In a recent paper, Yi and Neumark [Phys. Rev. B 48, 17 043 (1993)] determined the effective radii of donors and acceptors in ZnSe:Na from the time decay of the close pairs. The pair decay rate used is proportional to an overlap integral between donor and acceptor wave functions. We show that such an integral as well as the hopping matrix can be used for both donor-acceptor and donor-donor systems. The density of states, calculated from these integrals, is capable of predicting the shift of conductivities observed in experimental results. The calculation for the metal-nonmetal transition of Si: P,B is in good agreement with recent experimental findings.

Conductance measurements of ${\mathrm{C}}_{60}$ films in the range 100–350 K reveal the existence of hysteresis in the photoconductance, but not in the accompanying dark conductance. The light-induced photoconductance changes are almost completely annealed at the 260-K phase transition, but a peak and partial annealing occur at 150–160 K, lending support to the existence of a weak structural anomaly or phase transition at 155 K. A phenomenological model is proposed for the observed hysteresis in terms of a light-induced redistribution of recombination centers in the gap of the semiconductor.

We present results from low-angle neutron and x-ray scattering experiments on hydrogen and deuterium loaded (001)-oriented single-crystal Mo/V superlattices. The individual Mo and V layer thicknesses in the samples were determined by simulations of the x-ray reflectivity curves. The intensity of the first-order neutron-diffraction peak for the hydrogenated sample is 31% larger than from the dehydrogenated sample. When deuterated, the intensity of the first-order peak is extremely small, only 0.5% of the dedeuterated sample. The distribution of hydrogen in the layers was derived by simulations of the neutron and the x-ray diffraction patterns. The results unambiguously demonstrate that the hydrogen isotopes are exclusively located in the V layers, but at the interfaces there is a region with reduced hydrogen concentration.

We study the response of a recently proposed global-coupling model of charge-density waves to a joint ac plus dc drive. The model is the standard Fukuyama-Lee-Rice model with an additional global-coupling term to account for interaction with (uncondensed) quasiparticles. We find that traditional mode locking is accompanied by hysteresis on mode-locked steps. The experimentally observed asymmetry in the size of hysteresis is reproduced and explained by drawing attention to the importance of the pinning force. For large values of global coupling, low-dimensional behavior leads to vanishing of subharmonic steps consistent with experiments.

The electronic and the optical properties of β-${\mathrm{C}}_3$${\mathrm{N}}_4$ are studied by means of first-principles local-density calculations. The structural properties obtained via total-energy calculations are in good agreement with the results of Liu and Cohen [Phys. Rev. B 41, 10727 (1990)]. The frequency-dependent optical properties are calculated after an approximate self-energy correction scheme is applied to the energy spectrum. It is found that the band gap in β-${\mathrm{C}}_3$${\mathrm{N}}_4$ increases linearly with pressure. The absorption spectrum shows two major peaks at 7.9 and 13.9 eV with a static dielectric constant of 3.2 predicted.

Electronic-structure calculations are reported for ${\mathrm{IrSb}}_3$, ${\mathrm{CoSb}}_3$, and ${\mathrm{CoAs}}_3$ in the skutterudite structure. The band structures show a pseudogap around the Fermi level. The single band, which crosses the pseudogap, touches the conduction-band minimum at the Γ point in ${\mathrm{CoAs}}_3$ and ${\mathrm{IrSb}}_3$ and almost touches it in ${\mathrm{CoSb}}_3$ due to a different ordering of the conduction bands. The dispersion of this gap-crossing band is quadratic at the Γ point as expected, but for the antimonides it remarkably crosses over to linear behavior extremely close to the band edge, so that for doping levels as low as 3×${10}^{16}$ holes/${\mathrm{cm}}^3$ the properties are determined by the linear dispersion. This yields interesting transport effects, such as a completely off-diagonal inverse mass tensor and unusual doping dependencies of the hole mobility and Seebeck coefficient. The Seebeck coefficients calculated from the band dispersion are in excellent agreement with experiment.

Angle-resolved photoemission studies of single-crystal ${\mathrm{C}}_{60}$(111) films grown on GeS(001) reveal changes in valence feature line shape with emission angle and photon energy that are indicative of band dispersion. For an excitation energy (hν) of 10 eV, normal emission spectra show four sharp structures within the ∼1.1-eV-wide valence feature derived from the second highest molecular orbital (HOMO-1) of ${\mathrm{C}}_{60}$. For hν=8.1 eV, the 1-eV-wide HOMO-derived feature exhibits changes with emission angle mainly due to dispersion of 0.6 eV in the unoccupied bands. The distribution of electronic states underlying HOMO and HOMO-1 indicates that vibronic loss structures are not necessary to explain the width of these valence features.

It has been suggested that excitation of poly(p-phenylene vinylene) (PPV) by photons above the absorption edge produces principally polaron pairs or split excitons. We show that such polaron pairs can account very well for the peaks in the observed picosecond photoinduced absorption, the width of the absorption, and other phenomena associated with photon absorption in PPV.

