We investigate the linear-response conductance through a pair of coupled quantum dots. The conductance spectrum under ideal conditions is shown to consist of two sets of twin peaks whose locations and amplitudes are determined by the interdot coupling and the intradot charging. We will show that the qualitative features of the spectrum survive against experimental nonidealities such as (1) detuning of the individual dots, (2) interdot charging, (3) inelastic scattering, and (4) multiple lateral states. The effect of higher lateral states depends strongly on the nature of the interaction potential, screening lengths, and exchange terms, but the lowest set of twin peaks is largely unaffected by these details.

Symmetric GaAs/AlAs multiple quantum wells of 44 and 99 Å well and barrier widths were studied by means of resonant LO-phonon Raman scattering in high magnetic fields. Experiments were performed in Faraday geometry with the field along the growth direction. LO-phonon Raman intensity profiles versus magnetic field were recorded in the four backscattering configurations (${\mathrm{\sigma }}^{+}$,${\mathrm{\sigma }}^{+}$), (${\mathrm{\sigma }}^{\mathrm{-}}$,${\mathrm{\sigma }}^{\mathrm{-}}$), ${\mathrm{\sigma }}^{+}$,${\mathrm{\sigma }}^{\mathrm{-}}$), and (${\mathrm{\sigma }}^{\mathrm{-}}$,${\mathrm{\sigma }}^{+}$). Raman resonances in the specimen with narrower well width are mainly related to electron–heavy-hole interband magneto-optical transitions conserving the Landau quantum number n. A more complex structure is found in the magneto-Raman profiles of the sample with wider wells due to the proximity of light and heavy mass valence subbands. Due to the mixing of these hole subbands and the resulting anticrossing of levels, some resonances related to allowed transitions vanish with increasing magnetic field and other features appear. In both samples we observe magneto-Raman oscillations in incoming and outgoing resonance. From fits of a theoretical model to the data, we determine values for electron effective masses ${\mathit{m}}_{\mathit{e}\mathrm{\perp }}^{\mathrm{*}}$ along the growth direction and ${\mathit{m}}_{\mathit{e}\mathrm{\parallel }}^{\mathrm{*}}$ in the planes of the quantum wells. We also deduce heavy- and light-hole masses and discuss their variation with magnetic field. Experimentally determined magneto-optical transition energies are related to wave vectors ${\mathbf{k}}_{\mathrm{\parallel }}$ of the zero-field in-plane quantum-well dispersion. A comparsion to band structure results confirms the anticrossing of heavy- and light-hole valence bands. We analyze magneto-Raman intensity profiles using a previously developed model, which allows a classification of the resonances.

Auger-electron diffraction (AED) and surface-extended x-ray-absorption fine structure (SEXAFS) have been used to obtain a complete description of the atomic structure of a two-dimensional epitaxial Er silicide layer on Si(111). AED reveals that a monolayer of Er is located underneath a buckled Si double layer. The relevant Er-Si interlayer spacings are determined by means of single scattering cluster simulations and a R-factor analysis to be 1.92±0.05 Å to the first and 2.70±0.05 Å to the second Si top layer. Er near-neighbor bond lengths and coordination numbers are obtained independently from polarization-dependent SEXAFS. The SEXAFS data, when combined with the Si top-layer geometry inferred from AED, permit the determination of the atomic positions at the silicide/Si(111) interface. The Er is found to reside in relaxed ${\mathit{T}}_4$ sites of Si(111) with a single Er-Si distance of 3.09±0.04 Å to the first- and second-layer Si atoms of the substrate.

We present full-scale relativistic pseudopotential calculations of the first-order susceptibility in p-type Si-SiGe structures with a view of exploring the suitability of such systems for infrared (10–15 and 3–5 μm wavelengths) applications. The frequency dependence of the linear response due to transitions between valence minibands is calculated and the microscopic origin of the peaks determined. We also explore the effect of temperature on various structures. We show that simple particle-in-a-box models are unable to correctly describe the observed peak positions or the mechanisms involved. In particular, many important contributions come from areas of the Brillouin zone away from the zone center. We aim to show that normal incidence absorption is possible using SiGe structures.

