Table of contents

The authors report here high-resolution electrical resistivity studies on single-phase quasicrystalline samples of the series Ti₅₆Ni_28-xFe_xSi₁₆(0<or=x<or=15). They have also compared them with this data on amorphous and crystalline samples. The authors quasicrystalline samples show some interesting features which are not reported for any other quasicrystalline systems. The value of the resistivity and its temperature dependence, the observation of resistance minimum at fairly high temperatures and their variations with x have been discussed in the framework of the modified Ziman-Faber theory.

The structural environment of Ti and Ni ions incorporated into LiNbO₃ single crystals during the growing process has been studied by using X-ray absorption spectroscopies: extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopies. It has been found that for the impurity concentration used (1% of TiO₂ and 0.1% NiO in the melt) both ions, Ti⁴⁺ and Ni²⁺, are incorporated at the Li site of the LiNbO₃ lattice.

A new infrared absorption band at a wavenumber of about 3545 cm^-1 has been observed in LiNbO₃ crystals implanted with energetic H⁺ ions. It is found that the peak wavenumber of this band seems to be independent of the stoichiometry, dopant species and doping level in the investigated samples. The band, polarized perpendicular to the c axis, disappears after the sample was annealed in air at 500 degrees C for 0.5 h. The new absorption band is interpreted as a distributed OH stretching vibration in a H⁺-implanted LiNbO₃. The displacement of Nb⁵⁺ ions from their regular sites (which was caused by the ion implantation) may expand the oxygen triangle to result in a longer O-O bond this can explain the higher wavenumber of the OH band in the H⁺-implanted LiNbO₃.

The ground state properties of fluorite (CaF₂) have been studied using CRYSTAL, an ab initio periodic Hartree-Fock program. Twenty-two and thirteen atomic orbitals (represented as contracted Gaussian-type functions) are used for the calcium and fluorine atoms, respectively. The binding energy (BE), the equilibrium lattice parameter (a), the elastic constants (C_ij) and the central zone phonon frequencies nu (IR) and nu (Raman) have been evaluated, and a good agreement obtained with experiment (for instance the error is +2.0, +1.7, -0.5% for BE, a and C₁₁, respectively). The calculated C₄₄ elastic constant reduces from 48 to 44 GPa (experimental value: 37 GPa) when the fluorine atoms are allowed to displace under strain, indicating the importance of inner deformation for shear elasticity. Electron density maps, density of states and band structure plots are reported which confirm the fully ionic nature of fluorite.

Self-avoiding magnetic chains (SAMCs) were observed in annealed Fe-Cu multilayers. Scaling analyses showed that these structures were statistically self-similar and satisfied the power-law relationship well in their stretched dimension. Compared with the self-avoiding walks, the SAMCs were found to have smaller global fractal dimensionality and less geometrical entropy, which were attributed to the existing magnetic interaction and local lattice coherence during their growth, respectively. A modified tip-to-tip model was proposed to simulate the aggregation processes, and the results were in agreement with the observations.

Scattering experiments using Mossbauer gamma rays and X-rays have been performed on ordered and disordered Pb(Sc_0.5Nb_0.5)O₃ (PSN), and Pb(Fe_0.5Nb_0.5)O₃ (PFN) which exists only in a disordered state. All three materials are ferroelectric at room temperature and undergo a change to a paraelectric form close to 100 degrees C. No evidence of an increase in inelastic scattering was found as the transition was approached in any of the materials studied. Rather large differences were found in the magnitude of Debye-Waller factors calculated from integrated intensities obtained using X-rays and Mossbauer gamma rays. These differences arise because part of the atomic displacement from positions of high symmetry is dynamic.

For pt.I see Ibid. vol.2, p.7143 (1990). A previous investigation of the stability of static one-dimensional solutions of the Landau-Ginzburg equation with a quartic non-linearity is extended. Exact spatially varying solutions are modified by small amplitude, time-dependent perturbations. In contradiction to the case of part I of this study, these are not assumed to have small frequency omega and small decay rates gamma . The authors show that all periodic solutions, as well as the solitary waves, are unstable with respect to this new type of perturbations. The kink solution is stable with respect to all perturbations considered. When the results of both parts of this paper are put together, they obtain an extensive stability analysis of static solutions to the Landau-Ginzburg equation. This equation is important in fluid dynamics, solid state and superconductivity theory, as well as other branches of physics. The paper is self-contained and can be read independently of part I.

