Table of contents

Searching for a charge density wave (CDW) in pure chromium, the satellite reflection intensity mainly due to a periodic lattice distortion (or strain wave, SW) was carefully studied at 150 K by means of X-ray diffraction. The intensity ratio of satellite peaks around the main Bragg peak does not fit the simple SW model. The discrepancy is resolved by consideration of the coexistence of the SW and CDW having an amplitude of 0.003-0.02 electrons/atom. The phase relation between the SW and CDW is also determined. The charge density distribution of the CDW seems to have greater extent in real space than that of the total electrons.

An amorphous Zn₆₀Zr₄₀ alloy was synthesized for the first time by mechanical alloying of pure elemental powders of Zn and Zr using a planetary high-energy ball mill. The structure and thermal behaviour were analysed by X-ray diffraction and differential thermal analysis. The result indicates that the large negative enthalpy of formation of Zn-Zr provides the necessary driving force for solid-state amorphization. In addition, the realization of the amorphization of Zn-Zr shows that Zn is a fast-moving species in host Zr although the ratio of V_imp(Zn)/V(Zr) is larger than the critical value of the Zr-TM (3d) system, 0.58. The activation energy of crystallization of the glass is about 3.068 eV.

The authors report time-resolved photoluminescence spectroscopy of highly porous silicon. Their results show that the luminescence is due to localized quantum-confined excitons in undulating crystalline silicon wires. The resonantly excited photoluminescence spectrum exhibits satellite structure due to momentum-conserving phonons of crystalline silicon. This provides a clear signature of the crystalline-silicon electronic band structure. The spin states of the localized exciton are split by the electron-hole exchange interaction. This splitting is manifested both in the strong dependence of the luminescence lifetime on temperature, and as an energy gap in the resonantly excited photoluminescence spectrum. The experimental splitting is in good agreement with the value calculated for a localized exciton in crystalline silicon.

Reorientation of the Fe spins in YbFeO₃ has been studied at 4.2 K using ⁵⁷Fe Mossbauer spectroscopy and applied magnetic fields up to 10 T. In addition to the expected reorientation (spin flop) induced by a field applied along the easy antiferromagnetic axis, unusual orientations were observed for fields applied perpendicular to the easy axis. It is suggested that these are due to breaking of the Fe-Yb coupling.

The authors carried out a nuclear quadrupole resonance study of ¹³⁹La in La_2-xSr_xNiO_{4+ delta} ( delta =0.00-0.10; x=0.0-1.3) in the temperature range T=1.4-200 K, paying particular attention to its relationship to the superconducting cuprates. In lanthanum nickel oxide, the occupation of holes on both Ni 3d(x³-y³) and 3d(z²) orbitals with parallel spins (S=1) causes some characteristic properties. (1) The large internal magnetic field (H_int=26-18 kOe) at the La site indicates the existence of a strong transferred exchange interaction between Ni 3d(z²) and La 6s orbitals through the 2p_z orbital of apex oxygen. (2) Discontinuities in the magnitude of H_int and electric field gradient observed at delta approximately=0.02 are attributed to the crystal phase transition from a low-T orthorhombic to a high-T tetragonal phase. (3) Up to high Sr doping (close to the phase transition at x approximately=1.0 from antiferromagnetic to paramagnetic), H_int remains almost constant, suggesting that the doped holes first go into the 3d(x²-y²) orbital without producing Ni-Ni spin frustrations. (4) The T dependences of H_int and relaxation rate T₁^-1 at low T are explained basically by the spin-wave theory for three dimensional antiferromagnets, and the sublattice magnetic moments of Ni²⁺ are close to the localised spins. (5) The dependence of T₁^-1 proportional to T^2.6 indicates that the ¹³⁹La relaxation can be attributed to the short-wavelength antiferromagnetic fluctuations notwithstanding the geometrical cancellation at the La site.

