It is well known that an antiferromagnetic (AF) spin arrangement within dimers provides a simple explanation of the gap in the electronic structure of the Si(100)-(2×1) reconstructed surface. Taking this AF description at its face value, we explore the implications of strong Si on-site repulsions (U) for the alkali-metal-covered surface in the coverage (θ) range up to two potassium atoms per dimer. As coverage (electron doping) increases, the substrate undergoes an insulator-metal transition which, in contrast with the U=0 situation, is gradual and due to weakening and final destruction of the AF ordering and its associated gap. The adsorption bond is strongly ionic in the low-coverage regime, with fractional ionic character (FIC) about 0.9. At θ=0.5 (one-half of the first monolayer), however, a crossover between one-dimensional and two-dimensional behavior is found, with a steep rise of the potassium charge accompanied by a steep drop of the FIC of the bond, both signaling the formation of an occupied potassium band increasingly charged and hybridized with the Si band. This process goes on, but now slowly, along the rest of the coverage range, up to saturation, leading finally to a rather low FIC of the bond (0.28) at θ=2. The above crossover separates the low-coverage regime, insulating and highly ionic, from the high-coverage regime where both the substrate and the adsorbate overlayer are metallic and the bonds predominantly covalent. The present model, therefore, does not sustain either the strong ionic picture along the whole coverage range or the opposite view that the transferred charge on the Si atoms is small even at low coverage.

An exactly solvable electron model of a confined system with inverse-square interaction is presented. The ground state is given by the Jastrow-product wave function of power-law form. We then apply the results to conductance oscillations in semiconductor nanostructures by generalizing the approach of Johnson and Payne to single-electron charging effects. Due to the internal spin degrees of freedom, there appear two independent periods of the conductance oscillations in very narrow channels even at zero temperature.

We reported the photoluminescence properties of a high-density electron gas in GaAs-${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As modulation-doped quantum wells by using cw and subpicosecond excitation correlation photoluminescence techniques. The gain or loss of photoluminescence intensity was found to be closely related to the excitation intensity vs carrier concentration. There is a significant distinction between the relaxation processes of minor carriers and of the doped two-dimensional electron gas in the wells. The former takes about a constant time duration (∼70 ps), while the latter is sensitive to the excitation intensity, and theoretically can be attributable to phonon reabsorption.

Under an intense laser field, a uniform-field-modulated interfacial-phonon amplification effect is discussed. The rate of change of phonon population is sensitive to the field strength of the uniform field. A basic relation between the rate and the field strength is obtained.

Dynamics of a two-level system with Ohmic dissipation driven by a time-dependent cw field in the noninteracting-blip approximation are studied. It is found that at low temperatures when α, a friction parameter, lies between 1/2 and 1, the localization predicted by Chakravarty [Phys. Rev. Lett. 49, 681 (1982)] and Bray and Moore [Phys. Rev. Lett. 49, 1546 (1982)] disappears, and the rate constant for tunneling of the particle increases with the intensity and frequency of the field. For 0<α<1/2 quantum coherence is observed, and the transition temperature from the coherent to the decay regime depends on the field intensity. When the intensity parameter approaches the zeros of the zeroth Bessel function coherence disappears. This transition from coherent to incoherent motion oscillates with the intensity of the field, giving rise to a cascade of transitions at different temperatures.

The cluster model ${\mathrm{As}}_2$-(${\mathrm{GaH}}_2$${)}_4$ is used for ab initio complete active space self-consistent-field (CASSCF) molecular-orbital calculations, to investigate the electronic mechanism of ${\mathrm{As}}_2$ desorption from the fourfold-hollow Ga dangling-bond (DB) site. The electronic state of the adsorbed ${\mathrm{As}}_2$ is revealed to be the ${\mathrm{\sigma }}_{\mathit{g}}^2$${\mathrm{\pi }}_{\mathit{u}}^2$${\mathrm{\pi }}_{\mathit{g}}^2$ doubly excited configuration, where the free ${\mathrm{As}}_2$ molecule is characterized by ${\mathrm{\sigma }}_{\mathit{g}}^2$${\mathrm{\pi }}_{\mathit{u}}^4$ with the triple bond. Namely, the four π orbitals of the ${\mathrm{As}}_2$ unit are used to form the four Ga-As covalent bonds with the Ga-DB site and a weak σ bond remains between the two As atoms. The calculated CASSCF wave functions indicate that the desorption process could be just the chemical reaction of multiple bond conversions having strong electron correlations due to the near degeneracy. The single-configurational picture is completely invalid in understanding the ${\mathrm{As}}_2$ desorption. The transition state is found to be of the ‘‘late-barrier’’ type, and from this the ‘‘cold’’ vibrational distribution of the desorbed ${\mathrm{As}}_2$ is predicted.

We identify the contributions to the terahertz (THz) signals from both the creation of polarized uncorrelated electron-hole pairs and polarized excitons in GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As quantum wells. We find that the THz radiation from the creation of polarized excitons is comparable in magnitude to that from the creation of polarized uncorrelated e-h pairs. Unlike the THz transient from uncorrelated e-h pairs, the signal from polarized excitons is also strongly influenced by the combined effects of detuning and finite pulse duration.

