The analysis of small-capacitance tunnel junctions is complicated by the fact that the connecting leads play a significant role. The most successful approach to the single-junction case utilizes the quantum Langevin equation, a technique pioneered by Cleland, Schmidt, and Clarke [Phys. Rev. B 45, 2950 (1992)]. Here, we apply a generalization of the same method to the double-junction case, obtaining satisfactory agreement with existing experiments.

Inelastic cotunneling of holes through double-tunnel-junction systems fabricated in δ-doped silicon-germanium layers was studied for bias voltages below the Coulomb blockade threshold. The inelastic, linear, finite-temperature cotunneling term dominates for lower voltages while the zero-temperature, cubic cotunneling term dominates for higher voltages below the Coulomb blockade threshold for temperatures above 4.2 K.

A single-electron transistor with a RC circuit between its central electrode and the controlling voltage source is considered. The coupling resistance is assumed to be much greater than the resistance quantum. Several metastable charge states are possible within the Coulomb blockade range. This leads to the hysteresis of the central electrode potential as a function of the gate potential. Another important feature is that the blockade threshold is nonvanishing for any gate potential. If the coupling resistance is much greater than tunnel resistances, the I-V curve can contain several branches and exhibits hysteresis.

The electronic structure of the clean and the hydrogen-chemisorbed Ge(001) surface has been studied with photoelectron spectroscopy. Angle-resolved valence-band and high-resolution core-level spectra were recorded for both the room-temperature, 2×1, and the low-temperature, c(4×2), reconstructions of the clean surface. The electronic structure for the two phases was found to be very similar. The Ge 3d core-level spectra are decomposed with two surface-shifted components and the different origins of the components are discussed. The valence-band electronic structure of the clean, room-temperature, 2×1 and the hydrogen-induced 2×1:H reconstructions has been studied in detail by polarization-dependent angle-resolved photoemission. The surface-state dispersions E(k${\mathrm{¯}}_{\mathrm{\parallel }}$) in the [010] and [110] directions are mapped out. For the clean surface these dispersions are compared with surface-state bands obtained from a self-consistent calculation, using the local-density approximation and scattering theory, applied to a semi-infinite crystal with a 2×1 reconstructed surface consisting of asymmetric dimers. In spectra recorded from freshly annealed samples a structure is observed just above the Fermi level. This structure can be explained by thermal excitation to an empty dangling-bond band. The valence-band maximum was determined to be less than 0.1–0.2 eV below the Fermi-level for the clean 2×1 surface. On the hydrogen-chemisorbed 2×1:H surface two strong hydrogen-induced surface states are observed in the energy region 4.4–5.5 eV below ${\mathit{E}}_{\mathit{F}}$. The symmetry properties of these states, investigated by their polarization dependence, were found to be similar to the two corresponding states of the monohydride Si(001)2×1:H surface.

Transmission ion channeling has been used to investigate the bonding geometry of Ge on the Si(100)-2×1 surface at a coverage of 0.6 ML. It is shown that this surface is terminated by asymmetric Ge-Ge dimers with a bond length of 2.6 Å and a tilt of 12°, in agreement with previous results. The structure of the surface is shown to vary with Ge coverage, which is possibly attributable to the strain induced during growth by the 4% lattice mismatch between Ge and Si.

Angle-resolved inverse-photoemission spectra have been measured on the nonreconstructed As:Si(111)-1×1 surface along the two main directions Γ¯K¯ and Γ¯M¯ of the surface Brillouin zone. We observed three empty bands, the lower one of which is characteristic of a surface state partly resonant with bulk states in good agreement with theoretical calculations. We also observed a marked asymmetry in the dispersion along the M¯’Γ¯M¯ direction.

The transmission probability of electrons traversing a quantum well subject to a harmonic driving force ${\mathit{V}}_1$ cos(ωt), matched between two static barriers, is shown to exhibit a rich spectrum of sidebands in addition to the common central resonance. The intensity of these bands is an oscillatory function of the driving amplitude ${\mathit{V}}_1$ and frequency ω, with a strong simultaneous reduction of the transmission probabilities of all bands at characteristic ratios ${\mathit{V}}_1$/ħω. It is shown that such a quenching is related to a vanishing amplitude of the central band, and analytical results, nonperturbative in ${\mathit{V}}_1$, are presented.