The electronic structure of the CoSi₂(111) surface is studied within the tight binding approximation coupled with the decimation technique. The different geometries considered have either a Co- or a Si-bulk terminated surface and are called CoSi₂-Co and CoSi₂-Si respectively. The CoSi₂-Co surface shows a sharpening of the Co partial d density of states in relation with the reduction in the number of Co neighbours. In contrast, CoSi₂-Si presents a surface state with a Co-d character at 2.3 eV below the Fermi level. This result is compared with the surface state determined by Pirri et al.

The authors present two-dimensional angular correlation of (polarized positrons) annihilation radiation (2D ACAR) results measured on a single crystal of Ni in the (100) plane below and above the Curie temperature (at 4.2, 300, 600 and 660 K). The experimental distributions (total momentum density) measured at different temperatures do not exhibit any significant differences, in contradiction to the one-dimensional (non-spin-polarized) results of Kontrym-Sznajd et al. They conclude that the Fermi surface topology of Ni essentially does not change between 4.2 and 600 K. This can be related to the fact that the exchange splitting Delta , remains almost constant up to the Curie temperature. This would confirm the spin-polarized electron-energy-loss spectroscopy results of Kirschner and Langenbach.

The author calculates the temperature dependence of the inelastic scattering rate at the mobility edge in a disordered system in which diffusion is characterized by a wavevector and frequency dependence, and compare it with that obtained when only conventional diffusion occurs. The author finds that the inelastic scattering rate exhibits a linear temperature dependence in both cases. He discusses this result in the context of low-temperature transport measurements in integral quantum Hall samples and point out certain theoretical inconsistencies which need to be resolved.

In the present paper the authors continue the recent investigations of Tomaszewicz et al on the influence of spatial non-homogeneity of the trap distribution on thermally stimulated currents (TSC) due to transport of carriers in insulating layers. They consider here-in contrast to the previous work-the case of bulk initial generation of carriers. For both non-dispersive and dispersive transport, the analytical formulae obtained agree satisfactorily with the results of Monte Carlo simulation. The possibility of determining the spatial trap distribution on the basis of the measured TSC is discussed.

The authors calculate the ground state energies and wavefunctions for small systems of fermions on a lattice using singly and doubly periodic boundary conditions. They find minima in the ground state energy when the magnetic flux per plaquette corresponds to filling fractions with odd denominators. They interpret these minima in terms of fractional quantum Hall states. The pair correlation function at nearest and next-nearest neighbours is calculated as a function of the repulsive interaction strength between particles.

Applying a magnetic field B to a double-barrier resonant-tunnelling diode, perpendicular to the layer structure, introduces oscillations in current density and capacitance that are periodic in 1/B. A derivation of this periodicity is given, based on coherent wave propagation. Two magneto-periods are found, corresponding to the electron concentration in emitter and well, respectively. Numerical calculations are presented for a semiconductor model with self-consistently determined electron potential.

The authors use the 2D quantum percolation model to simulate the mesoscopic transport properties of disordered metallic films. The conductance of a mesoscopic sample is calculated by the multichannel Landauer formula, with the transmission and reflection coefficients evaluated numerically with a recursive Green-function approach. The distribution of the conductance is found to exhibit three distinct types: normal, log-normal and between normal and log-normal, depending on the sample size L and metal fraction p. The variation of the mean with L yields the localization length and the root-mean-square conductance fluctuation Delta g is shown to have different regimes, including the universal-conductance fluctuation regime where Delta g is constant as a function of L and of the order of e²/h. Based on these results, they propose the use of mesoscopic samples of disordered metallic films for the direct experimental observation of Anderson localization for electrons. Two possible sample systems are discussed.

