Using time-resolved photoluminescence, we have studied the nonradiative recombination of excess charge carriers in oxygen-implanted GaAs/${\mathrm{Al}}_{0.4}$${\mathrm{Ga}}_{0.6}$As single quantum wells. In order to reveal the recombination mechanism, the carrier lifetime is measured as a function of temperature and well width. We observe a decrease of the measured lifetime with increasing implantation dose and the nonradiative part of the recombination rate increases with increasing temperature. We find that the nonradiative recombination is thermally activated and the activation energies increase with decreasing well width. Under the assumption of strongly localized deep impurities we can explain this behavior with multiphonon capture processes and an analysis of the well-width dependence of the activation energies allows for the determination of the coupling strength for electron capture. In agreement with deep-level transient-spectroscopy (DLTS) measurements we determine a deep level 430 meV below the conduction band.

By use of the molecular-dynamics method we study the high-temperature nonequilibrium and equilibrium properties of a metallic overlayer on a compact metal surface. The system is modeled by the embedded-atom method, and consists of a Pt slab with a large ideal (111) surface (cross section of 930 atoms) on which Ag atoms are randomly distributed with coverages FTHETA=0.10, 0.25, 0.30. We simulate the short-time (τ∼1 ns) process by which the adatoms form dynamically stable aggregates in local equilibrium with a dilute two-dimensional vapor. We analyze the equilibrium properties of the adlayer as a function of T and FTHETA. We compute the cluster size distribution, the adatom radial-distribution function, the diffusion coefficient, and we characterize the cluster shape by the ratio of its principal moments of inertia. At all FTHETA and T the Ag islands grow pseudomorphically on Pt(111). At high temperatures (T∼1000 K) the clusters are fluidlike. There is no evidence of peculiar stability at the sizes corresponding to the filling of two-dimensional close-packing shells (‘‘magic numbers,’’ N=7, 10, etc.). At intermediate temperatures (T=600–800 K) the size distribution is shifted toward larger aggregates and, at low coverage, starts to display stability peaks at the magic numbers. The larger clusters develop a solidlike core surrounded by a fluidlike boundary. At the lowest temperature of our study (T=400 K) the clusters display a high degree of local order. The size distribution function is restricted to small N’s by the slowing down of the cluster growth beyond the nucleation stage. At FTHETA=0.10 the stability peaks at N=7 and 10 are apparent. We discuss the influence of a step on the behavior of the system.

A study of ordering and segregation in bimetallic clusters has been carried out using the embedded-atom method. Icosahedral and cuboctahedral clusters with up to 147 atoms have been considered. The model has been applied to the Cu-Ni and Cu-Pd systems and we have found that the icosahedral structure is favored over the cuboctahedral one. Cu segregates to the surface in the Cu-Ni clusters, and a subtle interplay between ordering and segregation is observed in the Cu-Pd clusters.

A theoretical study is presented of the equilibrium domain microstructure expected for a thin film grown epitaxially onto a single-crystal substrate as a function of temperature, film thickness, and lattice misfit for the case when the film can undergo a structural phase transition. The theory treats the epitaxial interface at the level of the Frenkel-Kontorova model and takes into account domain-wall energies and bulk-elasticity effects in both the film and the substrate. Existing predictions for this problem by Roitburd are reproduced when the epitaxial interface is commensurate. Results are obtained for the general case when the interface is incommensurate. The corresponding domain structure generically involves multiple phases.

