The temperature behavior of the charge-density-wave (CDW) state in one-dimensional conductors is studied within the framework of a mean-field approach using the exact solution of the self-consistent equations. The properties of the CDW condensate are found to be dependent on the value of the dimensionless parameter which includes an electron-phonon coupling constant and the density of electrons. The thermal behavior of the CDW is shown to be similar to that of the Bardeen-Cooper-Schrieffer (BCS) superconducting condensate only at sufficiently weak electron-phonon coupling or relatively high electron density. In the case of comparatively strong coupling or sufficiently low electron density, the correct mean-field consideration leads to violation of the BCS relation between the phase transition temperature and electronic gap at T=0, and also to a variation of the CDW period, which decreases with increasing temperature.

Photocreated metastable states in organopolysilane (PSi) solids are observed using the electron-spin-resonance (ESR) technique. Two types of light-induced ESR (LESR) spectra are found depending on the excitation photon energy. These two types of LESR centers are annihilated by thermal annealing. Based on a first-principle electronic calculation, the origins of these LESR centers have been discussed. The lowest photoexcitation (∼3.5 eV) in PSi induces the Si skeleton stretching forces, which creates a weak bond (WB) in several places of the Si skeleton. Electronically, this WB acts as a self-trapping center for the photoexcited σ electron. The other higher photoexcitation (over 4.8 eV) causes side-pendant dissociation, which creates a dangling bond (DB) and causes the localized midgap state. The four lines found in the higher-energy excitation are considered to arise from hyperfine interaction between this DB electron and a sodium impurity nucleus.

The unusual galvanomagnetic properties of the monoclinic semimetal ${\mathrm{SrAs}}_3$, first-order longitudinal Hall effect and magnetoresistivity in Hall geometry, are quantitatively described within a two-band model. The ten model parameters for T→0 K are obtained from a fit to 14 low-field coefficients in combination with high-field Hall data and Shubnikov–de Haas oscillations. The temperature variation of the galvanomagnetic coefficient can be explained most convincingly with three temperature-dependent parameters, i.e., electron and hole scattering times and electron concentration, while the anisotropy ratios are kept constant. An energy gap of 38 meV for the temperature activation of the electrons is derived from this procedure.

The dynamic process of light illumination of GaAs is studied numerically in this paper to understand the photoquenching characteristics of the material. This peculiar behavior of GaAs is usually ascribed to the existence of EL2 states and their photodriven metastable states. To understand the conductivity quenching, we have introduced nonlinear terms describing the recombination of the nonequilibrium free electrons and holes into the calculation. Though some photoquenching such as photocapacitance, infrared absorption, and electron-paramagnetic-resonance quenching can be explained qualitatively by only considering the internal transfer between the EL2 state and its metastability, it is essential to take the recombination into consideration for a clear understanding of the photoquenching process. The numerical results and approximate analytical approach are presented in this paper for the first time to our knowledge. The calculation gives quite a reasonable explanation for n-type semiconducting GaAs to have infrared absorption quenching while lacking photoconductance quenching. Also, the calculation results have allowed us to interpret the enhanced photoconductance phenomenon following the conductance quenching in typical semi-insulating GaAs and have shown the expected thermal recovery temperature of about 120 K. The numerical results are in agreement with the reported experiments and have diminished some ambiguities in previous works.

This paper addresses some important questions concerning hopping conduction in doped conducting polymers, i.e., dimensionality, homogeneity, adiabaticity, Coulomb interactions, and multiphonon character. It will be shown that electron transport in these materials is due to correlated hopping between polaronic clusters. We propose a model which describes quantitatively this process. Conductivity and magnetoresistance measurements performed on disordered polypyrroles doped with polyelectrolytes are presented in support of these ideas. This model emphasizes the role of the dopant as an attractive potential and as a tunneling bridge between neighboring chains. The dopant ions are the source of both the polaronic clusters stability and of the transverse conduction.

The atomic structure of the intrinsic EX defect naturally generated in thermal ${\mathrm{SiO}}_2$ on Si has been analyzed by electron spin resonance on 51% ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ enriched (111)Si/${\mathrm{SiO}}_2$ structures. Although the hyperfine structure could be only partly observed, the results indicate that the EX center is an excess-oxygen defect, the defect core incorporating several O atoms which appear not to be entirely equivalent sites with regard to hyperfine interaction. The emanating working model is a Si vacancy in ${\mathrm{SiO}}_2$, where an unpaired electron is delocalized over the four bordering O atoms, resulting in an effectively pure s state, which is subject to ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ superhyperfine interaction with neighboring Si sites.

