Designed to refute arguments supporting a previously suggested cesium-centered quantum-mechanical model for the electride electrons in the crystalline electride [Cs(18-crown-6${)}_2$], the anionic-hole-centered quantum-mechanical model for these species described in the preceding Comment is shown to be seriously flawed and to exhibit an inherent nonuniqueness.

We study the temperature dependence of persistent near-infrared absorption bands induced in single crystals of [Pt(en${)}_2$][Pt(en${)}_2$${\mathrm{Cl}}_2$](${\mathrm{ClO}}_4$${)}_4$, where en=ethylenediamine, by irradiation with 488-nm light. If the temperature is changed between 10 and 200 K in dark after irradiation, these bands show significant changes in the spectral position, line shape, and intensity. The behaviors of the mean energy and second and third moments of the A band are interpreted consistently in terms of a quantum-fluctuation model of solitons bound by shallow, periodic potential wells.

We have studied the electronic structure, atomic geometry, and formation energies of native defects in GaN using first-principles total-energy calculations. Our results reveal the vacancies to be the dominant defects in GaN, whereas antisites and interstitials are energetically less favorable. In p-type GaN the nitrogen vacancy (a donor) has the lowest formation energy, in n-type GaN the gallium vacancy (an acceptor). Our results show that the vacancies may be important for compensation. However, isolated point defects and particularly the nitrogen vacancy can be excluded as the source for the n-type conductivity in as-grown GaN, contrary to the generally accepted picture.

We present the results of a photoluminescence study of interdiffusion on the group-III sublattice in the ternary In-Ga-As/In-Ga-As and the quaternary In-Ga-As-P/In-Ga-As-P systems, both grown off InP substrates. We have shown that the diffusion obeys Fick’s law, and that over the temperature range 950 °C–650 °C, the diffusion coefficients of both the ternary and quaternary materials can be described by the equation D=${\mathit{D}}_0$exp(-${\mathit{E}}_{\mathit{A}}$/kT), where ${\mathit{E}}_{\mathit{A}}$=3.4±0.2 eV and ${\mathit{D}}_0$=3.9 ${\mathrm{cm}}^2$/s. Within experimental error these values are identical to those previously measured for interdiffusion on the group V in the same material system.

We discuss the origin of zero-dimensional states, which give rise to resonant structure at the current onset in tunneling devices. The states can be identified as being due to random pairs of shallow donors.

We calculate the instanton–anti-instanton action ${\mathit{S}}_{\mathit{MM}\mathit{¯}}$(τ) in the gauge theory of the half-filled Landau level. It is found that ${\mathit{S}}_{\mathit{MM}\mathit{¯}}$(τ)=(3-η)[${\mathrm{\Omega }}_0$(η)τ${]}^{1/\left(3\mathrm{-}\mathrm{\eta }\right)}$ for a class of interactions v(q)=${\mathit{V}}_0$/${\mathit{q}}^{\mathrm{\eta }}$ (0≤η<2) between electrons. This means that the instanton–anti-instanton pairs are confining so that a well-defined ‘‘charged’’ composite fermion can exist. It is also shown that ${\mathit{S}}_{\mathit{MM}\mathit{¯}}$(τ) can be used to calculate the spectral function of electrons from the microscopic theory within a semiclassical approximation. The resulting spectral function varies as ${\mathit{e}}_0^{\mathrm{-}[\mathrm{\Omega }}$(η)/ω${]}^{1/\left(2\mathrm{-}\mathrm{\eta }\right)}$ at low energies.

Ballistic-electron-emission microscopy (BEEM) has been used to investigate the effects of strain on ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ alloys. Lifting of the degeneracy of the conduction-band minimum of ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ due to lattice deformation has been directly measured by application of BEEM spectroscopy to Ag/Si structures. Experimental values for this conduction-band splitting agree well with calculations. In addition, an unexpected heterogeneity in the strain of the ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ layer is introduced by deposition of Au. This effect, not observed with Ag, is attributed to species interdiffusion and has important implications for metal-semiconductor devices based on pseudomorphic ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$/Si material systems.

Ensembles containing a few hundred quantum dots, ∼17 nm in diameter, were prepared by etching μm-size mesas in ${\mathrm{Al}}_{\mathit{y}}$${\mathrm{In}}_{1\mathrm{-}\mathit{y}}$As/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum-dot samples grown using the spontaneous island formation during molecular-beam epitaxy. The visible photoluminescence spectra display the reproducible statistical fluctuations of the ground states, which are emitting individual lines a fraction of a meV in width.

The magnetic-field-dependent conductivity of a square antidot lattice is calculated from the experimentally determined magnetoresistance and Hall resistance. Close to the commensurability conditions where the magnetoresistance always displays pronounced maxima we find a crossover from minima to maxima in the conductivity as a function of antidot size. We interpret this behavior in terms of different classes of electron trajectories whose relative importance depends on the size of the antidots with respect to the lattice period.

Coherently strained GaX/InX interfaces (X=P, Sb) lattice matched to a (001)-oriented substrate are predicted to have a type-I band-gap alignment, with both the valence-band maximum and the conduction-band minimum (CBM) located on the In-rich material. At the same time, the CBM wave function of short-period (GaX${)}_{\mathit{n}}$/(InX${)}_{\mathit{n}}$ superlattices is predicted to have larger amplitude on the GaX layers, leading to a type-II alignment. We show that (i) a type-II→type-I transition occurs around the period n=4; (ii) this transition has a different origin with respect to the well-known case of GaAs/AlAs superlattices; (iii) the band structure of ultrathin superlattices cannot be explained in terms of a simple effective-mass theory; (iv) the wave-function localization in short-period superlattices is determined by the atomic orbital energies.

