The self-assembled network of conducting polyaniline (PANI), protonated by camphor sulfonic acid (CSA), in a matrix of insulating polymethylmethacrylate (PMMA) has a remarkably low percolation threshold. The critical volume fraction (f) of the PANI-CSA phase segregated in PMMA is inferred from the concentration dependence of the conductivity, ${\mathit{f}}_{\mathit{c}}$≊0.003 (0.3%). The conductivity at room temperature near the percolation threshold is quite high, 3×${10}^{\mathrm{-}3}$ S/cm. Transmission-electron microscopy (TEM) results are in agreement with the percolation threshold inferred from the transport data; the TEM micrographs show that the connectivity of the PANI-CSA network decreases rapidly for f<0.005. Near room temperature, the positive temperature coefficient of resistivity (ρ), a feature typical of the intrinsic metallic nature of PANI-CSA, is retained in the networks. At lower temperatures, ρ(T) exhibits a temperature-dependence characteristic of variable range-hopping transport, ρ(T)∝exp[(${\mathit{T}}_0$/T${)}^{\gamma }$], with the exponent increasing from γ=0.25 to 1 upon decreasing the volume fraction of PANI-CSA from f=1 to ${\mathit{f}}_{\mathit{c}}$. This systematic increase in γ results from transport on the fractal structure and to the related superlocalization of the electronic wave functions. Below the percolation threshold, the temperature dependence of the resistivity is like that of granular metals with γ≊0.5, consistent with the morphology and microstructure seen in the TEM micrographs. The positive magnetoresistance shows a maximum upon decreasing the volume fraction of PANI-CSA in agreement with effective-medium theory. Analysis of the magnetoresistance indicates that the localization length near the percolation threshold is approximately 25 Å at 4.2 K.

Metal-insulator transitions in the three-band extended Hubbard model are studied within the mean-field theory of the Kotliar-Ruckenstein slave-boson formalism. It is shown that the charge-transfer (CT) insulator transition (Δ≪U) takes place when twice the CT-energy loss 2Δ exceeds eight times the average kinetic-energy gain per site E, contrary to the Mott-Hubbard (MH) transition (U≪Δ) which occurs when the Coulomb-energy loss U exceeds E. The positions of the renormalized d levels of quasiparticles formed on dilute doping are analyzed. A dilute hole doping into the insulator phase gives rise to several qualitatively different doping regimes, e.g., those with d-hole or d-electron doping determined by some simple rules, in which dilute hole doping just inside the boundary always induces d electron doping. We will show that the dilutely hole-doped phase diagram is remarkably symmetric with respect to U and Δ, i.e., the MH and CT insulators. The optical conductivity gap ${\mathit{E}}_{\mathrm{gap}}$ of the MH and CT insulators is interpreted as a jump in the chemical potential Δμ between dilute electron and hole doping, and is strongly renormalized from the bare Hubbard gap U or CT gap Δ near the metal-insulator boundary, while it is roughly given by the bare gap well inside the insulator phase.

The transport of electrons, reflected from solids while suffering elastic scattering only, is investigated in detail. The angular path length and the depth distributions of the reflected electrons are calculated by solving the relevant half-space transport problem in the ${\mathit{DP}}_1$ approximation and with detailed Monte Carlo simulation. The incentive for the investigations is to calculate the energy spectrum of the reflected electrons in the vicinity of the elastic peak. According to a suggestion of Tougaard and Sigmund, this can be obtained from the path-length distribution of the elastically scattered electrons and a suitable energy-loss function. The purpose of the present paper is to determine the path-length distribution. It has been found that for certain elements, notably aluminium, carbon, and copper, at certain energies the path-length and depth distributions are not monotonic. The ${\mathit{DP}}_1$ approximation is in general better than the ${\mathit{P}}_1$ approximation, but for the nonmonotonic distributions, it is not satisfactory. The nonmonotonic behavior is shown to be related to certain properties of the scattering kernel.