The external-electric-field effect and the doping effect on a modulation-doped finite-length GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As superlattice have been studied by solving the Schrödinger-Kohn-Sham equation and the Poisson equation self-consistently. In the heavy-doping case, the external electric field makes electrons sequentially tunnel through several potential barriers, and accumulates electrons near one side of the finite-length superlattice. The collective motion of the electrons results in high- and low-field domains in the superlattice. We have found that Wannier-Stark localization exists only in the high-field domain, and that the electronic states are not evenly spaced. The electric-field-induced (doping induced) depopulation (population) of subbands has been studied. We have also found oscillatory behavior of the calculated conductivity near the Fermi energy as a function of the external electric field.

A numerical study is performed on the conductance in quantum wires and their junctions in strong magnetic fields. For the lowest Landau level, the energy and system-size dependence of transmission probabilities and their fluctuations is quite universal and characterized by a single parameter within the numerical accuracy. The critical exponent obtained from the width of the transition region between adjacent quantized plateaus is in good agreement with that of localization of bulk Landau states. For the first excited Landau levels, strong mixing between the edge state of the lowest Landau level and bulk states destroys the universality.

We report deep-center photoluminescence of GaAs grown at low temperatures between 200 and 400 °C by molecular-beam epitaxy. Changes of temperature and excitation intensity were used in addition to heat-treatment and photoluminescence-excitation measurements. The type of photoluminescence transition strongly depends on the growth temperature and the [${\mathrm{As}}_4$]/[Ga] ratio. The layers grown at 200–300 °C show a dominant emission at 0.68 eV (EL2) and an emission at 1.1 eV related to a ${\mathit{V}}_{\mathrm{Ga}}$ center. The layers grown at 325–400 °C show one transition at 0.75–0.81 eV (designated as the 0.8-eV emission). We attribute the 0.8-eV emission to the ${\mathrm{As}}_{\mathit{i}}$-${\mathit{V}}_{\mathrm{Ga}}$ center. The center shows the large lattice relaxation exemplified by the Franck-Condon shift of 0.34 eV. The thermal ionization energy is determined to be 0.36 eV. We discuss our assignment of the ${\mathrm{As}}_{\mathit{i}}$-${\mathit{V}}_{\mathrm{Ga}}$ center to the well-known EL6 center.

We study transport properties of phase-coherent quasi-one-dimensional disordered conductors in the diffusive regime, in terms of the eigenvalue distribution of the two-terminal transmission matrix. Using an expansion in inverse powers of the classical conductance, we calculate the average transmission eigenvalue density and the two-point correlation function for fluctuations in the density. A formula for the average value and the variance of a general linear statistic on the transmission eigenvalues is obtained. Our results confirm an earlier hypothesis, based on random-matrix theory, that fluctuations are universal in the diffusive regime and ultimately determined by level repulsion.

Interactions between magnetoplasmons are found for a two-dimensional electron gas with filled Landau levels, in analogy to the phonon-phonon interactions in liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. This interaction lowers the magnetoplasmon excitation energies, so that the magnetoroton mode lies close to or even below the cyclotron resonance energy. The calculations show that such interactions must be taken into account in the study of the inter-Landau-level excitations.

The band structures of various semiconductors in both wurtzite (WZ) and zinc-blende (ZB) structures are calculated using the first-principles pseudopotential method within the local-density approximation, and then the band offsets at (111) WZ/ZB interfaces are evaluated for the band-edge states around fundamental gaps. We found that the band offsets are larger for ‘‘zone-boundary states’’ having large wave numbers perpendicular to [111] than for ‘‘zone-center states’’ having small wave numbers perpendicular to [111]. These phenomena occur because the former offsets are caused by the phase matching of a wave function through the first-nearest-neighboring site, whereas the latter are caused by the difference between the position of the third-nearest-neighboring site in the WZ and ZB structures. In addition, we show that the band offset increases with decreasing ionicity of the constituent semiconductor material, i.e., the magnitude of the band offset is determined by the competition between ionicity and covalency.

The tetragonal symmetry of epitaxial ${\mathrm{FeSi}}_2$ grains on Si(111) has been revealed for thin films (<40 Å) issued from two very different processes, which had in common a growth temperature of about 500 °C. The structure has been investigated by out-of-plane x-ray diffraction and transmission electron microscopy. An atomic model is proposed derived from the high-temperature α phase.

