Table of contents

The results of X-ray diffraction measurements of a lead magnoniobate single crystal-PbMg₁3/Nb₂3/O₃, performed on a four-circle diffractometer with Mo K alpha radiation, are described. It is confirmed that even at relatively high temperatures lead ions are shifted from the high-symmetry position. It is shown that these shifts are random both in length and direction. A spherical layer model is proposed for the adequate description of the experimental data. The correspondence between the crystal structure and the physical properties of the crystal is discussed.

The conductivity of poly(3-dodecylthiophene), which decreases after a heating-cooling cycle, again increases to the original value in a period of several days, while ESR and the optical properties showed no marked relaxation phenomenon. The crystal structure of poly(3-dodecylthiophene) studied by X-ray diffraction measurements also showed marked changes in its lattice parameters a and b in the same period after the heat treatment. Relaxation to the original smaller b-value which corresponds to twice the distance between the neighbouring two conjugation planes was observed after the heat treatment. This is regarded as the origin of the relaxation in the conductivity. Thus, the increase in conductivity during the relaxation is interpreted in terms of the increase in the probability of inter-chain hopping of charge carriers and therefore carrier mobility with decreasing lattice parameter b. Conformational relaxation from gauche type to trans type was also observed in an FTIR analysis of the side alkyl chains.

We simulated the diffusion-limited aggregation (DLA) on a pentagonal lattice and on an octagonal lattice; the fractal dimensions of the DLA on these two lattices are found to be 1.53 and 1.57 respectively, exhibiting a weak lattice dependence. We also calculate the vibrational densities of states for the two DLA clusters; the fracton dimensions of the two DLA clusters appear to follow d_s=2d_f/(d_f+1).

The initially ordered intermetallic compound Co₂Si with orthorhombic structure was mechanically milled in a high-energy ball mill. The structural changes with milling time were followed by X-ray diffraction, high-field magnetization measurements and differential scanning calorimetry (DSC). Ball milling results in a continuous increase of the magnetization at 4.2 K in a field of 21 T with milling time. This gives evidence that by mechanical milling anti-site disorder is generated in Co₂Si, similar to the case of orthorhombic Co₂Ge. X-ray diffraction reveals that the number of diffraction peaks decreases with increasing milling time and that all diffraction peaks broaden. The absence of a large number of diffraction lines supports the creation of atomic disorder. The average crystallite size derived from the fine width shows a drastic decrease in the early stage of milling and tends to become constant with a value of 8-10 nm upon further milling. The material remains in the same structure as the starting compound even after prolonged periods of milling. Exothermic heat effects due to both atomic reordering and nanocrystallite growth are evident in the DSC scans of ball-milled samples. Well defined atomically disordered nanocrystalline Co₂Si is produced by ball milling. Since the amount of heat released in reordering and grain growth is comparable, it can be concluded that both anti-site disorder and grain boundaries are the important sources of energy storage in Co₂Si during high-energy ball milling.

We report an investigation of the local modes of hydrogen and deuterium atoms by means of inelastic neutron scattering at 10 K and 100 K in some hydride and deuteride monohalides of the rare-earth elements, REXH_x, with RE=Y and Tb, X=Cl and Br and x approximately=0.8. These compounds contain hydrogen/deuterium atoms in distorted tetrahedral metal-atom voids. The hydride spectra exhibit two modes centred at about 125 meV and 90 meV for YBrD_x. For the deuterium compounds YBrD_x (x approximately=0.8) second-harmonic transitions are observed. The energy splitting of the two modes is of the order of 6-10 meV. The low-energy mode is about twice as intense as the high-energy mode. The splitting of the modes is consistent with local C_3v symmetry and analysed in terms of a model suggested recently by Bennington et al. Broadenings of the modes are observed and attributed to a weak splitting of the doubly degenerate low-energy mode due to a reduction of local symmetry. To relate the spectra to crystallographic properties a powder neutron-diffraction experiment and a refinement of the crystal structure of YBrD_0.79 have also been performed.