We study a simple model of conducting polymers, such as polyacetylene, which consists of a quasi-one-dimensional coupled electron-phonon system with short-range electron-electron repulsions. The problem is solved in the Hartree-Fock approximation. A variety of experimentally measurable properties of the system, especially the infrared absorption spectrum, are computed. (i) As a function of doping, the interchain coupling causes the system to undergo a phase transition from a correlated soliton-lattice state to a metallic state, described by a weakly interacting but highly anisotropic three-dimensional Fermi liquid. The result is consistent with a previous renormalization-group treatment. (ii) When interactions with counterions are included, the nonmetal–metal transition is very sensitive to the arrangement of counterions relative to the polymer chain: a minor change in the arrangement will either stabilize or destabilize the soliton state allowing the coexistence of the metallic and the soliton states. (iii) For realistic three-dimensional band structures, a semimetallic phase intervenes the nonmetallic (Peierls) and metallic (Fermi-liquid) phases.

The nature of the chemical bond in three titanium oxides of different crystal structure and different formal oxidation state has been studied by means of the ab initio cluster-model approach. The covalent and ionic contributions to the bond have been measured from different theoretical techniques. All the analysis is consistent with an increasing of covalence in the TiO, ${\mathrm{Ti}}_2$${\mathrm{O}}_3$, and ${\mathrm{TiO}}_2$ series as expected from chemical intuition. Moreover, the use of the ab initio cluster-model approach combined with different theoretical techniques has permitted us to quantify the degree of ionic character, showing that while TiO can approximately be described as an ionic compound, ${\mathrm{TiO}}_2$ is better viewed as a rather covalent oxide.

We study an interaction of a running longitudinal wave with a metal. It is shown within a quadratic response that a single wave creates a dc electric current even if the creation of electron-hole pairs is prohibited. As a consequence, a macroscopic electric current depends on an instant distribution of bosons, e.g., plasmons. Beside a formal theory, two practical formulas are provided: the random phase approximation formula for a general electron band structure, and a general formula for a single parabolic band.

The search for electronic energy bands in full-potential Korringa-Kohn-Rostoker (KKR) band theory requires the evaluation of the scattering matrices at many different energies for a certain number of wave vectors. We develop Chebyshev-function approximations for the scattering matrices, which can save a factor of 2 to 3 in computing time for the case of silicon. Moreover, the Chebyshev coefficients of the scattering matrices can be used in a polynomial expansion of the KKR secular matrix, which transforms the search for the energy bands to an eigenvalue problem. In a test calculation for silicon it turns out that if one wants to reproduce energy bands to within 1 mRy one needs to include cubic terms in the polynomial expansions. Calculations with polynomial coefficients from a Chebyshev series expansion are found to give more precise energy eigenvalues than calculations where the polynomial coefficients were obtained from a Taylor series expansion without using any additional computing time.

A method is described for calculating the magnetotransport properties of a two-component composite conducting medium which has a periodic microstructure. The method is based on a Fourier series expansion of the local electric potential. Because a large number of expansion coefficients needs to be used in order to get reliable results for the bulk effective behavior, a special approach is developed that does not require solving a large set of coupled linear algebraic equations for them. Results are presented for the magnetoresistance and Hall resistance as functions of the magnetic field in a number of models where periodic arrays of insulating inclusions of various shapes are embedded in a uniform host material. These samples include cases where the inclusions are well separated as well as cases where they touch and where they overlap.

The half-filled extended Hubbard model (including nearest-neighbor interaction) is considered in perturbation theory at weak coupling on a d-dimensional hypercubic lattice (d≳3). It is shown that the exact critical temperature and the exact order parameter in the weak-coupling limit differ from the corresponding Hartree results by a renormalization factor q of order unity. This renormalization factor q is calculated systematically up to O(1/d) in a 1/d expansion. The results depend sensitively on dimension and on the relative strength of the nearest-neighbor interaction. The renormalization factor in d=3, estimated from the 1/d expansion, is q≃0.282 for the pure Hubbard model, q≃0.179 near the spin-density-wave to charge-density-wave transition and q≃0.830 in the limit of pure nearest-neighbor interaction.

We write the Hamiltonian of the Bose gas with two-body repulsive δ-function potential in a pseudoparticle-operator basis which diagonalizes the problem via the Bethe ansatz. In this operator basis the original bosonic interactions are represented by zero-momentum forward-scattering interactions between Landau-liquid pseudoparticles. We find that this pseudoparticle-operator algebra is complete: all the Hamiltonian eigenstates are generated by acting pseudoparticle operators on the system vacuum. It is shown that one boson of vanishing momentum and energy is a composite of a one-pseudoparticle excitation and a collective pseudoparticle excitation. These excitations have finite opposite momenta and cannot be decomposed. Our formalism enables us to calculate the various quantities that characterize the static and dynamic behavior of the system at low energies.

