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The feasibility of using the Monte Carlo (MC) approach to orientation distribution function (ODF) approximation from experimental pole figures (PFS) is demonstrated. The MC simulation is combined with the local projection method developed recently for generation of the PFS from a model ODF. First, a set of ideal orientations represented by a sum of Gaussians is assumed and the corresponding set of points describing the texture is located within the orientation space by means of the MC algorithm presented. Next, this set of data points is projected upon the PFS by means of the local projection technique. The validity of this procedure for texture simulation is tested on the example of cold-rolled steel.

We report the existence and characterization of the stable ternary phase Tb₃(Fe_1-xTi_x)₂₉. The structural characterization by X-ray powder diffraction is evidence for a monoclinic structure (P2₁/c space group) with refined lattice parameters a=10.583(1) AA, b=8.5116(7) AA, c=9.6736(9) AA and beta =97.018(5) degrees . A large magnetovolume effect has been observed in the volume thermal expansion at the order temperature T_c=455 K. The anisotropy has been measured by using the singular point detection technique. Two distinct anisotropy fields of relatively high intensity have been detected. The H_A values measured at 6.4 T and 1.9 T at 293 K and both increase markedly with decreasing temperature. Below 200 K, the approach to saturation corresponding to the highest H_A develops into a first-order magnetization process.

Tracer diffusion of oxygen in CoO-SiO₂ silicate melts has been studied in the composition range of 0.3<or=X(SiO₂)<or=0.45 (X=mole fraction) at 1450 to 1550 degrees C. Experiments were carried out using a capillary-reservoir method, involving a specially designed capillary to avoid convection effects. ¹⁸O diffusion profiles were obtained from secondary-ion mass spectrometric analysis. Effective diffusivities, D*_O,eff, were obtained by fitting these profiles to the appropriate solutions of Fick's second law. The effective diffusivity of oxygen was found to be smaller than that of cobalt, but larger than that of silicon. The composition dependence of the activation energies for oxygen diffusion, E_A, is significantly different from those of silicon or cobalt: the variation of E_A with the composition was less in the case of oxygen. Experimentally determined diffusion profiles have been compared to those of computer simulations that are based on a kinetic model. The agreement between the shapes of the experimental and simulated concentration profiles, as well as the similarity in the composition dependence of experimental and simulated D*_O,eff, is evidence for the strong tendency towards polycondensation in cobalt silicate melts.

In this work we have improved the kinetic model of Kieffer and Borchardt for diffusion in silicate melts, which takes into account both the diffusion of individual polyanions and the reaction of different polyanions through condensation and splitting events. Making assumptions for the individual condensation reactions, similar to those by Masson, the equilibrium polyanion distributions in silicate melts were calculated by solving a system of coupled first-order differential equations, describing the reactions between polyanions of various sizes. The concentration dependence of the SiO₄^4- monomers, determined by our approach, is identical to that resulting from the thermodynamic treatment by Masson. Furthermore, by allowing for local changes in the isotope distributions of the elements, and by assuming that migration of all species present in the system occurs due to random motion, the self-diffusion of silicon and oxygen in CoO-SiO₂ melts has been simulated. The parameters for the model have been estimated by fitting me simulation results to our experimental data of Si and O tracer diffusion. While the simulated concentration profiles were in good agreement with our experimental tracer diffusion measurements in the CoO-SiO₂ system, their shape could not be described by the standard solution of Fick's law. Conversely, for the CaO-SiO₂ and PbO-SiO₂ systems, the simulated profiles were in much better agreement with the standard solution. This difference in diffusional transport properties can be qualitatively interpreted as due to the structural differences of CoO-SiO₂ as compared to the two other systems.

The configurational entropy of a binary mixture of unequal hard spheres is analysed over a wide range of densities and concentrations as a sum of two contributions: a pair term, s₂, arising from two-body spatial correlations, and a residual term, Delta s, that is due to correlations involving more than two particles. The validity of a one-phase ordering criterion, based on the non-monotonic behaviour of Delta s as a function of the total packing fraction, is investigated in relation to the phase transformations undergone by the model.