We report results obtained by a systematic study of Sb chemisorption on the relaxed GaAs(110) surface, using density-functional theory within the local-density approximation and norm-conserving, fully separable, ab initio pseudopotentials. The GaAs(110) surface is simulated by a slab geometry wherein the atomic structure of the Sb atoms at the preferred adsorption positions and the top three substrate layers is optimized by minimizing the total energy. Sb coverages of FTHETA=1/2 and FTHETA=1 are considered, corresponding to one or two Sb atoms per surface unit cell, on the average. We study nine different bonding configurations in detail. The results are interpreted in terms of the strong adsorbate-substrate bonds and the Sb-Sb interaction. For the energetically favored epitaxial continued layer structure in the FTHETA=1 case, the atomic positions are found in good agreement with results of low-energy electron diffraction and x-ray standing wave analyses. However, the epitaxial on top structure, which seems to fit somewhat better to the scanning tunneling microscopy (STM) data, is some tenths of an eV higher in energy. In the FTHETA=1/2 case we give a detailed analysis of the total-energy surface of the Sb/GaAs(110) system and identify stable and metastable adsorption sites. The resulting adsorption energies and equilibrium geometries indicate a tendency to form two-dimensional Sb clusters for submonolayer coverage. The accompanying electronic properties (surface band structure, photothreshold, etc.) are discussed within the context of experimental data available from STM, photoemission spectroscopy, etc.

We have analyzed the influence of the topmost two layers on the properties of the collective excitations of a finite superlattice structure. For semi-infinite superlattices the effect of the topmost layers on the collective excitations was discussed by Streight and Mills [Phys. Rev. B 29, 6337 (1987)]. We extend the discussion to the case of a superlattice with a finite number of periods, in which the thickness and the dielectric constant of the topmost two layers may differ from those of the layers below. A general dispersion relation for the finite superlattice is derived and applied to finite GaAs-AlAs superlattices. The dispersion curves and the electrostatic potentials show that several modes change in character with varied thickness of the topmost two layers. This change of the mode character is reflected in the electron-energy-loss spectra calculated in the framework of the dielectric theory.

We address the discrepancies between measured values of the persistent current in mesoscopic devices in the diffusive regime, and theoretical expectations. We investigate analytically and numerically the influence of the (screened) Coulomb interaction between electrons, of the impurity scattering of electrons, and of the periodic lattice potential acting on each electron, on the persistent current in mesoscopic rings at zero temperature. Our analytical arguments apply to one and two dimensions while the numerical simulation is restricted to one- and two-channel rings. We pay particular attention to the symmetry properties of the many-body wave functions but find that these do not qualitatively influence our results. Without Coulomb interaction, the impurity potential strongly suppresses the current both in the absence and in the presence of a periodic potential. We show that the Coulomb interaction very effectively counteracts the impurity suppression of the persistent current. This is our central result. There is evidence that the Coulomb interaction restores the impurity-suppressed average persistent current to a value close to the one in the absence of impurity scattering. This is a strong effect even for an interaction strength which is quite small compared to realistic estimates of the screened Coulomb interaction. Moreover, we show that in the absence of one-body potentials, the persistent current is exactly [one-dimensional (1D)] or essentially (2D) the same without and with the Coulomb interaction. A periodic potential alone does not affect the persistent current; when combined with the Coulomb interaction, it tends to reduce the current.

The resistivity and thermopower of a high-mobility two-dimensional electron gas at a heterojunction have been measured as a function of in-plane magnetic field in the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ temperature range. The sharp negative peak previously found as the upper subband just empties is identified with phonon drag; at low temperatures it is replaced by a sharp positive peak which is due to diffusion thermopower. By assuming that the measured resistivity can be used to obtain the density of states of the upper subband, the form and magnitude of the diffusion peak can be calculated and the results are found to be in good agreement with experiment. This suggests that the density of states obtained in this way is quantitatively accurate and that the method gives a good measure of both the form of the disorder broadening, as well as the energy width. Following sample illumination, the phonon-drag peak is found to be time dependent, more so than any other quantity measured; this might suggest that the peak is connected with disorder, but we have no consistent explanation of the behavior. The effect of slight misalignment of the field on both the thermopower and resistivity has also been investigated.

The initial stages of formation of the Ge/GaAs(100) interface have been investigated by monitoring, using low-energy electron diffraction, and scanning tunneling microscopy (STM), the structural changes of the GaAs(100) surface after submonolayer germanium deposition and annealing. The distribution of Ge atoms on the GaAs(100)-(2×4) surface is random when the substrate temperature is below 600 K. After annealing at about 700 K, a poorly ordered (2×1) LEED pattern is observed which is attributed to Ge-As dimerization. When annealed above 825 K, a well-ordered, stable surface with a (1×2) superstructure is obtained, suggesting the formation of Ge-Ga dimer bonds. These results demonstrate the usefulness of STM in monitoring changes in the interfacial atomic structures during the initial stages of heteroepitaxy, which is an essential step in understanding and controlling other important interfacial properties, such as energy band offset.

Linear and nonlinear reaction transport in GaAs/AlAs cylindrical quantum wires is investigated with the help of the balance-equation transport theory. Confined phonons, surface phonons, and their Fro¨hlich couplings with electrons in these quasi-one-dimensional electron-phonon systems are described using the dielectric continuum, hydrodynamic continuum, and Huang-Zhu optic-phonon models, respectively. Both intrasubband and intersubband transitions of up to 21 electron subbands are taken into account in the numerical calculation. The comparison of the results from these three models with those obtained by using bulk phonon approximation shows that the bulk phonon approximation is accurate enough to describe the Fro¨hlich interaction when calculating the linear and nonlinear electron transport even in ultrafine GaAs/AlAs quantum wires.