Table of contents

Structural correlations in molten trivalent metal chlorides are evaluated as functions of the metal ion size R_M across the range from LaCl₃ (R_M approximately=1.4 AA) to AlCl₃ (R_M approximately=0.8 AA), using a charged soft-sphere model and the hypernetted chain approximation. Main attention is given to trends in the local liquid structure (partial radial distribution functions, coordination numbers and bond lengths) and in the intermediate range order (first sharp diffraction peak in the number-number and partial structure factors). The trend towards fourfold local coordination of the metal ions, the stabilization of their first-neighbour chlorine cage and the growth of medium-range order are found to proceed in parallel as the size of the metal ion is allowed to decrease at constant number density and temperature. A tendency to molecular-type local structure and liquid-vapour phase separation is found within the hypernetted chain scheme at small metal ion sizes corresponding to AlCl₃ and is emphasized by decreasing the number density of the fluid.

The behaviour of the coherent scattering functions S(Q, omega ) (where h(cross)Q and h(cross) omega are the momentum and energy transferred in radiation scattering experiments) in the limit of small Q approaching the hydrodynamic limit has been discussed in many theoretical papers. However, experimental results have mainly been confined to liquid metals and near-quantum fluids. This paper describes the observed behaviour of S(Q, omega ) as Q is increased away from the hydrodynamic region for a dense gas sample at constant density. The data are compared to theoretical predictions, for which there are essentially no adjustable parameters, and the region where the theory is successful as well as the initial departures from it are identified.

Samples of Co₆₈Mn₃₂ and Co₆₃Mn₃₇ were examined with neutron polarization analysis over a scattering vector range that encompasses the first Brillouin zone in the FCC structure. The atomic disorder cross section was separated and fitted for short-range-order Cowley parameters and the magnetic cross section was separated. The magnetic cross sections could be satisfactorily compared with those calculated for zero scattering vector from previous susceptibility measurements using the quasistatic approximation. However, the scattering vector dependence of the magnetic cross section is quite different to that expected from a localized moment model, and indicates that the average moment per atom is small.

The I to or from II phase transition of ammonium chloride has been studied using energy-dispersive powder diffraction with a synchrotron source of X-radiation. The equilibrium transition temperature was found to be 184.5+or-0.5 degrees C and the activation energy for the transformation was found to be 213+or-15 kJ mol^-1. The transition was satisfactorily described using the Avrami equation with a shape parameter of 3.

A detailed calculation of the momentum distributions in paramagnetic (BCC) chromium is carried out using the linear combination of Gaussian orbitals (LCGO) method. Using the same electron wavefunctions, theoretical results are obtained for Compton scattering and positron annihilation, and are presented in the form of electron momentum distributions rho (p), reciprocal form factors B(z), two-photon momentum distributions rho ^{2 gamma} (p), one-dimensional angular correlation of positron annihilation radiation, Lock-Crisp-West distributions, etc. These results are analysed in terms of the Fermi surface topology of paramagnetic Cr. In particular, the present results for rho ^{2 gamma} (p) are compared with those reconstructed by Kubota et al (1991) from their two-dimensional angular correlation of positron annihilation radiation measurements. Observed differences are ascribed to the effects of e⁺-e^- many-body correlations and the reconstruction procedure.

The formula for the correlation factor in tracer diffusion has been derived by a simple and elementary calculation in the case where the diffusion takes place by ions jumping alternately between two kinds of site with equal spacing. The correlation factors for the vacancy and interstitial mechanisms are deduced from this formula. The formula has been applied to alpha -Ag₂Te in which Ag ions diffuse by changing alternately between octahedral sites and tetrahedral sites in the anion FCC sublattice.

The transmittance and reflectance spectra of polymorphous PbTe films grown by laser-assisted deposition on KCl substrates were investigated in the range between 400 and 4000 cm^-1. The refractive index dispersions n(E) were obtained from the spectra. The energy gaps E_g of the high-pressure (HP) PbTe phases were determined from the maxima in the dependences n(E)-E_g=0.23 eV for the BCC HP PbTe phase and E_g=0.42 eV for the orthorhombic HP PbTe phase. The additional maxima in the n(E) dependences were related to transitions across the heterophase junctions formed in the process of growth. The energy profiles of films grown under different technological conditions were obtained.

For pt.I see ibid., vol.4, p.8997 (1992). Picosecond photoconductivity measurements of polymorphous PbTe films have been performed. Excess carriers were excited in the films by means of a picosecond YAG:Nd laser with pulse duration t=12 ps. The instantaneous relaxation times of photoconductivity were obtained as a function of the excess carrier concentration Delta N from the kinetic analysis. The heterophase junction nature of the comparatively thin (d approximately=1 mu m) films was confirmed by their high photosensitivity. The junction thickness was evaluated from the interpretation of the instantaneous lifetime dependence on the excess carrier concentrations.

LMTO calculations have been done in order to obtain electronic and positronic states in YB₂Cu₄O₈. The annihilation rates have been calculated including the enhancement effects. Theoretical momentum density spectra are used for prediction of Fermi surface signals.