We calculate the fermion Green function and particle-hole susceptibilities for a degenerate two-dimensional fermion system with a singular gauge interaction. We show that this is a strong-coupling problem, with no small parameter other than the fermion spin degeneracy N. We consider two interactions, one arising in the context of the t-J model and the other in the theory of half-filled Landau level. For the fermion self-energy we show that the qualitative behavior found in the leading order of perturbation theory is preserved to all orders in the interaction. The susceptibility ${\mathrm{\chi }}_{\mathit{Q}}$ at a general wave vector Q≠2${\mathbf{p}}_{\mathbf{F}}$ retains the Fermi-liquid form. However, the 2${\mathit{p}}_{\mathit{F}}$ susceptibility ${\mathrm{\chi }}_{2\mathit{p}\mathit{F}}$ either diverges as T→0 or remains finite but with nonanalytic wave-vector, frequency, and temperature dependence. We express our results in the language of recently discussed scaling theories, give the fixed-point action, and show that at this fixed point the fermion-gauge-field interaction is marginal in d=2, but irrelevant at low energies in d≥2.

We have observed cyclotron resonance in the infrared wavelength range between 10.6 and 16.9 μm with pulsed high magnetic fields up to 220 T. The band-edge mass of n-type ZnS is ${\mathit{m}}^{\mathrm{*}}$=0.20±0.01${\mathit{m}}_0$ and the Fröhlich electron-phonon coupling constant α is 0.63 from a comparison with the experimental results of cyclotron resonance and a theoretical calculation based on the Wigner-Brillouin second-order perturbation theory. We have also observed phonon-assisted cyclotron resonance at the low-field side of the cyclotron resonance.

The semimagnetic semiconductors ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}\mathrm{-}\mathit{y}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Mg}}_{\mathit{y}}$Te grown by molecular-beam epitaxy have been studied by photoluminescence and photoluminescence excitation spectroscopy. It is shown that the Zeeman splittings of exciton states depend on the Mn content only and are independent of the Mg content, i.e., the carrier–Mn spin-exchange constants are not affected by the Mg incorporation. Exciton magnetic polarons have been studied using the method of selective excitation of the exciton luminescence. The partial substitution of Cd by Mg leads to a strong increase of the magnetic polaron energy from 12 meV in ${\mathrm{Cd}}_{0.85}$${\mathrm{Mn}}_{0.15}$Te to 28 meV in ${\mathrm{Cd}}_{0.70}$${\mathrm{Mn}}_{0.14}$${\mathrm{Mg}}_{0.16}$Te. This result emphasizes the strong dependence of the polaron energy on the conditions of the primary localization of the excitons. Time-resolved photoluminescence measurements yield polaron formation times of about 100 ps without pronounced differences between the quaternary and ternary alloys. But the gain in polaron energy within a fixed time interval increases with increasing Mg content. The independently controllable variation of magnetic and nonmagnetic properties in ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}\mathrm{-}\mathit{y}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Mg}}_{\mathit{y}}$Te alloys allows us to demonstrate the importance of the primary localization of excitons for the magnetic polaron formation in ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}\mathrm{-}\mathit{y}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Mg}}_{\mathit{y}}$Te as well as in ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te alloys.

In a high magnetic field, such that the distance between the Landau levels exceeds the binding energy of an exciton, an exciton gas in a semiconductor is capable of forming the Bose-Einstein condensate as well as a superfluid state even at a relatively high temperature. We consider the problem of excitonic interaction in a semiconductor in its multielectron formulation, starting from the second-quantization representation of the Hamiltonian of interacting electrons and holes in a high magnetic field. The expressions for the ground-state energy, the chemical potential, and the spectrum of elementary excitations of the system are obtained in a linear approximation in the concentration of excitons. It is shown that a system of excitons in a high magnetic field is similar to a weakly nonideal Bose gas. The existence and the stability of the Bose condensate due to an essential decrease of the interaction between excitons and an increase of their binding energy in a high magnetic field are established at a high density of excitons. The results obtained show that the excitation spectrum vanishes linearly, with a slope equal to a macroscopic speed of sound which depends on the direction of the magnetic field. Thus this spectrum satisfies the Landau criterion for superfluidity. The possible observable effects of the Bose condensate and the superfluidity of excitons in a high magnetic field are also discussed. It is shown that Bose condensation leads to unusual optical properties of a semiconductor, e.g., anomalous absorption of light, and to another mechanism of light amplification.

