We report the dependence of the microwave-frequency dielectric constant and dc conductivity on the crystallinity and local order of doped polyaniline. The dramatic increase of the dielectric response in the three highest crystalline physically crosslinked polyaniline samples provides evidence for the formation of three-dimensional coupled mesoscopic metallic regions. We report the measurement of a negative dielectric constant in polyaniline (doped with camphor sulfonic acid) demonstrating the intrinsic metallic nature of polyaniline.

We present a study of the electrical transport in porous Si layers prepared by anodic etching of two different kinds of (100) p-type Si substrates. It is shown that by choosing a sufficiently thick layer, the problem of injection from the contacts can be eliminated. In this way we measure the intrinsic transport properties. The results suggest that a Poole-Frenkel type of mechanism accounts for the observed electric-field-enhanced conduction.

Various microscopic mechanisms for Si diffusion in GaAs and Si-induced interdiffusion in GaAs/AlAs superlattices are investigated by ab initio molecular dynamics. The dominant mechanism involves the motion of negatively charged ${\mathrm{Si}}_{\mathrm{III}\mathrm{-}}$${\mathit{V}}_{\mathrm{III}}$ pairs through second-nearest-neighbor jumps. This mechanism explains both the ability of Si to disorder superlattices regardless of whether it was introduced during growth or in-diffused afterwards and the suppression of interdiffusion by compensation doping. The computed activation energies are in very good agreement with the experimental data.

We present data from Mo/a-Si/Mo tunnel junctions together with calculations that show that hopping transport via localized states in amorphous silicon is highly correlated. Localized states whose single-particle energies lie well below the Fermi level participate in transport due to the large on-site Coulomb interaction U. The results also imply that the density of these states is roughly constant over a wide energy range of order U.

We report measurements of the homogeneous linewidth of an excitonic resonance in a quantum wire. Transient time-integrated and time-resolved four-wave-mixing experiments are used to obtain the homogeneous linewidth by measuring the dephasing time. A biexponential decay is observed and attributed to the presence of free and localized excitons. The homogeneous linewidth for both species is measured as a function of temperature and excitation density. Quantum beats are observed due to interference between the m=1 and m=2 center-of-mass-quantized n=1 heavy-hole exciton states.

The mechanism of strain relief for compressively stressed Ag layers epitaxially grown on Pt(111) is studied by scanning tunneling microscopy. The strain in the compressed commensurate (1×1) Ag monolayer on Pt(111) is relieved in the bilayer by the formation of two weakly incommensurate structures: the metastable striped incommensurate phase, which converts into a trigonal network of domain walls upon annealing. This network is a general case for isotropic strain relief realized by the crossing of domain walls. The introduction of a phase shift of one set of domain walls allows the system to account for the energy difference of fcc and hcp stacking. The stability of unidirectional via isotropic strain relief and the respective structures of domain walls is generally discussed for surface reconstructions and metal epitaxy of hexagonal-close-packed surfaces.

The possible ground states of a ternary fcc lattice model with nearest- and next-nearest-neighbor pair interactions are investigated by constructing an eight-dimensional configuration polytope and enumerating its vertices. Although a structure could not be constructed for most of the vertices, 31 ternary ground states are found, some of which correspond to structures that have been observed experimentally.

We propose a tiling-and-decoration model of the icosahedral phase of Al-Mn-Si, in which a 126-atom ‘‘double’’ Mackay icosahedron and a 54-atom ‘‘simple’’ Mackay icosahedron alternate on the even and odd nodes of the tiling of four canonical cells. The model structure exhibits weak six-dimensional fcc ordering introduced by Al-vacancy alternation, similar to that observed in the related α phase. The density of the model is consistent with experimental data and calculation of the x-ray- and neutron-diffraction intensities leads to good agreement with experimental data in both cases: R factors are 0.09 and 0.16, respectively.

We report the results of inelastic neutron-scattering measurements on ${\mathrm{Rb}}_{2.6}$${\mathrm{K}}_{0.4}$${\mathrm{C}}_{60}$. Librational modes were observed as broad peaks with maxima between 4.1 and 4.7 meV, as the temperature is lowered from 300 to 12 K. As in ${\mathrm{K}}_3$${\mathrm{C}}_{60}$, no change in the width or position of the librational peak was observed when the sample was cooled through the superconducting transition. Thus any coupling of the librations to electronic states is small. The magnitude of the orientational potential barrier was estimated from the librational peak frequency. A flat background observed in the low-energy inelastic-scattering spectra is ascribed to two-phonon scattering. The density of states of intramolecular modes is similar to that of previously studied ${\mathit{M}}_3$${\mathrm{C}}_{60}$ compounds; modes at 53 and 66 meV in pure ${\mathrm{C}}_{60}$ are not observed in ${\mathrm{Rb}}_{2.6}$${\mathrm{K}}_{0.4}$${\mathrm{C}}_{60}$.

