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Conformations and packings of poly(n-butyl-ethyl silane), poly(n-propyl-methyl silane), and poly(iso-propyl-methyl silane) have been proposed on the basis of experimental and theoretical x-ray diffraction patterns. The silicon backbone of poly(n-butyl-ethyl silane) has a helical conformation, whereas the backbones of poly(n-propyl-methyl silane) and poly(iso-propyl-methyl silane) have all- trans conformations. Side-chain conformations and packings have also been proposed, and the similarities of and the differences between the structures of the polysilanes having symmetrical and asymmetrical side-chains are pointed out.

Energy-level statistics in a quasiperiodic system possessing a metal - insulator transition is investigated. For this purpose, the integrated level-spacing distribution I(s) for the Harper model with an incommensurate potential is numerically calculated. It is shown that there are only three possible distributions. In the metallic regime , I(s) is of the form , where A is a constant. In the insulating regime , I(s) follows the same law as it does in the situation where the energy levels are equal to . At the critical point , I(s) is described by an inverse power law with . The derivative of I(s) indicates that level-spacing distributions for the Harper model in the metallic and insulator regimes are different from the well known Wigner surmise and Poisson law for disordered systems.

perovskites (R = rare earth) provide a remarkable opportunity to study the relationship between structural and physical properties since, by moving along the 4f rare earth series, the evolution of several transport and magnetic properties can be nicely correlated to the steric effects associated with the lanthanide contraction. The most appealing example is probably the metal - insulator transition discovered for the compounds with , whose critical temperature increases with decreasing size of the rare earth ion. On the other hand, several experimental results show that in perovskites the degree of Ni 3d - O 2p hybridization is extremely high. The purely ionic configuration, first suggested by neutron diffraction experiments, has been then substituted by a ground state where the and configurations are strongly mixed. In this paper, a summary of the scientific work performed on the system during the last 6 years is presented. The results of the different experimental techniques are successively reviewed, with special emphasis on the relationship between the structural and the physical properties. The possible mechanisms responsible for the metal - insulator transition, which is the major open question for perovskites, are also discussed.

A quantum mechanical treatment of the effects of stacking faults on dechannelling is given. A simple harmonic model for the planar potential due to two planes surrounding the channel, and the corresponding bound states in the potential are considered. At the stacking fault boundary, these states make transitions for which the probabilities have been calculated, using the `sudden approximation'.

The low-order moments of the one-phonon spectral function of a crystal are shown to be related directly to the self-energy of the phonon. These relationships are generalizations of the Placzek sum rule and of a well-known high-temperature extension. The rules can be used to simplify a lattice-dynamical calculation of the moments, or, if the moments have been calculated by some other method, such as a Monte Carlo simulation, the rules can be applied in the other direction to yield information about the self-energy of the mode. This information can, in turn, be used to test the quality of a lattice-dynamical approximation. As an illustration, the rules are applied to transverse and longitudinal zone-boundary phonons in argon at 81 K.

The effect of oxygen deficiency on a phase transition from cubic to tetragonal phase has been investigated for the spinels with by measurements of differential scanning calorimetry (DSC), magnetic susceptibility nuclear magnetic resonance ( NMR) and electronic resistivity . According to the DSC measurements, the transition temperature for a stoichiometric compound is determined to be K. As increases from zero to 0.066, the magnitude of is found to decrease by about 4 K; then, the magnitude of increases by about 41 K with further increase of up to 0.10; thus, the curve exhibits a broad minimum at around . This relationship between and is also observed by the measurements of NMR and . The dependence of on is considered to be a result of a competition between a decrease in the average valence of Mn ions and a dilution of the nearest-neighbour interaction between Mn ions. Both effects are caused by the oxygen deficiency; the former raises , whereas the latter reduces it.

We report on a study of the electronic, chemical and lattice structure of carbon films and of the microstructural modifications induced by implantation. Correlations were made with the new mechanical properties of the implanted films. The C films were r.f. magnetron sputtered from a graphite target with Ar discharge. implantations were performed with energies ranging from 30 to 160 keV and at a fluence of . The effects of N on the structural and chemical properties of the C films were studied by means of x-ray photoelectron spectroscopy (XPS) and Raman scattering. Nuclear reaction analysis (NRA) and elastic recoil detection (ERD) were used to determine the atomic composition and density of the films. The carbon films consisted of two phases: microcrystalline graphitic domains dispersed in an amorphous matrix. The ratio of the hardness to the Young modulus, H/E, is comparable to that of diamond. implantation produces new bondings in these films, enhances the amorphicity and increases the disorder of the graphitic phase. The diamond-like features of the electronic valence band (strong s - p-orbital mixing) are preserved in the implanted films, together with the Young modulus of the network. Only the top of the valence band was affected by the ion irradiation. The new microstructure of the surface region of the implanted films resulted in an improved friction behaviour. implantation decreased the ratio H/E to a value similar to that of metallic hard coatings.

The electronic properties of the wide-band-gap semiconducting ordered alloys (for x = 0.0, 0.25, 0.50, 0.75 and 1.0) and the random alloys have been investigated by using a full-potential self-consistent linear muffin tin orbital (LMTO) method. The calculated direct band gap for random distribution of cation nearest-neighbour tetrahedral clusters in the alloys for any arbitrary concentration x is seen to show a quite linear variation in agreement with the experiment. On the other hand, the indirect band gap remains invariant. We observe a direct to indirect band gap crossover at x = 0.59. The band gap bowing is seen to be very small.

Transport properties of one-dimensional Kronig - Penney models with binary correlated disorder are analysed using an approach based on classical Hamiltonian maps. In this method, extended states correspond to bound trajectories in the phase space of a parametrically excited linear oscillator, while the on-site potential of the original model is transformed to an external force. We show that in this representation the two-probe conductance takes a simple geometrical form in terms of evolution areas in phase space. We also analyse the case of a general N-mer model.

