A phenomenological model is proposed here to show that the pinning of the charge-density-wave (CDW) phase by individual impurities yields an energy gain even for very small values of the Fukuyama-Lee-Rice (FLR) parameter. In three-dimensional systems the individual pinning is energetically more favored than the collective (weak in the FLR sense) pinning in all situations that can be described within a phase-only theory. The present results agree well with those of numerical simulation. Unlike the collective pinning, the individual pinning can account for the +2${\mathbf{k}}_{\mathit{F}}$/-2${\mathbf{k}}_{\mathit{F}}$ asymmetry of the x-ray satellite lines recently reported in many CDW compounds. In three dimensions, two regimes are found for the depinning by an applied electric field, characterized by a different dependence of the threshold field on the CDW gap and CDW-impurity coupling strength. The extension to the spin-density wave is straightforward.

Metal-insulator and valence transitions in the spinless Falicov-Kimball model are studied exactly by numerical calculations and perturbation theory. We have found that the model can describe the following valence transitions: (i) Transitions from an integer-valence ground state (the density of the localized f electrons ${\mathrm{n}}_{\mathrm{f}}$=1) into an inhomogeneous intermediate-valence ground state (${\mathrm{n}}_{\mathrm{f}}$≠1). (ii) Transitions from one inhomogeneous intermediate-valence state with ${\mathrm{n}}_{\mathrm{f}}$ into another inhomogeneous intermediate-valence state with ${\mathrm{n}}_{\mathrm{f}}^{\prime }$≠${\mathrm{n}}_{\mathrm{f}}$. (iii) Transitions from an inhomogeneous intermediate-valence state into a homogeneous intermediate-valence state. In the general case these transitions have a staircase structure and for sufficiently strong interactions U they are very sharp.

We have determined the Cu local density of states in CuAu I by utilizing the Cooper minimum phenomenon in the Au 5d photoemission cross section. To assist the analysis we carried out fully relativistic x-ray photocurrent calculations using the potential functions and densities of states from electronic structure calculations of CuAu I based on the relativistic Korringa-Kohn-Rostoker method. The good agreement between the experiment and theory suggests that the latter scheme provides a realistic description of the Cu local density of states in CuAu I.

The steady-state and conditional-probability distributions, as well as the power spectrum, of electron fluctuations are studied on the basis of stochastic equations. It is shown that the distribution of fluctuations has, in general, a binomiallike form, rather than Gaussian, with the third central moment of the same order as the variance. When rates of electron transitions are linear functions of electron number, the steady-state distribution is purely binomial. A procedure is proposed to calculate the power spectrum without previously calculating the conditional probability. It is useful when the nonlinear dependence of the rates on electron concentration is important. This occurs when the number of electrons is relatively small. A transition to the continuous limit, from discrete stochastic equations to the single, infinite-order partial differential equation, is carried out using Kolmogorov’s method. It is shown that the fundamental solution of the differential equation is equivalent to the solution of the corresponding stochastic equations. In Kolmogorov’s approximation the equation becomes a Kolmogorov (or Fokker-Planck)-type equation in the continuous space of particle numbers.

We investigate the band-gap narrowing in silicon caused by the introduction of additional electron carriers together with a neutralizing uniform positive background charge. The local-density approximation (LDA) is shown to be inadequate for calculating the band-gap narrowing. Using a first-principles technique and the GW approximation for the self-energy operator, we show that the change in the screening of the electron-electron interaction is the dominant effect. By employing the nonlocal inhomogeneous and energy-dependent dielectric function of the intrinsic material, we obtain significant corrections at high doping densities to previous model theories that use the simple static dielectric constant. While the inclusion of local-field effects is shown to give a negligible contribution, it is the energy dependence of the dielectric function that gives the main improvement. The inclusion in the screened Coulomb interaction of the ‘‘true’’ wave functions (calculated using the LDA), which we assume the additional carriers to occupy, is found to have no significant effect compared with the results obtained from a calculation using plane-wave functions. This is due to the fact that the changes in the electron-electron interaction occur predominantly at long wavelengths, so that the microscopic details of the wave functions are unimportant.

