Table of contents

We present a unitary cluster approach to the calculation of several electron and photon spectroscopies, ranging from core and valence level photoelectron diffraction and absorption to electron, Auger and anomalous diffraction. Electron energy loss and Auger–photoelectron coincidence spectroscopies can also be treated in the same frame. This approach is based on multiple-scattering theory with a complex optical potential of the Hedin–Lundqvist type and is valid for all electron kinetic energies. Similarities and differences between these diffraction techniques are examined and cluster size convergence is discussed in connection with the electron mean free path. Applications to selected problems are presented to illustrate the method, both for structural and electronic analysis.

The suppression of the ferromagnetic behaviour of metal–organic chemical vapour deposition grown Ga_1−xMn_xN epilayers by silicon co-doping, and the influence of the Fermi level position on and its correlation with the magnetic and optical properties of Ga_1−xMn_xN are reported. Variation in the position of the Fermi level in the GaN bandgap is achieved by using different Mn concentrations and processing conditions as well as by co-doping with silicon to control the background donor concentration. The effect on Mn incorporation on the formation of defect states and impurity induced energy states within the bandgap of GaN was monitored by means of photoluminescence absorption and emission spectroscopy. A broad absorption detected around 1.5 eV is attributed to the presence of a subband introduced by Mn induced energy states due to temperature independent transition energies and linewidths. The intensity and the linewidth of the absorption band correlate with the Mn concentration. Similarly, the magnitude of the magnetization decreases as the Fermi level approaches the conduction band, as the Fermi energy is increased above the Mn (0/−) acceptor state. Silicon concentrations>10¹⁹ cm⁻³ caused the complete loss of ferromagnetic behaviour in the epilayer. The absorption band at 1.5 eV is also not observed upon silicon co-doping. The observed spectroscopic data favour a double-exchange-like mechanism rather than an itinerant free carrier mechanism for causing the ferromagnetism. This behaviour significantly differs from the properties reported for widely studied (Ga, In)MnAs.

We apply the exact numerical diagonalization technique to study two interacting electrons in a one-dimensional parabolic quantum dot. We consider a modified Coulomb interaction potential between the electrons with a truncation parameter that serves to regularize the behaviour of the bare Coulomb potential in one dimension. We report the dependence of ground and excited state energies for several values of the truncation parameter as the strength of electronic correlations is varied relative to the confinement energy. The similarity of this quantum dot system to the ammonia molecule is pointed out.

By using a microscopic mean field theory we investigate the effects of competing order (against the superconducting one) and inter-layer tunnelling on the transition temperature T_c of cuprate superconductors. We obtain the theoretical phase diagrams of mono- and bi-layer systems. For multi-layer systems we investigate T_c as a function of the layer number N (within a unit cell). It turns out that near optimal doping T_c(N) is non-monotonic and peaks at N = 3, in qualitative agreement with existing experimental data. This arises from the cooperative effects of competing order, charge redistribution and inter-layer tunnelling. However, the dropping of T_c with increasing N in this case is not as significant as in experiments. We also predict that in the very under- and over-doped regimes T_c becomes a monotonic function of N and saturates for . This is because the competing order is either robust against thermal effects near T_c (in the very under-doped regime) or entirely absent (in the very over-doped regime), so that T_c as a function of N is solely enhanced by inter-layer tunnelling.

We show that the antisymmetric planar Hall effect in Fe and Fe₃Si layers grown on GaAs(113)A substrates and the symmetric intrinsic planar Hall effect in Fe₃Si on GaAs(001) are closely related to each other. These effects appear in conjunction with additional contributions to the anomalous Hall effect, which reflect the magnetic field-induced crystalline anisotropy upon atomic ordering of the crystal. In the context of recent theoretical studies based on the Berry phase and the spin chirality, we explain the behaviour of the planar Hall effect with a microscopic model that takes into account dynamic non-coplanar spin configurations. We analyse such non-coplanar spin configurations, which are in accordance with the symmetry of the investigated systems, and find a good correspondence with experimental results.

In the problem of a vortex escaping from a metastable potential, we derive the localization criterion of a vortex at finite dissipation and temperature by analysing both the crossover temperature formula and the escape rate formula. In the absence of a pinning potential in the stable direction, this criterion shows that a vortex will be localized in a metastable potential when the Magnus force is strong enough. Using the concept of this localization criterion, the effective mass of a vortex can be defined and interpreted. Moreover, the role of pinning and dissipation in the process of vortex escape can also be discussed.

The current–voltage (I–V) characteristics of Au/Polypyrrole/p-Si/Al contacts have been measured at temperatures ranging from 70 to 280 K. The I–V characteristics of the device have rectifying behaviour with a potential formed at the interface. The high values of the ideality factor n depending on the sample temperature may be ascribed to a decrease of the exponentially increasing rate in current due to space-charge injection into the PPy thin film at higher forward bias voltage. The experimental reverse bias I–V characteristics of the device followed the Schottky-like conduction model or Poole–Frenkel effect formulae. A linear temperature dependence of the barrier height Φ_b from the reverse bias I–V characteristics was observed, and the Φ_bo value decreased with lowering temperature, ranging from 0.69 eV at 280 K to 0.22 eV at 70 K.