Extended x-ray-absorption fine-structure measurements have been made on amorphous Ge and crystalline Ge at temperatures up to 450 °C. The first-neighbor distance, variance of the bond-length distribution, and third-order cumulant increase with increasing temperature. Temperature dependence of the variance of the bond-length distribution is in agreement with Filipponi’s calculation. The amorphous-to-crystalline transition takes place at 450 °C. During the crystallization, these structural parameters decrease. The radial bond-energy change on crystallization is estimated to be one-tenth of the crystallization enthalpy. Anharmonicity effects are discussed by investigating the relationships between the first-neighbor distance, variance of the bond-length distribution, and third-order cumulant.

Positron-lifetime measurements performed at high temperature have been used to investigate vacancylike defects in various samples of GaAs and in GaP. A detailed three-state trapping model, which allows for detrapping, is utilized to show that monovacancylike defects trap positrons with a binding energy of close to 0.3 eV, while vacancy clusters constitute traps deep enough such that no detectable detrapping occurs up to 800 K.

The phenomenon of stimulated Brillouin scattering of a large-amplitude electromagnetic wave by the electron acoustic wave has been analytically investigated in a piezoelectric semiconductor plasma in the presence of an external uniform magnetic field. The Boltzmann equation with a Krook model for the collisional term has been solved in the guiding center coordinates for the nonlinear response of the plasma electrons. It is noticed that the magnetic field enhances the growth rate of the parametric instability in contrast with the fluid-model results.

The temperature dependence of the photoquenching of the EL2 level in semi-insulating GaAs is studied by photocurrent and thermally stimulated currents. The observations made in these experimental procedures together with other results reported in the literature reveal that the metastable transformation of EL2 cannot be fully achieved when the sample temperature is above 85 K. This observation is compared with the thermal recovery of the EL2 ground state from its metastable configuration EL$2^{\mathrm{*}}$, which is known to take place between 120 and 130 K; showing thus the existence of a conspicuous thermal hysteresis between both transitions, EL2→EL$2^{\mathrm{*}}$ and EL$2^{\mathrm{*}}$→EL2. This is analyzed in terms of the existence of a level that would play the role of an actuator of the metastable transformation of EL2. The charge state of this level can be altered by both optical excitation and temperature. Above 85 K it would be thermally emptied, being in such a charge state configuration unable to activate the metastable transformation of EL2.

We have measured the room-temperature Raman spectra of the coupled LO-phonon–damped-hole-plasmon modes of ten p-type, Be-doped, molecular-beam-epitaxy-grown GaAs films with hole concentrations from 6.3×${10}^{17}$ to 2.9×${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$. Although coupled-mode theory predicts the existence of two modes, we observed only one, with an asymmetric line shape. A line-shape analysis combining the deformation-potential and electro-optic mechanisms shows that the theory reproduces the data very well. It also suggests that one mode is suppressed by the large damping of the hole plasmon. The asymmetry of the observed peak is shown to come from interference between Raman scattering from atomic displacement fluctuations and that from macroscopic electric-field fluctuations. The ratio between Hall mobilities of the holes and optical mobilities of the holes from the Raman measurements is ${\mathrm{\mu }}_{\mathrm{Hall}}$/ ${\mathrm{\mu }}_{\mathrm{optical}}$=1.5–2.6 for different hole concentrations.

We have used Raman spectroscopy to study the lattice-constant dependence of the optical-phonon frequencies in AlAs and GaAs. From these studies, we deduce the dynamical effective charge for AlAs to be ${\mathit{e}}_{\mathit{T}}^{\mathrm{*}}$=2.20±0.01-(5.8±0.1)(-Δa/${\mathit{a}}_0$). For GaAs we find a dynamical effective charge ${\mathit{e}}_{\mathit{T}}^{\mathrm{*}}$= 2.18±0.01-(4.8±0.7)(-Δa/${\mathit{a}}_0$)-(28±16)(-Δa/${\mathit{a}}_0$${)}^2$. First-principles calculations, recently confirmed by experiment, indicate that the interatomic force constants (related to the second derivative of the interatomic potential) are virtually identical for GaAs and AlAs. The results reported in this paper indicate that higher-order derivatives of the interatomic potential are also remarkably similar for these two materials.