The diffusion of titanium in crystalline lithium niobate is studied within the framework of the Nernst-Planck theory for ionic diffusion. Under this conceptualization, a concentration-dependent diffusivity emerges naturally from the mathematical model as a direct consequence of the electric field induced by ion dislocation in the lattice. Numerical simulations of the one-dimensional case show strong evidence of enhanced diffusive transport of mass. This might explain the anomalous lateral penetration of Ti in LiNbO₃ which has been observed in previous experiments.

The thermal conductivity of nominally pure KTaO₃ exhibits an unexpected strong phonon scattering around 8 K. The question arises whether this scattering is related to the distorted microregions observed by other experiments. We have investigated a set of undoped single crystals grown by different methods. By comparison with doped specimens, it is suggested that acoustic phonons are coupled to the soft optic mode via the local disorder. From complementary dielectric permittivity and electron paramagnetic resonance experiments, oxygen vacancies and rhombic Fe³⁺ appear to be incontestable candidates to account for the distorted microregions.

We have studied the spectroscopic properties of Pb²⁺ in a CdCl₂ host crystal. By excitation in the absorption band due to the ¹A_1g to ³T_1u(³P₁) transition, two emission bands are observed. At very low temperature a strong UV luminescence with a very long lifetime (2.5 ms) is detected. As the temperature increases, the lifetime of the UV emission drastically decreases to a few nanoseconds at 100 K, and the UV intensity declines. Meanwhile a strong green fluorescence appears. It is assumed that this emission is of the impurity-trapped exciton type, as in the case of CdCl₂:Cu⁺ and BaF₂:Eu²⁺. A detailed study of the kinetics of both fluorescences is presented.

Inelastic neutron scattering has been employed to study the crystalline-electric-field interaction of the distorted perovskite PrGaO₃. The observed energy spectra show several well defined inelastic peaks of magnetic origin in an energy transfer range up to 86 meV which are attributed to crystalline-electric-field transitions within the ground-state J multiplet ³H₄ of the Pr³⁺ ions. The spectra are analysed in terms of single-particle crystal-field theory based on geometrical coordinations associated with the C_s site symmetry of the Pr³⁺ ions. J mixing as well as the imaginary part of the crystal-field Hamiltonian are found to be important to reproduce the experimental data. The resulting crystal-field parameters are close to the values extrapolated from a similar study of NdGaO₃. Our approach turns out to constitute a useful extrapolation scheme for other structurally related perovskite-type compounds.

We present the results of theoretical and experimental studies of GaAs/GaAlAs Fibonacci superlattices. The normal-incidence reflectance was measured for a Fibonacci superlattice in the temperature range from 20 to 300 K. We develop a simple model to predict the reflectance theoretically. The energy levels are calculated with the help of the envelope-function approximation and the Kohmoto-Kadanoff-Tang renormalization-group method. The model predictions are in good agreement with the experiment. The electronic structure of Fibonacci superlattices exhibits multifractal properties, which manifest themselves also in the experimental reflectance spectra.

Complex investigations of the photoinduced changes in YBa₂Cu₃O_{7- delta} ( delta >0.5) single crystals have been carried out using the time-resolved dependences of the photocurrent, photoresistivity, photovoltage and optical second-harmonic generation (SHG). Simultaneously the spectroscopic parameters were monitored by time-resolved Raman and UV-visible reflection spectra. A time delay shift between the SHG and photocurrent dependences was found. The contributions of two different mechanisms in the formation of the superconducting behaviour were unambiguously shown. It was clearly shown that only by simultaneously taking into account the anharmonicity due to the specific electron-phonon interaction together with the traditional Anderson metal-insulator transition we can clarify the main physical mechanisms of the high-temperature superconductivity. Magnetic fields of up to 1 T suppress the SHG signal. The different delay times for the electronic and phonon spectra indicate the essential role of the electron-phonon anharmonic coupling in the high-temperature superconductivity. Some different approaches to explain the observed phenomena are discussed.

