A gapped double electron waveguide that contains a gap in the hard middle wall between two waveguides is proposed, and transport properties of the gap, which is also an electron waveguide, are analyzed. Noninteger conductance steps are found. Similar to what happens in a quantum point contact, conductance steps are found to arise whenever an additional propagation mode in the gap is excited as its width is increased; however, the contribution of each mode to the conductance is found to be less than 2${\mathit{e}}^2$/h due to the current branching at the gap. It has also been found that when the width of the input waveguide is modulated, conductance for the current leaked out of the gap shows strong oscillations lined up with the 2${\mathit{e}}^2$/h steps of the input waveguide conductance, which can also be characterized by current spectroscopy of the waveguide as del Alamo and Eugster suggested for a leaky electron waveguide.

Resonant electron capture in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/AlAs/GaAs quantum-well structures is systematically investigated by means of both continuous-wave and time-resolved photoluminescence. The capture efficiency and the capture time of barrier electrons into quantum wells both exhibit a clear oscillation as a function of well width. The enhanced capture efficiency in the oscillation is ascribed to the resonant electron capture. It is also revealed that the resonant effect is drastically enhanced by the insertion of AlAs tunnel barriers at both heterointerfaces and controlled by their widths. The resonant well width can be well described within the framework of the effective-mass approximation. An oscillating capture time due to the resonant electron capture is observed between 175–480 and 43–270 psec for 20- and 10-Å tunnel barriers, respectively. Investigation of the temporal profile of the quantum-well luminescence suggests ambipolar capture of the carriers due to the charge buildup effect.

Pseudomorphic iron-silicide phases grown on Si(111) have been studied by means of low-energy electron-diffraction, x-ray photoelectron-diffraction, and surface extended x-ray-absorption fine-structure experiments at the Fe K edge (7110 eV). These silicides have been epitaxially grown by codeposition of Fe and Si onto a room-temperature Si(111) substrate with silicide stoichiometry ranging from FeSi to ${\mathrm{FeSi}}_{\mathit{x}}$ (x∼2). In all cases, they were found to be epitaxial as attested to by a (1×1) low-energy electron-diffraction pattern observed after room-temperature deposition. X-ray-absorption measurements reveal that Fe atoms are coordinated with eight Si atoms for all metastable silicides with bond lengths of 2.34–2.37 Å and with Fe atoms with bond lengths in the 2.68–2.75-Å range. Fe-Si and Fe-Fe bond lengths as well as coordination numbers are found to increase with Fe contents within the silicide. These experiments confirm the formation of an epitaxial cubic and metastable CsCl-type FeSi upon deposition of Fe and Si in the 1:1 ratio onto a room-temperature Si(111) substrate. Furthermore, all data recorded from ${\mathrm{FeSi}}_2$ grown at room temperature or annealed at high temperature, are consistent with CsCl-derived or α-derived ${\mathrm{FeSi}}_2$ structures. Finally, the present data are inconsistent with the formation of a ${\mathrm{CaF}}_2$-type ${\mathrm{FeSi}}_2$ structure.

In this paper it is shown that diffuse-scattering experiments within the region of total external reflection can be explained quantitatively using the distorted-wave Born approximation for layer systems. Three Si/Ge samples with different degrees of complexity were investigated. The simultaneous analysis of the specular reflected intensity and the diffuse scattering leads to one consistent set of interface and layer parameters, which is able to fit both the shapes and the locations of all dynamic peaks in the off-specular scans and the characteristics of the reflected intensity. Therefore the distorted-wave Born approximation seems to give a correct and complete description of the diffuse scattering in the region of total external reflection.