The site conversion of adsorbed CO between the terminal site and the bridged site on Ni(100) was studied by means of infrared reflection absorption spectroscopy (IRAS). The temperature dependence of the relative occupation for two sites was measured from 80 to 266 K in detail, where the binding-energy difference was determined to be 11 meV. The driving force for the predominant occupation of the terminal site at higher temperature is ascribed to the vibrational entropy of the low-energy degenerate-hindered translational mode of the terminal CO. The kinetics of approaching the equilibrium was studied by time-resolved IRAS combined with a pulsed gas dose. Following a rapid dose, CO molecules are initially adsorbed at the terminal site and the bridged site with the a priori ratio of 1:2, indicating that gas-phase CO molecules are directly trapped by the potential minima initially, are thermalized, and migrate on the surface to approach the equilibrium occupation ratio. The microscopic hopping rate from the terminal site to the bridged site was estimated to be 0.02 ${\mathrm{s}}^{\mathrm{-}1}$ and that from the bridged site to the terminal site was estimated to be 0.005 ${\mathrm{s}}^{\mathrm{-}1}$ at 83 K. A random-walk model assuming the microscopic hopping rates gives a self-diffusion coefficient of 3.1×${10}^{\mathrm{-}19}$ ${\mathrm{cm}}^2$ ${\mathrm{s}}^{\mathrm{-}1}$ at 83 K, which is in good agreement with the previously reported macroscopic results. Thus, the elementary step of surface diffusion is ascribed to the hopping between the terminal site and the bridged site. The difference between the estimated barrier by assuming a harmonic potential and the activation energy for diffusion suggests the presence of anharmonicity in the potential between the terminal site and the bridged site.

In this paper we discuss an interesting feature of the system In on InP(001), i.e., that the shape of a significant number of large In clusters on the surface, formed during thermal decomposition of the compound semiconductor, undergo shape cycles between spherical and rectangular during cluster growth. Indications of repeated shape cycles are shown and a detailed theoretical discussion of the effect is given. The shape cycles are caused by enhanced substrate etching below clusters which results in the formation of square- or rectangular-shaped depressions. The depressions modify the energetic equilibrium condition governing cluster shapes such that cluster shapes follow the shape of the expanding depression for some time. We further demonstrate that the global cluster size distribution follows a self-similar behavior.

Electron differential inverse mean free paths for volume-plasmon excitations and differential probabilities for surface-plasmon excitations have been calculated using dielectric response theory. A model dielectric function which satisfied sum rules and agreed with optical data was established for these calculations. In surface-plasmon calculations, we considered electron-impact emissions of the reflected electron-energy-loss spectroscopy. Formulations were made for obliquely incident electrons with the recoil effect and without the small-scattering-angle assumption. For volume-plasmon excitations, we evaluated corrections due to the exchange and ${\mathit{Z}}_1^3$ effects. Comparison between calculated results and experimental data extracted from reflected electron-energy-loss spectra showed good agreement. Calculated differential cross sections have been used to solve the transport equation for the angular and energy flux spectra of reflected electrons. Contributions to the spectra from single and plural plasmon excitations were analyzed. It was found that calculated spectra were in good agreement with measured data.

We have calculated the surface energy and the work function of the 4d metals by means of an energy functional based on a self-consistent, spherically symmetric atomic-sphere potential. In this approach the kinetic energy is calculated completely within the atomic-sphere approximation (ASA) by means of a spherically symmetrized charge density, while the Coulomb and exchange-correlation contributions are calculated by means of the complete, nonspherically symmetric charge density within nonoverlapping, space-filling Wigner-Seitz cells. The functional is used to assess the convergence and the accuracy of the linear-muffin-tin-orbitals (LMTO) method and the ASA in surface calculations. We find that the full charge-density functional improves the agreement with recent full-potential LMTO calculations to a level where the average deviation in surface energy over the 4d series is down to 10%.