The reactivation kinetics of passivated Mg acceptors in hydrogenated InP during unbiased annealing of a Schottky diode is reported. The reactivation is found to slow down gradually with annealing time and this phenomenon is attributed to substantial retrapping of H at the acceptor sites. It is found from the concentration profiles and the kinetics data that the reactivation is most likely limited by ${\mathrm{H}}_2$ molecule formation processes for longer annealing times; for shorter annealing times, contributions from in-diffusion of H also become significant. The diffusion of H during the initial period follows an Arrhenius relation with an activation energy for the effective diffusion coefficient of 1.13±0.10 eV. In the ${\mathrm{H}}_2$ formation regime, the reactivation is thermally activated with an activation energy for the annealing parameter of 1.71±0.10 eV. The ${\mathrm{H}}_2$ formation-limited regime of reactivation occurs sooner as the annealing temperature is increased.

Polyaniline doped with camphor sulphonic acid (PANI-CSA) has been shown to yield a material that, after casting from solution in meta-cresol, exhibits a temperature-independent magnetic susceptibility [Y. Cao, P. Smith, and A. J. Heeger, Synth. Met. 48, 91 (1992); N. S. Sariciftici, A. J. Heeger, and Y. Cao, Phys. Rev. B 49, 5988 (1994)]. We report recent ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR experiments on uniformly ${}_{}{}^{13}{}_{}{}^{}\mathrm{enriched}$ PANI-CSA in which the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ spin-lattice relaxation rates are shown to obey a modified Korringa relation for relaxation via the hyperfine coupling to conduction electrons. This observation of Korringa relaxation in polyaniline provides strong evidence for a metallic state in this material. An estimate is made of the Korringa enhancement factor that provides a measure of the degree of electron-electron correlations present. Two-dimensional spin-exchange experiments are also reported, which show that the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR signal results from a heterogeneity in the sample over at least a 30-Å distance scale. These results are discussed in terms of the spatial extent of the doping-induced defect.

Following the time-dependent perturbation technique for the density matrix, the phenomenon of stimulated Brillouin scattering (SBS) has been analytically studied in weakly noncentrosymmetric (NCS) crystals like InSb immersed in a moderately strong magnetostatic field. The twofold role of the magnetostatic field B in terms of the sharpening of the density of states and Landau-level splitting have been examined quantum mechanically. The extremely large value of the spectroscopic splitting factor (${\mathit{g}}_{\mathit{c}}$-${\mathit{g}}_{\mathit{v}}$) and the very low electron and hole effective masses play the key roles in the significant enhancement in Brillouin susceptibility ${\mathrm{\chi }}_{\mathbf{B}}$, even when B is reasonably low. The origin of the NCS effect has been assumed to be in the parity indefiniteness of the energy states in the crystals exhibiting lack of inversion symmetry. We have considered near-resonant band-to-band electron transition in the direct-gap crystals to reduce to a simple one-dimensional problem in the presence of the magnetostatic field. Numerical estimates have been made for the InSb crystal at 77 K, duly irradiated by a pulsed 5.3 μm CO laser. The SBS threshold is found to be well below the crystal damage threshold and ${\mathrm{\chi }}_{\mathbf{B}}$ is found to be appreciably large. The results support strongly the candidacy of InSb for establishing itself as a promising crystal for strong SBS and optical-phase conjugation while immersed in a moderate magnetostatic field.

We present a theoretical method to calculate the phase shifts of the carrier wave function resulting from intraband and interband scattering in the two valence bands of p-type semiconductors. Using the phase-shift method, it is possible to calculate separate ionized-impurity-scattering relaxation times for carriers in the heavy- and light-hole bands and, thereby, obtain a new computation for the mobility of holes in heavily doped materials. Numerical computations of the ionized-impurity-scattering mobility are performed for p-type GaAs over the hole density range p=${10}^{17}$–${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$. It is found that the use of the Born approximation results in a small overestimation of the scattering rates at low-carrier densities, but in a significant underestimation for degenerate-carrier densities. For p>${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$ the mobility calculated at 300 K by the phase-shift method is approximately 40% lower than the value obtained from the Born approximation.

