The carrier density dependence and the temporal evolution of the ground level parameters of a quasi-one-dimensional electron-hole plasma confined in GaAs quantum wires have been studied by a line-shape analysis of the time-resolved luminescence. The obtained data are compared with the available theoretical calculations.

In this paper, we investigate the effect on the transport in a quantum wire of short-ranged irregularities in the boundary. The results obtained are generalized to the case of a small hole in the boundary. Via such a hole, the wire is coupled to free two-dimensional space or to a second wire. Some interesting resonance features in the transmissivities are found and it is shown that the flux through the orifice exhibits narrow peaks at energies where quasibound states in one of the wires are possible. These fluxes are on the order of the currents inside the wire and thus should be measurable by experiment. We use a multiple-scattering formalism in terms of renormalized scattering amplitudes. This method turns out to be well suited for this kind of problem because it allows a very compact formulation, as has been shown in previous papers.

We have developed a theory that explains the spectral distribution and the different time-decay behaviors of the signals observed in picosecond excitation-correlation measurements of semiconductor materials. The results display good quantitative agreement with experiments on multiple quantum wells and show that band filling has an important role in determining the characteristics of the correlation signals. Two limit cases are found and mathematically characterized: the nondegenerate regime, where the signals are positive with exponential-like decays, and the degenerate regime, where band filling gives rise to negative signals. It is shown that the technique is a sensitive probe of the carrier dynamics not fully exploited in previous work.

We describe a continuum model of optical phonons in GaAs/AlAs superlattices which consists of a linear combination of LO, TO, and interface modes. The model describes the mixing of confined and interface modes as the in-plane phonon wave vectors are varied. We derive an analytical expression for the dispersion relation and a detailed comparison is made with two different microscopic models. Excellent agreement is found for [111] superlattices and good agreement is found with [100] superlattices, especially if we take into account that the mechanical vibrations are not quite confined to the nominal GaAs layers.

The redistribution of electron density caused by magnetic gaps is explored for both dot and antidot lattices. It is assumed that the saddle points in the electron-density distribution determine the magnetotransport in these periodic systems. We show that the quantum Hall plateaus are shifted to lower magnetic fields in the case of antidot lattices and to higher magnetic fields in the case of dot lattices. Our theory is in a good quantitative agreement with the experimental data by Ensslin and Petroff for antidot systems. The results are obtained by computer simulation taking into account nonlinear screening by the electrons of two Landau levels. For one-level screening, we present an analytical solution of the nonlinear electrostatic problem.

A fully dynamical one-step theory of photoemission for several atoms per unit cell is presented. The theory is a relativistic generalization of the original work due to Pendry and allows us to calculate photoemission and bremsstrahlung isochromat spectra for pure elemental solids and compounds. The key aspect of the model consists of the formulation of the dipole matrix elements in the velocity form to guarantee a maximum amount of numerical stability in the computational implementation. In this contribution we apply our theory to spin-integrated and spin-resolved photoemission calculations for the GaAs(110) surface and observe an excellent agreement between theoretical results and experimental data for the spin-integrated spectra. Further, good agreement is obtained for the maximum calculated degree of spin polarization and the corresponding experimental values measured in energy and angle-integrated photoemission (photoyield) experiments.

The persistent current for an arbitrarily filled strongly interacting one-dimensional ring is studied using variational and numerical exact diagonalization methods. For an ordered ring a difference between the commensurate and the incommensurate cases is observed. While for the commensurate case the current in the limit of strong interactions always tends to zero, in the incommensurate case there are situations (termed the frustrated cases) in which the current is finite. For a disordered ring the current always tends to zero in the limit of strong interactions. The implications on persistent current experiments are discussed.

