The electronic structure and chemical bonding in α-${\mathrm{TeO}}_2$ are investigated by x-ray-photoemission spectroscopy and first-principles cluster calculations. The calculated valence-band spectrum is in good agreement with the experimental one. The partial-density-of-states analysis shows an s-p hybrid on a Te atom in the bonding between Te and O atoms. Mulliken overlap populations indicate that four strong and two relatively weak Te-O bonds are formed around the Te atom.

The effective-medium approximation (EMA) is employed to study the effective nonlinear response of a two-component composite. The first component, of fraction p, is nonlinear and obeys a current-field (J-E) characteristic of the form J=${\mathrm{\sigma }}_1$E+${\mathrm{\chi }}_1$‖E${\mathrm{\Vert }}^2$E while the second component, of fraction q, is linear with J=${\mathrm{\sigma }}_2$E. Near the percolation threshold (${\mathit{p}}_{\mathit{c}}$ or ${\mathit{q}}_{\mathit{c}}$), we examine the conductor-insulator (C/I) limit (${\mathrm{\sigma }}_2$=0) and the superconductor-conductor (S/C) limit (${\mathrm{\sigma }}_2$=∞). For the C/I limit and p>${\mathit{p}}_{\mathit{c}}$, the effective linear- and nonlinear-response functions behave as ${\mathrm{\sigma }}_{\mathit{e}}$≊(p-${\mathit{p}}_{\mathit{c}}$${)}^{\mathit{t}}$ and ${\mathrm{\chi }}_{\mathit{e}}$≊(p-${\mathit{p}}_{\mathit{c}}$${)}_2^{\mathit{t}}$, respectively. For the S/C limit and q<${\mathit{q}}_{\mathit{c}}$, ${\mathrm{\sigma }}_{\mathit{e}}$ and ${\mathrm{\chi }}_{\mathit{e}}$ are found to diverge as ${\mathrm{\sigma }}_{\mathit{e}}$≊(${\mathit{q}}_{\mathit{c}}$-q${)}^{\mathrm{-}\mathit{s}}$ and ${\mathrm{\chi }}_{\mathit{e}}$≊(${\mathit{q}}_{\mathit{c}}$-q${)}_2^{\mathrm{-}\mathit{s}}$. Explicit calculations are done in two dimensions and generalized to d dimensions. The exponents are found to be s=t=1 and ${\mathit{s}}_2$=${\mathit{t}}_2$=2; ${\mathit{p}}_{\mathit{c}}$=1/d and ${\mathit{q}}_{\mathit{c}}$=(d-1)/d within EMA. For a finite-conductivity ratio h and at percolation, ${\mathrm{\sigma }}_{\mathit{e}}$ and ${\mathrm{\chi }}_{\mathit{e}}$ are found to cross over from the fractal (h=0) to homogeneous (h=1) behavior. In the limits of small h and p-${\mathit{p}}_{\mathit{c}}$ (or ${\mathit{q}}_{\mathit{c}}$-q), the EMA results can be rescaled to collapse onto a universal curve. The scaling function is extracted and compared to a general scaling theory and an excellent agreement is found.

Hot-electron cooling and the hot–longitudinal-optical-phonon generation is studied theoretically in GaAs and InP. The electron-energy collision broadening and the electronic screening in the dynamical random-phase approximation are taken into account. These two mechanisms together are shown to lead to an enhancement of the electron cooling rate in a certain range of the electronic densities. Support for the recent interpretation of the hot-electron cooling measurement by Hohenester et al. in terms of the collision broadening is provided. Also, an alternative interpretation of the earlier Raman measurements of the hot-phonon population in the long-wavelength region of the Brillouin zone is suggested, based on the above mechanism.

It is demonstrated that the real-space Green’s-function method provides a reliable and efficient framework to treat disorder in alloys. The method has been used to calculate the frequency spectrum of ${\mathrm{Si}}_{0.5}$${\mathrm{Ge}}_{0.5}$. The results obtained by this method not only reproduce the three-mode behavior which cannot be obtained by mean-field approximations such as the coherent potential approximation but also correctly predicts the intensities of the peaks associated with the three modes which cannot be easily obtained by the supercell approach.

A new way to study the two-electron state in DX centers at atmospheric pressure is reported. It is based on the idea of codoping a ${\mathrm{GaAs}}_{0.6}$${\mathrm{P}}_{0.4}$ sample with a uniform background of Te shallow donors (as source of free carriers) and a Gaussian distribution of sulfur DX centers by ion implantation. Using both capacitance-voltage profiling and deep-level transient spectroscopy measurements, we demonstrate that the ground state of the sulfur DX center traps two electrons and therefore has negative U.

