Table of contents

The quantum Hall effect (QHE) is shown to be a natural property in high magnetic fields of any system possessing both quasi-two-dimensional and quasi-one-dimensional Fermi-surface components. Such Fermi surfaces are known to occur in some organic charge-transfer salts (e.g. with M=K, Tl, ). Whilst the QHE in two-dimensional semiconductor systems such as heterostructures and MOSFETs relies on the existence of localized states at the edges of the Landau levels, in organic metals the quasi-one-dimensional portion of the Fermi surface is shown to provide the necessary reservoir for pinning the chemical potential between the completely filled and empty Landau levels of the quasi-two-dimensional Fermi-surface section.

Photon-stimulated tunnelling (PST) of electrons at Si - and SiC - interfaces is shown to be insensitive to the density and energy distribution of electrons in the semiconductors. Instead, the observed relation of PST to the type of the oxide layer indicates a defect level in the oxide as the initial state of electrons contributing to PST. The defect's energy level was found to be positioned 2.8 eV below the conduction band of , the defect density increasing with silicon enrichment of the . The observed correlation between PST yield and dark conductance of the oxide suggests the isolated defect states to be involved in the high-field electron transport of layers.

The relationship between the open volume defects and the deposition conditions of superconducting thin-film was studied by the position lifetime technique. Using a low-energy pulsed positron system, positron lifetime as a function of implantation energy was measured on epitaxial superconducting thin-film deposited on yttrium stabilized cubic zirconia substrates (YSZ) with pulsed laser deposition in a partial pressure of air under different conditions. The results show that the type of open volume defect is independent of deposition conditions such as the substrate temperature, , and the air pressure, . The defect concentration increases with decreasing and increasing .

We present a review of the basic theoretical ideas concerning unconventional superconductors. These are superconductors with an order parameter which breaks the rotational symmetry of the normal-state crystal; in some cases, time-reversal symmetry is broken as well. The resulting novel broken-symmetry state should display many unusual properties, which can be analysed at a variety of theoretical levels. We pay particular attention to the Ginzburg - Landau type of theory, and to the more microscopic theory of a superfluid Fermi liquid.

The temperature dependence of the diffusion coefficient of particles is studied on lattices with disorder. A model is investigated with both trap and barrier disorder that was introduced earlier by Y Limoge and J L Bocquet (1990 Phys. Rev. Lett. 65 60) to explain an Arrhenian temperature dependence of the diffusion coefficient in amorphous substances. We have used a generalized effective-medium approximation (EMA) by introducing weighted transition rates as inferred from an exact expression for the diffusion coefficient in one-dimensional disordered chains. Monte Carlo simulations were made to check the validity of the approximations. Approximate Arrhenian behaviour can be achieved in finite temperature intervals in three- and higher-dimensional lattices by adjusting the relative strengths of the barrier and trap disorder. Exact Arrhenian behaviour of the diffusion coefficient can only be obtained in infinite dimensions.

Heat capacities C(T) of C15-type have been measured in the temperature range 5.3 - 293 K. For all H- (D-) doped samples the low-temperature linear C(T) term is found to contain considerable contributions of non-electronic origin. This feature is consistent with the existence of a glass-like low-energy excitation spectrum. The excess heat capacity strongly changes with hydrogen concentration, being nearly isotope independent. In the range 15 - 50 K marked deviations from the Debye behaviour of C(T) are observed for all H- (D-) doped samples. In addition, for the samples with the heat capacity shows a peak near 60 K which can be attributed to H (D) ordering.

X-ray diffraction, thermoconductivity and nuclear magnetic resonance experiments have been carried out in order to study the diffusion mechanism of N atoms in the lattice. These experiments reveal that a small number of N atoms which enter the lattice, but do not occupy the octahedral interstitial sites, are mobile at the nitrogenation temperature, while most of the N atoms, which enter the octahedral sites, are immobilized. Based on the characteristics of nitrogenation observed in (were R is the rare-earth element) systems, a tripping diffusion model has been proposed and discussed in detail. The previously proposed free diffusion model is the high-temperature approximation of the trapping diffusion model. The experiments and theoretical analysis show that it is the N - lattice interaction, instead of N - N interaction, that leads to the formation of the observed nitrided - unnitrided configuration, causes a tremendous decrease of the apparent diffusion frequency factor and is responsible for the N uptake stability and irreversibility. Also, this work shows how the N - lattice interaction affects the N uptake in various nitrogenation conditions.

The excitonic reflectivity spectra and the wavelength derivative reflection spectra have been investigated. The n = 1, n = 2 and n = 3 states of the A, B and C exciton series are determined. The exciton binding energies and exact values of the band gaps of all three intervals - , - and - have been calculated. Data on the splitting due to the crystal-field and the spin - orbit interaction are determined as well.