The single-particle dynamics of a charged test particle in a background fluid under a magnetic field are studied using molecular-dynamics computer simulations. The off-diagonal as well as the diagonal components of the velocity autocorrelation functions (VAFs) are computed. The model interaction between the particles is a hard sphere. The diffusion constants and the Hall coefficients are also calculated at various densities. These are compared with a theory based on the projection-operator formalism. In the low density of a background fluid, the VAF can be well approximated by an exponential form. The Hall coefficients with respect to the volume fraction nu show a peak around nu =0.4, reflecting the fact that the ratio D/D_E should decrease to the value of unity from above where D_E is the Enskog diffusion constant.

The ternary mixed crystal system Mg_xCd_1-xTe has recently attracted interest as a basic material for the blue laser diode. We use norm-conserving pseudopotentials in connection with the local-density approximation for ab initio calculations of the stable crystal structure and the lattice constant. Our results are in quantitative agreement with structural properties. The electronic band structure is calculated in dependence on the composition x of the system by the virtual-crystal approximation (VCA). We find a transition from direct to indirect energy gap for x=0.7 in accordance with experimental data.

Using the molecular dynamics technique, porous glasses of several densities have been simulated. Lattice distortions and development of non-bonding oxygens number are analysed. We emphasize the role of these characteristics for the doping of glasses. Europium-doped silica glasses have also been simulated. The comparison between glasses of different densities allows us to understand the doping of silica by europium. In particular, we can explain the low coordination (<4) obtained in this glass. Several peculiarities of the rare earth spectroscopy are explained by the simulation results: broadening of lines and variation of the Stark splitting.

Molecular dynamics and lattice dynamics calculations have been used to study crystalline KReO₄ which has the tetragonal scheelite structure. The charge distribution on the anion and repulsive potential parameters were adjusted to fit the unit cell dimensions at low temperature and zero stress, and were then used to study the properties of the salt over a range of temperature. The results agree satisfactorily with experimental crystallographic and other data over a range of temperature.

Ammonium scheelites (of which ammonium perrhenate is an example) show several anomalies in their behaviour (including thermal contraction of the a lattice parameter, large thermal expansion of the c lattice parameter, enhanced nuclear quadrupolar relaxation, increased specific heat) which has been attributed to the onset of orientational disorder of the ammonium ions. Unlike in the ammonium halides there is no actual phase transition. Extensive molecular dynamics simulations of this material are reported. The model shows anomalies in the thermal expansion which are associated with a broadening and shift of the orientational distribution of the ammonium ions. There is no evidence of more than one preferred orientation. The ammonium ions also begin to jump between the twelve equivalent orientations, and we present evidence that this is primarily by rotation about one of the bonds. The activation energy for this agrees well with experiment. Angular and linear velocity-time correlation functions for the ammonium ion show rapid dephasing at higher temperatures, which is consistent with the observed broadening of the corresponding lines in the Raman spectrum. The origin of the anomalous thermal expansion is discussed in terms of various contributions to the entropy of the crystal.

A study of the rotational dynamics of NH₄ ions in the paraelectric phase of mixed crystals of K_1-x(NH₄)_xH₂PO₄ by quasielectric neutron scattering is reported. For high NH₄ concentrations (x=0.9) no anomalous behaviour was found and the model of NH₄ ion jumps between equivalent orientations in the crystal gave a good description of the experimental data. In contrast the crystal with x=0.6, which forms a protonic dipolar glass state at low temperatures, revealed an unusual temperature dependence of the elastic incoherent structure factor far above the glass formation boundary. This temperature dependence is analysed in the framework of the model of dynamical cluster formation in the paraelectric phase. The order parameter for the gradually condensing NH₄ groups is obtained. The results are compared with data obtained by other methods on related compounds.