A model is proposed which describes the evolution of exciton oscillator strength in a quantum well with lateral potential fluctuations. Four different types of exciton lateral confinements are evidenced: (type A) the strongly localized quantum-dot-like states, (type B) excitons localized as whole particles in long-scale potential fluctuations, (type C) free excitons at the mobility edge, and (type D) free-exciton resonances above the mobility edge. This model provides the continuous changes of the calculated oscillator strength during the transition between these localization regimes. We show that the inhomogeneous broadening of the quantum-well exciton line in the absorption spectra should be rather symmetric, while the higher energy wings of the spectra are caused by the resonances in the continuum of free states.

A structural investigation of the (1×1)-Bi/GaAs(110) interface by grazing-incidence x-ray diffraction is presented. The (1×1) structure has been prepared by depositing 1 ML of bismuth at room temperature on a GaAs(110) substrate grown by molecular-beam epitaxy. The structural model for the (1×1) symmetry chemisorption of Bi and the in-plane structural parameters are obtained and compared with previous experimental results. We find that the epitaxial geometry corresponds to a shifted epitaxial continued layer structure (ECLS), contrary to the almost perfect ECLS geometry of the Sb/GaAs(110) interface. We determine a 2.85-Å Bi-Bi bonding length, lower than the bulk Bi value of 3.1 Å, and consistent with the presence of a compression of the Bi atoms within the overlayer.

A calculation of the exciton binding energy (${\mathit{E}}_{\mathit{x}}$) and oscillator strength (OS) for quantum wires in type-II semiconductor systems is presented. These structures consist of a cylindrical wire of one semiconductor embedded in a second semiconductor. In the type-II exciton systems in quantum wells the electron is confined in one semiconductor and the hole in the other is due to band lineups in the two materials, which make this arrangement energetically favorable. We use an approach which initially decouples the ρ and z components. We use a variational approach for motion in the ρ direction which allows for correlation of the free particle, and solve for the one-dimensional exciton in the z direction using an effective Coulomb interaction. The solutions are then coupled self-consistently to produce ${\mathit{E}}_{\mathit{x}}$, where ${\mathit{E}}_{\mathit{x}}$(ρ,z)=E(ρ)+E(z), and a product wave function which is used to calculate the OS. We consider the idealized situation of infinite confining barriers and the more realistic case of finite barriers for the system GaAs/AlAs, where the electron is confined in the X state in the AlAs while the hole is confined in the Γ state in the GaAs wire (for a wire diameter of less than 43 Å). For finite barriers ${\mathit{E}}_{\mathit{x}}$ peaks at 33 meV for a wire diameter of 16 Å, which represents a large enhancement over the infinite-barrier case due to the strong wave-function correlation possible in two dimensions.

We present the results of experimental determination of the fundamental band gap (${\mathit{E}}_0$) and the spin-orbit split-off energy gap (${\mathrm{\Delta }}_0$) of zinc-blende CdSe using photomodulation spectroscopy. The single-crystal CdSe film was grown by molecular-beam epitaxy on a (100) GaAs substrate with a ZnTe buffer layer. Photoreflectance (PR) measurements were performed on the sample at various temperatures from 10 K to room temperature. The sharp derivativelike spectral features associated with the interband ${\mathrm{\Gamma }}_8^{\mathit{V}\mathrm{-}}$${\mathrm{\Gamma }}_6^{\mathit{C}}$ and ${\mathrm{\Gamma }}_7^{\mathit{V}\mathrm{-}}$${\mathrm{\Gamma }}_6^{\mathit{C}}$ transitions in PR spectra allow us to determine the ${\mathit{E}}_0$ and ${\mathit{E}}_0$+${\mathrm{\Delta }}_0$ band-gap energies. We found that zinc-blende CdSe has a fundamental band gap ${\mathit{E}}_0$ of 1.661 eV and a spin-orbit split-off gap ${\mathrm{\Delta }}_0$ of 0.42 eV at room temperature (295 K). The fundamental band gap ${\mathit{E}}_0$ of zinc-blende CdSe has been mapped out as a function of temperature and the Varshni thermal coefficients have been determined for this material. The results yield ${\mathit{E}}_0$(T)=1.766–6.96×${10}^{\mathrm{-}4}$ ${\mathrm{T}}^2$/(281+T) eV.

We report direct measurement of hot-electron temperatures and relaxation dynamics for peak electron temperatures between 3400 and 11 000 K utilizing two-pulse-correlation femtosecond (fs) thermionic emission. The fast relaxation times (<1.5 ps) are described by extending RT characterizations of the thermal conductivity, electron-phonon coupling, and electronic specific heat to these high electron temperatures.