With both longitudinal optical (LO) and surface optical (SO) modes included, the polaronic effects on the quantum-confined Stark effect in quantum wells is investigated by means of the variation technique. For an infinite and a finite well the significant corrections attributed to LO and SO modes are calculated. It is found that the Stark shift will be weakened by the LO-phonon effect and enhanced by the SO-phonon effect. It is also shown that the influence of electron-phonon interactions depends not only on the well width but also on the potential barrier.

Magnetic circular dichroism (MCD) spectra of the C band of KCl:Sn²⁺, and RbCl:Sn²⁺ crystals have been investigated experimentally. Coupling to the T_2g vibrational mode is confirmed to be present in the C band of the Sn²⁺ centre. The observed inversion-asymmetric MCD lineshape is understood in terms of a quadratic electron-lattice interaction.

The phonon drag contribution S_gxx to the thermopower is calculated for a 2DEG by using Boltzmann equations for both electrons and phonons. The system has a parabolic confining potential and is subjected to an in-plane magnetic induction field B and the temperature gradient is parallel to B. A quasi-phonon model is proposed which greatly simplifies the calculation of S_g. Qualitative comparison is made between the theoretical predictions and experimental data.

The temperature dependence of the electrical resistivity of some magnetic intermetallic compounds is analysed using a model which takes into account the changes in the electronic structure with the variation of the long-range order neglecting chemical and magnetic short-range order (SRO) effects. The values of atomic and magnetic long-range order (LRO) parameters are obtained using a magnetic-cluster variation method (CVM) approach with the interaction parameters fitted to the equilibrium phase diagrams. The three principal components of the resistivity, namely the chemical disorder, the magnetic disorder, and the electron-phonon scattering contributions are reproduced well for several paramagnetic (NiPt, Fe_0.7Al_0.3) and ferromagnetic (CoPt, Co_0.3Pt_0.7) compounds. The analysis indicates an important change in the electronic structure with the variation of the LRO parameter, this effect being very pronounced in Fe_0.7Al_0.3.

The novel mechanisms of polaron pairing in complex oxides and the possibility of high-temperature superconductivity as a result are reported. The peculiarity of complex oxides is taken into account by considerations of a medium with two optical phonon branches satisfying the adiabatic condition of the polaron theory and having different dispersions. The system of nonlinear motion equations for the coupled fields of carriers and polarization is solved. It is shown that two carriers localized because of their interaction with one phonon branch form a bipolaron owing to the interaction with the real phonons of another branch under the resonance condition. Such a novel bipolaron does not need a high static dielectric constant (SDC) for its formation, distinct from other bipolaron models. The novel bipolaron is energetically favourable compared with other bipolarons, when SDC is not high. By taking into account the peculiarity of bosons having a low maximum velocity, the expression for their condensation temperature is modified. As a result, the high-temperature Bose condensation of the novel bipolarons is shown to be possible at lower concentrations even if their effective mass is very large.

The spectrum of specific elementary excitations in a many-electron system, i.e., magnetoexcitons, is investigated theoretically. It is shown that in finite-size quantum wells there exist additional branches of magnetoexcitons in comparison with ideal two-dimensional systems. The new branches differ from the known branches in their dispersion laws. The general features of the spectrum for all types of magnetoexciton are obtained.

The dependence of the optical and spin-Hamiltonian parameters of NiF₆^5- and NiF₄^3- D_4h units, with a b_1g* ( approximately x²-y²) unpaired electron, upon the equatorial (R_eq) and axial (R_ax) Ni⁺-F^- distances has been studied through multiple-scattering X alpha and self-consistent charge extended Huckel methods. Both methods lead to the following main conclusions: (i) A charge-transfer transition like e_u( pi + sigma ,eq) to b_1g*( approximately x²-y²) (termed E_u) is more sensitive than a crystal-field one like b_2g* ( approximately xy) to b_1g*( Delta ₁) or e_g* to b_1g*( Delta ₂) to variations in R_eq. The change experienced by E_u mainly reflects that of V_el(M)-V_el(L), where V_el(M) and V_el(L) are the electrostatic potentials experienced by an electron placed on metal and ligands, respectively, (ii) As regards the unpaired spin densities onto n_Lp and n_Ls ligand valence orbitals (termed f_sigma and f_s respectively), it is found that f_sigma >>f_s but f_s is much more sensitive than f_sigma to changes in R_eq. The microscopic origin of this relevant fact is explained in detail. (iii) The removal of axial ligands, keeping R_eq constant, induces a decrease of E_u as a result of the diminution of the electrostatic repulsion, V_el (M), and produces a slight increase of Delta ₁ and a more important one of Delta ₂. (iv) The dependence of _g//-g₀ and g_{perpendicular to} -g₀ on R_eq essentially reflects that of Delta ₁^-1 and Delta ₂^-1, respectively, because of the low covalency (f_sigma approximately=2%).

