximately reduced to the sine-Gordon equation. The sine-Gordon equation can, therefore, be considered of rather general validity when modelling the structure of incommensurate phases irrespective of their particular thermodynamic potential. The introduction of the theta modulation along the internal space further simplifies the equation; its parameters become system independent, being a function of only the soliton density.

We present a formulation of ab initio molecular dynamics with a mixed basis. The mixed basis is constructed by a plane-wave set and localized basis functions which are defined by spatially localized functions. The basis functions are orthonormalized with each other, and the equations of motion for wavefunctions of electrons are simplified. Forces acting on atoms include the Pulay terms, which are explicitly represented in the present formulation. This method is tested on diamond and applied iron as a first step in studying the effect of spin polarization.

Matrix elements of the propagator in a spherical wave representation are the key quantities in most scattering problems. Here, we review the existing methods to compute them and propose a new one-a set of recurrence relations-which is faster than the currently used ways of calculating them at high energy.

The topological excitations (kinks) in a one-dimensional one-third-filled band charge density wave system (i.e. a pi electron system) and their electronic spectra are studied. The geometry is obtained by finding the self-consistency between bond lengths and pi electron densities in the middle of each bond. The pi electron density distributions are evaluated with the step potential model. It is shown that the ground state of the perfectly trimerized system has a long-short-long bond sequence as expected from the anti-integrable limit and in contrast to the Su-Schrieffer approach. Nevertheless the kinks obtained are fractionally charged. Type 1 kinks have the (spin, charge) relation (¹/₂, -e/3) and (0, +2e/3), type II kinks (¹/₂, +e/3) and (0, -2e/3).

Results are presented of ab initio periodic unrestricted Hartree-Fock calculations which show the density of states for M_xMg_1-xO(M=Mn, Ni) to be essentially independent of dopant (M) concentration x. The small variations of the projected densities of states with dopant concentration are of the same magnitude as those resulting from changes in magnetic order. This is consistent with the marked similarity of the low-energy optical spectra of NiO, Ni_xMg_1-xO and aqueous solutions of Ni(II) salts, which suggests 'independent ion' behaviour in these systems. It is thus argued that the unrestricted Hartree-Fock approach correctly reproduces the qualitative features of the ground state of magnetic insulators such as MnO and NiO which result from strong on-site Coulomb interactions between essentially localized electrons.

The energy level scheme of Sm²⁺ in CaFCl is determined in detail from the fluorescence spectra of the ⁵D_J to ⁷F_J transitions at ambient pressure and under pressures up to 8 GPa. The crystal-field (CF) parameters B_q^k are derived directly from CF energy levels and their variations under pressure. The comparison of the CF interactions for Sm²⁺ in different MFCl (M=Ba, Sr, and Ca) host crystals shows very similar behaviour under pressure. With the superposition model (SM), the local structure around the Sm²⁺ ions in the CaFCl host is quantitatively determined and the four local structural parameters for the local coordination and the atom positions as well as their pressure dependences are derived. It is thus shown that the present method results in a valuable tool for the investigation of local distortions around optically active impurities.

The breakdown of conductance quantization in a quantum point contact in the presence of a random long-range impurity potential is discussed. It is shown that in the linear response regime a decisive role is played by the indirect backscattering mechanism via quasilocalized states at the Fermi level, this can provide a much higher backscattering rate than any direct backscattering process. For realistic contact lengths (<or=2000 nm) the scattering processes prove to be independent, in spite of the coherence of the electron wave. The distribution function of conductance fluctuations is obtained by direct numerical calculations as well as being estimated within an analytical model for the first time. It is shown to be a generalized Poisson distribution. Estimates are made for quantum point contact performance at different choices of parameters. In particular, it is better the larger the intermode distance is compared to the amplitude of the random impurity potential.

In the present paper the results of studies of the deviations from Matthiessen's rule (DMRs) in single-crystal Al-Y alloys in magnetic fields up to 8 T are represented. The results obtained bear witness to the anisotropic behaviour of the mechanisms of conduction electron scattering from photons and impurity atoms. In the analysis of experimental data, a correlation was revealed between the behaviour of the temperature dependences of the DMRs and the direction of the magnetic field relative to the crystallographic orientation of the specimen, and an explanation of the negative value of the DMR observed in the experiment is given.

We propose a phenomenological picture for the mechanism of the metamagnetic transition often observed in intermetallic alloy compounds near the paramagnetic side of the ferromagnetic instabilities such as YCo₂ with Al doping. The present treatment assumes the magnetization process is described in terms of the spin fluctuation spectrum, in clear contrast with the previous approaches based on the Hartree-Fock approximation assuming the fine structure of the density of states curve around the Fermi energy.