The inverse microscopic dielectric matrix of crystalline silicon is evaluated within the time-dependent density-functional theory using one-electron energies and wave functions obtained by all-electron modified augmented-plane-wave band-structure calculations. Local fields are taken into account by inverting the dielectric matrix. The dependency of the exchange and correlation on the frequency and momentum is described using different approximations. We find that the polarization of the 2s and 2p core electrons changes the inverse dielectric function by up to 8%. In the long-wavelength limit our results for the region below 5 eV deviate from optical experiments by an almost constant energy shift, which is the same size as the band-gap error in local density approximation. Our results correspond with inelastic x-ray scattering spectroscopy measurements.

With the use of a variational method to solve the effective-mass equation, we have studied the electronic and hydrogenic impurity states in a corner under an applied electric field. The electron energy levels and the impurity binding energies are calculated. Our results show that, with the increasing strength of the electric field, the electron energy levels increase, and the impurity binding energy in the ground state increases at first, to a peak value, then decreases to a value which is determined by the impurity position in the corner. The dependence of the impurity binding energy on the applied electric field and impurity position is discussed in detail.

Introducing F into the lattices of the copper oxides and is accomplished via a low-temperature fluorination reaction using as the fluorinating reagent. The oxyfluorides retain the structures of their precursors, but striking lattice expansions are observed. No trace of was detected in the fluorinated products. F doping was successfully used to induce superconductivity in the oxyfluoride with a reduced plane and in the presence of apical anions, but failed to optimize the carrier density and induce superconductivity in and .

Samples of the system with different oxygen contents have been obtained by annealing the as-grown samples at different temperatures and oxygen partial pressures. The results of resistivity and thermoelectric power (TEP) measurements of these samples are reported here. Some anomalous phenomena in transport for the underdoped sample of were observed, i.e., below a characteristic temperature, the TEP exhibits a significant enhancement and the temperature dependence of the resistivity deviates downward from linearity. These results imply that a spin gap exists in the La doped Bi-2201 system. By comparison with the results reported previously, we suggest that the microstructural characteristics are another important factor in determining the spin excitation spectrum.

The non-resonant magnetically modulated microwave response measurements of powdered Bi - Sr - Ca - Cu - O samples using the conventional EPR spectrometer are presented. After cooling in a near zero magnetic field, all samples exhibited a sharp (about ) microwave absorption with applied magnetic field, superimposed on the widely observed and well explained broader minimum. The width of the absorption maximum is found to be dependent on the particle size. It becomes broader with decreasing particle size. The effects of particle size and field history on the peak are given in detail and some possible mechanisms to account for the observations are presented.

We use a `first-principles' concentration-wave approach based on a finite-temperature, electronic density-functional, mean-field, grand potential of the random alloy to investigate the atomic short-range order (ASRO) in some FeV and FeAl solid solutions in both ferromagnetic and paramagnetic phases. Thermally induced spin fluctuations are modelled in terms of local moments on the Fe sites. This picture produces satisfactory estimates of all of the alloys' Curie temperatures, . We compare our calculations with the ASRO deduced from neutron and x-ray diffuse scattering measurements on single crystals carried out either in situ at or quenched from temperatures above and below . Our calculations describe the measured ASRO well. Both alloy systems exhibit B2-type ordering correlations in their paramagnetic states which strengthen in the case of FeV and weaken for FeAl as the temperature is lowered into the ferromagnetic state. We extract electronic mechanisms for these effects. The ASROs of the ferromagnetic alloys also show intensity around (1/2, 1/2, 1/2) which is traced to a Fermi surface feature and may be a precursor to the ordering in FeAl. Finally, suggestions for further polarized neutron, in situ measurements are made.

We propose a self-consistent approximate solution of the s - f model for describing the exchange coupling of a local moment system with a partially filled energy band. Induced electronic correlations account for the characteristic quasiparticle band effects which become manifest via striking temperature dependencies, band deformations and splittings. For weak s - f exchange interactions a `Stoner-like' spin splitting of the conduction band proportional to the f magnetization occurs. As soon as the coupling exceeds a critical value an additional spin splitting of the quasiparticle dispersion sets in, which is due to different elementary excitations. One of these appears as a repeated emission and reabsorption of a magnon by the conduction electron, resulting in an effective electron - magnon attraction. This gives rise to a polaron-like quasiparticle (a `magnetic polaron'). Other elementary processes are connected to magnon emission or absorption by the conduction electron (`scattering states'). The polarization of the conduction band due to the s - f exchange interaction J feeds back to the localized spin system leading to an indirect coupling between the spins. For weak s - f coupling the RKKY mechanism dominates , but with remarkable deviations for intermediate and strong couplings. The Curie temperature saturates with increasing J, where the saturation value is strongly dependent on the band occupation n. The oscillating behaviour of the effective exchange integral connecting the localized spins restricts ferromagnetism to special regions for n. The magnetization curve, the spin polarization of the itinerant electrons, and f - f as well as s - f spin correlation functions are worked out for a simple cubic lattice and discussed in terms of the band occupation n and the s - f exchange coupling J.