The lattice sites of a number of trivalent co-dopants (In, Nd, Gd, Er, Tm, and Lu) have been determined in LiNbO₃ crystals also with Mg, using the Rutherford back-scattering and proton-induced X-ray emission channelling techniques. In, Er and Lu have been found mainly on Li sites, but a small fraction of In and Lu may substitute for Nb ions. Gd and Tm ions also replace mainly Li, but some of these ions seem to be present in other positions, possibly related to some kind of aggregate, e.g. self-compensating pairs. A fraction of Nd ions probably occupies interstitial sites in structurally empty oxygen octahedra, while other Nd ions may substitute for host cations. In addition to the already known effect of Cr, Fe and Nd ions, In, Lu and also Sc give rise to an OH^- absorption band between 3499 and 3523 cm^-1, which is attributed to Mg²⁺ (Li site)-OH^-(O site)-M³⁺ (Nb site) complex defects. No such band is observed for Y, Gd, Er and Tm co-doped crystals.

The author reports on self-consistent calculations for the electronic structure of three hydrogen-phosphorus-vacancy complexes in crystalline silicon, based on a semiempirical tight-binding theory. These complexes, denoted by V-1H-3P, V-2H-2P, and V-3H-1P, consist of a vacancy and substitutional phosphorus impurities on the nearest-neighbour sites of the vacancy. The remaining silicon dangling bonds in the complexes are all saturated by hydrogen atoms. Each of the complexes is described by a large repeated supercell and the authors use the recursion method for computing local densities of states and orbital electron occupancies. They find that the three complexes do not introduce energy levels into the fundamental band gap, revealing that the electrical activity of the silicon dangling bonds can be well passivated by hydrogen atoms at bonding positions through strong orbital interactions and by phosphorus atoms at substitutional positions through Coulomb attractions. However, the complexes can have defect resonant states outside of the band gap. The authors show that these resonant states can be well described in terms of the symmetric combinations of the phosphorus orbitals and the symmetric combinations of the hydrogen and the hydrogen-saturated silicon orbitals.

In this work the author reports Davydov soliton dynamics at 300 K using the partial dressing ansatz state introduced by Brown and Ivic. As in the case of Davydov's mod D₁) ansatz they found that the window for the appearance, in the partial dressing case, of slowly dispersing solitary waves in the parameter space is shifted to smaller non-linearities with increasing temperature. However, in the partial dressing approximation no stable solitons show up. Since the results of studies on the thermal stability of Davydov solitons using different models disagree with each other even qualitatively, the authors give some comparisons between results obtained using four different approximations to the dynamics with quantum Monte Carlo (QMC) results found in the literature. They find that Davydov's method ( mod D₁) state, averaged Hamiltonian) leads to quantitatively incorrect results, but reproduces the qualitative trends correctly. All other models considered by the authors ( mod D₂) state with thermal phonon population, Langevin equation and partial dressing state) failed to reproduce QMC results even qualitatively.

For pt.II see ibid., vol.5, p.803 (1993). The author shows numerically that Scott's suggestion to use revised parameter values in order to simulate interchain coupling effects on Davydov soliton dynamics with one-chain simulations is correct also for excitations within the mod D₂) ansatz state, for which his analytical derivation does not hold. However, in the case of the mod D₁) state he finds that the equations of motion for the symmetric mode reduce exactly to the equations for one chain without any renormalization of parameters. Further, he presents numerical simulations for three coupled chains including temperature within different ansatz states and for one chain using Scott's revised parameters. The author found that Davydov solitons should exist in three-chain systems at physiological temperature for reasonable parameter values.

Profile analysis of powder diffraction lines was used to determine the variations in lattice parameters as a function of temperature. Using this procedure, phase diagrams of (K_1-xNa_x)MnF₃ and (K_1-xLi_x)MnF₃ were obtained in the concentration range x<or=0.02. From the splitting of cubic (220) and (400) Bragg's lines the temperatures of the structural phase transitions and the types of distortion were determined. The following sequence of distortions was observed: cubic to tetragonal to orthorhombic to monoclinic.

The authors investigate the structural stability of the various random and intermetallic phases observed in the experimental phase diagrams of the LiAl system. A second-order pseudopotential theory is used in conjunction with the virtual crystal approximation to describe the structural energies of the random substitutional alloys. On the other hand the authors employ the augmented-spherical-wave (ASW) method on the same footing to calculate the total energies for the corresponding ordered phases. Even though these two methods are based on entirely different approaches, it is interesting to examine their separate roles in explaining the relative stability of different phases of LiAl alloys.

Likely mechanisms for proton transfer in alpha-helical proteins are explored. The fundamental structural element is assumed to be continuous chains of hydrogen bonds formed from the peptide groups and a molecular example is presented. Using the PM3 semi-empirical molecular orbital method, the authors study the potential energy surfaces for the proton transfer processes in such systems. The results obtained can be used to explain some basic transport phenomena in biological systems.