GaAs-AlAs superlattices have been investigated by combining high-resolution electron-energy-loss spectroscopy (HREELS) and Raman spectroscopy. The superlattices were grown on GaAs(100) substrates by molecular-beam epitaxy. The sample surface was protected by an As capping layer formed in situ after growth, so that a well-ordered surface could be obtained by heating the sample mildly in UHV prior to HREELS measurements. The experimental results were compared with the dielectric theory of HREELS reported by Lambin et al. [Phys. Rev. B 32, 8203 (1985)]. The TO phonon frequencies and damping constants of GaAs and AlAs of individual samples, required for the theoretical calculations, were determined by Raman-scattering measurements. Excellent agreement was obtained between the theoretical and experimental results for all the samples with different layer thicknesses and for all the electron impact energies (from 4 to 35 eV) observed.

A two-dimensional electron liquid interacting with free or localized valence-band holes located a distance d away from the electron layer is studied by exactly diagonalizing the Hamiltonian of a finite-size system. For a coplanar electron-hole system, our calculation does not show the usual sign of an imcompressible state or a fractional quantum Hall effect. For the case where the separation d is about 1.5 times the magnetic length, pronounced cusps are revealed in a plot of the ground-state energy versus the Landau-level degeneracy. Detailed analysis suggests that the ground state responsible for the cusps consists of anyonic ions (composed of Laughlin quasielectrons bound to a hole) weakly coupled to an incompressible fluid of the remaining electrons. The hypothesis of stable anyonic ions is further supported by the structure displayed in the photoluminescence (PL) spectrum. Three PL systems, namely, electron–free-hole (e-h), electron–localized-hole (e-${\mathit{A}}^{+}$), and electron–neutral-acceptor (e-${\mathit{A}}^0$), have been investigated as functions of separation d, filling factor ${\nu }_{\mathit{e}}$, and temperature T. Multipeaked spectra are obtained for all three systems; they can be understood in terms of quasielectron-hole recombination processes and the ‘‘band structure’’ of two-dimensional electrons in the fractional quantum Hall effect regime. The connections between the calculated spectrum and some recent experimental results are also discussed.

A phenomenological theory of the quantum Hall effect in a large homogeneous sample based on the local-conductivity approach is proposed. We argue that a correlated electron system in the vicinity of a diagonal resistivity peak represents a random mixture of two phases: the quasielectron and the quasihole phase originating from two neighboring points of incompressibility. The two phases have different values of the local Hall conductivity which are quantized at low temperatures due to localization of quasiparticles by disorder. The effective diagonal resistivity of this system is found to be small everywhere except in a narrow interval of filling factors near the percolation threshold where the crossover between plateaus in the Hall resistivity takes place. Using an exact symmetry transformation, we show that both components of the resistivity tensor are related by a universal dependence which represents a semicircle. At low temperatures, the maximum value of the diagonal resistivity is finite and is determined by the heights of adjacent plateaus only.

A maximum-entropy model describing the conductance statistics of disordered mesoscopic quantum wires is presented. The model, closely related to the Laguerre ensemble of random matrices, predicts a weak localization effect and universal conductance fluctuations, in qualitative agreement with microscopic theory. A simple interpretation of the maximum-entropy model is given and the relation with other maximum-entropy models is discussed.

Energy levels of ground and excited shallow acceptor states in the presence of an external magnetic field have been calculated for center-doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells (QW’s). The impurity states are calculated within a four-band effective-mass theory, in which the valence-band mixing as well as the mismatch of the band parameters and the dielectric constants between well and barrier materials have been taken into account. The g factors of the shallow acceptor 1${\mathit{s}}_{3/2}$ ground states and the 2${\mathit{p}}_{3/2}$ excited states are obtained for QW’s with different well widths. The infrared optical transitions corresponding to the G, D, and C lines of acceptors in bulk GaAs have also been calculated versus magnetic fields up to 16 T for the case of acceptors confined in QW’s.

The semiclassical (local) approximation of mass discontinuity in the Wigner theory, when applied to resonant-tunneling diodes (RTD) with wide or high barriers, is shown to lead to unphysical results such as negative valley currents and negative peak-to-valley ratios (PVR). To deal with such problems, we propose to include the full nonlocal effect of mass discontinuity and restore the positive definiteness of the diagonal part of the density matrix. We have studied several examples and established the following points: (i) in the low narrow-barrier case, we obtain substantially lower PVR than that with the local approximation; (ii) in the wide- and/or high-barrier case, we obtain positive valley currents and hence meaningful PVR, indicating the applicability of the present theory to a wide range of RTD; specifically those which are actually in use; (iii) the peak currents are reduced substantially compared to that in the local approximation. In addition, effects of the mass discontinuity on ac characteristics of resonant-tunneling diodes are also presented for the ac admittance and the second-harmonic coefficients.