We present real-time Raman measurements on graphite under 3-keV rare-gas ion irradiation. The graphite lattice disorder is estimated by the relative intensity ratio of the disorder-induced peak (∼1360 ${\mathrm{cm}}^{\mathrm{-}1}$) with respect to the Raman active ${\mathit{E}}_{2\mathit{g}2}$ mode peak (∼1580 ${\mathrm{cm}}^{\mathrm{-}1}$), and the peak intensity ratio is found to be proportional to the square root of ion irradiation time in the beginning of ion irradiation at doses below 2.5×${10}^{17}$ ions/${\mathrm{m}}^2$. At the higher doses the peak intensity ratio reaches saturation. The lattice disordering rate constant steeply increases for lower mass as ion mass increases, slowly reaching a saturation value for the higher mass. Ion-mass dependence of the disordering rate is well explained by the interdefect distance model calculation.

The continuous n-state Potts model is reinvestigated. The partition function is cast in a form allowing the limit, n→1, to be taken prior to perturbation-theoretical expansions. Proceeding in this way, we free the theory of pathologies known to plague other approaches. Several alternatives are presented for a Feynman-diagrammatic expansion of the theory, two of which are identical to those utilized in previous analyses of the mdoel. One of the introduced diagrammatic approaches is used to recover known results of one-loop renormalization calculations. It is also utilized to calculate the partition function of the (1+ε)-state Potts model, and, hence, the generating function for bond percolation, on a finite lattice. This generating function for cluster statistics exhibits the scaling features proposed by Privman and Fisher for systems of finite size.

Elastic and vibrational properties of lacunary square networks with first-neighbor central forces are shown to reduce, in the framework of the harmonic approximation, to longitudinal modes determined by a fragmentation in independent blocks, i.e., segments. Vibrational spectra are thoroughly analyzed with evidence for a transition from continuous spectra to Dirac singularities via singular continuous spectra which occur near the linear percolation threshold. Random lacunary networks, as well as fractal ones, are studied.

${\mathrm{Ni}}^{+}$ centers produced by room-temperature x-ray irradiation of ${\mathrm{CsCaF}}_3$ single crystals have been studied by EPR. Three types of centers have been detected, all of them with tetragonal symmetry and the ‖${\mathit{x}}^2$-${\mathit{y}}^2$〉 ground state. The models for these centers, which are similar to those observed in other fluoride crystals, consist of a ${\mathrm{Ni}}^{+}$ ion surrounded by a Jahn-Teller elongated fluorine octahedron [${\mathrm{Ni}}^{+}$(I)] and the same model, but with one and two nearest-neighbor fluorine vacancies, for ${\mathrm{Ni}}^{+}$(II) and ${\mathrm{Ni}}^{+}$(III). The spin-Hamiltonian parameters (g and superhyperfine tensors) have been measured and compared with the values predicted from molecular-orbitals calculations of the ${\mathrm{NiF}}_6^{5\mathrm{-}}$ and ${\mathrm{NiF}}_4^{3\mathrm{-}}$ complexes. The results allow us to determine the values of the ${\mathrm{Ni}}^{+}$-${\mathrm{F}}^{\mathrm{-}}$ distances. An anomalous behavior is found for ${\mathrm{Ni}}^{+}$(I). The orthorhombic character of the superhyperfine interaction with the equatorial fluorines also has been theoretically estimated and is in good agreement with the experimental results. Besides, an explanation based on the spin polarization of filled orbitals has been proposed to account for the superhyperfine interaction with the axial fluorines. Dynamical effects observed between 20 and 30 K in the ${\mathrm{Ni}}^{+}$(I) EPR signal have been discussed on the basis of a dynamical Jahn-Teller effect.

Inelastic light scattering by magnetic-energy fluctuations has been observed in the one-dimensional Heisenberg antiferromagnet ${\mathrm{KCuF}}_3$ using a conventional Raman spectrometer. The scattering, having a peak in the cross section at zero frequency and completely distinguished from Rayleigh scattering, is observed over an extremely wide range of temperature above ${\mathit{T}}_{\mathit{N}}$=39 K. The scattered light is strongly polarized and its line profile around zero frequency is well fitted on a Lorentzian curve. The cross section increases with increasing temperature above ${\mathit{T}}_{\mathit{N}}$. Our experimental data are well explained by the theory which Halley developed introducing the hydrodynamic form for the correlation function of magnetic-energy density given by Halperin and Hohenberg. The absence of such inelastic scattering in ${\mathrm{K}}_2$${\mathrm{CoF}}_4$ (a two-dimensional Ising antiferromagnet) and ${\mathrm{K}}_2$${\mathrm{CuF}}_4$ (a two-dimensional Heisenberg ferromagnet) in a Raman spectrum is also discussed.