Resistance and reflection phase distributions are obtained from the exact asymptotic solution of the invariant embedding equations for arbitrary disorder for samples of length L<<(2k₀)^-1<L_c (with k₀ the incident electron wavenumber and L_c the localization length). The resistance moments differ quantitatively from earlier results for the range (2k₀)^-1<<L<<L_c and from results obtained by assuming uniformly distributed phases. The phase distribution reduces, for weak disorder, to a binary form with possible phases pi /2 and 3 pi /2. The conductance moments diverge more strongly than the results based on uniformly random phases in 1D.

This paper presents novel results on the electrical properties of amorphous carbon films prepared by ion-beam sputtering. The conductivity is found to be similar to that seen in evaporated carbon. Over the range approximately 40-300 K, it is well described by the variable range hopping equation with a hopping exponent of 1/2. This indicates that the density of localized states around the Fermi energy is parabolic. An analysis of the conductivity data in terms of variable range hopping in a Coulomb gap is quantitatively consistent, indicating that this could be the origin of the parabolic shape. A second possibility is that the parabolic gap is a remnant of the graphitic density of states, which is very small at the Fermi level. Field effect measurements on the carbon films were not able to distinguish between the two models, as both could be reconciled with the observed results, although in the case of the 'graphitic' gap a large density of surface traps is required to obtain a consistent interpretation.

We present a quantitative analysis of the temperature dependence of the quasi-two-dimensional electron concentration in Al_xGa_1-xAs/GaAs heterostructures taking into account the fact that in the bulk Si-doped Al_xGa_1-xAs two types of donor coexist, i.e. deep and shallow, which independently, and by different mechanisms, provide electrons to the bulk Al_xGa_1-xAs different conduction band minima and to the quasi-two-dimensional electron gas (Q2DEG). We calculate the electronic states, the ionized-donor concentrations, the Q2DEG and the bulk-electron concentrations and the corresponding mobilities as a function of temperature. Our numerical results are in very good agreement with the experimental data.

The electron paramagnetic resonance spectrum of Mn⁴⁺ in single-crystal lead titanate has been studied at 9.8 and 33.8 GHz. An axial sextet with g_{perpendicular to} ^eff=3.7742(1) to g/sub ///^eff approximately=2.00 was measured at 9.8 GHz and with g_{perpendicular to} ^eff=2.7048(15) at 33.8 GHz; a second sextet was observed at high field at 9.8 GHz. The spectr um was fitted using spin .Hamiltonian parameters S=³/₂, I=⁵/₂, g/sub ///=1.99, g_{perpendicular to} =1.987, A/sub ///=79.46*10^-4 cm^-1. A_{perpendicular to} =71.05*10^-4 cm^-1 and mod D mod =0.3166 cm^-1. The electronic spin value and the magnitude of the hyperfine constant, mod A mod =73.9 cm^-1, confirm the Mn⁴⁺ assignment. A superposition model analysis using available b₂(R) functions for 3d³ configuration ions in oxide hosts was unable to reproduce the observed axial crystal field magnitude, mod D mod .

An effective-medium theory is developed for a composite medium consisting of nonlinear coated spherical grains embedded in a linear host. By means of a field-dependent T-matrix approach, exact expressions for the effective permittivity and nonlinear susceptibilities of the composite medium with coated spherical grains is obtained.

Magnetic properties have been investigated for amorphous Er_xCu_100-x (x=25, 30, 40, 50, 60 and 70) alloys prepared by high-rate DC sputtering. The paramagnetic Curie temperature theta _p of these alloys is positive and it means that ferromagnetic exchange interactions are predominant. However, a steep decrease of theta _p occurs below about x=50, suggesting a development of some antiferromagnetic exchange interaction. These alloys exhibit spin glass-like behaviour, and the spin freezing temperature T_f decreases with decreasing Er content, but an abrupt change occurs around x=40 approximately 50 in analogy with that of theta _p. The spin freezing is dominantly caused by the RMA in high Er concentration ranges above about x=50. On the other hand, not only the RMA but the competition of ferromagnetic and antiferromagnetic interactions due to the exchange fluctuations contribute to the magnetic properties in low Er concentration ranges below about x=50. Consequently, RMA becomes weaker, whereas the competition of the exchange interaction becomes stronger with decreasing Er content. The concentration dependence of magnetization per Er atom is also explained by considering both the random magnetic anisotropy and the competition of the exchange interaction.

Measurements of the decay kinetics of regular Ce³⁺ emissions in CeF₃ single crystals have been performed under photo-excitation and gamma (²²Na radioisotope) excitation. While flashlamp-excited photoluminescence decay is distorted mainly by the influence of the 'near-surface' traps, the multiexponential nature of the scintillation decay is explained using pair interaction of close-lying Ce³⁺ excited ions, which account for the discrete character of their distances. Measurements of the same photoluminescence decay under high-intensity laser excitation show a similar departure from exponentiality as in the case of scintillation decay and support the proposed model.