Within the random-phase approximation, we demonstrate the existence of multiple undamped acoustic plasmons in a two-component three-dimensional plasma with an isotropic momentum distribution.

Photoexcited carrier behavior in modulation-doped multiple narrow GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum-well structures which vary in quantum-well width and doping concentration is investigated with Monte Carlo simulations. The model includes scattering of Γ-valley subband electrons by confined slab and interface-polar-optical phonons and between Γ-valley and L-valley electrons via optical-deformation-potential phonons. The model closely predicts time constants for electron relaxation found experimentally and shows that the effect of including the L-valley transitions in the Monte Carlo simulation is essential.

The piezomodulated-, electromodulated-, and photomodulated-reflectivity spectra of a pseudomorphic ZnTe epilayer, grown on an InAs epilayer by molecular-beam epitaxy, exhibit heavy- and light-hole excitonic signatures split by the lattice mismatch induced biaxial compressive strain. This splitting in the pseudomorphic epilayer is studied as a function of applied hydrostatic pressure using photomodulated reflectance spectroscopy at 80 K. With increasing hydrostatic compression, the compressive strain is progressively compensated by the pressure-induced tensile strain. At ∼55 kbars the epilayer becomes strain-free, and is under a biaxial tension at higher pressures. The separation between the heavy- and light-hole signatures is superlinear in pressure, suggestive of a strain or volume deformation-dependent shear deformation-potential constant. We also compare the pressure dependence of the Raman LO phonon of the ZnTe epilayer on InAs with that of a bulk ZnTe sample at 13 K. The pressure-dependent strain is found to be linear. Accurate values of the first-order strain derivatives of the LO phonons and mode Grüneisen constants are obtained.

In this paper, we present a simple theory relating the unit-cell size of a heterointerface to the alloy-film composition (x) and we apply it to ${\mathrm{Si}}_{\mathit{x}}$${\mathrm{Ge}}_{1\mathrm{-}\mathit{x}}$/Si(111)-7×7 and a-Si/${\mathrm{Si}}_{0.8}$${\mathrm{Ge}}_{0.2}$(111)-5×5 heterostructures. The results are analyzed in the light of existing experimental data.

Magnetotransport behavior of two-dimensional X-band electrons created in modulation-doped very thin (25–75 Å) AlAs quantum wells was studied. Effective-mass analysis based on temperature-dependent magnetoresistances clearly showed ${\mathit{X}}_{\mathit{z}\mathrm{-}}$${\mathit{X}}_{\mathit{x}\mathit{y}}$ crossover at an AlAs thickness of about 45 Å. This value is much smaller than that in an AlAs/GaAs type-II superlattice (60 Å). Origins of this difference were discussed in terms of the effect of doping and degeneracy change and of the change of strain due to the structure difference.

We have performed a full self-consistent many-body study, within the framework of the random-phase approximation, on the intrinsic bistability in double-barrier resonant-tunneling structures. We found that the exchange-correlation effect can suppress the bistability which may be produced by the electrostatic feedback to the potential by the charge accumulated in the quantum well. Our self-consistent numerical results explain the existing contradiction between the affirmative theoretical prediction and the negative experimental observation on the occurrence of bistability in certain samples.

We demonstrate that, by enhancing the E1-H1 wave-function overlap to maximize the interband interactions in semimetallic InAs/${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$Sb superlattices with thin ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$Sb layers, one can induce a band structure which is strikingly similar to that for semimetallic HgTe/CdTe superlattices. The indirect semimetallic alignment which results from a holelike dispersion of E1 and electronlike dispersion of H1 leads to the possibility of observing a wide range of phenomena such as mass broadening, double cyclotron resonance, a second semiconducting regime at large well thicknesses, and the magnetic generation of electron-hole plasmas above a critical field. Recent experimental data indicating the coexistence of multiple electron species in a semimetallic InAs/${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$Sb superlattice are readily interpreted in terms of the predicted arrangement of the bands.

We employ ab initio total-energy-minimization techniques to study the relaxation of Ge dimers on Si(100). We find that the dimers adopt an asymmetric configuration, in agreement with recent experimental results [E. Fontes et al., Phys. Rev. Lett. 70, 2790 (1993)]. The average dimer height displacement and tilting angle are found to be 0.79 Å and +19°, respectively. Our calculations predict that the (2×2) and c(4×4) reconstructed structures have the same energy, and suggest that either one of these two could be the ground-state structure.

We compute the quantum correction δA due to weak localization for transport properties A= ${\mathit{tsum}}_{\mathit{n}}$ a(${\mathit{T}}_{\mathit{n}}$) of disordered quasi-one-dimensional conductors, by integrating the Dorokhov-Mello-Pererya-Kumar equation for the distribution of the transmission eigenvalues ${\mathit{T}}_{\mathit{n}}$. The result δA=(1-2/β)[1/4a(1)+${\mathcal{F}}_0^{\mathrm{\infty }}$dx(4${\mathit{x}}^2$+${\mathrm{\pi }}^2$${)}^{\mathrm{-}1}$a(${\cosh }^{\mathrm{-}2}$ x)] is independent of sample length or mean free path, and has a universal 1-2/β dependence on the symmetry index β∈{1,2,4} of the ensemble of scattering matrices. This result generalizes the theory of weak localization for the conductance to all linear statistics on the transmission eigenvalues.

