We present a theoretical study on various kinds of short-period (GaP${)}_{\mathit{m}}$/(AlP${)}_{\mathit{n}}$ superlattices based on first-principles calculations. Our calculations show that the direct band gap is realized for (GaP${)}_{\mathit{m}}$/(AlP${)}_{5\mathrm{-}\mathit{m}}$ systems grown on (001)GaP (m=3,4) substrates, whereas this system has an indirect gap in the case of m=n (≤5). An alternating change of the symmetry is observed between the lowest and second-lowest states in the conduction band as m increases from 1 to 4. We also examine some effects, e.g., strain (GaAs substrate) and modulated structure, to obtain luminescence efficiency.

A recent photoreflectance (PR) experiment on a GaAs/${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As modulation-doped quantum-dot array shows that at 77 K the quasi-two-dimensional ‘‘2C-2H’’ interband transition develops a series of evenly spaced oscillations. Such features are due to the quantization of the energy levels related to the in-plane paraboliclike potential for such reduced-dimensional sytems, e.g., evenly spaced conduction and valence subbands. However, for the other dominant feature, i.e., ‘‘1C-1H’’/‘‘1C-1L,’’ no fine structure is observed since the first electron subband is occupied. We present a self-consistent field theory and numerical calculations for the intersubband absorption coefficient of an array of modulation-doped GaAs/${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As quantum dots with lateral parabolic confining potentials for electrons and holes. Our numerical results for the derivative of the absorption coefficient have features which are quite similar to those observed in the PR experiment.

Confined and interface acoustic phonon modes in a cylindrical quantum wire embedded in another material are analytically investigated based on the elastic continuum model by means of the potential theory. Confined acoustic phonon modes are coupled modes of bulk-longitudinal and transverse acoustic waves, classified into torsional, dilatational, and flexural modes due to the rotational symmetry of the modes. Dispersions of the confined modes have subband structures with finite cutoff frequencies owing to quantization of wave vectors in the lateral direction. The density of confined phonon states becomes, as a result, a staircaselike structure. As for the interface modes, regions of material parameters for the possible existence of interface modes are investigated. We found that the existence of interface modes in a quantum wire–surrounding system becomes more sensitive to the combinations of materials than that for a plane interface.

The many-electron ground state is studied in a quantum wire with several populated spin and orbital subbands. The many-body effect is treated in the lowest order. The phase diagram of the subband population is obtained as a function of magnetic field and the strength of the confining potential. If the magnetic field is small, there is a phase boundary between spin-polarized states and partially spin-polarized states. Using these results, transport characteristics are calculated using Landauer’s formula and a dip is predicted in the conductance plateau of 2${\mathit{e}}^2$/h.

We examine the effects of atomic-scale imperfections on the transmission properties of a quantum dot resonant tunneling structure. For this purpose, we employ a three-dimensional model of quantum transport. We find that variation in the surface roughness of quantum dots leads to substantial fluctuations in the transmission properties. Impurities in a quantum dot are studied as a function of impurity strength and location, and it is found that an attractive impurity near the center of a dot can reduce fluctuations caused by surface roughness. Nevertheless, the presence of more than a single impurity can give rise to a complex resonance structure that varies substantially with impurity configuration.

We employ time-dependent photocurrent measurements to study the temperature dependence of the tunneling dynamics of electrons in GaAs-AlAs superlattices with similar coupling but different subband spacings. At low temperatures the transport characteristics are governed by resonant tunneling as well as longitudinal optical phonon-assisted tunneling out of the lowest subband. However, for a smaller subband spacing, resonances due to elastic tunneling out of the thermally populated second subband are observed at higher temperatures. At room temperature these resonances dominate in comparison with the inelastic transport channels out of the ground state. The thermal population of the second subband is directly observed by electroluminescence spectroscopy.