An experimental and theoretical study of the electronic structure and the X-ray Si K and Si L emission bands of 4:2-coordinated alpha -quartz and 6:3-coordinated stishovite is presented. The nature of atomic states participating in the X-ray valence band spectra is identified. In particular the long-standing controversy about the role of Si 3d-like states is analysed; evidently, Si 3d-like electrons essentially contribute to the intensity of Si L emission bands. The authors have calculated the total density of states, the local partial densities of states, the ratios of the s, p and d charge components inside silicon and oxygen sited spheres, and the Si K and Si L emission bands. There is good agreement between their measured and calculated spectra. Reasons are given why in contrast with the Si K emission bands the Si L emission bands are insensitive to the local crystal structure of alpha -quartz and stishovite.

The linear muffin-tin orbital method is used with the atomic-sphere approximation to study the ground-state properties, band structures and pressure dependence of the band gaps of zincblende GaN. The equilibrium lattice constant calculated is smaller than the experimental value by about 2%. It is found that the fundamental band gap is direct from Gamma _15v to Gamma _1c and that the value (3.90 eV) is in good agreement with the experimental value. The linear pressure coefficient for this band gap is much smaller than those for GaAs and GaP. The total density of states and local density of states are discussed. Moreover, the band structure of wurtzite GaN is also studied. It is found that the band gap of wurtzite GaN is smaller than that of zincblende GaN, which agrees well with the experimental results.

The linear muffin-tin orbital method is used with the atomic-sphere approximation to study the electronic properties of beta -SiC. The equilibrium lattice constant, bulk modulus and band structures calculated are in good agreement with the experimental results. The strain effect on the band structures is discussed; the results show that, when the lattice constant parallel to the surface is expanded by 8% or more the indirect fundamental gap might become direct.

The linear muffin-tin orbital method in the tight-binding representation is adopted to study the electronic structures of the native defects in beta -SiC with the Green function approach. The original potential parameters are obtained by the standard band calculation of bulk beta -SiC and the change in the potential parameters is obtained self-consistently. It is found that the silicon vacancy acts as an acceptor and the carbon vacancy acts as a donor with the split-off states at 0.45 and 1.67 eV above the valence band maximum. For the antisite defects, there is no split-off state in the band gap.

This study investigates the effects of dissipation on the quasi-particle transport in a degenerate molecular trimer in which the time evolution is described by the discrete non-linear Schrodinger equation. The standard procedure of introducing dissipation as a phase-destroying stochastic parameter through the corresponding Liouville-von Neumann equation is employed. A closed second-order integrodifferential equation describing the time evolution of the site occupation probability difference is derived in the simultaneous presence of non-linearity and dissipation for trimers in compact geometry subjected to specific initial conditions. The Painleve property of the evolution equation is examined and it is found that the system is not of p type. A conflict arising from the two opposing views existing in the literature on the effects of dissipation on non-linear dynamics in a closely related system is focused on. Both the resolution of this conflict and a description of trimer dynamics are provided by elucidating the effects of the interplay between dissipation and non-linearity through numerical experiments.

The polarized Raman spectra from Bi_2.4Sr_2.6Fe₂O_{9+ delta} , which is isostructural to the superconducting Bi₂Sr₂CaCu₂O_{8+ delta} , are reported. The crystals have been characterized by TEM analysis, confirming the existence of commensurate modulation and giving evidence for a weak misorientation of the a-b planes. The number and symmetries of Raman phonon modes have been analysed within the Amaa space group. In close comparison with previous reports on the scattering from Bi₂Sr₂CaCu₂O_{8+ delta} , the Raman lines of Bi_2.4Sr_2.6Fe₂O_{9+ delta} , at 116, 157, 190, 247, 285-300, 310-330, 365, 460 and 614 cm^-1 have been assigned to definite modes of Ag symmetry. Much weaker lines of B_1g (160 and 290 cm^-1) and B_3g (about 240 cm^-1) symmetries have also been observed. The commensurate modulation of the structure seems to have no effect on the Raman spectra.