We have studied one-hole dynamics in a quantum antiferromagnet by use of the anisotropic three-dimensional t-J model in the simple cubic lattice within the self-consistent Born approximation. The hole spectral function, the density of states, the quasiparticle dispersion relationship, the effective mass, the quasiparticle spectral weight, and the incoherent multiple spin-wave background are determined as functions of the Heisenberg exchange and hopping amplitude in the thermodynamic limit.

A simple approximate equation of motion for the longitudinal component of the magnetic moment of a single-domain uniaxial superparamagnetic particle is proposed. This equation is used to evaluate the spectral density function of the interparticle magnetic motion at finite temperatures and to describe the stochastic resonance effect. The correspondence of the results obtained with those of previous calculations is discussed.

The antiferromagnetic structure of the high-temperature superconductor HoBa₂Cu₄O₈ with T_c=77 K was determined by means of neutron diffraction at T=9 mK. Imperfect three-dimensional antiferromagnetic ordering of the Ho³⁺ ions corresponding to propagation vector K=(0, 1/2 , 0) is found below T_N=100 mK with magnetic moments of magnitude 1.5 mu _B at saturation and oriented parallel to the a-axis. The antiferromagnetic sublattice is characterized by stacking faults corresponding to a finite correlation length xi _c=18 AA along the c-axis. The majority of the 17 crystal-field level within the ground-state J-multiplet of the Ho³⁺ ions is established by inelastic neutron scattering, which allowed an unambiguous determination of the nine assignable crystal-field parameters. The observed temperature dependence of the order parameter is found to be in excellent agreement with mean-field calculations including crystal-field, exchange and hyperfine interactions.

The absorption of electromagnetic radiation by small conducting discs is calculated for the case of ballistic electron motion, using a semiclassical analysis valid at frequencies small compared to the plasma frequency omega _p. For a smooth walled disc there are complicated resonance structures, and the absorption coefficient has a low-frequency cut-off at a critical frequency omega _c, which corresponds to the frequency of a circumferential classical orbit of an electron at the Fermi surface. For frequencies satisfying omega _c<< omega << omega _p, the absorption coefficient is proportional to omega if fluctuations due to resonances are averaged over. We also consider a rough-walled disc and some more general shapes: the proportionality to omega rather than the expected omega ² is a general feature.

The afterglow and thermoluminescence (TL) properties of several Ce³⁺ doped Lu₂SiO₅ crystals are reported. Both properties are caused by the presence of charge traps in the crystals. At least six different glow peaks are observed in the TL glow curve. Each is related to a specific charge trap. The parameters for these charge traps, such as the trap depth and the frequency factor, were obtained from first-order kinetics peak analysis of the TL glow curve. A charge trap with a depth of 1.0 eV is responsible for the afterglow observed at room temperature. Ce³⁺ ions appear to be the luminescence centres in the recombination process of the trapped charge carriers. It will be shown that optical excitation in the 5d levels of Ce³⁺ produces trap filling. The possible nature of the charge traps will be discussed.

A regular method for finding solutions of the equation for the distribution of energy deposited by atomic particles in elastic collisions is developed, using the model of an infinite random medium and power cross section. It is shown that in neglecting threshold energy the distribution derivative has a singularity at the target surface. The calculated distributions and other functions connected with the problem are given for the case of equal masses of the projectile and target atoms.

The atomic momentum distribution of lithium-7 in natural Li has been determined as a function of temperature from 20 to 300 K. The experiments were undertaken within the impulse approximation using the electronvolt spectrometer at ISIS. Final state effects were present above the statistical accuracy of our data and were accounted for by symmetrizing the neutron Compton profiles. The mean atomic kinetic energy of the Li nucleus was obtained directly from our measurements and was found to be approximately 10% higher than that predicted by a harmonic model incorporating a density of states derived from previous phonon measurements. A fit of the data to the Debye model gave a Debye temperature of 419+or-3 K.