The Li diffusion in Li₂S at high temperatures was investigated by quasielastic neutron scattering techniques on a single crystal of ⁷Li₂S. It is shown by using polarized neutrons that the scattering is almost purely incoherent. The data were analysed in terms of an extended Chudley-Elliott jump diffusion model. The Li diffusion process takes place by hopping between regular tetrahedral and interstitial octahedral sites. The mean residence times on the regular Li sites were estimated to be 17.3 ps at T=1173 K, 6.7 ps at 1273 K and 4.3 ps at 1363 K.

Powder neutron diffraction measurements were made on CuCl and CuBr solid ion conductors over large ranges of temperature in the cubic gamma phase (sphalerite structure). Anharmonic data treatment based on the Gram-Charlier expansion showed significant contributions up to fourth order for the thermal parameters of Cu. The thermal motion of Cu is strongly elongated along the tetrahedral diagonals. The corresponding probability density maps and the derived potential functions illustrate the characteristics.

We present a database of 171 aluminium structures with coordination number ranging from 0 to 12 and nearest-neighbour distance from 2.0 AA to 5.7 AA. The purpose of the database is further to test and refine empirical and semi-empirical models of metallic bonding. Each structure is specified by the atomic positions and the unit cell used and a total energy per atom is given. Full details of the first-principles total energy calculations are given along with the estimated errors involved. Examples of densities of states are also given for a few of the structures.

The ordered alloy, Pt₂Cr has the L1₂ structure and is ferrimagnetic with T_c=170 degrees C. Angle-resolved photoemission experiments using synchrotron radiation (18-45 eV) have been made in normal take-off geometry from the (110) surface in order to elucidate the band structure. The valence band energy-momentum relationship in the Gamma Sigma M direction has been determined assuming a free-electron final state. The symmetries of these initial states have been determined using the polarization of the synchrotron radiation.

Superconducting fluctuation conductivities and magnetoconductances have been measured in thin granular Al-Ge films above the superconducting transition temperature. For very metallic films located well above the metal-insulator transition, the two-dimensional Aslamazov-Larkin and Maki-Thompson theories describe the conductivity and magnetoconductance data well. The Maki-Thompson expressions involve the temperature-dependent pair-breaking parameter delta . For films located near the metal-insulator transition, we have observed a dimensional crossover from three dimensions to two dimensions to a fractal dimension with decreasing temperatures. Values for the diffusion constant, ranging from 0.4 to 1.4 cm² s^-1, were obtained from the magnetoconductance fits, and these magnitudes compared favourably with values obtained from critical-field measurements taken below T_c.

Multiphoton exciton transitions induced by an intense optical wave in a semiconductor superlattice are analysed. It is assumed that the excitons can be treated as two-dimensional (2D) excitons localized in the heteroplanes and that the oscillating electric field of the optical wave is directed perpendicular to the heteroplanes. The superlattice potential barriers are modelled by a periodic chain of delta -type functions. Quasienergetic time-dependent states are used. The explicit dependence of the coefficient of the multiphoton absorption on the frequency and magnitude of the optical wave and on the superlattice and exciton parameters is obtained. The exciton absorption spectrum is shown to consist of a series of continuous absorption bands associated with the discrete levels of the exciton. The width of each band is equal to the sum of the widths of the electron and hole minibands. The distance between the neighbouring bands is equal to the separation of the exciton levels. The form of the exciton absorption essentially depends upon the parity of the number of absorbed photons and on the relationship between the separation of the exciton levels and the total width of the electron and hole minibands.

The additional resistivity of a quantum film due to an obstacle is investigated. The film is treated by a semi-classical formalism whilst the obstacle is characterized by its quantum mechanical scattering cross-section. The film intrinsic scattering mechanism allows for the adaptation of an arbitrary density distribution to a characteristic 'ideal' distribution over the channels. Special attention is paid to the multiple-scattering cycles between the obstacle and its surroundings, i.e., the scattering background of the film. The analysis of these backscattering processes leads to a self-consistent equation for the current density incident on the scatterer. For the general case of an arbitrarily strong obstacle and many conducting channels, this equation system can be solved only numerically. However, the formalism becomes handy if the obstacle scatters only weakly. A condition is found for the obstacle to be considered as weak. On the other hand, if one considers only one conducting channel it is possible to solve the transport problem analytically even for a strong obstacle. In this case, we find an expression for the resistivity which contains the scattering cross-section in a non-linear manner. This non-linearity was already predicted in 1957 by Landauer.