We have investigated the magnetotransport in InP sulphur-doped δ-layer structures. In contrast to GaAs-based systems, it was possible to measure single δ layers in the concentration range between ${10}^{15}$ and ${10}^{16}$ ${\mathrm{m}}^{\mathrm{-}2}$ where a metal-insulator transition is expected. At low magnetic fields perpendicular to the sample we observed a giant negative magnetoresistance (∼30% at 4.2 K), followed by an exponential increase in magnetoresistance at high fields. The behavior of the minimum shows certain features of the quantum Hall effect corresponding to a filling factor of 2. The experimental data are interpreted with the help of a strong-weak–localization transition-phase diagram in a disordered two-dimensional electron gas constructed using a simple lucid model.

The effect of high free-carrier concentrations on the formation of native defects is considered. It is shown that the onset of degenerate carrier statistics results in an abrupt reduction of the formation energy of charged native defects and an enhanced diffusion of impurities and host lattice atoms. The effects are especially important in very heavily doped semiconductors at reduced growth and processing temperatures. As an example, it is shown that the proper incorporation of the effects of the degenerate statistics is necessary for understanding the mechanisms determining the maximum attainable concentration of free carriers in p- and n-type ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As.

Thermalization, intervalley scattering, and cooling of photoexcited carriers are observed in the indirect-band-gap semiconductor Ge through time-resolved direct-band-gap transmission and luminescence with 100-fs resolution. The transmission experiment, which uses λ=1.53 μm, 120-fs pulses from a 76-MHz optical parametric oscillator takes advantage of the indirect-band-gap character of Ge to unambiguously determine the Γ→L intervalley scattering time at 295 K. The measured Γ→L transfer time for electrons at the Γ valley edge is 230±25 fs yielding 4.2±0.2×${10}^8$ eV/cm for the ${\mathit{D}}_{\mathrm{\Gamma }\mathrm{-}\mathit{L}}$ intervalley deformation potential. The luminescence experiments were performed at 10 K using a standard time and spectrally gated up-conversion technique employing an 82-MHz, 80-fs pulse width Ti:sapphire laser operating at λ=750 nm. From the luminescence experiments conducted with peak carrier densities of ${10}^{18–}$${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$, we observe that Γ→L,X scattering competes with carrier-carrier scattering so that nonthermalized carriers are observed for up to several hundred femtoseconds after an excitation pulse. Because the initial carrier kinetic energy is quite high we find that in addition to phonon-assisted L-L intervalley scattering, X-L and X-X processes must be included to account for a <1-ps carrier-cooling time. A value of 5×${10}^8$ eV/cm is obtained for the ${\mathit{D}}_{\mathit{X}\mathrm{-}\mathit{X}}$ deformation potential.

Samples of crystalline CdTe doped with two different concentrations of iron were prepared by the vertical high-pressure Bridgman method. Absorption and emission spectra were recorded at liquid-helium temperature in the region of the ${}_{}{}^5{}_{}{}^{}$${\mathit{T}}_2$(D)? ${}_{}{}^5{}_{}{}^{}$E(D) infrared transitions of substitutional ${\mathrm{Fe}}^{2+}$(${\mathit{d}}^6$) ions. Especially in the range between 2200 and 2300 ${\mathrm{cm}}^{\mathrm{-}1}$, a rich structure is resolved comprising more lines than predicted from plain crystal-field theory. The explanation of all the important lines is found after introducing a vibronic Jahn-Teller term to the Hamiltonian. A linear coupling between the double-degenerate vibrational mode ε (or ${\gamma }_3$) to the electronic orbitals of the atomic multiplet of symmetry ${}_{}{}^5{}_{}{}^{}$D leads to the diagonalization of the total Hamiltonian in a set of vibronic functions. Just one free parameter is used in the adjustment: the so-called Jahn-Teller energy representing the strength of the coupling. The corresponding value that we report here is 3 ${\mathrm{cm}}^{\mathrm{-}1}$. The energies thus found are in good agreement with the positions of the observed lines in the spectra. With the final wave functions we can calculate the relative intensities of the most important transitions and approximate theoretical line shape. This is also in good agreement with the experiment. Using these same energies and wave functions a calculation was performed to explain data existing in the literature about far-infrared absorption for the system CdTe:${\mathrm{Fe}}^{2+}$. Again, good agreement between experiment and theory is found.

