An analysis of the absorption spectra, distorted by an interference effect in 0.3-μm-thick hydrogenated amorphous-silicon (a-Si:H) films, is carried out. In the midgap region, the absorptance interferograms, as measured by photothermal deflection spectroscopy, reveal that a-Si:H films have a 10–30-nm-thick surface-defective layer, whereas silicon nitride covered films show a more uniform defect profile. Unlike the bulk layer, the surface density of states is independent of light soaking and thermal annealing, and has a similar absorption spectrum to doped a-Si:H films.

We investigated the degree of forward focusing and of multiple-scattering-induced defocusing for electrons in the keV range along regular chains of different atomic density in GaAs and GaP single crystals. The intensity angular anisotropy of elastic and inelastic (losses) Auger electrons was measured in the primary-beam diffraction-modulated electron-emission approach. In variance to the photoelectron-diffraction (PD) case, the possible influence of the energy-loss mechanism on the observed anisotropy can be disregarded and the in-depth distribution of the scattered intensity is independently determined. Both the focusing and defocusing lengths were found to be significantly larger than in previously reported PD results, and strongly dependent on the interatomic spacing. A comparative study of defocusing along the Ga [110] chains in GaAs and GaP gives experimental evidence of the influence of nearest-neighbor atomic rows (As and P, respectively) on the scattering processes.

We report our experimental results of optical second-harmonic generation (SHG) from H-terminated Si(100) surfaces. For the clean Si(100)-(2×1) and the Si(100)(3×1)-H surfaces, the SH intensity decreased monotonically with increasing temperature in the temperature range 350–700 K and 350–500 K, respectively. For the Si(100)(2×1)-H surface, with increasing temperature, the SH intensity increased in the temperature range 350 to about 470 K, and decreased in the range 470 to about 660 K. Transformation between three phases was found to be reversible. The surface structure and sample temperature could be monitored by the SHG measurements.

The characterization and interaction between Cl atoms on the Si(100)-2×1 surface have been studied by atom superposition and electron-delocalization molecular-orbital theory, using a 2×1 symmetric-dimer model for this surface. A model to explain the slowing of chlorine adsorption kinetics at a coverage of about 0.7 Cl/Si is proposed.

The photoacoustic technique is used to investigate the thermal properties of a two-layer system, of variable thickness, consisting of a semiconductor polycrystalline thin film and glass. It is shown that the thermal diffusivity and thermal conductivity are completely determined based upon the effective sample model proposed by Mansanares et al. [Phys. Rev. B 42, 4477 (1990)].

We present scanning-tunneling-microscopy images that support the single-dimer exchange mechanism proposed for surfactant-mediated epitaxial growth of Ge on the Si(001) surface [Phys. Rev. Lett. 72, 3190 (1994)]. Unusual features of the calculated images corresponding to the (meta)stable geometries during the exchange process are bright spots between the dimer rows and their variation depending on the bias voltage. The experimental image also shows the bright spots between the dimer rows.

We have measured low-temperature photoluminescence spectra of a ZnSe/${\mathrm{ZnS}}_{0.18}$${\mathrm{Se}}_{0.82}$ symmetric superlattice at hydrostatic pressures up to 8 GPa. A crossover of the Γ-like conduction-band states of barrier and well near 4.5 GPa is evidenced by the pressure dependence of the intensity of the main emission band which corresponds to the localized exciton of the ZnSe wells. Further support for a type-I to type-II conversion comes from the formation of a type-II exciton transition, where the excited electron state is confined in the alloy layers, and also from the observation of a ZnSe-like phonon sideband characteristic of the barrier material. The conduction-band crossover is an indirect confirmation of the small conduction-band offset in the ZnSe/${\mathrm{ZnS}}_{\mathit{x}}$${\mathrm{Se}}_{1\mathrm{-}\mathit{x}}$ superlattices.

We discuss far-from-equilibrium electron transport in quantum waveguide structures at low temperatures. On slowly cooling the devices in the dark, the current-voltage characteristics are found to be similar to those of a quantum point contact. Exposure to light at low temperature alters the characteristics dramatically, with one or more regions of current-controlled negative differential conductance occurring. The characteristics can be returned to their prelight condition by annealing the samples above 120 K, which indicates that the effect is associated with the occupancy of DX centers in the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As. We argue that the negative differential conductance arises from hot-electron bistabilities due to puddles of charge trapped in the waveguide by potential inhomogeneities associated with ionized DX centers.

We present a simple theoretical model for the diffusion of free carriers in layered semiconducting compounds, for which a small concentration of dopants (guest layers) disturb the system conductance, resulting in a strong anisotropy across and along the layers, which depends on the temperature of the system, as well as on the barrier lowering caused by the applied voltage. This model is based on diffusion via random walks of free particles, with the guest layers acting as barriers hindering free motion. By considering the probability of overcoming the barriers, we derive a formula for the current in terms of the guest concentration, barrier height, and temperature. Experimental results from previous studies for the system ${\mathrm{ZnIn}}_2$${\mathrm{S}}_4$:${\mathrm{Zn}}_2$${\mathrm{In}}_2$${\mathrm{S}}_5$ concerning the anisotropic conductivity, its temperature and voltage dependence, as well as computer simulations are in excellent agreement with this model.

While the binding energy and the ground-state wave function of a hydrogenic impurity in a strictly one-dimensional or three-dimensional system can be exactly calculated, the case of finite but small (a≤${\mathit{a}}_{\mathit{B}}$) or large (a>${\mathit{a}}_{\mathit{B}}$) wire radius a is not amenable to an exact treatment, and is usually left to the variational method as the only resort. However, the amount of computation involved in variational calculations is large and its accuracy is sometimes difficult to assess. We have developed reliable and physically motivated perturbative approaches specifically to treat the a≤${\mathit{a}}_{\mathit{B}}$ and a>${\mathit{a}}_{\mathit{B}}$ situations. The close agreement of our results with those from variational methods lends support for our simpler method as an alternative to the variational approach.