Quantum mechanics does not provide direct tools for calculating time quantities related to the motion of a particle. In this paper we introduce a meaningful ‘‘time,’’ the ‘‘stay time’’ in a space region, and we propose a method for calculating its statistical distribution. The stay time is obtained by a method based on Feynman’s path integrals, which is similar to the one devised by Sokolovski and Baskin. We add a perturbative potential to the region being considered in order to induce variations in the wave function from which we can draw information about the time spent in the region. Unlike Sokolovski and Baskin, however, we obtain a real stay time and real greater order moments of its distribution. We also analyze other two ‘‘event times,’’ the ‘‘time of presence’’ at a given position and the ‘‘time of passage’’ through a surface. These times, which were introduced by Olkhovski and Recami, are obtained directly from the time evolution of the probability density and the probability current density. We find some relations between such times and the stay time, which show the consistency of the proposed method. Our approach is internally self-consistent, allows a general analysis of the characteristic times in the motion of a quantum particle, and is effective in explaining the results of other studies, in particular in the field of the tunneling times of potential barriers.

Commensurability effects between the period of an artificial array of scatterers (antidots) in a two-dimensional electron gas and the classical cyclotron radius are found to depend strongly on the size of the antidots in comparison with the period. Magnetotransport experiments on tunable antidot arrays are reported and analyzed. In the region of large depletion zones we find strong deviations of the Hall effect from the Drude slope in magnetic fields up to several teslas. From this, we deduce undepleted regions of less than 60 nm in typical 500-nm period structures. The suppression of the commensurability motion around several antidots observed in the longitudinal magnetoresistance in the case of strong depletion has the same origin and allows us to determine independently and consistently the depletion radius of the antidots.

We investigate the temporal evolution and the density dependence of exciton bleaching in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells. We find that the carrier-induced reduction of exciton oscillator strength is not influenced by carrier cooling after creation of hot carriers with femtosecond laser pulses. Excitation density experiments suggest that either phase-space filling or Coulomb screening dominates the exciton bleaching, depending on the carrier density.

Using rotating analyzer ellipsometry, we determined at room temperature and at 30 K the doping dependence of the ${\mathit{E}}_1$ and ${\mathit{E}}_1$+${\mathrm{\Delta }}_1$ critical points of n-type highly Si-doped GaAs(100) (up to carrier concentrations of 1.9×${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$) exposed to air. For both temperatures the ${\mathit{E}}_1$ and ${\mathit{E}}_1$+${\mathrm{\Delta }}_1$ critical points shift to lower energies with increasing dopant concentration. This redshift, however, unexpectedly saturates at a dopant concentration of about 3×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$. We found these measurements to be strongly influenced by surface effects. UHV-cleaved GaAs(110) samples (n- and p-type), exhibiting flat bands at the surface, show a significantly smaller energy shift over the whole doping range following a power-law dependence corresponding rather well to the predicted effect of screened impurities, i.e., dopants. We believe that this represents the behavior of bulk doping. Fermi-level pinning introduced by hydrogen adsorption after cleaving produces a behavior similar to that observed for air-exposed surfaces. These measurements are influenced by band bending and carrier depletion in the surface region. Measurements of undoped material in an electric field indicate that the presence of this field does not account for all the band-bending effects. We propose that unscreening of dopants in the depletion region makes an important contribution.

In this study we set out to determine the structural parameters (e.g., well width, well asymmetry, etc.) leading to optimum second-order nonlinearities in p-type GaAs-AlAs quantum-well structures. Maximum confinement of all participating states is of paramount importance for optimizing such responses.

An exact calculation of the quantum conduction through a curvilinear constriction in a three-dimensional electron gas is presented. We show that the conductance behavior presents significant differences with respect to the two-dimensional case. Importantly, we find that the conductance of a circular point contact deviates from the classical Sharvin result and the conductance per unit area is not constant except in the limit of macroscopic areas. We show that quantum finite-size effects can be taken into account by a simple semiclassical correction to the Sharvin formula. Recent experiments and calculations on quantum constrictions formed in atomic-scale point contacts are discussed.