We calculate the electron elastic mean free path due to ionized impurity scattering in semiconductor quantum wires, using a scheme in which the screened ionized impurity potential and the electron screening self-consistently determine each other. By using a short-range-scattering potential model, we obtain an exact solution of the self-energy within the self-consistent Born and ‘‘noncrossing’’ approximations. We find that, compared to the mean free path for the bare unscreened potential ${\mathit{l}}_{\mathrm{bare}}$, the calculated mean free path including self-consistent screening ${\mathit{l}}_{\mathrm{sc}}$ is substantially larger, going as ${\mathit{l}}_{\mathrm{sc}}$∼${\mathit{l}}_{\mathrm{bare}}$[ln(${\mathit{l}}_{\mathrm{bare}}$)${]}^2$ for large ${\mathit{l}}_{\mathrm{bare}}$.

The electric-field-dependent intrinsic lifetimes of resonances in biased multiple qunatum wells are studied by using a complex-energy analysis based on an Airy-function transfer-matrix description of tunneling. Special attention is paid to the case when at some particular electric field two resonances belonging to different quantum wells resonantly align in energy. Two different characteristic behaviors were found in such electric-field sweeps: Long-lived resonances typically exhibit an anticrossing of their quasieigenenergies and a corresponding crossing of their lifetimes, usually associated with large changes in these lifetimes over many orders of magnitude, while short-lived resonances feature a crossing of their quasieigenenergies and an anticrossing of their lifetimes with less variation in these lifetimes. The parameters relevant for these two different regimes are discussed, and a simple model is derived to describe the crossover between these regimes.

We present a theoretical investigation of the collective excitations in doped tunneling semiconductor superlattices by calculating the loss functions describing inelastic electron scattering and Raman scattering experiments. The dynamical response of this inhomogeneous electron system is treated within the random-phase approximation. Our calculations indicate that in addition to the conventional plasma modes associated with the in-phase motion of all electrons in the system, as well as the single-particle-like transitions with nonvanishing oscillator strength, there exist plasma modes related with the charge depletion in the surface layers of typical systems. A tunneling plasma mode corresponding to the electron motion along the superlattice growth direction is also shown to exist. The dispersion relations for the different plasma modes are obtained together with their effective oscillator strength for both inelastic electron and light scattering experiments. The strongest intraminiband plasmon behaves like that in two-dimensional electron systems in the small ${\mathit{q}}_{\mathrm{\parallel }}$ regime, while the surface plasma modes exhibit an acousticlike linear dispersion due to the screening of the electrons in the bulk subbands. The behavior of the tunneling plasma mode is intermediate to these two. We show explicitly that the two experimental probes emphasize different modes in the various wave-vector regimes, and are therefore rather complementary.

We calculate self-consistently the spatial distribution of ionized silicon donors in the barrier of a nanostructured GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructure with a corrugated top gate, as well as the electron density of the corresponding modulated two-dimensional electron gas (2DEG). The geometry of the periodically corrugated Schottky gate gives rise to an inhomogeneous occupation of deep donor levels (DX centers) at room temperature, which freezes in when the device is cooled down. This charge pattern can nearly compensate the desired field effect near the GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As interface. On the other hand, moderate doping of the barrier is predicted to lead to an improvement of the confinement potential, which affects the electrons in the 2DEG. Our numerical treatment of the laterally modulated electron density combines a Fang-Howard variational approach describing quantum confinement in the growth direction and a Thomas Fermi approximation modeling the lateral inhomogeneities, and covers the gate-voltage-induced crossover from a weakly modulated 2DEG to isolated quantum dots.

We study both analytically and numerically phonon transmission fluctuations and localization in partially ordered superlattices with correlations among neighboring layers. In order to generate a sequence of layers with a varying degree of order we employ a model proposed by Hendricks and Teller as well as partially ordered versions of deterministic aperiodic superlattices. By changing a parameter measuring the correlation among adjacent layers, the Hendricks-Teller superlattice exhibits a transition from periodic ordering, with alternating layers, to the phase-separated opposite limit, including many intermediate arrangements and the completely random case. In the partially ordered versions of deterministic superlattices, there is short-range order (among any N consecutive layers) and long-range disorder, as in the N-state Markov chains. The average and fluctuations in the transmission, the backscattering rate, and the localization length in these multilayered systems are calculated based on the superlattice structure factors we derive analytically. The standard deviation of the transmission versus the average transmission lies on a universal curve irrespective of the specific type of disorder of the superlattice. We illustrate these general results by applying them to several GaAs-AlAs superlattices for the proposed experimental observation of phonon universal-transmission fluctuations.