The results of synchrotron-x-ray-scattering studies of stepped Pt(001) surfaces are reported. An orientational phase diagram is proposed for Pt surfaces between the cubic (001) orientation and orientations tilted 6° away from the [001] direction towards the [110] direction, and for temperatures between 300 and 1850 K. Four distinct orientational phases are identified. At the highest temperatures, we find a one-phase region in which the surface is rough with the local surface normal equal to the macroscopic surface normal. Below ${\mathit{T}}_0$=1820 K, hexagonally reconstructed (001) facets appear in coexistence with the tilted rough phase. The phase boundary of the rough phase decreases with temperature as a quadratic function of the tilt angle away from the [001] direction, consistent with simple theoretical ideas. Below ${\mathit{T}}_1$=1630 K, there are two symmetry-related 2° magic vicinal phases with azimuthal orientations symmetrically rotated away from the cubic [110] direction by ±4.8°. The 2° magic vicinals are reconstructed, so that the terraces between their constituent steps form a corrugated and rotated quasihexagonal structure. Remarkably, the steps are parallel to the troughs and crests of the corrugation. In addition, the tilt angle appears closely related to the period of the reconstruction, favoring an integer (3 or 4) or half integer (9/2 or 7/2) number of corrugation periods on each terrace between steps, depending on the temperature. Finally, below ${\mathit{T}}_2$=1590 K, we find the 6° magic vicinal phase. Its microscopic structure involves terraces upon which there is one period of the surface corrugation.

We present self-consistent ab initio total-energy calculations of the equilibrium relaxed structures and surface energies of the stoichiometric (1×1) (110), (100), (001), and (011) surfaces of ${\mathrm{TiO}}_2$. The relaxations of atoms on these surfaces are found to be substantial, and are responsible for a large reduction of the calculated surface energies. A Wulff construction is used to display the relative energetics of these surfaces. The (100) surface is found to be stable with respect to forming macroscpic (110) facets, while the (001) surface is nearly unstable with respect to forming macroscopic (1×1) (011) facets. These results shed light on published experimental results on the structures of these surfaces.

The convergence of the gradient expansion (GE) for the kinetic energy functional ${\mathit{T}}_{\mathit{s}}$[n] is tested on the basis of the spherical jellium model for metal clusters. By insertion of Kohn-Sham densities into the GE it is found that fourth-order contributions to the GE are more important for jellium spheres than for atoms, indicating that these corrections might also be relevant for the description of solids. By solution of the Euler-Lagrange equations resulting from the GE truncated at second or fourth order, it is demonstrated that the variational accuracy of the GE is considerably lower than that obtained by insertion of high-quality densities. Furthermore, it is shown that a GE to second order with an adjusted prefactor of the gradient term does not give variational results for jellium spheres superior to a fourth-order GE as in the case of atoms.

The effects of short-range electronic correlations on the properties of sodium clusters are studied using the Brueckner g matrix as an effective interaction which describes the scattering of two electrons in the presence of a many-electron medium. The associated cluster Hamiltonian is solved within the Hartree-Fock approximation for the ground state and the dipole plasmon resonance is studied using the self-consistent random-phase approximation. Effects due to ionic core electrons are considered within the pseudojellium model of metal cluster, which goes beyond jellium by using ionic pseudo-Hamiltonians.

Effects of Coulomb interactions and lattice fluctuations in the optical-absorption spectra of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ are theoretically investigated by using a tight-binding model with long-range Coulomb interaction and bond disorder. Anisotropy effects in ${\mathrm{C}}_{70}$ are also considered. Optical spectra are calculated by using the Hartree-Fock approximation followed by the configuration interaction method. The main conclusions are as follows. (1) The broad peaks at excitation energies, 3.7, 4.7, and 5.7 eV, observed in experiments of ${\mathrm{C}}_{60}$ molecules in a solution, are reasonably described by the present theory. Peak positions and relative oscillator strengths are in overall agreement with the experiments. The broadening of peaks by lattice fluctuations is well simulated by the bond disorder model. (2) The optical gap of ${\mathrm{C}}_{70}$ is larger when the electric field of light is parallel to the long axis of the molecule. The shape of the frequency dispersion also depends on the orientation of the molecule. These properties are common in the free-electron model and the model with Coulomb interactions. (3) The spectrum of ${\mathrm{C}}_{70}$ averaged over bond disorder and random orientations is compared with experiments in a solution. There is an overall agreement about the spectral shape. Differences in the spectra of ${\mathrm{C}}_{60}$ and ${\mathit{C}}_{70}$ are discussed in connection with the associated symmetry reduction.

