Table of contents

Sum rules for non-linear optical constants are given, using the concept of complex contour integral and a symmetry relation imposed on non-linear optical constants.

General formulae are given for providing sets of the special points and their weighting factors for k-space integration without the use of a recurrence process for cubic, hexagonal and tetragonal lattices. The formulae are allowed to be used for intermediate numbers of k-points not considered in the method of Chadi and Cohen (1973). It is shown that the special-point method is an open-type Lagrange quadrature of lowest order and that it gives accurate values for moderately varying functions but less accurate values for those with discontinuous derivatives at the Brillouin zone boundaries. In the present method it is possible to incorporate the second-order Lagrange quadrature and the Gaussian method. The efficiencies of these methods are discussed in comparison with the correctly weighted tetrahedron method.

The spin-dependent positron-electron pair momentum density in ferromagnetic cobalt was studied by a full-scale use of the two-dimensional angular correlation of polarized positron annihilation radiation (2D-ACAR) method. Ferromagnetic cobalt shows two different Fermi surfaces, for the majority spins and the minority spins, respectively. The positrons emitted in the positive-beta decay are partially polarized in the direction of their motions. In the present work this property has been utilized. The differences in the 2D-ACAR spectra were observed as a function of spin and their values reached a maximum value of 3%. After the reconstruction the three-dimensional electron momentum density in cobalt in the extended-zone scheme was obtained. The differences in the electron momentum densities represent the contributions of the majority spin and the minority spin, and they showed directional anisotropy. The electron momentum density in the reduced-zone scheme has been obtained by using the LCW-folding procedure. It was found that the electron in cobalt behaved in the k-space in a two-dimensional manner. The experimental results in the present work were discussed with the theoretical calculations. They showed agreement at the typical symmetry points of the first Brillouin zone except around the point K.

The cyclotron resonance of magnetopolaron in quantum well wires (QWWS) has been studied with the use of variational solutions to the effective-mass equation. The results that the authors obtained show that both the absolute value of the electron-phonon interaction energy and the cyclotron resonance frequency decrease with increase in the sizes of the QWWS, and also that the cyclotron resonance frequency increases with increase in the external magnetic field.

Using the extra-orbital model, the authors calculate the electronic structure of one or two atoms of Li, Be, B, C or N, represented by 2s-2p orbitals, intercalated between graphite layers. To do so, (i) they have regenerated the pi -band structure of 2p_z states in pure graphite within a Slater-Koster scheme; (ii) they have calculated the corresponding intersite Green functions within the corresponding D_3h symmetry point group; and (iii) they have computed the transfer integrals between the intercalated atom and a neighbouring carbon atom by using the basic two-centre atomic integrals. Within the authors' simple model they discuss the natures of the various electronic situations in terms of the various types of impurities considered. Finally they calculate the electronic binding energies of the corresponding diatomic intercalated molecules (Li₂, Be₂, C₂, N₂) with respect to the differing geometrical configurations.

PbTe films with various thicknesses, grown by laser-assisted deposition on KCl substrates, have been investigated. X-ray diffractograms have been taken in the angular range from 10 degrees to 70 degrees . FIR reflection spectra have been studied in the range from 20 to 5000 cm^-1. X-ray diffractograms indicated strained layers with HP PbTe phases: GeS and CsCl type. The frequencies of the infrared modes in these HP phases were determined by fitting the experimental spectra with the reflectance calculated for four media: air and two absorbing layers on absorbing substrate. It was found that the lineshape of the reflectance spectra was governed by a resonant-like frequency dependence of the electron damping parameter. The resonant-like shape of the damping parameter spectrum was attributed to electron scattering off potential wells, formed by dislocations.

The far-infrared (FIR) reflectance of PbTe:Cr films deposited on KCl substrates was investigated. The Raman spectra of samples with prevailing GeS-type phase in the strained sublayer were investigated. More comprehensive information about the optical phonons in PbTe/KCl films was obtained from the parallel investigation of the FIR and Raman spectra. It turned out that frequencies of the Raman-active modes in the GeS-type content of the films are consistent with those measured in bulk PbTe at a hydrostatic pressure of about 6 GPa.

The authors discuss the circularly symmetric soliton states and the I-V characteristics of an annular Josephson junction with a large width by direct numerical simulation. By assuming a soliton or multiple-soliton solution initially, they find that there exist a stable single-soliton (one-soliton) state, two-soliton state and three-soliton state in their system in the absence of an external applied field. The corresponding zero-field steps are obtained. A state that traps initially more than four solitons in the system turns out to be unstable. There exists a transition between the multiple-soliton states which suggests some physical points for the application of an annular junction. A suggestion of soliton excitation in a physical experiment is also discussed.