The interaction of atomic and molecular hydrogen with the (110) surface of silver has been studied using electron-energy-loss spectroscopy, thermal desorption spectroscopy, low-energy electron diffraction (LEED), and work-function measurements. No evidence for associative or dissociative chemisorption of ${\mathrm{H}}_2$ is observed at the substrate temperatures investigated (≥90 K). However, at 100 K, atomic hydrogen bonds in the [1 1¯ 0] troughs of the surface in tilted-trigonal sites. As a function of concentration, a sequence of lattice-gas superstructures is observed with LEED including (1×4), (1×3), (2×6), and (2×2) patterns. At saturation coverage, the work function increases by 0.22 eV. However, this phase is metastable; upon annealing, hydrogen desorption is accompanied by an irreversible transition to a new bonding geometry in which LEED shows a dim (1×2) superstructure. The desorption of molecular hydrogen is characterized by two overlapping peaks centered at ∼155 K [6.9 kJ/mol] and ∼180 K [9.9 kJ/mol] which obey first- and second-order kinetics, respectively. Various structural models and hydrogen site assignments are discussed in comparison with data for similar systems.

Using a tight-binding many-body potential we estimated classical ground states for gold clusters of various structures and sizes. Cluster contractions are observed as expected. We obtained cluster vibrational properties at very low temperature by evaluating phonon spectra in the pure harmonic approximation. Finally we simulated extended x-ray-absorption fine structure for both the icosahedral and the fcc structures. At sizes corresponding to the closed shells in the icosahedral and cuboctahedral packages, the icosahedral clusters turn out to be more cohesive than the corresponding fcc ones. Critical comparison with available experimental data suggests the presence of such structures in samples obtained by evaporation.

We report results of Monte Carlo simulations of the formation of multilayer films on a model heterogeneous surface composed of parallel strips of sites characterized by different adsorbing potentials. It is shown that depending on the difference in the strength of surface potentials for adjacent strips, there is a certain limiting strip width ${\mathit{L}}_{\mathit{s}}^{\mathrm{co}}$ such that for broader strips the surface of weakly adsorbing sites is not covered by admolecules until the surface coverage on the strongly adsorbing part of the surface exceeds a certain critical value. For higher surface coverages of highly adsorbing strips, the entire surface exhibits uniform film thickness and surface nonuniformity does not play any role in the further growth of adsorbed film.

The structure of two thin films of Nb grown epitaxially on sapphire has been studied using high-resolution x-ray-scattering techniques. The films, of nominal thickness 400 and 150 Å, were grown using molecular-beam epitaxy on a (112¯0) sapphire substrate. They were of exceptionally high quality, with rocking curves of ≊0.004° for the (110${)}_{\mathrm{Nb}}$ Bragg peak. The x-ray scattering observed in transverse scans of the wave-vector transfer through the (110${)}_{\mathrm{Nb}}$ Bragg peak exhibits two other unusual features. First, weak satellite Bragg peaks are observed. These are shown to arise from a regular sinusoidal distortion of the Nb thin film. Using a frozen phonon model for this distortion, we relate the position of the satellites to the length of a terrace on the vicinal sapphire surface, and the intensity of the satellites to the lattice mismatch between the two materials in the surface normal direction. Second, in addition to the sharp Bragg peaks, there is a broader, diffuse component. This is isotropic in the plane of the film and has a Lorentzian-squared line shape. The diffuse component is shown to arise from a random network of two-dimensional Nb domains, with the size of domain related to the in-plane lattice mismatch. By combining these results with x-ray-reflectivity measurements from the thin films, and x-ray-reflectivity and crystal-truncation-rod measurements from the bare substrates, we develop a detailed model of the structure of Nb films on sapphire.