A laser-induced band-bending variation on ultrahigh vacuum cleaved CdTe(110)1×1 surfaces kept at room temperature has been observed both in angle-resolved photoelectron spectroscopy (ARPES) and, through the Franz-Keldish (FK) effect, in surface-differential reflectivity. Through ARPES spectra, the shift of the Fermi level, between the sample obscured and illuminated with an ${\mathrm{Ar}}^{+}$ laser power of 500 mW has been determined to be 0.2 eV. The FK effect has been observed at the ${\mathit{E}}_0$ bulk critical point and studied as a function of laser power, showing a linear increase in its oscillation amplitude up to a power of 400 mW. Upon switching off the laser, the FK effect disappears exponentially with a time constant of about 1300 s. A surface photovoltage induced by the helium lamp has also been observed.

A one-electron effective Hamiltonian, previously found to describe quite accurately quantum-well energy levels of both free electrons and donor electrons in magnetic fields, is generalized to account for localized two-electron centers. The two-electron effective Hamiltonian derived is applied to the problem of finding energies for optical transitions originating from the ground state of ${\mathit{D}}^{\mathrm{-}}$ centers in seven different quantum wells with widths ranging from 9.5 to 52.0 nm. Central-cell corrections for the ${\mathit{D}}^{\mathrm{-}}$ centers are included. Comparison with experiments in magnetic fields below 10 T gives agreement to within 2 ${\mathrm{cm}}^{\mathrm{-}1}$ for all well widths without any adjustable parameters.

We present calculational studies of the electronic and geometric structure of an ordered monolayer deposition of Bi on III-V(110) surfaces. The technique which we have applied to these systems relies on the complete self-consistent solution of the Kohn-Sham equations for both the electronic and ionic degrees of freedom. An ab initio pseudopotential method, within the local-density approximation, is used in a supercell approach. From a given initial set of atomic positions, a conjugate-gradient technique is used to achieve the equilibrium geometry by moving along the Born-Oppenheimer subspace. The calculated relaxed geometries of the clean and Bi-covered (110) surface of GaAs, InP, and InAs compare well with available low-energy electron-diffraction and x-ray standing-wave studies, and the electronic band structures agree with angle-resolved photoemission results. The orbital nature of states that might participate in Schottky-barrier formation at Bi-covered surfaces is also discussed.

Pulsed nuclear-quadrupole-resonance (NQR) experiments have been performed at 77 K on rapidly drawn fibers of ${\mathrm{As}}_2$${\mathrm{Se}}_3$. The NQR line shape at 77 K was compared to the line shape of bulk glassy ${\mathrm{As}}_2$${\mathrm{Se}}_3$ and to thin-film ${\mathrm{As}}_2$${\mathrm{Se}}_3$. The fiber was found to have a line shape that is intermediate between the glassy and thin-film line shapes. The line shape of the fiber was also compared to the line shapes of glassy ${\mathrm{As}}_{0.5}$${\mathrm{Se}}_{0.5}$ at 77 K. The absence of a peak near 78 MHz in the fiber spectrum indicates that there are no As-As bonds in this sample.

A perturbation method has been used to study the interface polaron in a magnetic field with a two-dimensional approximation. The results show that there are two splittings of the polaron cyclotron mass occurring due to the coupling of the electron with two branches of interface phonon modes. In general, one splitting caused by the phonons with higher frequencies will be more important experimentally. However, both of the two splittings might be observable for some semiconductor heterostructures, for instance GaAs/AlAs.

Ballistic-electron-emission microscopy (BEEM) has been used to study the Schottky-barrier formation at the Au/n-ZnSe interface. Spectroscopy measurements indicate that the Schottky-barrier heights present unusually large variations and range from 1.53 to 2.15 eV. The lowest values are in good agreement with the barrier heights obtained from macroscopic current-voltage (I-V) measurements. The possible reasons for the existence of large Schottky-barrier variations are presented and discussed in this paper. These include the effect of variations of the work function and the behavior of the contact as a metal-insulator-semiconductor structure due to microclusters of different phases at the interface. Contrasts in the BEEM image are also found to follow the gold grain boundaries and are assumed to depend both on the grain orientation and interface quality.