The positron two-dimensional angular correlation of annihilation radiation (2D-ACAR) technique has been coupled with optical excitation to study the native point defects in semi-insulating GaAs. The As vacancy was observed below ∼170 K when illuminated with 1.41±0.07 eV photons. The temperature dependence of the 2D-ACAR spectra, with and without illumination, was studied. Data were also collected at 25 K as a function of the intensity of infrared light. The 2D-ACAR spectra reflect the ${\mathit{e}}^{+}$-${\mathit{e}}^{\mathrm{-}}$ pair momentum distribution at or near the vacancy and provides symmetry and electronic structure information, which can be used as a unique defect signature for the vacancy.

Strong enhancements of the exciton binding energy (${\mathit{E}}_{\mathit{x}}$), compared to bulk GaAs, are deduced from high-resolution spectroscopic studies of shallow GaAs-${\mathrm{Al}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{y}}$As quantum wells, with aluminum concentrations of (1–4.5)%. A clear increasing trend in ${\mathit{E}}_{\mathit{x}}$ from 5.9 meV at 1% aluminum to 7.0 meV at 4.5% is deduced, in good agreement with the predictions of variational calculations. Even at 1%, the value of ${\mathit{E}}_{\mathit{x}}$ represents an enhancement of 40% over that in three-dimensional GaAs. The similarity of all the spectra supports strongly the marked two dimensionality of the lowest exciton states, even for very low barrier heights.

We propose and use surface acoustic waves to control single-electron tunneling through an array of small tunnel junctions. The first experiments show that the wave traveling on the GaAs substrate couples to the islands of the array by modulating the potentials of their gates. We see cusps in the slope of the I-V curve at f=290 MHz around I=±ef. At 48 MHz and zero voltage biased to the ends we observe modulation of current by 3 pA around zero as a function of equal potential on the islands. Advantages of our method and problems, in particular the offset charge on GaAs, are discussed.

Specially designed single GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterojunctions with two monolayers of acceptors located at different distances from the interface have been studied by luminescence experiments. Using a time-resolved technique we distinguished the two channels of luminescence corresponding to radiative recombination of two-dimensional electrons with holes bound to acceptors from these two monolayers. Comparison of the fractional-quantum-Hall-effect (FQHE) energy gaps derived from these two recombination channels allows us to measure the corrections to the FQHE gaps associated with the finite e-h spatial separation.

The entire distribution is computed of the conductance of a quantum dot connected to two electron reservoirs by leads with a single propagating mode, for arbitrary transmission probability Γ of the mode. The theory bridges the gap between previous work on ballistic leads (Γ=1) and on tunneling point contacts (Γ≪1).

Using the supersymmetry technique we calculate the distribution of local amplitudes of electron wave functions in quantum dots in the regime of chaotic motion. The exact formula for an arbitrary magnetic field is obtained from first principles; this provides the possibility of describing the crossover between the orthogonal and unitary ensembles. The distribution function of chaotic electron waves can be the basis for a calculation of the resonance conductance distribution function.

A kinetic model for surfactant-mediated epitaxial growth is suggested. The surfactant atoms join kinks predominantly at steps as the latter are the energetically most favored sites on the crystal surface. The deposited atoms must displace them in order to join the crystal lattice. This gives rise to additional kinetic barriers the atoms have to overcome when they join the ascending steps. The asymmetry of incorporating atoms in ascending and descending steps is reversed and a diffusion gradient that drives the atoms to join the descending steps appears. The thermodynamic driving force for the two-dimensional–three-dimensional transformation to occur is suppressed and layer-by-layer growth takes place.

We consider from a theoretical viewpoint the direct imaging of atoms at and near the surfaces of solids by both x-ray-fluorescence holography (XFH) and electron-emission holography (EEH). The more ideal nature of x-ray scattering makes XFH images superior to those in single-energy EEH. The overlap of real and twin features for pairs of atoms at ±a can cause their XFH or EEH atomic images to cancel for certain combinations of wave vector and ‖a‖. The relative merits of XFH and EEH for structure studies are considered.

The effective-medium theory of electrons by Norskov and Lang suggests that hydrogen adsorbed on transition metals is affected by only one-third of the electron densities when compared with interstitial hydrogen in the metals. As an important consequence, we show that the hydrogen-electron coupling parameter κ (as defined by Kondo) on W and Ni surfaces is close to 1/2 as opposed to 0.2 for interstitial hydrogen and positive muons in metals. It means that the electron-controlled under-barrier tunneling rate of hydrogen on W and Ni, proportional to ${\mathit{T}}^{2\mathrm{\kappa }\mathrm{-}1}$, varies weakly with temperature. Our numerical calculation shows that the over-barrier hopping can directly cross over to electron-controlled tunneling. We thus believe that the conduction-electron mechanism is a strong candidate to explain the weakly temperature-dependent tunneling of hydrogen on W and Ni.

The interaction of ${\mathrm{O}}_2$ with Na–(polycrystalline Pt and Cr) interfaces at room-temperature Na saturation coverage has been studied with projectile- and target-autoionization electron spectroscopy using low-energy ${\mathrm{Ne}}^{+}$-ion impact. Our results show that for both systems oxygen is initially incorporated beneath the Na atoms. For Na/Pt, further deposited oxygen is uniformly distributed on top of the Na layer, whereas for Na/Cr large patches are formed on the surface.