Theoretical expressions for the height-height correlation function of self-affine fractal surfaces are discussed in comparison with scanning tunneling microscopy, correlation and surface-width data obtained from rough silver and gold films. Fourier transformations are used to compare with equilibrium phenomena, and lead to a correlation model with an associated roughness spectrum of analytic form.

We have obtained fluorescence and atomic-force-microscopy images of the liquid and solid phases of Langmuir-Blodgett monolayers of L-α-dimyristoylphosphatidic acid, some containing ∼1 mol % of L-α-phosphatidylethanolamine-N-4-nitrobenzo-2-oxa-1,3-diazole. The fluorescence microscopy images are similar to those of monolayers at the air-water interface, but slight differences indicate that changes occur in the monolayers as a result of the deposition process. The atomic-force-microscopy images show the clean glass substrates to be very much rougher than when the monolayers are deposited on them. We propose that a water layer provides a smooth surface on which the Langmuir-Blodgett monolayers rest. We measured a ∼25-Å height difference between the liquid and solid phase region, one explanation being that the alkyl chains of the molecule are standing up in the solid phase and lying down in the liquid phase. This result is consistent with the fluorescence and atomic-force-microscopy images which show an increase in the amount of solid phase domains as a result of the deposition process. Further, the atomic-force-microscopy images of the monolayers show features which have not been described before and which could not be attributed to the presence of the dye. Finally, we showed that if the Langmuir-Blodgett monolayers are not stored in a humid environment, they begin to collapse into multilayers.

We have observed the optical second-harmonic generation (SHG) from a polycrystalline film of copper phthalocyanine (CuPc) with thicknesses between 40 and 2000 Å. The nonlinear susceptibility of this film is ${\mathrm{\chi }}_{\mathit{Z}\mathit{Y}\mathit{Y}}^{\mathrm{}\left(2\right)\mathrm{}}$=1.89×(3/4π)×${10}^{\mathrm{-}7}$ esu where Z and Y are the directions perpendicular and parallel to the film surface, respectively. This value is unexpectedly large, approaching a quarter of the value for the well-known nonlinear optical crystal ${\mathrm{LiNbO}}_3$. The finite value of the nonlinear susceptibility indicates that the structure of this film lacks inversion symmetry. The polarization properties of the second-harmonic (SH) light show that the film lacks inversion symmetry perpendicular to the film surface. This result contradicts the observation by x-ray diffraction that indicates that the structure of this film is centrosymmetric. Our data indicate that the true crystal structure of this film is slightly deformed from the one determined by x-ray diffraction. The slight deformation makes SHG possible, and the observed strong SHG arises from a resonance of the SH light to an electronic transition. We propose a mechanism of this deformation-induced SHG that is based on the electronic orbitals of this molecule. In order for the SHG to be possible, the molecular plane of CuPc must be nearly perpendicular to the substrate.

We propose a spectroscopic method to determine the existence of an electronic Wigner lattice at a given electron density n. We derive the ‘‘perpendicular’’ excited states of the two-dimensional Wigner lattice on a dielectric layer with a metallic substrate, and compare them with the corresponding excited states of the two-dimensional electron gas at the same electron densities. The transitions from the ground to the first excited state are studied in detail and we have found a significant difference between the lattice and the gas excitation energies for n≳${10}^8$ ${\mathrm{cm}}^{\mathrm{-}2}$. In that sense, the spectroscopy of electronic transitions can prove the existence of a Wigner lattice and a rapid change in the excitation spectrum can serve for determination of the lattice melting temperature. A linear Stark effect in the case of a Wigner lattice is also analyzed.