We present theoretical studies of absorption spectra associated with above-barrier excitons in semiconductor superlattices. Using a k-space sampling technique combined with the recursion method, we obtain absorption spectra of both type-I and type-II superlattices. Excitonic effect, valence-band mixing, and Fano resonances due to the interaction of discrete exciton states with continua are all properly taken into account. We find that electrons and holes localized in the barrier region of a superlattice do form quasibound exciton states that appear as prominent features in the absorption spectra. Our calculation offers a direct comparison between theory and experiment.

Calculations of far-infrared optical absorption for ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As perturbed parabolic quantum wells (PQW) with a magnetic field in the plane of the electron slab are presented within the linear response theory. The nonparabolicities associated with the perturbation allow the coupling of long-wavelength radiation to collective excitations of the electron gas other than the center-of-mass mode, otherwise forbidden in pure PQW’s by virtue of Kohn’s theorem. We employ a quantum-mechanical self-consistent-field approach which makes use of the density functional formalism within the local-density approximation. We study two different types of samples employed in recent magneto-optical absorption measurements. The first consists of PQW’s with controlled δ-planar perturbations located at the center of the well or forming superlatticelike periodic arrays. These samples were recently used in an experimental study aimed at indirectly measuring the magnetoroton dispersion relation of a three-dimensional electron gas. We construct a magnetoplasma dispersion relation and critically discuss whether the experimental results are consistent with the bulk magnetoroton picture originally invoked to understand the data. The second system we study is asymmetric parabolic wells, obtaining good agreement with experiment at high and low areal densities, but quantitative discrepancies at intermediate density.

We present measurements of the electronic tunneling density of states of ultrathin films with sheet resistances at 8 K in the range 100 Ω<R<100 kΩ. We find that in the strongly disordered regime the Coulomb anomaly in the tunneling density of states grows in strength with R in a manner which depends strongly on film structure. These data demonstrate that the effective electron-electron interactions in disordered films depend on film structure in this regime. We discuss the implications of this behavior for the properties of superconductors near the two-dimensional superconductor-to-insulator transition.

The ac Stark effect can shift initially nonresonant minibands in semiconductor superlattices into multiphoton resonances. This effect can result in strongly enhanced generation of a particular desired harmonic of the driving laser frequency, at isolated values of the amplitude.

We consider a charged quantum particle in a random magnetic field with Gaussian, δ-correlated statistics. We show that although the single-particle properties are peculiar, two-particle quantities such as the diffusion constant can be calculated in perturbation theory. We map the problem onto a nonlinear σ model for Q matrices of unitary symmetry with renormalized diffusion coefficient for which all states are known to be localized in d=2 dimensions. Our results compare well with recent numerical data.

Magnetotransport measurements have been carried out under hydrostatic pressure on [001]- and [111]-oriented InAs/GaSb structures grown by metal-organic vapor-phase epitaxy. The system is semimetallic at zero pressure, but as the pressure is increased, electrons, located in the InAs layers, transfer back into the valence band of GaSb and a semimetal-to-semiconductor transition is observed. In double heterojunctions the zero band overlap condition occurs at 14 and 17 kbar for the [001] and [111] A orientations, respectively. The band crossing at the interface has been calculated self-consistently for the two orientations, taking into account the intrinsic and extrinsic origins of the carriers. This band overlap is found not only to be strongly orientation dependent, approximately 60 meV larger in [111] A compared with [001], but also to decrease at rates of 10 and 12 meV/kbar for the [001] and [111]A orientations, respectively. These rates are much larger than those determined from molecular beam epitaxy samples, but closer to the variation with pressure of the band gaps of the bulk materials.

We present a comparative study of the absorption spectra for a series of systems with a 50:50 Si/Ge content. These are as follows: a random alloy, a perfect interface ${\mathrm{Si}}_5$${\mathrm{Ge}}_5$ superlattice, and the same superlattice with random alloy layers between the volumes of Si and Ge. It is found that the presence of any random structure reduces the peak absorption (just above the band gap) by an order of magnitude. Evidence is also given that experiment should be able to detect the difference in the absorption coefficient, between structured and random arrangements of atoms.