For pt.I see ibid., vol.4, p.7307 (1992). The R₁ line of Cr³⁺ in an antiferromagnetic garnet crystal Gd₃Sc₂Al₃O₁₂ (GSAG) is broadened inhomogeneously and homogeneously. Site-selective techniques, such as fluorescence line narrowing (FLN), can remove the inhomogeneous broadening created by strain and compositional disorder. The FLN spectrum then reflects the broadening due to processes including spin-spin exchange interaction between Cr³⁺ ions in octahedral positions and Gd³⁺ ions in dodecahedral positions in the GSAG unit cell. A magnetic field can remove the homogeneous broadening due to antiferromagnetic exchange. The R₁ line is associated with the spin-forbidden ²E to ⁴A₂ transition. In general, the selection rule on the R₁ line breaks down through spin-orbit interaction on the Cr³⁺ ion. Simulations of the FLN spectra of Cr³⁺:GSAG as a function of both temperature and magnetic field have been used to estimate the magnitude of the spin-spin exchange coupling constant, J approximately=0.25-0.33 cm^-1. Such simulations also show that for the ²E to ⁴A₂ transition to antiferromagnetic GSAG the Gd³⁺ spin in the Cr³⁺-Gd³⁺ spin exchange system is conserved in the presence of a magnetic field (B>2T), whereas the selection rule breaks down in the absence of a magnetic field.

The specimens of Fe_1-xCo_x alloys obtained by electrodeposition onto graphite were tested by Mossbauer spectroscopy and X-ray diffraction. Two structural transformations from the body-centred cubic to the face-centred cubic and further to the hexagonal, as the Co concentration increases, were observed. How the hyperfine field depends on the occupancies of the 3d and 4s states was deduced. Changes in the preferred direction of domain magnetization caused by the variable Co concentration have also been observed.

At low temperatures, TbPO₄ shows two phases with antiferromagnetic ordering in immediate succession, the upper one with tetragonal and the lower one with monoclinic crystal symmetry. Magnetisation and magnetic susceptibility were measured in the region of these phases for different external field directions. One main aspect was to study the anisotropy of the susceptibility for a single-domain crystal in one of the low-symmetry phases. Unexpectedly, it turned out that the susceptibility is isotropic in the basal plane.

For pt.I see ibid., vol.5, p.935 (1993). Studies of the Faraday rotation in TbPO₄ confirmed the known properties of the different low-temperature phases. Unexpected oscillations, however, were observed in the temperature dependence of the measured rotation in one of the low-symmetry phases. Model calculations of the rotation for a material possessing both Faraday rotation and birefringence did allow an interpretation of the findings, but rather unusual properties of the substance have to be assumed. These calculations additionally showed that the method of measurement influences the results in a typical way so that the optical properties of the substance cannot always be derived directly from the experimental curves.

For pt.II see ibid., vol.5, p.945 (1993). Measurements of the linear optical birefringence were used to determine the phase diagram of TbPO₄ at temperatures down to 1.4 K. At least one new structural and magnetic phase transition was found that is not detectable in magnetic investigations. Calculations using an appropriate mean-field Hamiltonian, including crystal distortions of monoclinic symmetry, allowed the authors not only to reproduce the experimental phase diagram very satisfactorily but also to give further insight into the magnetic structures and the distortions of the crystal in the various phases.

Positron lifetime and Doppler broadening measurements have been performed on the layered semiconductors GaS, GaSe and GaTe in the temperature range 8-320 K. The temperature dependences of the annihilation parameters in GaS and GaSe are analysed in terms of the thermal expansion coefficient of the lattice and a volume coefficient of the annihilation parameter. The results reveal a noticeable volume effect on the positron annihilation characteristics in the bulk of GaS and GaSe; no evidence for positron trapping at grown-in defects was found in these samples.

The spherically averaged Compton profile (CP) for gold in calculated on the basis of the relativistic density-functional theory in the local-density approximation (LDA) with a warped muffin-tin potential. In the all-electron self-consistent procedure, the core levels are calculated by means of the Dirac operator while the valence levels are determined by a relativistic generalization of the modified augmented plane-wave (MAPW) method using the reduced 2*2 matrix Hamiltonian according to Foldy and Wouthuysen. The CPs of the core electrons are found to be in surprisingly good agreement with the orbital CPs of the relativistic Hartree-Fock calculation for atomic gold by Biggs et al. The relativistic corrections on the valence CP due to mass-velocity and the Darwin term are investigated separately and comparisons with non-relativistic calculations are made.