In the present work the clean Ge(001)-(2×1) surface is studied with scanning tunneling microscopy. Many different dimer-vacancy (DV) defects and complexes are seen on the surface. Among them the complex consisting of a missing dimer, a twin missing-dimer, and a dimer between them, i.e., the (1+2)-DV complex, comprises a great majority. Our line scan of the (1+2)-DV complex strongly supports the structural model proposed recently by Wang, Arias, and Joannopoulos for the (1+2)-DV complex on the Si(001) surface. In addition, some information on the interactions between neighboring dimers are also obtained and discussed, and some differences between Ge(001) and Si(001) surfaces are noted.

The results of a semiempirical tight-binding calculation of conduction bands in (001) ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/${\mathrm{Al}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{y}}$As/GaAs double-barrier superlattices are described. The parameters chosen are appropriate to the design of an ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As/AlAs/GaAs structure for quantum-well infrared-photodetector (QWIP) applications in the 3–5-μm band. The dependence of the superlattice conduction-band energy levels on slab thicknesses, alloy compositions, and wave vector are examined. The tight-binding method has the ability to describe states far away from the center of the Brillouin zone and band mixing is shown in the characteristic energy-dependence curves. States can be identified as Γ-valley-like or X-valley-like, due to Γ or X electron localization in the wells formed by the GaAs, ${\mathrm{Al}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{y}}$As, or ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As slabs. The Γ-valley-like states of GaAs wells in the ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As/ AlAs/GaAs structure (the transition states for QWIP applications) have energies close to those of a simple AlAs/GaAs square superlattice when the number of AlAs layers is greater than 4. In addition to the GaAs well states, the results show that the X-valley-like bound states from the surrounding AlAs slabs consist of two closely spaced energy eigenvalues, the doublet splits when the two AlAs slabs become closer as the thickness of the intervening GaAs (or ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As) slab is reduced.

Band mixing is evident from the results obtained by varying the Al mol fraction of ${\mathrm{Al}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{y}}$As or ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As barriers. Crossing and anticrossing behavior is noted at specific compositions x and y where states interfere. The energy-dispersive relations along wave vectors parallel to the interface have also been calculated. Curvatures differ due to the different effective masses of Γ-valley- and X-valley-like states.

We have performed electroreflectance measurements on a five-period ${\mathrm{In}}_{0.24}$${\mathrm{Ga}}_{0.76}$As(3 nm)/GaAs(10 nm) strained-layer superlattice as a function of electric field. The applied electric field was exactly determined by the Franz-Keldysh oscillation of GaAs. We have resolved transitions between electrons confined in the ${\mathrm{In}}_{0.24}$${\mathrm{Ga}}_{0.76}$As layer and light holes localized in the adjacent layers. The type-II Stark-ladder transitions are clearly observed. The observed field evolution of the transition energy shows a nonlinear behavior originating from an anticrossing by coupling resonantly between the first light-hole subband and unconfined energy states of light holes.

Degenerate-four-wave-mixing experiments have been performed on crystalline silicon at 620 nm using 100-fs pulses. Symmetry analysis has been applied to monitor the fourfold (bulk) symmetry of the [100] direction. Deviations from orthogonal polarization of the pump-and-probe beam with respect to the diffracted-signal beam have been studied in detail. The rotation patterns exhibit the transformation behavior of the relevant ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ tensor. Preliminary time-resolved measurements reveal distinct variations of the signal occurring on a time scale of 10 fs upon excitation with 100-fs pulses, which cannot be detected by standard correlation techniques.

We fabricate quasi-one-dimensional electron wires by low-energy ion-beam exposure in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs heterojunctions and study their transport properties in the low- and high-frequency domain. Typical widths of our electron wires range from 100 nm to 1 μm. The static and far-infrared (FIR) conductivities are measured at low temperatures (T≤4.2 K) and high magnetic fields (B≤12 T). The experiments demonstrate that in these wires a large proportion of the boundary scattering events is diffuse. The FIR conductivity exhibits intersubband and intrasubband plasmon resonances. The linewidths of these excitations reflect an intriguing anisotropy that arises from a mode-dependent sensitivity of these high-frequency excitations on the boundary roughness. Photoluminescence experiments indicate that the confinement is accompanied by creation of defects in the plane of the electron system that strongly reduce the mobility of the electrons in the areas exposed to the ion beam.