Several atomic processes enhancing layer-by-layer growth have been identified in simulated deposition of Pt atoms onto Pt(111) at 275 K. After depositing the equivalent of one and two monolayers, only ca. 10% of the atoms sit atop the primary growing layer. Impinging atoms accelerate strongly towards the surface and can displace peripheral island atoms. Excess kinetic energy is generated that can effect annealing, even several atoms away from the impact site. Adatoms can descend at island edges only by an exchange diffusion mechanism, particularly at kink sites and vacancy islands.

It is shown that the results of calculations using a scattering-matrix approach based on empirical pseudopotentials support the use of the usual envelope-function interface connection rules at GaAs/AlAs heterojunctions, contrary to the claims of the above-cited paper, provided one uses an energy-dependent effective mass.

We point out that the standard diffusion model for the motion of thermalized positron does take into account multiple encounters of the positron with the surface, contrary to what could be inferred from a recent paper by Kong and Lynn [Phys. Rev. B 44, 13 109 (1991)]. In this context we discuss recent measurements on positron-surface interactions and positron Auger spectroscopy.

In replying to Jensen and Walker’s Comment, we point out the inconsistency that exists in theoretical modeling and experimental data analysis for positron remission measurements. For the modeling aspect, Jensen and Walker have correctly emphasized that the diffusion-equation theory includes the multiple-encounter effects for positrons at surfaces.

Kondo insulators are a class of materials with a nonmagnetic ground state characterized by extremely narrow charge-excitation gaps induced by strong electron correlations. Prompted by suggestions that FeSi may be strongly correlated in a similar class to the Kondo insulators, we present results for the electronic structure and optical properties for FeSi calculated with a self-consistent augmented spherical wave method. The calculated indirect gap (≊0.1 eV), is roughly consistent with resistivity measurements which indicate a gap of about 50 meV. We calculate the optical response incorporating simple finite-temperature effects, namely, electron-phonon scattering of intrinsic carriers. We find that this provides a poor direct fit to recent infrared measurements. The number of intrinsic carriers is low compared to experimental observations, even with an ad hoc renormalization of the gap, and a strong scattering mechanism with attendant disordering of the band structure is required as well.

The usual application of the Lei-Ting balance equation method for treating electron transport problems makes use of a Fermi distribution function for the electron motion relative to the center of mass. It is pointed out that this presumes the existence of a moving frame of reference that is dynamically equivalent to the rest frame of reference, and this is only true for electrons with a constant effective mass. The method is thus inapplicable to problems where electrons governed by a general energy-band dispersion E(k) are important (such as in miniband conduction). It is demonstrated that this difficulty can be overcome by introducing a distribution function for a drifting electron gas by maximizing the entropy subject to a prescribed average drift velocity. The distribution function reduces directly to the usual Fermi distribution for electron motion relative to the center of mass in the special case of E(k)=${\mathrm{ħ}}^2$‖k${\mathrm{\Vert }}^2$/2${\mathit{m}}^{\mathrm{*}}$. This maximum entropy treatment of a drifting electron gas provides a physically more direct as well as a more general basis for the application of the balance equation method.

The observation of shake-up satellites assigned to simultaneous excitation of 1s and 2p Mg core electrons in magnesium oxide and fluoride is reported. The presence of well-defined features due to KL double-electron excitations in x-ray photoemission spectra of these solid-state systems is assessed. Possible multiplet splitting and effect of crystalline structures in double-electron line shapes are discussed. Reference to previous works in atomic and molecular systems is given. The relevance of this problem in x-ray-absorption spectroscopy data analysis is pointed out.

A microscopic tight-binding model of the spin-dependent coupling between effective-mass electrons and localized spins in the IV-VI diluted magnetic semiconductors is proposed. Contributions from the direct Coulomb exchange as well as from the hybridization between band states and resonant levels associated with magnetic ions are taken into consideration. The exchange integrals are evaluated quantitatively and are shown to compare favorably with experimental values for both the Mn- and the Eu-based compounds. Some peculiarities of IV-VI magnetic alloys, which may render the virtual-crystal approximation invalid, are noted and linked to the previously unexplained experimental results.

We report a systematic study of the dc-electric-field dependence of THz radiation from a bulk GaAs sample optically excited by femtosecond pulses. The experimental results reveal that both displacement and transport currents contribute to THz radiation and the relative contributions are shown to be very sensitive to the dc electric field. This observation agrees qualitatively with a simulation based on a fully quantum-mechanical treatment of the transient photoconductive response of GaAs.

Intense laser irradiation has been used to anneal porous silicon. In situ Raman spectroscopy and photoluminescence (PL) spectroscopies have been used as probes of the silicon particle sizes and temperatures, and the resulting light emission of porous silicon. Once annealed, the PL of porous silicon while at 830 °C exhibited no PL energy shift from its 300-K position. Since the gap of silicon redshifts by 0.29 eV at 830 °C, this red PL cannot be the property of the Si particles. A model involving the presence of nonbridging oxygen hole centers has been suggested to account for this PL.

We show that the kinetics of recombination of holes bound at acceptors from a δ-doped monolayer in a heterostructure with magnetically frozen two-dimensional (2D) electrons evolves according to the power law I(t)∝${\mathit{t}}^{\mathrm{-}1}$. This behavior is universal for any kind of 2D electron insulator (ordered or disordered) in the ultra-quantum-limit and follows over a wide time range after the photoexcitation pulse. The difference between ordered and random insulating phases shows up at the longest time delays: The ordering of electrons into a Wigner lattice yields an asymptotical single lifetime decay tail, with a recombination rate whose temperature dependence is described by the characteristic Debye-Waller-type factor.