We study nonequilibrium noise in the transmission current through barriers in one-dimensional Luttinger liquids and in the tunneling current between edges of fractional quantum Hall liquids. The distribution of tunneling events through narrow barriers can be described by a Coulomb gas lying in the time axis along a Keldysh (or nonequilibrium) contour. We show that the charges tend to reorganize as a dipole gas,which we use to describe the tunneling statistics. The dipole-gas picture allows us to have a unified description of the low-frequency shot noise and the high-frequency Josephson noise. The correlation between the charges within a dipole (intradipole) contributes to the high-frequency Josephson noise, which has an algebraic singularity at ω=${\mathit{e}}^{\mathrm{*}}$V/ħ, whereas the correlations between dipoles (interdipole) are responsible for the low-frequency noise. We show that an independent or noninteracting dipole approximation gives a Poisson distribution for the locations of the dipole centers of mass, which gives a flat noise spectrum at low frequencies and corresponds to uncorrelated shot noise. Including interdipole interactions gives an additional 1/${\mathit{t}}^2$ correlation between the tunneling events that results in an ‖ω‖ singularity in the noise spectrum. We present a diagrammatic technique to calculate the correlations in perturbation theory, and show that contributions from terms of order higher than the dipole-dipole interaction should only affect the strength of the ‖ω‖ singularity, but its form should remain ∼‖ω‖ to all orders in perturbation theory. A counting argument also suggests that the leading algebraic singularity at ${\mathrm{\omega }}_{\mathit{J}}$ should be ∝‖ω-${\mathrm{\omega }}_{\mathit{J}}$${\mathrm{\Vert }}^{2\mathit{g}\mathrm{-}1}$ to all orders in perturbation theory.

Scanning tunneling microscopy has been used to provide the atomic view of the reaction and diffusion of Ti and Si(001) at the monatomic adsorption stage. Two monatomic adsorption structures below and above 440 K have been found. The characteristic adsorption below 440 K is a Ti atom at the pedestal site on a dimer row. The high-temperature adsorption structure above 440 K is adsorption at a dimer vacancy induced by a dimer ejection process on the structural conversion path. The high-temperature adsorption structure shows one-dimensional hopping motion along a dimer row. Measurement of the hop rate gives the activation energy for diffusion and the prefactor of ${\mathit{E}}_{\mathit{D}}$=1.8±0.1 eV and ${\nu }_{\mathit{H}0}$=${10}^{14\pm 1}$ ${\sec }^{\mathrm{-}1}$, respectively. The growth features and the diffusion mechanism can be interpreted in terms of Ti-Si bond formation.

The theory on the Coulomb blockade in ultrasmall double junctions with external circuits is proposed, which self-consistently describes tunneling currents and charged states of islands without resorting to the stochastic technique. The island Hamiltonian in the theory contains the chemical potential (charged state) of the island to be determined self-consistently through the current continuity conditions. A general expression for the tunneling current is obtained together with self-consistency equations for the charge state of the island. Current-voltage characteristics are discussed analytically at zero temperature in the limits of high and low impedance environments. Coulomb gaps and the difference in current-voltage characteristics between symmetric and asymmetric double junctions (Coulomb staircases) were also analytically discussed in connection with the concept of neutral and charged islands. At finite temperatures current-voltage characteristics and the number of charges on the island under equilibrium configuration is calculated numerically for various junction parameters and temperatures. The theory describes reasonably well the fundamental features of the ultrasmall double junctions with external circuits.

Surface-shift low-energy photoelectron diffraction has recently been revealed as a powerful tool for the determination of surface structures. We have applied this method to study clean and hydrogenated (1/4 of monolayer) GaAs(110) surfaces. A recently developed renormalized multiple-scattering theory [M. Biagini, Phys. Rev. B 48, 2974 (1993)] has been applied to the interpretation of As 3d polar spectra. Numerical computations have been carried out for several surface geometries, corresponding to different buckling angles and surface bond lengths. We have obtained a buckling angle ω=26° and 6° for clean and hydrogenated surfaces, respectively. As far as surface bond lengths are concerned, changes within +2% for the clean surface and ideal bond lengths for the hydrogenated surface were found.