Magnetization and ESR measurements have been performed on Si-doped (y = 0.01 and 0.02) single crystals. The y = 0.01 sample shows a spin-Peierls transition at as well as undoped material and additionally show an antiferromagnetic state below . When an external field is applied to the c-axis below , two magnetization jumps due to the spin-flop and the transition from dimerized spin-Peierls phase to magnetic phase are observed at 0.98 T and 11.5 T, respectively, which means the coexistence of spin-Peierls and antiferromagnetic states. On the other hand, the y = 0.02 sample shows only an antiferromagnetic transition at .

The novel compound has been successfully synthesized. Its x-ray pattern can be indexed with a monoclinic symmetry and the space group. Thermomagnetic analysis gives a magnetic ordering temperature of 400 K. The saturation magnetization of at 4.2 K and room temperature are and and the anisotropy fields at 4.2 K and room temperature are 5.2 T and 1.7 T, respectively. A systematic analysis of the magnetic properties of novel compounds (R = Y, Ce, Pr, Nd, Sm, Gd, or Tb) at 4.2 K has been performed on the basis of their saturation magnetization. This suggests that the saturation magnetization of compounds with a low stabilizing element concentration can be calculated roughly from a combination of those of the rhombohedral or hexagonal and tetragonal units in a ratio of 1:1.

We have used resistivity measurements to study the magnetic phase diagram of the itinerant antiferromagnet in the temperature range from 0.3 - 300 K in magnetic fields up to 16 T. In contrast to theoretical predictions, the incommensurate spin-density-wave phase is found to be stable at least up to 16 T, with an estimated critical field of . We have also studied the low-temperature magnetoresistance in the [100], [110], and [001] directions. The transverse magnetoresistance is well described by a power law for magnetic fields above 1 T with no saturation observed at high fields. We discuss our results in terms of the magnetic structure and the calculated electronic band-structure of . We have also observed, for the first time in this compound, Shubnikov - de Haas oscillations in the transverse magnetoresistance with a frequency of for a magnetic field along [001].

The spin dynamics of the hexagonal -type quasi-one-dimensional antiferromagnet is investigated by means of an inelastic neutron scattering technique. In good qualitative agreement with a recent spin-wave calculation including higher-order terms, a large scattering cross-section arising from two-magnon excitations is observed at the one-dimensional antiferromagnetic zone centre. In addition, we measured spin-wave excitations between the chains precisely and revealed that the spin-wave dispersion curves are modified in energy and in intensity on account of the anticrossing between the one-magnon branches and two-magnon continuum. These results demonstrate that anharmonic terms are important in the spin dynamics of even at low temperatures. We also measured the temperature dependence of the magnetic excitations and found that far above the Néel temperature the two-magnon process gives a considerable contribution to the inelastic spectrum.

The Raman scattering, vibrational, electron spin-resonance, electron and luminescence spectra of the ion in a sodium bromide crystal host are reported. The results indicate that the complex ion is trapped in two nonequivalent symmetries, which have been identified and characterized by the parameters and D of the spin Hamiltonian and and B of the crystal field. The IR and Raman spectra are discussed on the basis of possible symmetries of the trapped ion. Following vibrational analysis the vibronic assignment of the phosphorescence spectrum is proposed. The role and influence of the charge-compensational configurations as regards the spectroscopic properties of the chromium (III) ion are analysed.

Phases in ball milled for different lengths of time have been investigated by means of x-ray diffraction, Mössbauer spectroscopy and Raman scattering spectroscopy. It was found that the high-temperature phase can be formed by ball milling at room temperature. In the milling procedure the compounds FeSi and were also formed.

Due to a printing error, the wrong figure 2 appeared in this paper. The correct figure is reproduced below. The World Wide Web version of the paper contains the correct figure 2.

Figure 2. A schematic representation of the ideal structure of a rare earth superlattice. The epitaxial relationships in the growth direction are: (110) A1₂O₃ || (110) Nb || (001) Rare earth.

Due to a printing error, the wrong figure 2 appeared in this paper. The correct figure is reproduced below. The World Wide Web version of the paper contains the correct figure 2.

Figure 2. An STM image obtained after irradiation with laser pulses of a chlorine-saturated Si(111) surface [14]. The photon energy is 4.7 eV and the sample bias voltage is +3.0 V. A white rhombic indicator shows a unit cell surrounded by corner holes (`c') and dimers. Bright spherical protrusions (`p') on an atomic scale are 7 × 7 ordered as the Si rest atoms of the DAS model. A number of depressions (`d') are found on the surface especially around corner holes and dimers.

In addition, referring to figure 1, the third sentence of the second paragraph of the Introduction should read:

A unit cell in the DAS model includes 12 adatoms (large solid circles), 42 rest atoms (small solid circles), 9 dimers, and 1 corner hole.