An ab initio quantum chemical cluster study of the neutral vacancy-oxygen or (V-O${)}^0$ center in silicon, is presented, with emphasis on its metastable triplet state. We calculate a singlet-triplet splitting of 0.16 eV for (V-O${)}^0$. By calculation of the anisotropy of the nonradiative triplet life times we prove that the nonradiative decay occurs through combined spin-orbit and vibronic interaction. The observed ${\mathit{C}}_{2\mathit{v}}$ off-center displacement is shown to be an effect of chemical bonding. Relaxation effects of nearest-neighbor Si and local vibrational modes are studied. As a check on the approach we study the effect of cluster size and compare our theoretical values for the hyperfine interactions with ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ and the three nearest ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ neighbor shells to experiment.

We have carried out molecular-dynamics simulations of neutral hydrogen in crystalline silicon using tight-binding (TB) quantum-mechanical total energies and forces. A well-established model due to Goodwin et al. is used for the Si-Si interactions. For the Si-H interactions, we have fit our Harrison-type TB model to the known properties of silane and to both theoretical and experimental information about hydrogen in crystalline silicon. Several simulations performed in the temperature range of 1050 to 2000 K yield a diffusion curve in good agreement with experiment. The H diffusion is found to be jumplike between bond-centered (BC) sites, and the trapping of H at the BC site is mediated by a metastable onefold coordinated H configuration which weakens adjacent Si-Si bonds, allowing the H to enter the BC site. Vibrational frequencies for hydrogen at the BC site are also calculated and isotopic frequency shifts are discussed.

A general formalism for evaluating the effects of the large magnetic exchange interaction between single carriers and the magnetic ions in a diluted magnetic semiconductor, in the presence of an external magnetic field, is developed. In particular, a justification is provided for incorporating a modified Brillouin function into the description of the resulting magnetic effects.

Theoretical calculations of magnetic polaron energies resulting from charge carriers bound to donor or acceptor ion sites in diluted magnetic semiconductors have been performed. The dependence of the polaron energy on the magnetic field, the temperature, and the concentration of the magnetic ion has been evaluated. Similarly the effects of central cell corrections on polaron energies have been determined. The calculated values of the polaron energies are compared with experimental results and the implications of the comparison are discussed.

We study the two-dimensional electron gas in the presence of a random perpendicular magnetic field. We examine, in particular, the limit in which the correlation length of the random field is large compared to the typical magnetic length. In this limit, a semiclassical approach can be used to understand a large part of the energy spectrum. To investigate localization, we introduce a simplified model, in which electrons propagate coherently on a random network derived from the classical trajectories. The same network model (with different parameters) also represents electron motion in a uniform magnetic field and a random scalar potential, in a spin-degenerate Landau level. Requiring that the global phase diagram of our model be consistent with Khmelnitskii’s scaling flow for the quantum Hall effect, we argue that all electron states in a random magnetic field are localized in the semiclassical limit. We present the results of numerical simulations of the model in support of this conclusion.

The electronic structure and optical properties of narrow-gap PbSe/${\mathrm{Pb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se quantum-well structures are investigated experimentally and theoretically. The samples were grown by molecular-beam epitaxy along the (111) growth direction and were characterized by high-resolution x-ray diffraction, using triple-axis diffractometry. Their optical properties were investigated by transmission and photoconductivity experiments in the midinfrared range using Fourier-transform spectroscopy. The experimental data show that PbSe/${\mathrm{Pb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se has a staggered band lineup which is in agreement with recent data on coherent anti-Stokes Raman-scattering experiments (Geist et al.), from which it was concluded that holes are confined in the PbSe and electrons in the ${\mathrm{Pb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se layers. The experimental absorption data are compared with calculated data based on a k⋅p envelope-function approach for the type-II superlattice band structure which yields the energy eigenstates, eigenfunctions, and oscillator strengths. The absorption constant is obtained from an integration over k space, and the refractive index from a Kramers-Kronig transformation. The calculated fundamental absorption coefficients are in good agreement with the experimental data. Agreement between experimental and theoretical results is best for a normalized conduction-band-offset range of Δ${\mathit{E}}_{\mathit{c}}$/Δ${\mathit{E}}_{\mathit{g}}$=-0.4 to -0.6 (±0.2).