The current-voltage characteristics of porous-silicon structures are presented and discussed. Evidence is given that the forward and reverse currents show Schottky-junction-like behavior. Experimental measurements of reverse I-V curves at different ambient humidities demonstrate that the reverse current depends strongly on this parameter. The results are discussed in the light of generation-recombination processes in the porous-silicon depletion region. The presence of an inflection point in the reverse I-V curves is explained by the energy-band-gap difference between porous silicon and the crystalline silicon substrate.

We report on the observation of the boson peak in amorphous (a) GaAs formed by high-energy ion bombardment of the crystalline lattice. The experimental data are analyzed according to the theory of inelastic light scattering from fractons. The correlation length ξ and the spectral dimension d̃ of the fractal are determined. In comparison to a-Si:H the crossover frequency from the phonon to the fracton scattering regime, ${\mathrm{\omega }}_{\mathrm{co}1\mathrm{}}$, is lower and scales according to mass law, confirming the vibrational character of the boson peak. The origin of the fractals in tetrahedral amorphous semiconductors is discussed in terms of strained nanometer blobs of host atoms whose overcoordination is relaxed through bond percolation. The intensity increase of the boson peak relative to the amorphous component during the process of amorphization of GaAs, and the increase in the carbon content in a-Si:H, shows a composite structure that consists of a strained fractal region in the relaxed network. Some experimentally observed anomalies such as low values of the sound velocity in tetrahedral amorphous semiconductors, and transformation of the vibrational spectrum by quenching in ${\mathrm{As}}_2$${\mathrm{S}}_3$ are qualitatively explained by the fractal model. On the basis of maximum positions of the boson peak and the first sharp diffraction peak observed in the structure factor of inelastic x-ray or neutron scattering of amorphous semiconductors, the correlation lengths of medium-range order (MRO) are determined and compared for different tetrahedral and vitreous amorphous semiconductors. The observed three to four times shorter value of MRO in tetrahedral relative to vitreous amorphous semiconductors is used to explain a number of differences between their properties.

In this paper we study the spin correlations in a two-dimensional electron gas interacting via a Coulomb-type potential in the self-consistent scheme of Singwi et al. The spin-antisymmetric pair-correlation function is combined with the spin-symmetric pair-correlation function to determine the spin-dependent pair-correlation function. We also calculate the spin-dependent effective potentials from the knowledge of spin-symmetric and spin-antisymmetric effective potentials. It is found that ${\mathit{V}}_{\mathrm{eff}}^{\mathrm{\uparrow }\mathrm{\uparrow }}$ is always less than ${\mathit{V}}_{\mathrm{eff}}^{\mathrm{\uparrow }\mathrm{\downarrow }}$. Results are compared with the Monte Carlo simulation data, wherever available. We conclude that the electron correlation effects are somewhat more pronounced in two dimensions than in three dimensions.

The nature of the chemisorption bond between alkali metals, Li to Cs, on the Si(111) surface has been studied by means of the ab initio Hartree-Fock cluster-model approach. A comparative and systematic study has been carried out for a variety of cluster models simulating the high-symmetry sites of this surface. In all cases we found the bond highly ionic with a small participation of covalent effects to the interaction energy, which ranges from ≊20% for chemisorbed Li to less than 10% for Rb and Cs above the different active sites. This result is consistent with several analyses of the interaction focused on the interaction energy, the final Hartree-Fock wave function, the analysis of the dipole moment, and of its variation with the adsorbate-surface distance. We show that the dipole moment for chemisorbed alkali metals is smaller than the one expected from an ionic bond because of the substrate polarization. Consequently we argue that changes in the measured work function are not adequate to extract information about the ionicity of a given interaction. This is in agreement with previous works considering a metal substrate. Here we show that the same mechanism holds for semiconductor surfaces as well.

The atomic and electronic structures of Ga chemisorption on the Si(100)-(2×1) at low coverage have been investigated by a combined molecular-cluster total-energy and atomic-force approach and x-ray standing-wave (XSW) measurements. Four possible Ga-dimer adsorption geometries are calculated using cluster models with up to 77 atoms. By comparing the total energies for these optimized geometries, it is found that Ga dimers are centered at valley bridge sites with the Ga-dimer bond oriented parallel to the underlying Si-dimer bonds—a geometry strongly supported by XSW measurements of the Si(100)-(2×2)-Ga surface structure. The calculated Ga-dimer height above the ideal Si(100) is in excellent agreement with the XSW result. The stability of this parallel-dimer model is explained through the bonding between Ga-Si and Si-Si and by analyzing the Si-surface distortion when Ga is present.