The phenomena of Schottky barrier creation in ${\mathrm{RbAg}}_4$${\mathrm{I}}_5$ are studied. Luminescence with high spatial resolution is used as a tool for investigation of the process on the blocking-electrode–solid-electrolyte boundary. The profile of the distribution of electronic centers near the contact region is measured. The presence of electrons and ions in the depletion region at the interface between the ${\mathrm{RbAg}}_4$${\mathrm{I}}_5$ and the graphite electrode is established and interpreted within the framework of the modified Schottky model. It is shown that the small concentration of electrons determines the extent of the depletion region. The kinetics of the creation of Schottky barriers is measured and described by taking into account the presence of self-trapped electrons and mobile silver ions in ${\mathrm{RbAg}}_4$${\mathrm{I}}_5$. It is shown that the small value of the self-trapped electron diffusion coefficient (${\mathit{D}}_{\mathrm{st}}$≃1×${10}^{\mathrm{-}8}$ ${\mathrm{cm}}^2$/sec) limits the kinetics of the process of creation of Schottky barriers.

This is a study of phase-coherent conduction through a ballistic point contact with disordered leads. The disorder imposes mesoscopic (sample-to-sample) fluctuations and weak-localization corrections on the conductance, and also leads to time-dependent fluctuations (shot noise) of the current. These effects are computed by means of a mapping onto an unconstricted conductor with a renormalized mean free path. The mapping holds in both in the metallic and the localized regimes, and permits a solution for an arbitrary ratio of mean free path to sample length. In the case of a single-channel quantum point contact, the mapping is onto a one-dimensional disordered chain, for which the complete distribution of the conductance is known. The theory is supported by numerical simulations.

When silicon implanted with >${10}^{16}$ He ${\mathrm{cm}}^{\mathrm{-}2}$ is annealed at 700 °C or above, the He forms bubbles and then diffuses out leaving voids (nanocavities). We have prepared n-type and p-type Si samples with nanocavity layers and characterized their structure and their effect on the local band structure and the transport of charge. The ambipolar Si dangling orbitals at the nanocavity walls trap majority carriers in both types of silicon and form quasi-one-dimensional potential barriers which impede transport of charge across the cavity layer. Using dc conductance and high-frequency capacitance techniques we have characterized the height and width of these electrostatic barriers. Capacitance measurements have also been employed to study the evolution of trapped dangling-bond charge as the void containing layers are depleted of carriers in n-type and p-type Schottky-barrier structures. With transient capacitance techniques we have characterized the emission of holes from the unoccupied dangling-bond localized states and also the emission of electrons from the doubly occupied states. The lower dangling-bond level is 0.17 eV above the valence-band maximum, while the upper level lies 0.38 eV below the conduction-band minimum; these energies are qualitatively consistent with broader spectral features observed in ultrahigh-vacuum photoemission experiments on clean reconstructed Si surfaces.

Electron paramagnetic resonance (EPR) has been employed to observe unpaired spins associated with dangling orbitals on the cavity walls. The strength of the EPR signal corresponds to ∼0.1 unpaired spin per dangling orbital, and this reduced amplitude is interpreted in terms of charge redistributions among inequivalent sites which are well known to occur on reconstructed Si surfaces. A simple, one-electron model yields semiquantitative agreement with much of the experimental data on electrical properties and helps explain some of the unusual emission-rate prefactors seen in the capacitance-transient experiments. We nevertheless conclude that a more realistic treatment, including electron-electron repulsion within the cavities as well as charge redistributions on the neutral surface, is probably needed for quantitative prediction.

${\mathrm{Sr}}_{0.61}$${\mathrm{Ba}}_{0.39}$${\mathrm{Nb}}_2$${\mathrm{O}}_6$:Ce exhibits pronounced dichroism which is strongly correlated with the cerium concentration. This property triggers effective hologram recording with mutually orthogonal polarized writing beams via self-diffraction. We measure the saturation diffraction efficiency, using an extraordinarily polarized readout beam, versus the polarization of the recording beams. The experimental results are compared to straightforward theoretical models based on linear dichroism including self-diffraction. We show that the saturation diffraction efficiency is strongly enhanced by self-diffraction effects.

A theoretical investigation of coupled quantum-wire–quantum-dot–quantum-wire resonant-tunneling diodes is presented, with particular emphasis on the basic mechanisms producing the fine structure in the current-voltage characteristics. The possibility of having two different types of fine structure is remarked upon, and an explanation is proposed for the relative lack of thermal smearing observed in certain devices that could be important for the use of low-dimensional devices at high temperatures. The I-V curves are calculated numerically for two typical structures and very good agreement with experiments is obtained.