The authors present conductivity and magnetoresistance data for a range of a-Si_1-yNi_y:H alloys near the metal-insulator transition. In the metallic regime, weak-localization and interaction effects are identified, the latter being of normal sign. In the activated regime, conductivity results imply variable-range hopping in a parabolic density of states but quantitative analysis shows that if this density of states results from a Coulomb gap then the localized states extend over large clusters of nickel atoms. The magnetoresistance data generally show positive magnetoresistance attributed to spin splitting in interaction effects, but negative magnetoresistance attributed to suppression of weak-localization-type interference, is seen on the insulating side of the transition at not too low temperatures where interaction effects are partially suppressed.

The authors generalize the linear response theory of magnetoconductivity of disordered two-dimensional electron gases to include the case where a confinement potential is present in one of the two spatial directions. The theory allows studies of the effect of suppression of backscattering due to the existence of spatially separated edge states in high magnetic fields. Explicit calculations of the conductivity are carried out for weak scattering in two limits: the weak and the strong confinement limit.

Using the Mossbauer effect and NMR, and phase compositions of rapidly quenched Nd₄Fe_77.5B_18.5 alloy under different heat treatment conditions were studied. It was shown that the sample which was annealed at 670 degrees C for a short time and has the optimal magnetic properties has a main BCT Fe₃B phase containing Nd atoms and a small amount of alpha -Fe (6%). It is concluded that the hard magnetic properties of crystallized melt-quenched Nd-Fe-B with a low Nd concentration do not originate from the presence of the hard magnetic Nd₂Fe₁₄B phase but from the BCT Fe₃B phase containing Nd atoms.

The modern theory of high-field spin-lattice relaxation is applied to methyl group rotation. It is shown that the same time correlation functions are observed in nuclear magnetic resonance (NMR) relaxation experiments and in inelastic neutron scattering (INS) experiments. A linear relationship is derived between the spectral function S( omega ) observed in INS and the spin-lattice relaxation rate T₁^-1( omega ) as a function of the Larmor frequency. The similarities between the two methods are pointed out. For CD₃, NMR experiments on systems with high tunneling frequencies yield the width of the quasi-elastic E^a to or from E^b line of INS. For CH₃, the effect of intermolecular dipolar interactions on the spin-lattice relaxation time T₁ is calculated using a series expansion in r/R_i, where r is the radius of the methyl group and R_i is the distance of the considered proton i from the centre of the CH₃ group. It is shown that for systems with high tunneling frequencies T₁ is mainly determined by intermolecular dipolar interactions, whereas these have a negligible effect on spin conversion. T₁ experiments on diluted CH₃-containing systems yield the width of the inelastic A to or from E line, also in the so-called 'quasi-classical' temperature regime.

The low-temperature nuclear orientation of ⁵⁴Mn in ferromagnetic Pt-10 at.% Fe has been studied in the temperature range 4-50 mK and in the external magnetic field range 0.5-8.5 T. Considerable noncollinearity of Mn magnetic moments with respect to the bulk magnetization direction has been found in the whole B_ext range. A model based on the Heisenberg Hamiltonian in the first-neighbour approximation is proposed, which fully describes the experimental data. The authors found that the first-neighbor Mn-Fe exchange interaction is antiferromagnetic with the value J_Mn-Fe=-21(1) K, which lies in the range of the nearest-neighbor Mn-Mn or Fe-Fe exchange interaction parameters for Pt-based alloys.

Mossbauer spectra of (NH₄)₂FeCl₅.H₂O have been measured over the temperature range 1.5-297 K for powder and various single-crystal absorbers. This antiferromagnet has shown complicated magnetic behaviour but, contrary to a previous suggestion, the spectra show that below the ordering temperature the Fe spins are aligned along the alpha axis. The antiferromagnetic spectra may be fitted with a small number of components coupled with relaxation broadening due to critical fluctuations. The data suggest two closely spaced ordering temperatures. Details of the hyperfine electrostatic interaction have been deduced from the paramagnetic spectra. No evidence was found for a reported structural phase change in the paramagnetic region.

The authors propose a theory of the electron-phonon drag effect in an ideally pure 2D system under a quantizing magnetic field. Two situations are considered, i.e. when phonons are (i) those of a diffusive heat flow and (ii) those of a focused ballistic beam. In the first case, the authors calculate the drag current for different asymptotical temperature intervals and give its relation to the phonon-scattering-induced conductivity. For the 2D gas illuminated by a hot beam of phonons, they analyse the manifestation of their cyclotron absorption both in the drag and in the beam-activated conductivity.

Resonance Auger processes (AP) in solid samarium after 4d to 4f excitations have been studied using electron spectroscopy, with synchrotron radiation. The N_4,5O_2,3O_2,3 and N_4,5O_2,3N_6,7 resonance AP were found to pay a significant role in excitations corresponding to the low-energy side of the resonance region. Kinetic energy shifts relative to the normal N_4,5O_2,3O_2,3 and N_4,5O_2,3N_6,7 AP were estimated to be 1.0 eV and 1.5 eV, respectively. In excitation corresponding to the main giant resonance the normal AP were found to be stronger than in the case of heavier rare earths indicating stronger continuum-like character for the main resonance in samarium.