We consider an inverse problem of multiple scattering for fast charged particles propagating in an inhomogeneous medium. The scattering processes are described by the diffusion-type equation in the small-angle approximation. It is shown that by using the scattering data given on some small interval, it is possible to recover the spatial dependence of the density of the medium. This inverse problem is ill posed in the sense that small noise in the data may lead to large perturbations in ε(z) if no a priori assumptions are made about ε(z). This is clear from our presentation, since an analytic continuation of ε(z) is involved. One hopes that the proposed method can be applied to thin foils and to mesoscopic systems.

We have developed a method for adding the effect of the surface barrier to the Van Hove–Tong suite of low-energy electron-diffraction programs enabling us to analyze very-low-energy electron-diffraction (LEED) spectra from complex reconstructed systems. The method models the effect of the surface-potential barrier on the intensities and accurately replicates the fine-structure features found at low energies on many surfaces. It is free from many of the simplifying assumptions made by others attempting to model similar systems. We find that the fine-structure peak positions depend on the inelastic scattering in the barrier region as well as the shape of the real part of the potential barrier. This limits the accuracy of measurement of the shape of the barrier, but useful shape information can be derived from these analyses. We have studied the effect of oxygen overlayers on Cu(001) as well as the clean Cu(001) surface. These surfaces are well described by the model for a range of incident-beam directions. On the oxygen-exposed surface, it is necessary to assume that a c(2×2) structure exists as a precursor to the (√2 ×2 √2 )R45° in order to explain all the data. Further, it is found that the surface-potential barrier moves away from the surface with establishment of the precursor c(2×2) structure, but then moves back towards the surface as the final (√2 ×2 √2 )R45° forms with additional exposure to oxygen. This accounts for the changes in work function with exposure to oxygen found on this surface.

Using the method of molecular quantum electrodynamics, we investigate the polarization dependence of multipolar optical second-harmonic and third-harmonic generation including up to two multipolar interactions from an isotropic medium with a surface.

The growth of ultrathin platinum films on a rutile ${\mathrm{TiO}}_2$(110) surface has been studied using low-energy ion scattering and x-ray photoelectron spectroscopy. In the temperature range between 160 and 420 K, Pt grows in three-dimensional islands on the ${\mathrm{TiO}}_2$ surface with the size of the islands increasing with increasing surface temperature and without indication of an interface reaction. Evaporation of Pt in the presence of CO leads to a significantly higher degree of wetting. This behavior is attributed to a chemisorption-induced surfactant effect.

X-ray standing-wave (111) and (111¯) measurements have been performed on the room-temperature and high-temperature phases of a Pb layer on Ge(111). At room temperature, our results support a four-atom–unit-cell surface structure. On passing into the high-temperature phase, we find that there is an increase in the Fourier coefficient along the surface [112¯] direction for the Pb density distribution. Our model, which requires a 0.6-Å in-plane thermal vibration amplitude, is similar to one previously proposed by Hwang and Golovchenko and disagrees with a strictly two-dimensional-liquid interpretation.

The hexagonal structure of ferromagnetic Fe in ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Ru}}_{\mathit{y}}$ superlattices, x (y) being the number of Fe (Ru) atomic planes in one layer, is still controversial. From the x-ray-absorption near-edge-structure results of Beaudelet et al. it has been suggested that the Fe hexagonal stacking is not hcp but that the local environment of Fe atoms is a distorted bcc structure, despite the epitaxy on hcp ruthenium. This unusual Fe structure may allow an understanding of its ferromagnetic properties and thus is supported by recent theoretical investigations. To verify its existence, we have undertaken an x-ray-diffraction study performed in transmission mode for in-plane Bragg reflections and with grazing incidence angle for out-of-plane measurements. Unexpectedly, all our results convincingly support the regular hcp structure and seem to rule out the ‘‘bcc-like’’ model.