We analyze the eigenstates in one dimension of a semi-infinite ordered lattice in contact with a semi-infinite disordered lattice described by an Anderson model. Specifically, we study the site wave functions in the disordered region at distances N from the interface which are small compared to the localization length ${\xi }_0$. The wave functions at these length scales are relevant for conduction in the metallic regime of a disordered lattice of finite size. From a perturbation expansion for weak disorder we obtain qualitatively different results for the random (N-dependent) rates of exponential growth of wave functions in different domains corresponding to the energy band of an infinite nondisordered chain. Their mean values are anomalous near the band center and near the band edges, while corresponding to a fixed central limit (${\xi }_0^{\mathrm{-}1}$) between these limits, up to oscillatory terms. The study of the relative rms deviations of the above rates shows that they are self-averaging in a range of finite N≫1, at any energy. At the intermediate energies the weak disorder expansion is valid for any length scale, while near the band center and near the band edges it ceases to converge beyond a characteristic length ${\xi }_1$. The length ${\xi }_1$ defines the border between scales N≪${\xi }_1$, where the wave functions are weakly perturbed Bloch amplitudes, and scales N≫${\xi }_1$, where the weak disorder acts as a strong perturbation, leading to localized states with stationary positive Lyapunov exponents (${\xi }_0^{\mathrm{-}1}$). We find that ${\xi }_1$ scales with the disorder in the same way as does ${\xi }_0$, while being less than an order of magnitude smaller than ${\xi }_0$. Finally, we relate our results for wave-function growth rates at finite length scales to the resistance of a quasimetallic sample, using a simple Ansatz for the transmission coefficient. The resistance is found to be Ohmic, but anomalous, near the band center and strongly non-Ohmic near the band edges.

The LO-phonon resonant Raman scattering is studied in GaAs/${\mathrm{Al}}_{0.32}$${\mathrm{Ga}}_{0.68}$As Bragg-confining structures at low temperatures. Comparative studies are made on a GaAs/${\mathrm{Al}}_{0.32}$${\mathrm{Ga}}_{0.68}$As superlattice and on a bulk ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As crystal. A strong scattering intensity is observed in the spectral range of the (${\mathit{e}}_{\mathit{B}}$:${\mathrm{hh}}_{\mathit{B}}$)1S, (${\mathrm{e}}_{\mathrm{B}}$:${\mathrm{lh}}_{\mathit{B}}$)1S Bragg-confined excitions and the (${\mathit{e}}_1$:${\mathrm{hh}}_1$)1S superlattice exciton. The Raman profiles, namely, the scattering intensity versus exciting laser energy (in the respective spectral ranges) are characterized by a large outgoing-beam/incoming-beam intensity ratio. These profiles are analyzed in terms of a model based on exciton confinement, on the anisotropy of its 1S wave function (which defines its dimensionality), and on its scattering by interface and alloy potential fluctuations. The (${\mathit{e}}_{\mathit{B}}$:${\mathrm{hh}}_{\mathit{B}}$)1S exciton is thus found to be virtually two-dimensional, while the (${\mathit{e}}_{\mathit{B}}$:${\mathrm{lh}}_{\mathit{B}}$)1S and (${\mathit{e}}_1$:${\mathrm{hh}}_1$)1S excitons are intermediate between two and three dimensions. The dimensionality is found to be related to the degree of confinement.

We have studied the electronic properties of ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$Sb/InAs quantum wells, including the negative persistent photoconductivity effect, the magnetic-field-induced semimetal-to-semiconductor transition, and the sources of electron accumulation. We have also demonstrated that the negative persistent photoconductivity effect observed in these quantum wells can be used as a tool to investigate not only the electron-density dependence of the semimetal-to-semiconductor transition but also the alloy dependence of ionized deep donors in the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$Sb barrier layers. We found that negative persistent photoconductivity is a general property in this quantum-well system and the saturated reduction of the electron density due to the negative persistent photoconductivity effect increases with Al composition. From the alloy dependence of the electron density, we believe that the electrons removed from the InAs well after the illumination are captured by the ionized deep donors in the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$Sb layer, and that the deep donor level is not the only source of electron accumulation in the InAs well.

The second-order optical nonlinear susceptibilities ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ of biased ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells (QW’s) are theoretically estimated. The contributions of Wannier excitonic states and nonexcitonic excited states to ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ are discussed. It is shown that the transitions between a Wannier excitonic state and nonexcitonic excited states contribute dominantly to ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ of biased ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As QW’s. The ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ values are calculated for various QW widths and electric fields. The strong resonance of exciton energy and pumping-light energy, the strong oscillator strength of Wannier exciton states, and the large dipole of Wannier excitons result in large ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ values greater than 5×${10}^{\mathrm{-}10}$ m/V for QW’s with well widths of less than 70 Å. Because QW’s have large ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ values in the wavelength range near 1.55 μm, they are good candidates for nonlinear optical devices for communication systems.