The magnetic susceptibility of the new compound Cs₅Eu(N₃)₈ has been measured in the temperature range from 77 to 300 K. From excitation and luminescence spectra the compound is known to contain two distinct but very similar Eu³⁺ sites with approximately D_4d microsymmetry. The experimental susceptibility data are interpreted in terms of the Van Vleck theory for closely spaced multiplets. A perfect fit to this model is obtained on the assumption that the two sites with energies as observed in the emission spectra are present in equal proportions.

Magnetic ordering in NdMn₂ has been studied by magnetization measurements and powder X-ray and neutron diffraction experiments. Giant magnetoelastic effects giving rise to both a large magnetovolume effect and a monoclinic crystal distortion at the magnetic ordering are found. A magnetic structure with two different propagation vectors and an unusual decoupling phenomenon is deduced. All these effects are discussed and shown to lead to reduction in the high frustration of the Mn magnetism in NdMn₂. A reorientation process involving the Nd moments and only a quarter of the Mn moments is also found with decreasing temperature and is discussed in terms of the Mn and Nd anisotropy.

An adapted spin-wave model has been developed to explain spin reorientation (SR) processes in rare-earth ferromagnetic (or ferrimagnetic) systems with competing axial-planar anisotropies. The model involves only second order in the effective spin S and single-ion crystal electric field (CEF) interactions, and several results of interest have been achieved, in particular the temperature dependence of the SR angle theta . The conditions required to have an SR transition have been established. Of importance is that the 0 K SR angle theta (0) becomes a consequence of the frozen-in uniform zero-point quantum spin fluctuations, and depends only on the ratio Delta =-(D_p/D_a) between the planar and axial second-order CEF strengths. A consequence is that the SR temperature T_SR and the 0 K second-order magnetic anisotropy free energy, K₁(0) sin² theta (0), become proportional. The angle theta fluctuates critically for T<or approximately=T_SR and its exponent has been calculated, beta =1/2, as well as the dependence of T_SR with Delta and the exchange strength. At low temperatures the dependence of sin² theta becomes of T³2/ Bloch-like type. A comparison of these results with current experimental findings in hard magnetic intermetallics, such as (RE_x^'RE_1-x)₂Fe₁₄B and RE_x^'RE_1-xCo₅,has been made. The effective anisotropy constant K₁(0) and the magnon energy at the Brillouin zone boundary for some of the above series of intermetallics have been obtained from previous experiments, the K₁(0) values being in good agreement with theory.

The longitudinal (parallel to the c axis) and transverse (to the c axis) spin correlation functions are calculated for ferromagnetic systems with crystalline axial symmetries, and undergoing spin reorientation (SR) transitions under the effects of competing crystal electric field (CEF) anisotropies of axial and planar character. The structure factor is a combination of a long-range ferromagnetic one, F( theta ) delta (k), with amplitude dependent on the SR angle theta , plus a finite contribution at finite wavevectors k. This small-angle neutron scattering (SANS) structure factor is contributed by a Lorentzian (L) term, due to the magnon scattering, plus a squared Lorentzian (L²) term produced by zero-point uniform quantum spin fluctuations. This SANS scattering in the canted magnetization phase is produced within the magnetic domain walls, and therefore has a correlation length of the order of the domain wall width. The prefactors of the L and L² SANS factors are complex functions of the spin reorientation angle and of the CEF strength parameters and spin-wave stiffness constant. These results are briefly discussed in connection with recent SANS experiments performed in the tetragonal (Er_xNd_1-x)₂Fe₁₄B hard intermetallic pseudoternary compounds.

An overview on paramagnetic defects in BaTiO₃ single crystals, used in photorefractive applications, is given. New results concerning cation defects include the investigation of double quantum transitions in isolated F³⁺, identification of the parameters of Fe⁴⁺-V₀, analysis of Co²⁺, the report of Co⁴⁺-V₀, Jahn-Teller-induced spin-lattice relaxation effects of Pt³⁺, identification of Rh²⁺, Rh⁴⁺, Ir⁴⁺ and Pt⁵⁺. In the field of anion defects O^- trapped hole centres associated with unknown acceptors and with Al³⁺ are reported. The role of the identified defects in light-induced charge transport processes is discussed.

The Curie temperature of ferroelectric films described by the transverse Ising model was studied under the mean-field theory. The film layer number, the surface interaction and the surface layer number dependence of the Curie temperature were obtained. There is a critical surface interaction strength, which is J_SC=1.25 J for single-surface-layer films, and J_SC=1.078 J for multiple-surface-layer films. If the surface interaction strength exceeds the critical value, there exists an optimum film thickness which possesses the maximum Curie temperature; then the surface interaction strength is weaker than the critical value, the Curie temperature decreases monotonically with increasing film thickness, and there exist critical sizes or critical thicknesses at which the ferroelectricity will disappear if the surface interaction is weak enough.