The eigenvalue problem of a time-periodic driven two-level system is investigated. The closed-form solutions for the quasienergy and the Floquet states are obtained with the help of SU(2) symmetry. A distinct advantage of this approach is that the calculations of physical quantities can be made up to any order approximations straightforwardly. To illustrate the validity of our method, we present detailed calculations for a biased double-well heterostructure driven by a laser field. Various situations for the ratio of the energy gap of the undriven system to the laser frequency are discussed, among which it is found that the exact quasienergy crossing emerges if this ratio becomes a nonzero even number. Such an effect of localization should be observable in experiments.

We show that the well-known 0.15-eV donor in bulk GaAs quenches under IR-light illumination and that the quenched (metastable) state has an electronic transition energy about 0.14 eV deeper than the ground state and can be observed by temperature-dependent-resistivity and Hall-effect measurements. The quenched state thermally recovers by an Auger-like process at a rate of r=2.3×${10}^{\mathrm{-}12}$ ${\mathit{nv}}_{\mathit{n}}$ exp(-0.18/kT). Many of the properties exhibited by this donor are similar to those predicted theoretically for the complex defect ${\mathrm{As}}_{\mathrm{Ga}\mathrm{-}}$${\mathrm{V}}_{\mathrm{As}}$.

Strong exciton-photon coupling can be achieved in a semiconductor microcavity. In this regime, the Fabry-Pérot photon mode of the cavity and the exciton electronic mode are no longer the eigenstates of the system. Both states are strongly coupled and cavity polaritons occur. The splitting of the cavity polariton is also known as vacuum-field Rabi splitting in atomic physics. We observe this regime up to room temperature. This may have important implications from a device application point of view because perturbative approaches of the light-matter interaction such as Fermi’s golden rule are no longer valid. The magnitude of the splitting depends on the exciton oscillator strength and provides an accurate measurement of it.

By incorporating the hierarchy picture with Laughlin’s quasiparticle wave functions, trial wave functions for the incompressible fractional quantum Hall states are obtained. It is shown, by comparison with experiment, that the hierarchy level of a state can serve as a measure of its stability when the trial wave functions are rewritten in an alternative form. Numerical study for finite systems has been performed to confirm the theory.

Magnetoluminescence spectroscopy of the photopumped stimulated emission, together with systematic photoluminescence studies as a function of the photogeneration rate, were used to identify the main recombination mechanisms occurring in shallow ${\mathrm{Zn}}_{0.10}$${\mathrm{Cd}}_{0.90}$Se/ZnSe multiple quantum wells for blue-green laser applications. Although the main spontaneous recombination processes have excitonic character, we found that lasing originates from free-carrier transitions.

An exact solution for the single charge soliton, in a one-dimensional array of N gated junctions with equal junction capacitances C and equal gate capacitances ${\mathit{C}}_{\mathit{g}}$, is presented. Analytical expressions for the total energy, as well as the injection threshold voltage of a charge soliton in a biased array, are derived. Based on the exact solution, we analyze the effects of N and ${\mathit{C}}_{\mathit{g}}$/C on the charge soliton, to provide an understanding of the existing experiments.

The phonon-assisted mechanism for relaxing the polarized nuclear spins in the clean limit of a two-dimensional electron-gas (2DEG) system is studied under the quantum-Hall-effect condition. This mechanism consists of two processes. In the first process, electron and nuclear spins are simultaneously reversed by the hyperfine interaction. In the second process, on the other hand, only electron spins are reversed by their coupling to the lattice strain. The quantum nuclear-spin relaxation rate (${\mathit{T}}_1^{\mathrm{-}1}$) is computed as functions of magnetic field and temperature. A possibility that ${\mathit{T}}_1^{\mathrm{-}1}$ can be used as a magnetic probe in studying the electron-electron interaction in high-mobility 2DEG systems is discussed.