KY₃F₁₀ () is an anion-excess fluorite-related superstructure, which is employed as a room-temperature laser, when doped with rare-earths. Earlier conductivity measurements have revealed two thermally activated processes below and above 600 K. In this work we studied the high-temperature behaviour of KY₃F₁₀ in the temperature range from 293 K up to 800 K at ambient pressure using x-ray powder and single crystal diffraction. No structural phase transition is observed in this temperature range and the structure determinations reveal that the change of structural parameters, interatomic distances, and angles with temperature is very small. A prominent maximum observed in the difference Fourier maps, which corresponds to the centre of an empty F₈ cube, might be one of the interstitial sites involved in the conduction mechanism.

As known, the magnetic properties of AGCMs are determined by the radial, azimuthal and axial internal stresses induced during both the preparation process and the suitable thermal treatments of these microwires. In this paper we have proposed a theoretical model in order to determine the total internal stresses induced during the dc joule-heating thermal treatment (heating–crystallization–cooling). In this view (i) we have started from the internal stress values obtained in the preparation process, (ii) we have considered the supplementary axial tensile stresses due to the mechanical drawing of the microwire during the preparation process and (iii) we have taken into account the difference between the thermal expansion coefficients of metal and glass. We have found that (i) the maximum value of the axial stresses obtained after the thermal treatment is bigger than that obtained in the preparation process, the difference being about 450 MPa, (ii) the maximum values for the azimuthal and radial stresses decrease by and  MPa respectively, (iii) the dimensions of the cylindrical inner core increase significantly (by ), which involves an increase of the degree of magnetic order in the AGCMs and consequently leads to the appearance of a large Barkhausen effect (LBE) in low axially applied magnetic fields and (iv) the reduced remanence increases from 0.90 to 0.95.

Recent ab initio calculations have shown that the theory of the spin density wave (SDW) based on the nesting property of the chromium Fermi surface is not able to explain solely the stability of the incommensurate SDW. Using the model Hamiltonian approach, we develop a method which allows self-consistent calculation of SDWs with different wavelengths. The magnetic profile and the energy of the system agree well with ab initio results. To check the stability of the SDW we introduce a local external magnetic field which induces transitions towards SDW or antiferromagnetic (AF) states. A very small energy which is enough to create the nodes of the SDW determines the new mechanism of SDW formation without supposition about the nesting property of the Fermi surface. The individual nodes of the SDW are found to be quite stable. They can be considered as low-energy excitations, which do not interact at large distance and annihilate when they approach close to one another. About 10% of non-magnetic Cr atoms randomly distributed in a few (100) layers leads to the formation of the SDW state with the nodal plane inside these layers. A similar trend arises when non-magnetic V impurities are introduced in bulk Cr. The increase of SDW wavelength with temperature and the transition from an SDW to a layered AF state when magnetic impurities (Fe, Mn) are introduced into bulk Cr are discussed on the basis of the developed theory.

The cotunnelling is systematically studied in comparison to sequential tunnelling in Coulomb blockade metallic double quantum dot structures, using the standard master equation approach. In the case of zero gate voltage, we are able to derive analytical expressions of threshold voltages for both tunnelling processes: V_th^(s) for the sequential tunnelling and V_th⁽²⁾ for the lowest-order inelastic macroscopic quantum tunnelling (cotunnelling). Taking into account the gate and temperature effects, numerical solutions of the master equation show that an increase of the inter-dot capacitance leads to a decrease of the ratio V_th⁽²⁾/V_th^(s) and at the same time to a decrease of cotunnelling conductance compared to the sequential one. An oscillation of cotunnelling conductance is observed in classical Coulomb blocked regions. In comparison with the sequential tunnelling conductance spectroscopy, the peak height of cotunnelling conductance is about three orders of magnitude smaller and the peak spacing distribution is far from regular. Increase of temperature raises the current and destroys the Coulomb gap. Its relative influence is more important at lower bias.

The electrical and optical properties of n-type CdGeAs₂ crystals doped with indium have been studied at room temperature using van der Pauw Hall measurements, infrared absorption, and infrared reflectance. Free-electron concentrations (n) in the sample set range from 2.3 × 10¹⁷ to 4.3 × 10¹⁸ cm⁻³ at 300 K. For the higher levels of free carriers, conduction-band non-parabolicity causes significant increases in electron effective mass m^*. Infrared absorption due to free carriers is consistent with ionized impurity scattering, and absorption coefficients α_f for different samples agree with theoretical predictions when the dependence of m^* on n is included. A relation between α_f and n has been determined for CdGeAs₂:In that can be applied to large-sized samples used in nonlinear optical applications. The optical effective mass m_opt^* obtained from reflectance data is also in good agreement with predicted m^* values for CdGeAs₂.