Scanning tunneling microscopy (STM) has been used to study GaAs(001)-(2×4) surfaces grown by molecular-beam epitaxy. Although reflection high-energy electron diffraction always showed a characteristic (2×4) pattern, STM images indicated significant differences in the composition of the surfaces depending on the nature of the quenching conditions. A previously unreported, locally disordered (1×2) structure was observed under As-deficient conditions. This was found extending from step edges and in missing-dimer holes, where the top layer of As had been removed to expose the second layer. The atoms in this exposed layer are identified as Ga and they form short rows in the [1¯10] direction. The twofold periodicity is due to a vacancy structure, with Ga atoms located at alternate sites along the [110] direction. These results may rationalize some of the recent controversies regarding the composition and structure of the As-terminated (2×4) surface prepared by decapping methods.

We calculate the core-level shift of the Si 2p levels for atoms near the H/Si(111)-(1×1) surface. We show that a simple first-order perturbation theory using pseudopotentials and the local-density approximation gives good results for the photoemission spectra of the core electrons. The electric-dipole matrix elements for the levels are also calculated. The results are in good agreement with a recent high-resolution angle-resolved photoemission-spectroscopy measurement and allow us to interpret some previously unexplained experimental peaks.

The coherent motion of electronic wave packets in the Wannier-Stark regime of externally biased GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As superlattices is investigated by a time-resolved electro-optic technique. As predicted by the semiclassical theory, electronic wave packets undergo Bloch oscillations. They are observed via anisotropic changes in the refractive index associated with a intraband polarization, which is set up by the coherent spatial motion of electronic wave packets relative to localized holes. The oscillation amplitude depends strongly on the initial excitation conditions. Frequency, amplitude, and dephasing of Bloch oscillations are investigated over a wide range of electric fields. Complementary results to four-wave mixing and THz emission experiments are obtained.

The dynamics of carrier-induced absorption changes in CdSe quantum dots are investigated with femtosecond spectroscopy. After excitation with 4-eV photons a redshift of the lowest optical transition is observed in the initial phase of carrier relaxation. This shift is attributed to a biexciton effect where two electron-hole pairs interact via the Coulomb potential.

The effects of disorder and many-body interaction on the four-wave-mixing signal in semiconductor quantum wells are studied experimentally and theoretically. For weakly disordered samples it is shown that the combined influence of Coulomb and disorder effects leads to a characteristic photon-echo signal in time-resolved measurements and to oscillations in the time-integrated signal. Convincing agreement between experiment and theory is demonstrated.

We demonstrate that a supercurrent can flow through a high-mobility two-dimensional electron gas (2DEG) between two superconducting contacts 1 μm apart. The ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs channel length is over 10 times the proximity-effect-induced coherence length in the semiconductor, and electrons travel ballistically between the contacts. The supercurrent is destroyed when the mobility of the 2DEG is reduced by electron-beam irradiation, suggesting that the supercurrent results from coherent Andreev reflection and ballistic transport between the superconducting electrodes.

We propose a mechanism for the origin of RHEED intensity oscillations during the growth of (Y,Dy)${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ thin films. Surface relaxation and RHEED intensity recovery observed during the growth of these materials are ascribed predominantly to the diffusion of already formed (Y,Dy)${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ units rather than to the chemical reaction of the constituents to form a unit cell. The validity of the model is demonstrated by comparing the surface step densities determined from results of Monte Carlo simulations with RHEED intensity oscillations observed during pulsed laser deposition.

The adsorption geometries of the p(2×2) and (√3×√3)R30° Rb/Ru(0001) phases at coverages of 0.25 and 0.33, respectively, were investigated using low-energy electron diffraction. The structure analyses revealed that Rb resides in the threefold fcc site in the p(2×2) phase while for the (√3×√3)R30° phase the hcp site is favored. The effective radius of Rb does not change with the Rb coverage within the error bars of this analysis. The concept of ‘‘split positions’’ has been used in order to take the enhanced motion of the adsorbate atoms parallel to the surface properly into account.

Spin-orbit effects in normal photoemission from ferromagnetic cubic (001) surfaces with in-plane magnetization M→ have been studied by symmetry considerations and by relativistic one-step-model Green-function calculations for d-band emission from Ni(001). Spin-averaged spectra due to s-polarized light in two orthogonal states show an intensity asymmetry, i.e., magnetic linear dichroism (MLD). For off-normally-incident p-polarized light, there is another type of MLD, which is maximal for M→ normal, and zero for M→ parallel to the plane of incidence. In the latter case, the electron spin polarization P→ has components exclusively due either to spin-orbit coupling or exchange. Individual spectral features are explained by the relativistic bulk band structure and by analytical considerations.

A model for small helical carbon tubules is presented. A heptagon-pentagon defect in a perfect fullerene tubule, which is made up entirely of hexagons formed by nearest-neighbor carbon-carbon bonds, bends the tubule at a 30° angle, which is the angle between the axes of the two halves. The tubule circumferences should roughly match. Pentagons and heptagons should be isolated and separated as far as possible. These effects would favor long-range order along the helix, given a driving force for curling during the formation process.