The present work investigates the effect of the Gamma -X crossover at low temperatures on the donor binding energies in single-quantum-well (QW) microstructures. It is found that, at a constant QW thickness, the predicted donor binding energies are enhanced with increasing pressure and then decreased when approaching the crossover pressure between the QW Gamma states and the barrier X states. It is noticed that the pressure-dependent donor binding energies are mainly related to the changes in the dielectric constants and the effective masses of both materials.

We study the resonant tunnelling in double-barrier heterostructures via Landau levels in the sequential tunnelling approach. The effects of the space-charge formation in the electrodes and in the well together with inelastic-scattering broadening are taken into account. Under appropriate approximations, the tunnelling current and differential conductance can be written as simpler forms which analytically reproduce the main features of experiments. Then the dependences on the temperature and magnetic field are studied numerically; these are also in accordance with experiments.

We report a theoretical investigation on the frequency-dependent small-signal mobility in the vertical conduction in the presence of a DC bias electric field for cylindrically confined superlattices having transverse diameters ranging from that for an extremely confined (purely one-dimensional) limit to that for an almost unconfined system. In all the cases the differential mobility exhibits peculiar frequency dependence at finite DC bias: the real part shows a broad bump before finally approaching zero at high frequency, and the imaginary part experiences a marked dip to negative values before going through the conventional maximum. Such behaviour of the frequency-dependent differential mobility stems from a decrease of the average inverse effective mass of carriers in conjunction with an increase of their average longitudinal energy, and is characterized approximately by a momentum and an energy relaxation time. It is found that the effective energy relaxation time increases significantly with increase of the transverse diameter of the superlattice.

The critical current density of an aligned Ag doped Bi₂Sr₂Ca₂Cu₃O_x tape has been measured from 10 K up to T_c (105 K) in magnetic fields up to 15 T. Measurements of the critical current density (J_c,(B, T)) have been completed for three different configurations of magnetic field, transport current, and superconducting tape: (i) J perpendicular to c axis, J perpendicular to B, and B perpendicular to c axis; (ii) J perpendicular to c axis, J perpendicular to B, and B//c axis; (iii) J perpendicular to c axis, J//B, and B perpendicular to c axis. In all three configurations at high magnetic fields up to 15 T, we find the critical current density can be described by J_c(B, T)= alpha ( phi , T) exp(- mu ₀H/ beta ( phi , T)) where ( beta phi , T) is of a separable variable form beta ( phi , T)= beta /sup /(T)/f( phi ) where beta /sup /(T) is a function of temperature alone and f( phi ) is a constant dependent only on the orientation of B, J, and the c axis. A similar but not identical exponential magnetic field dependence has been found in Bi₂Sr₂Ca₂Cu₃O_x, Bi₂Sr₂CaCu₂O_x and YBa₂Cu₃O_x, thin films. We discuss the evidence that the exponential magnetic field dependence suggests that a pair breaking mechanism operates at barriers in these materials. We conclude that the grain boundaries determine J_c(B, T) in the Bi₂Sr₂Ca₂Cu₃O_x textured tape.

The Ising model with both random longitudinal and transverse fields is studied by combining the pair approximation with the discretized path integral representation. The phase diagrams of the spin S= 1/2 system are obtained; the relations between the critical temperature and the local structure as well as the tricritical and re-entrant phenomena are discussed.

The nature of three-magnon excitations in general spin-S quantum spin chains is studied using the recursion method. The asymptotic behaviour of the recurrence coefficients can be used to identify the presence of bound states in the spectrum. Special features of integrable models are easily identified.