The early stages of silicon solid-phase epitaxy (SPE) on the Si(111) surface are studied by scanning tunneling microscopy (STM). It is shown that an amorphous layer with thickness around two bilayers deposited on the silicon surface transforms in a polycrystalline layer with extremely small grain size (about 2–3 nm) after heating to 430 °C. The atomic-resolution STM imaging of such a disordered surface allows us to extract lateral coordinates of the upper-layer atoms. The application of the pair-distribution-function formalism reveals anisotropy in the orientational order that might indicate that the SPE occurs preferably in the step direction.

We present a study of electron transport in coupled quantum-well structures controlled by both front and back gates. The resonant resistance enhancement is systematically investigated by varying the mobility in each quantum well. We show that a large mobility ratio between the two wells gives a large resonant resistance enhancement only when the level broadening is smaller than the symmetric-antisymmetric gap of the coupled system.

A decomposition approach is developed to study a two-dimensional electron system with an even-denominator Landau-level filling factor. This approach divides the electron system into two subsystems—a boson system with bosonic Laughlin ground state and a fermion system with a Fermi surface, interacting with a gauge field. A comparison to the Chern-Simons gauge-field theory is made.

For a two-dimensional electron gas (2DEG) with a nonabrupt edge electron-density profile, edge modes, analogous to the higher-multipole surface modes of a 3DEG, are found in addition to the regular edge plasmon. Several simple model edge density profiles are investigated. For a linear edge profile the integral equation for the scalar potential is solved by expanding in a complete orthonormal set of functions. The resulting secular equation is truncated and solved numerically. For a simple double-step edge profile, which can be solved analytically, the multipole edge modes can be understood in a simple intuitive way.

We apply the Hubbard Hamiltonian to describe quantum-dot arrays weakly coupled to two contacts. Exact diagonalization is used to calculate the eigenstates of the arrays containing up to six dots and the linear-response conductance is then calculated as a function of the Fermi energy. In the atomic limit the conductance peaks form two distinct groups separated by the intradot Coulomb repulsion, while in the band limit the peaks occur in pairs. The crossover is studied. A finite interdot repulsion is found to cause interesting rearrangements in the conductance spectrum.

We have studied the zero magnetic field resistivity ρ of unique high-mobility two-dimensional electron systems in silicon. At very low electron density ${\mathit{n}}_{\mathit{s}}$ (but higher than some sample-dependent critical value ${\mathit{n}}_{\mathrm{cr}}$∼${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$), conventional weak localization is overpowered by a sharp drop of ρ by an order of magnitude with decreasing temperature below ∼1–2 K. No further evidence for electron localization is seen down to at least 20 mK. For ${\mathit{n}}_{\mathit{s}}$<${\mathit{n}}_{\mathrm{cr}}$, the sample is insulating. The resistance is empirically found to scale with temperature both below and above ${\mathit{n}}_{\mathrm{cr}}$ with a single parameter that approaches zero at ${\mathit{n}}_{\mathit{s}}$=${\mathit{n}}_{\mathrm{cr}}$ suggesting a metal-insulator phase transition.

The phenomenology of spatiotemporal concentration patterns associated with the catalytic oxidation of CO on a Pt(110) surface for conditions of excitability as monitored by photoemission electron microscopy is affected by deposition of submonolayer quantities of Au. This is due to continuous variation of the intrinsic kinetic parameters while the degree of homogeneity of the medium on the mesoscopic scale remains unaltered.

The linearly polarized x-ray-absorption dichroism at the Dy 3d-4f edge is studied for three different kinds of interfaces: Dy on disordered Si(111), Dy on amorphous ${\mathrm{SiO}}_2$, and Dy on polycrystalline Ta. Strong dichroism is observed in all the three interfaces, despite the very different interactions of Dy with the substrates. It is therefore pointed out that the x-ray-absorption dichroism using linearly polarized light is a more widespread, but at the same time complicated, phenomenon than previously suspected.

We have measured the temperature dependence of the coefficients b and γ in the expression ${\mathrm{\sigma }}_{\mathit{p}}$=${\mathit{bF}}^{\gamma }$, where F is the light intensity and ${\mathrm{\sigma }}_{\mathit{p}}$ is the photoconductivity of ${\mathrm{C}}_{60}$ films. Interpretation of the results in terms of an exponential density of gap states, assumed to be generated by disorder in the electronic potential, indicates that this disorder decreases with decreasing temperature below 260 K.

Dye has argued that in the electride Cs(18-crown-6${)}_2$ the valence electrons are localized in interstitial regions, or traps, which are far from the cesium nuclei (forming a lattice of F centers). Golden and Tuttle (GT) have argued that this picture is incorrect, the electron distribution being located instead in the immediate vicinity of the Cs nucleus. Here we refute GT’s arguments. We also give arguments based on ab initio calculations for an electride-type molecule, which supports a picture like Dye’s versus one where the electron probability is mainly near the cesiums.