Table of contents

The effect of strong magnetic fields parallel to the GaAs/Ga_xAl_1-xAs interface on the subband structure of a 2D electron layer is investigated theoretically. A system with two levels occupied in zero magnetic field is considered and the magnetic-field-induced depletion of the second subband is studied. The confining potential and the electron energy dispersion relations are calculated self-consistently. The electron-electron interaction is taken into account in the Hartree approximation.

The thermogravitational separation for each of the binary liquid systems nitrobenzene+n-hexane and isobutyric acid+water as a function of composition was measured at temperatures above and near to their critical conditions. The first system, in which the component densities are very different, presents large separations near the critical composition. In contrast, in the second one, with similar densities of the pure components, no separation is observed. The extension to high Soret numbers of the authors' previously developed thermogravitational theory, which takes into account the effect of the buoyancy-driven convection induced by the density composition dependence allows them to obtain values of thermal diffusion factors of binary liquid mixtures in the vicinity of consolute points from separation measurements.

The long-time self-diffusion coefficient of a colloidal fluid interacting via a Yukawa potential and obeying Brownian dynamics is calculated by computer simulations for different parameters of the Yukawa potential over a broad range of densities. As two extreme cases, the hard sphere and the unscreened Coulomb interaction are also included. The simulation results are compared with different theoretical expressions. In general, it is found that a kinetic-like theory that incorporates both the exact short-time behaviour of the friction kernel and the two-particle dynamics describes the simulational data best. Also, simulations and theories for the self-diffusion in two-dimensional Brownian Yukawa fluids are compared, where the same qualitative behaviour is found.

The electro-phonon resonance effect is a consequence of a resonant interaction between two electric subbands mediated by an optical phonon. It occurs in a quasi-two-dimensional electron system each time the energy difference between two electric subbands equals the energy of a LO phonon. The authors study the influence of this effect on the electron mobility by using the momentum balance equation. The temperature and electron density dependences of the resonances are studied in the linear and non-linear response regimes.

The author proposes an improved quadratic simplex method for evaluating surface and volume integrals in the Brillouin zone. The scheme that is commonly used for deriving a piecewise quadratic interpolation of the dispersion misweights the gridpoints with respect to their symmetry-induced multiplicity. He suggests subdividing the simplices into smaller ones for which different interpolating functions are obtained. In addition, he derives analytical expressions for both the surface and volume integral in two dimensions which properly take into account the integrable Van Hove singularities. As an example, the author discusses the self-consistent calculation of the hole subband structure in a GaAs-AlGaAs heterojunction.

X-ray absorption spectroscopy has been applied to the study of local structural changes in amorphous tungsten trioxide thin films under colouration in an acid aqueous solution. It has been found that the formation of W⁵⁺ coloured centres is accompanied by the appearance of local deformations around them. It is shown that this effect can be associated with the formation of small-radius polarons.

The spin-extended Hartree-Fock approximation with localized overlapping orbitals is applied to the low-density electron gas problem. A procedure is outlined which in the case of small overlap makes the energy per electron convergent when the number of electrons tends to infinity. As a first approximation the perfect pairing case is considered. It is shown that the singlet ground state energy lies in between the energies of ferromagnetic and antiferromagnetic states for any perfect pairing structure in second order in the overlap approximation.

Investigations were carried out on low-temperature conductivity in the defect band of germanium formed due to the presence of large concentrations of extended defects introduced during high-temperature plastic deformation or irradiation with large integrated fluxes of high-energy neutrons and protons. It has been demonstrated that at relatively low concentrations of dislocations and low temperatures the electrical conductivity in the defect band occurs by hopping. At higher concentrations of dislocations the metal-insulator transition (MIT) takes place in the system of dislocations, the conductivity near the MIT showing a power-law variation, sigma (T) approximately T^x, with x<or=1 in the range 30 to 3*10^-2 K. It has also been shown that the dislocation conductivity has a quasi-two-dimensional character. The dynamics of the Coulomb gap variations was studied near the MIT in Ge with a high concentration of radiation defects. The experimental data presented show that Ge with a high concentration of dislocations can be used for simulating some physical phenomena in amorphous and vitreous semiconductors.