We show that the magnetic properties of frustrated itinerant systems can be described by a peculiar shape of the q-dependent susceptibility. We justify the fact that the spin fluctuation spectrum is especially broad in this case. This spectrum is used to calculate the spin fluctuation contribution to the specific heat; we show that this contribution can be very large close to a magnetic-non-magnetic instability. Comparison with recent experimental results on YMn₂ is made and the similarities of this compound with the 4f heavy-fermion compounds are emphasized.

Neutron diffraction results on single crystals of CsCuCl₃ in high magnetic fields are reported. The fields were applied parallel (up to about 6.5 T) and perpendicular (up to 8.5 T) to the c axis of the hexagonal crystal. For the parallel field a spin-flop-like change in the magnetic structure at fields above 5.6 T was observed close below the Neel temperature. The magnetic phase diagram has been determined in the temperature range between 9.6 K and T_N=10.675 K. In the perpendicular field the magnetic satellite reflections, originating from the helical magnetic spin structure of the Cu ions, shift their positions in rising field, indicating a stretching out of the helices. The results are interpreted in the light of recent theoretical predictions about the transitions being triggered by quantum or thermal fluctuations. In addition to determining the spin structures, we find that the critical exponent beta is close to 0.25 in the low-field phase and probably close to 0.125 in the high-field phase.

The specific heat of the heavy fermion compound CeB₆ has been studied under quasi-hydrostatic pressure up to 1.35 GPa in the region 0.7-4.5 K. We find an increase of the antiferromagnetic transition temperature T_N of 0.1 K GPa^-1 and a constant antiferroquadrupolar transition temperature T_Q with increasing pressure. Additionally, a remarkable change in the shape of the antiferromagnetic lambda anomaly is observed, whereas the antiferroquadrupolar transition is hardly affected by pressure. The electronic specific heat contribution gamma shows a non-monotonic variation. We discuss possible mechanisms for this anomalous behaviour of gamma and the deformation of the lambda anomaly.

Quasicrystalline and amorphous Al_85-xPd₁₅Mn_x (x=12,13,14,15) alloys have been prepared by rapid melt quenching and high-rate DC sputtering. The saturation magnetic moment M_s, the magnetic Mn atom ratio, the magnetic moment per magnetic Mn atom P_s and spin glass behaviour have been investigated for both these phases. The temperature dependence of DC magnetic susceptibility for these quasicrystalline and amorphous alloys follows a Curie-Weiss law over a wide temperature range. The value of M_s and the average magnetic moment per Mn atom (P_c) for the quasicrystalline alloys are much smaller than those for the amorphous alloys. The magnetic Mn atom ratio in the quasicrystalline phase is about 10%, being almost half that in the amorphous phase in a similar manner to the Al_80-xCu₂₀Mn_x (x=13, 14 and 15) alloy system. The Mn concentration dependence of P_c in the quasicrystalline phase is steeper than that in the amorphous phase, and the value of P_c of the quasicrystalline Al₇₀Pd₁₅Mn₁₅ alloy exceeds 5 mu _B of a bare moment of Mn. It is, therefore, considered that a giant moment is formed by polarization of 4d electrons of Pd in the quasicrystalline alloy. The present alloys in both phases are magnetically dilute alloys and spin glass behaviour has been observed. The spin freezing temperature T_f for the quasicrystalline phase is lower than those for the amorphous phase in accordance with the smaller Mn atom ratio. From these results, a marked difference in the magnetic properties between the quasicrystalline and amorphous phases has been confirmed, reflecting their different local atomic structure.

The nuclear magnetic resonance of ¹³³Cs in a CsMnCl₃ single crystal grown by the Czochralski method has been investigated by employing a Bruker FT NMR spectrometer. The ¹³³Cs resonances of two different groups were recorded, that is the spectra of two groups with different intensity ratios associated with two physically inequivalent positions of caesium atoms in the unit cell. Various transitions belonging to two caesium spectra of different intensity ratios are analysed. The quadrupole coupling constant of Cs(I), giving weaker signals than the other, is 0.153+or-0.014 MHz, and that of Cs(II) is 0.212+or-0.014 MHz. The asymmetry parameter is zero for both. The principal axes of the EFG tensors for these two sites are found to be the same. The Z axis, conventionally the largest component of the EFG tensor, is parallel to the crystallographic c axis.