The electronic states in a disordered solid may be localized or delocalized, depending on their energy and the disorder potential. By examining exciton dynamics with transient four-wave mixing experiments we show that low-energy excitons in CdTe/(Cd,Mg)Te quantum wells are localized, while higher-energy excitons are not. In the delocalized regime the excitonic dephasing rate decreases with increasing energy, a theoretically predicted, but experimentally unreported result.

Excitonic properties have been studied in cubic ${\mathrm{Zn}}_{0.42}$${\mathrm{Cd}}_{0.58}$S lattice matched to GaAs. At 2 K, the time-integrated photoluminescence spectrum was composed of single emission peaking at 2.863 eV and its LO-phonon replica. The linewidth of the main peak was 18.5 meV, which fairly well agreed with the theoretical value based on the disorder-induced broadening of exciton luminescence in alloys. In order to assess the emission mechanism, the transient luminescence decay was measured at various emission energies. At the high energy tail (2.883 eV), the luminescence showed exponential decay with a time constant of about 72 ps. On the other hand, the decay time increased with decreasing the detected emission energy. It was about 660 ps at the emission peak (2.863 eV). We interpret these features by means of the model of exciton localization.

The Larmor clock and the particle absorption approaches to the tunneling time are reexamined and extended to cover two- and three-dimensional systems and multichannel scattering. We show that both of them can be viewed as particular cases of the recent stay-time method. Furthermore, the barrier traversal time proposed by Büttiker on the basis of the Larmor clock is found to correspond to the root-mean-square value of the actual tunneling time, rather than to the mean tunneling time.

We have used femtosecond optical spectroscopy to observe directly the vacuum Rabi oscillations associated with the coupled exciton-photon mode splitting in a semiconductor quantum microcavity. When the microcavity is impulsively excited by a coherent short optical pulse, the time-resolved emission from the cavity shows beats corresponding to the oscillation between cavity and exciton modes, with a decay corresponding to approximately twice the cavity lifetime. Interferometric pump-probe measurements clearly show the coherent evolution of the cavity polarization.

The phonon-dispersion curves for a xenon monolayer adsorbed on Cu(110) have been measured using inelastic He scattering. The size and geometry of the substrate unit cell introduces an anisotropic distortion of the xenon monolayer, which is reflected in a strong deformation of the phonon-dispersion curves with respect to the floating two-dimensional (2D) xenon layer. This effect is reproduced in a 2D phonon calculation, based on the Lennard-Jones Xe pair potential. In this way a microscopic relationship between lattice strain, force constants, and surface stress can be established.

The electronic structure of the magnetized Fe(001)-Pt interface has been studied at submonolayer coverages by spin-resolved photoemission with synchrotron radiation (hν=60 and 115 eV). Evidence of ferromagnetic coupling between Pt and Fe is found and strong hybridization effects between the Fe 3d and Pt 5d states are suggested. Core photoemission from the Fe 3p core levels is also reported showing a broadening of the majority spectrum of the Fe 3p spin-resolved line after Pt evaporation (at hν=140 eV).

The interaction potential between neutral magic sodium clusters (consisting of 2, 8, 20, and 40 atoms) is calculated within a two-center jellium model for the ionic cores. The results extend the long-range van der Waals potentials for these systems as determined by Pacheco and Ekardt [Phys. Rev. Lett. 68, 3694 (1992)] to smaller cluster-cluster separations.

The potential energies per atom of pure graphite and of ${\mathrm{C}}_{60}$ stage-one graphite intercalation compound (GIC) are compared in a calculation based on summing pairwise a Lennard-Jones 12-6 potential between all mass points with parameters taken from previous calculations on pure graphite. The C atoms are treated as uniformly smeared onto rigid spheres and rigid planes. For graphite the potential has a minimum of -46 meV/atom at an interplanar spacing of 3.37 Å with a compressibility of 3.0×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^2$/dyn. For the GIC, assuming a triangular ${\mathrm{C}}_{60}$ net, the minimum is -22 meV/atom occurring at both a ${\mathrm{C}}_{60}$-graphene and a ${\mathrm{C}}_{60}$-${\mathrm{C}}_{60}$ nearest approach distance of 2.9 Å. This distance is comparable to fcc ${\mathrm{C}}_{60}$ at room temperature. The calculated compressibility is 3.8×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^2$/dyn.

We study the effects of electron-electron interactions on the localization properties of electronic states in the random-dimer model. We show at the mean-field level that, although disorder or interactions separately do not destroy the dimer resonances, the combination of both can lead to localization. However, numerical evidence suggests that the localization length of the electronic states is larger than the typical size of the polymers in which this model is relevant.

The number of operations required for conventional density-functional algorithms grows as the cube of the number of atoms, N. For large systems the computing requirements are unattainable. To overcome this limitation it is acceptable to approximate those variables with respect to which the free energy is stationary. We show that the stationarity of the free energy with respect to electron density, one-electron potential, chemical potential, occupation function, and temperature allows for very useful approximations leading to rapid and accurate determination of the free energy. Here we discuss approximations involved in calculating the finite temperature electron density needed to evaluate the Harris-Foulkes free energy. Of particular importance are (1) an electron density at each site that is based on exact solution of the Poisson equation combined with a solution of the multiple-scattering problem in which only scattering from a small cluster of sites surrounding the site in question is retained and (2) an approximate occupation function having a finite number of poles in the complex energy plane. The intention is to develop, within density-functional theory, an O(N) scalable first-principles scheme, based on spatially local multiple-scattering methods, for calculating free energies of large systems.