The complex AC conductance of carbon-wax mixtures was measured as a function of angular frequency omega from 7 Hz to 100 kHz for different concentrations p of carbon near the percolation threshold p_c. The scaling exponents x=0.72+or-0.01, y=0.23+or-0.05 were found to be remarkably close to the theoretical values expected from the intercluster polarization model. The characteristic frequency omega _c at which the real part of the AC conductance deviates from its DC value scales with the DC conductance sigma _DC as omega _c approximately sigma _DC^z, where z approximately=1.1. The nature of the variation of loss tangent tan delta with frequency was studied in detail. It was found that tan delta went through a minimum at a frequency which increased with p. The increase of tan delta at higher frequencies cannot be explained by the usual RC model. The authors consider a modified version, the R-RC model in which a capacitor is always in series combination with a resistor, i.e. capacitors are lossy. This model is shown to reproduce the observed behaviour of tan delta and believed to be more realistic than the RC model hitherto considered in that series resistors may be a more effective representation of the dangling bonds in the system.

The authors' develop a general formulation of the problem of elastic transport between two semi-infinite systems, connected by a system of finite size, and derive expressions for the current in and the differential conductance of such a circuit in the limit of zero interactions between the carriers. These expressions are exact in the applied voltage, the coupling of the components of the circuit, and the temperature of the circuit. They then apply their results in a tight-binding approximation to three specific cases: the one-atom contact, the finite, disordered one-dimensional chain, and the generalized stacking fault.

Compounds in the series Sm₂Fe_17-xAl_x have been investigated. Samples with x=0, 1, 2, 3 and 4 have been shown to be single phased with the Th₂Zn₁₇ structure. The Curie temperature is found to increase from T_C=391K for the x=0 sample to T_C=471 K for the x=3 sample. Further substitution of Al for Fe decreased T_C. X-ray diffraction studies of magnetic-field-oriented powders showed that alloys with x<2 exhibit easy plane anisotropy while samples with x>2 exhibit easy axis anisotropy. The x=2 sample showed a mixture of easy plane and easy axis behaviour. This observation represents the first case of clear evidence for room temperature uniaxial anisotropy in the R₂Fe₁₇ compounds without the addition of interstitial nitrogen or carbon.

The compound Er₂Fe₁₇C_2.5 formed by melt spinning has the rhombohedral Th₂Zn₁₇-type structure. The spin reorientation transition occurs at about 136 K. The ⁵⁷Fe Mossbauer effect was measured in the temperature range 12-300 K. From the acceptable fit the authors' find that the effective hyperfine fields of various sites decrease in the order 6c>18f>18h>9d. The anisotropy of the effective hyperfine field is observed to be different for different sites. The temperature dependence of the quadrupole splitting shows a large increase at around T_sr with increasing temperature.

The OH infrared absorption spectra of heavily Mg-doped LiNbO₃ codoped with a trivalent or a divalent cation impurity have been investigated, and some correlated with ESR measurements. The authors examine how the threshold effect in Mg doping determines the incorporation sites of the codopant and attempt to derive a general relationship between impurity lattice sites and the OH spectrum. A main absorption band that peaked at 3536 cm^-1 appeared in all the samples and is attributed to OH stretching vibration in the Mg_Li-OH-Mg_Nb complex. Those codoped with M³⁺ (M identical to Fe,Cr,Nd) exhibited additional OH bands within the 3500-3525 cm^-1 range, the actual peak position depending on the charge misfit and the ionic radius of the impurity cation. They are attributed to Mg_Li-OH-Me_Nb³⁺ complexes. The authors did not find any additional band for M³⁺ identical to Er³⁺, Ho³⁺, Tm³⁺. A simple scheme is proposed that depicts whether such a new feature should appear, depending on the ionic radius and the electronegativity of the trivalent ion. They observed no change in either the infrared or the ESR spectrum in the case of a Tr²⁺ codopant (Tr identical to Co, Ni or Cu), implying that its lattice site is unaffected by Mg doping.

The internal-friction (IF) technique is applied to study high-T_c superconducting Bi-Sr-Ca-Cu-O (BSCCO) system cuprates. IF measurements at five different frequencies from 10^-2 to 5 Hz in a temperature range from room temperature to 330 degrees C are carried out. Resonance-type behaviour of IF at a very low frequency (10^-2 Hz) appears once the sample is slightly deformed ( in approximately=10^-4). The IF characteristic of the high-frequency side of the IF-frequency resonance peak is thus determined. It is suggested that the resonance originates from two-dimensional defect vibration, and most probably the domain wall motion caused by dislocation translation.