Free-carrier absorption in n-type GaAs films has been investigated for the case where the free carriers are confined in a quasi-two-dimensional semiconducting structure with a non-parabolic energy band of electrons. It is assumed that the carriers in semiconductors are scattered by acoustic phonons via firstly the deformation-potential coupling and secondly the piezoelectric coupling. Results show that the free-carrier absorption coefficient depends upon the polarization of the electromagnetic radiation relative to the direction normal to the quasi-two-dimensional structure, the film thickness, the photon frequency and the temperature of the semiconductors. The free-carrier absorption coefficient could be complex owing to the interaction of photons, phonons and conduction electrons in piezoelectric semiconductors. Firstly, when the deformation-potential coupling is dominant, the absorption coefficient increases with decreasing photon frequency and increasing temperature for the radiation field polarized parallel and perpendicular to the layer plane. It is also shown that the absorption coefficient increases with decreasing film thickness. Secondly, when the piezoelectric scattering is dominant, the absorption coefficient increases with decreasing photon frequency and decreasing film thickness for the radiation field polarized parallel and perpendicular to the layer plane. However, the absorption coefficient increases with increasing temperature for the radiation field polarized parallel to the layer plane while, for the radiation field polarized perpendicular to the layer plane, the absorption coefficient increases with decreasing temperature.

We report lattice and electronical properties of BaTiO₃:La powders and ceramics with 0<x_La<20 at.%. Dielectric measurements and x-ray scattering evidence a continuous decrease of the ferroelectric transition temperature when the La content is increased. For x_La=20 at.%, no dielectric maximum is found for temperatures higher than 20 K. At the same time, an impurity mode at 840 cm^-1 increases linearly with x_La. We have modelled these data using a linear chain model with one polarizable ion. The electric excitations were probed using DC conductivity, EPR spectroscopy and optical absorption. This last technique was applied to BaTiO₃:La single crystals, which were grown for the first time in our laboratory. Our main result is that the conductivity maximum at x_La approximately=0.5 at.% is related to an impurity level lying 2.1 eV below the conduction band.

The depinning fields for the twinned orthorhombic domains in DyVO₄ and Dy(As_xV_1-x)O₄ crystals have been determined from optical birefringence hysteresis measurements. In this Ising model system the depinning field in a mixed crystal (x=0.15) is found to be much smaller than that in pure DyVO₄ at low temperatures. An impurity mechanism that assists the motion of domain walls appears to be more important than pinning by the random strain fields.

Optical absorption edge measurements of pyrite thin films prepared by flash evaporation have been made at different temperatures in the range 10-300 K. Absorption edge values vary from 1.05 to 0.99 eV in that temperature interval. Experimental data have been fitted to the Varshni formula and to an expression proportional to the statistical Bose-Einstein factor used previously by other workers. The main phonon energy calculated from the fitting of the experimental points to the Bose-Einstein expression is in good agreement with the average energy of the active optical phonons as determined by Raman spectroscopy carried out on the same pyrite thin film.

The electronic structures of the AlB₂-type transition-metal diborides TMB₂ (TM=Sc,Ti,V,Cr,Mn,Y,Zr,Nb,Mo,Hf,Ta) have been calculated by using the self-consistent LMTO-ASA method. The binding mechanism is discussed and the rigid-band model is shown to provide a fairly good description of their electronic structures. The existence of a pseudogap in the total density of states is found to be a common feature of these compounds. The variation of the chemical stabilities of these diborides is analysed and we find that the trends can be understood in terms of the band-filling concept of the bonding states. The results are compared with other theoretical and experimental work.