We report the results of a comprehensive study of localization and electron-electron interaction effects in thin Bi films, Bi/Co multilayers, and Bi/Co/Bi sandwiches. Measurements of the electrical resistance and magnetoconductance have allowed us to separate the contributions to the resistance due to localization and electron-electron interactions. These have been utilized to determine the different scattering times due to these interactions. The overall behavior of these scattering times, excepting the magnetic scattering, is in good agreement with theory. Inelastic scattering has been shown to arise due to electron-electron scattering and the absolute magnitude of this scattering rate agrees reasonably well with theory. An anomalously sharp decrease of the magnetic scattering with a lowering of the temperature is observed. An explanation of this behavior has been attempted through the spin-wave theory of thin films. The observed surface magnetic impurities behave quite differently from the bulk impurities in metals and constitutes an important finding of this work.

We have observed magnetic and electrical transport properties in giant magnetoresistive inhomogeneous cobalt-silver films. The material consists of two distinct magnetic phases: large clusters which dominate the magnetization and magnetoresistive processes at room temperature, between which cooperative behavior is observed; and small clusters which dominate the magnetization below 10 K, but make only a minor contribution to the magnetoresistance. Both the variation of the magnetoresistance with magnetization in the film and the difference in the magnetoresistance between the zero-field-cooled and field-cycled state are interpreted by invoking interactions between the active magnetic regions in the sample.

We study the two-dimensional classical Ising model that is equivalent, via the Suzuki-Trotter mapping, to the XXZ Heisenberg quantum-spin chain. By imposing appropriate boundary conditions to the Ising model, the spin waves of the quantum model are studied. We reproduce the entire energy spectrum of the two-spin-wave states and derive bound-state energies of the three-spin-wave states. Thus, the continuum energetics of the elementary excitations of a d-dimensional quantum model are contained in the equivalent (d+1)-dimensional classical model, even though the latter is a discrete-spin model.

The magnetic phases of erbium have been studied by resonance x-ray-scattering techniques. When the incident x-ray energy is tuned near the ${\mathit{L}}_{\mathrm{III}}$ absorption edge, large resonant enhancements of the magnetic scattering are observed above 18 K. We have measured the energy and polarization dependence of this magnetic scattering and analyzed it using a simple model based on electric dipole and quadrupole transitions among atomic orbitals. The line shapes can be fitted to a magnetic structure combining both c-axis-modulated and basal-plane components. Below 18 K, we have observed unusual behavior of the magnetic scattering as a function of energy, whose origin is not understood.

${\mathrm{Tl}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ and ${\mathrm{In}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ show spontaneous magnetization around 121 K indicating magnetic phase transitions. The related pyrochlores ${\mathrm{Y}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ and ${\mathrm{Lu}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ also exhibit spontaneous magnetization at their respective magnetic transition temperatures around 17 K. However, neutron-scattering studies do not provide any evidence for long-range order but suggest reentrant spin-glass behavior. In view of this we have performed small-angle neutron-scattering (SANS) investigations on ${\mathrm{Tl}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ and ${\mathrm{In}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$. SANS results substantiate the long-range magnetic ordering in ${\mathrm{Tl}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ as indicated by the magnetic behavior. Preliminary SANS data of ${\mathrm{In}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$, while more ambiguous, are not inconsistent with the presence of long-range order. Effective paramagnetic and saturation magnetic moments of ${\mathrm{Tl}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ and ${\mathrm{In}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ indicate the presence of ${\mathrm{Mn}}^{3+}$ ions on the ${\mathrm{Mn}}^{4+}$ sublattice as do the measured lattice constants. The occurrence of ferromagnetic ordering in these materials at high temperatures has been attributed to the double exchange interaction between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ spins.

The influence of quasilocal vibrations on the far-infrared spectra of glasses is investigated under the supposition that the quasilocal vibrations are directly inactive in the attenuation. It is shown that taking into account phonon-quasilocal-vibration coupling and both medium- and short-range structure of the inhomogeneities of a photon-phonon coupling parameter can give rise to an attenuation curve similar to one observed for a-${\mathrm{SiO}}_2$ and related materials.