Oxide superconductors in the Y-Ba-Cu-O and Bi(Pb)-Sr-Ca-Cu-O systems with T_c degrees of 93.6 K and 115 K, respectively, have been prepared by the co-precipitation process via the oxine route using nitrates as the starting materials. Chemical homogeneity, stoichiometry and fine particles of the precursor obtained by this process permit a relatively short time for the production of Y- and Bi-based superconducting compounds compared with that obtained by the conventional solid state reaction method. The resulting densities of the sintered samples were typically greater than 90%. X-ray diffraction analysis, energy dispersive spectral analysis and scanning electron microstructural analysis are also reported in this study.

Single-crystal neutron diffraction measurements on cupric oxide, CuO, using a zero-field neutron polarimeter have confirmed that the Cu²⁺ moments are parallel to the monoclinic b axis in the commensurate antiferromagnetic phase stable below 213 K. Any component of moment in the a-c plane must have a magnitude less than 0.03 of the total. The incommensurate antiferromagnetic phase, stable between 213 K and the Neel temperature of 230 K, has a propagation vector (0.506, 0, -0.483) as previously reported, but the elliptical envelope of the helical modulation at 215.5 K is shown to have its axes parallel to (010) and in the a-c plane making an angle of 28.2(8) degrees to (001) beta obtuse. The envelope is almost circular with the ratio of the b to a-c components of 1.03(1) and the plane of the moments makes an angle of 73.0(5) degrees to the propagation vector. The new magnetic model produces an equally good fit to the original unpolarized neutron diffraction intensities as the previous model, in which the Cu²⁺ moments were confined to the a-c plane with a more elliptical envelope, and gives a root mean square Cu²⁺ effective moment of 0.27(1) mu _B.

The set of acoustic mode frequencies associated with elastic constants C₁₁, C₁₂ and C₄₄ have been determined from high temperature Brillouin scattering measurements on the anti-fluorite structured compound Li₂S. These decrease linearly from 300 to 800 K, but above 800 K substantial reductions occur in C₁₁ and C₁₂. The results are interpreted in terms of normal thermal expansion and a diffuse superionic transition involving Frenkel disorder on the lithium sublattice.

A parametric description of the crystal-field splittings of Ho³⁺ in the three host lattices YVO₄, YAsO₄ and HoPO₄ is given in the energy range from 0 to approximately 28000 cm^-1. The influence of electron correlation is taken into consideration by five additional parameters. The crystal-field Hamiltonian was diagonalized using a basis including all terms up to 35000 cm^-1 (316 states). Differences between the experimentally obtained crystal-field components and the calculated ones are usually smaller than 4 cm^-1.

The authors have performed electron transport studies in the quantum Hall regime on high-mobility Si MOSFETS with an adjustable discontinuity in the density of the two-dimensional electron gas. To achieve this they etch a gap with submicron width in the gate electrode and bias the two parts independently. For equal and integer filling factors on both sides of the gap there is negligible reflection of electrons in edge states meaning that the barrier is low compared with the Landau-level separation. For unequal but integer filling factors the longitudinal resistance across the gap is quantized, indicating that in this case the transmission coefficient of the edge states is either zero or one, depending on the gate voltages.

The thermodynamics of the Wigner crystallization of two-dimensional electrons in a magnetic field is set out. The results relate the slope of the melting curve as a function of the Landau level filling factor nu to changes in magnetization, Delta M, and entropy, Delta S, on melting, and identify points where Delta M, Delta S and Delta E=0, with E being the internal energy. The phase diagram as revealed by recent experiments on GaAs/AlGaAs heterojunctions is thereby analysed. Remarkable features are inferred in the magnetization of the strongly correlated electron liquid.

The author calculates an exact expression for the Green function of a single hole in a Neel background, for finite Ising interaction J_z identical to 4t²/U where t is the hopping (overlap) integral in the Hubbard model and U the usual on-site interaction. In 1D for fixed boundary conditions, loops are absent and thus the expression is exact and independent of the retraceable path approximation, while in 2D the expression is exact to within the exclusion of loops. The spectral function A( omega ) shows delta-function peaks at well defined energy spacings, with damping only at frequencies omega >5/2J_z different from earlier results. These peaks are necessary for the existence of quasiparticles. The author presents an argument that, for sufficiently large U, the discrete spectrum in A( omega ) merges to a continuous 'band' as the weight varies as J_z of the poles vanishes in the U= infinity limit and hence that use of the latter does not necessarily imply incoherent behaviour in the system.