A wide variety of luminescence properties of porous silicon formed under certain conditions to provide the preferable one-dimensional structure was studied. Rough vertical homogeneity, as it was revealed by depth profiling of the photoluminescence, indicated a wirelike morphology of the porous layer studied. Steady-state excitation with ultraviolet light, and electron and ion beams led to a rather high similarity of emission spectra. These unambiguously indicate the common mechanism of luminescence in porous silicon, namely, the quantum confinement inside the wires. The so-called slow component of the transient photoluminescence was studied. The radiative lifetime and nonradiative one were derived strictly from nonexponential transients. An internal quantum efficiency as high as (4±1)% was evaluated. An important role of the excitation level was established in a band-shape formation. At a low level, the thinnest parts of the quantum wires are screened by surface recombination states. This effect is restricted at a high excitation power, and the blueshift of the emission band may be observed.

Starting from the Hofstadter butterfly, we define lattice versions of Landau levels as well as a continuum limit that ensures they scale to continuum Landau levels. By including a next-neighbor repulsive interaction and projecting onto the lowest-lattice Landau level, we show that incompressible ground states exist on the lattice at odd-filling fractions. Already for values of ${\mathit{l}}_0$/a∼2 where ${\mathit{l}}_0$ (a) is the magnetic length (lattice constant), the continuum ν=1/3 Laughlin state is reproduced with perfect accuracy. Hence, a is argued to be an irrelevant length scale.

A nonlinear regime of electron transport in quasi-one-dimensional quantum wires has been predicted and investigated by the Monte Carlo technique. The electron transport in this regime closely resembles electron streaming. However, in contrast to conventional electron streaming caused by periodic emission of optical phonons, the streaming reported here is due to periodic acoustic-phonon emission. This effect originates from strongly inelastic acoustic-phonon scattering in quantum wires. The streaming occurs at low temperatures in a wide range of electric fields and is characterized by an oscillating-velocity autocorrelation function and a nonlinear velocity-field dependence. Both the analytical model and the Monte Carlo simulations yield an ${\mathit{E}}^{1/5}$ field dependence of the drift velocity and an ${\mathit{E}}^{2/5}$ dependence of the mean energy as a function of electric field E in the streaming regime.

A general expression for the momentum matrix elements for both intrasubband and intersubband transitions in n- or p-type doped quantum wells, wires, and dots is derived within the envelope-function approximation, in the process unifying the description of optical absorption in n- and p-type heterostructures. The derivation is nontrivially extended to the case of wave-function penetration into the barrier in a way that satisfies the principle of microscopic reversibility. The contribution of the valence-band anisotropy is shown to contribute to normal-incidence absorption in p-type heterostructures.

We present a model calculation, using the tight-binding approach, of a one-dimensional organic superlattice consisting of regular alternation of low- and high-gap segments of conjugated polymers. The electronic band structure is investigated as a function of the polymer composition and the length of the potential well. We show that, depending on the well-barrier alternation sequence, the electronic properties of this one-dimensional heterostructure span from those of a copolymer possessing an overall new electronic structure, to a situation where confinement occurs and superlattice effects are detected.

High-field electron transport in GaAs-based p-i-n nanostructure semiconductors has been studied by transient picosecond Raman spectroscopy at T=80 K. Both electron-distribution functions and electron-drift velocities have been directly measured as a function of (1) excitation photon energy, (2) electron density, and (3) electric-field intensity. Our experimental results show that for incident photon energies ħω=1.951 and 1.92 eV, as the injected electron density was increased from n=${10}^{17}$ to ${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$, the measured drift velocity was found to decrease substantially; on the other hand, the observed electron-distribution function changed from an extremely nonequilibrium one to an almost shifted Fermi-Dirac distribution. When the electric-field intensity E was increased from 15 to 35 kV/cm, the electron-drift velocity did not change significantly. As the incident photon energy ħω was decreased to 1.83 eV, the electron-distribution function remained extremely nonequilibrium even for an injected electron density n=${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$. All of these experimental results were compared with ensemble Monte Carlo calculations.