We study the effect of an idealized tip-induced electric field on the electronic structure of a semiconductor surface containing defects. The embedding-potential method is used to connect the Green's functions of the solid and the vacuum. The corresponding localized perturbation potential permits us to derive the Dyson equation for the Green's function of the full system. This equation is solved in real space on a discrete mesh. The polarization of the surface with defects is determined inside the tunnelling barrier. It is found that the electronic density over the clean surface responds more strongly to the tip-induced electric field than the electronic density over the region containing defects.

We have numerically and analytically studied (ac+dc)-driven Josephson-junction arrays with a single vortex or with a single vortex - antivortex pair present. We find single-vortex steps in the voltage versus current (I - V) characteristics of the array. They correspond microscopically to a single vortex phase-locked to move a fixed number of plaquettes per period of the ac driving current. In underdamped arrays we find vortex motion period doubling on the steps. We observe subharmonic steps in both underdamped and overdamped arrays. We successfully compare these results with a phenomenological model of vortex motion with a non-linear viscosity. The I - V characteristic of an array with a vortex - antivortex pair displays fractional voltage steps. A possible connection of these results to present-day experiments is also discussed.

We study the antiferromagnetic spin chain with both dimerization and frustration. The classical ground state has three phases, a Néel phase, a spiral phase and a collinear phase, which we study through a nonlinear sigma-model approach. In the spiral phase, the field theory becomes and Lorentz invariant at long distances, a model which is exactly solvable. The low-energy spectrum is doubly degenerate with massive `elementary' spin-1/2 particles and `two-particle' triplet and singlet physical excitations. The field theory also supports solitons which lead to a double degeneracy of the ground state for half-integer spins (when there is no dimerization).

The heat capacities of a single crystal of the stoichiometric compound were measured over the temperature range 14 - 500 K with an adiabatic calorimeter. Three heat capacity anomalies were observed. The two anomalies taking place at around 80 K and 150 K were interpreted as being due to first-order and second-order structural phase transitions, respectively, potentially of the locked-in-commensurate - incommensurate - normal-commensurate type. The third anomaly at 330 K was indicated to be a three-dimensional antiferromagnetic phase transition found commonly in two-dimensional antiferromagnets of the type. The two-dimensional antiferromagnetic short-range order is discussed on the basis of the magnetic contribution to the heat capacity curve on the high-temperature side of the transition temperature.

A novel method was developed to prepare iron nanocrystallite by using coimplantation of Fe and C in copper and subsequent heat treatment. The formation process of iron carbide and iron granules was monitored by means of conversion-electron Mössbauer spectroscopy, and the particle size was determined by transmission electron microscopy. Cementite, , was formed after annealing at , then transformed to particles with a size of about 25 nm. Magnetic coercivities of 1.15 kOe and 0.37 kOe for the Fe granules have been observed at room temperature (RT) and 80 K, respectively. A retained carbon-enhanced effect was proposed to explain the strong coercivity at RT.

Electron - nuclear double-resonance (ENDOR) measurements were performed on the F centre in CsBr using the stationary ENDOR method. Five Cs and three Br shells could be resolved. The observed superhyperfine interactions cannot be explained by a spherically symmetric F-centre wavefunction. Lattice-ion overlaps and an admixture of -type functions (L = 4) have to be included in the description of the F-centre wavefunction.

Reflection matrices with the correct space - time symmetry properties may be obtained from the standard Maxwell boundary conditions only when the constitutive relations for the D and H fields satisfy covariance requirements. In this paper we present the first application of covariant multipole forms for D and H, to the order of electric quadrupole and magnetic dipole, to a magnetic crystal in order to compare the predictions obtained from this theory with recent experimental results. In particular, the theory is used to determine the Fresnel reflection amplitudes for antiferromagnetic when a monochromatic plane light wave is incident normally from a vacuum on a crystal face perpendicular first to the axis and then to the axis. The theory agrees well with experiment. In addition, it reveals a novel relationship between certain components of the frequency-dependent magnetoelectric tensor, which, with the aid of transmission and reflection data, would enable the surface and bulk contributions in reflection to be distinguished.

The electronic band structure of single-crystalline (100) has been probed by angle-resolved ultraviolet photoemission in the energy range . The bands were mapped as a function of along the direction, and compared with theoretical calculations. The area closest to the Fermi energy (binding energy ) has been mapped in greater detail. A large density of states has been detected here. Two different bands were resolved for distinct polarization conditions, located close to the X point. One of them is non-dispersing, while the other presents a band width of . The presence of these narrow, intense bands is discussed in view of a possible Kondo-insulator-based description of the electronic structure of .

The Heusler alloy was prepared by mechanical alloying and subsequent heat treatment at 973 K for 1 h. At saturation, the resulting material had an atomic magnetic moment of /Mn atom. Neutron diffraction experiments were carried out at room temperature, both with and without an applied magnetic field of 12 kOe. The Rietveld method was used to fit calculated curves to both sets of data. Nuclear and magnetic contributions were modelled simultaneously, using a large Debye - Waller thermal parameter to model the magnetic form factor. The results showed that the material was ferromagnetic. Cu atoms occupied approximately 10% of the Mn sites. Particle and magnetic domain sizes were determined from the results of the Rietveld analysis. These were respectively and . The nuclear contribution to the diffraction pattern was separated from the magnetic, by subtraction of the two sets of data. This was studied using the Rietveld method and the resulting structure was similar to that obtained when nuclear and magnetic components were modelled simultaneously.

Equation (9) of this paper appeared in its correct form, as follows:

but the definition of contained errors and should have read

These were typographical errors and do not affect the subsequent calculations.