We investigate interparticle correlations for two layers of charge-carrier liquid trapped in adjacent wells of a gallium arsenide heterostructure. We determine in the absence of appreciable tunneling the static polarizability, the pair correlation functions, and the local-field corrections both within a layer and between the layers. The liquid ground state has points of instability to states that are inhomogeneous in the density. An important result is that, in contrast with electron-electron layers, for electron-hole layers, the interlayer correlations favor the formation of the inhomogeneous states. The pair correlation functions display unusual behavior near the points of instability.

In this paper we present and discuss our results for the conductance and conductance fluctuations of narrow quantum wires with two types of disorder: boundary roughness (hard-wall confining potential) and islands of strongly scattering impurities within the bulk of the wire. We use a tight-binding Hamiltonian to describe the quantum wire, infinite perfect leads, a two-terminal Landauer-type formula for the conductance, and the recursive single-particle Green’s-function technique. We find that conductance quantization is easily destroyed by strong scattering. We also find that Anderson localization imposes a serious restriction on the high carrier mobility predicted in quantum wires. Conductance fluctuations in narrow quantum wires are not, in general, universal (as in the metallic regime), but can be independent of the wire length over a short range of lengths.

We develop a Hartree-Fock approach to the charged spin-texture excitations (CSTE’s) of the ferromagnetic incompressible ground state, which occurs in the quantum Hall effect at Landau-level filling factor ν=1. The CSTE’s are the appropriate generalization of skyrmions to the situation when there is a nonzero Zeeman coupling. We find for Coulomb interactions that the charged spin-texture excitation energies are always smaller than the excitation energies of localized spin 1/2 quasiparticles and quasiholes. However, the amount by which the energy is lowered is quite small for typical experimental situations. The net spin of the CSTE’s is always much larger than 1/2, suggesting that adding or removing charge from a filled Landau level rapidly degrades its spin polarization.

Hot electron effects in multiple-layer system are treated a a function of a parallel electric field on the basis of a microscopic-balance-equation theory. Dynamically screened intrasubband scattering on ionized impurities, acoustical phonons due to deformation potential and piezoelectric interaction, as well as polar optical phonons are taken into account. The real-space transfer of electrons into the region between the layers is described by transfer scattering on optical and acoustical phonons. The obtained field-dependent drift velocities, electron temperatures, and densities of the two electronic subsystems are analyzed by considering the energy loss rates and frictional-force contributions of the included scattering events. Analytical expressions are derived for the transfer-scattering contributions to the energy and force balance and the phonon-assisted electron-density transfer. Specific results are obtained for a multiple-quantum-well system, which we compare with our former, more phenomenological, approach applied to analyze experimental results of multiple δ layers in GaAs.

Particle emisison from InP and GaAs surfaces induced by picosecond laser pulses of photon energies above the band-gap energy has been studied by means of crater-depth measurements. Below the melting threshold, the amount of atoms sublimated per laser pulse indicates that nonthermal processes are dominant. The crater profile shows a strong power dependence of the yield on the laser fluence. Both emission yield and crater profile can be explained by nonradiative energy transfer from recombining electron-hole pairs to atoms at surface sites.

We have studied the electroluminescence (EL) emission from GaAs-AlAs superlattices embedded in a ${\mathit{p}}^{+\mathrm{-}}$i-${\mathit{n}}^{+}$ diode structure under forward bias. The occupation of higher subbands by sequential resonant tunneling between adjacent wells is demonstrated via interband emission. We observed emission lines due to transitions involving the second and third conduction subband as well as the third heavy-hole subband. The EL lines originating from higher subbands exhibit nearly a constant Stark shift indicating inhomogeneous electric field and carrier density distributions.

We use the Chern-Simons gauge theory for the fractional quantum Hall effect (FQHE) to calculate the Josephson current for two weakly coupled two-dimensional electron gas (2DEG) subsystems separated by a thin tunnel barrier, for the primary fractional quantum Hall states v=1/(2k+1) (k=0,1,2,...), at zero temperature. We find that the Josephson current amplitude is mainly suppressed by the long-range quantum fluctuations of the Chern-Simons gauge field, by a factor ∝exp{[-(2k+1)/4π](L/${\mathit{l}}_0$)} where L is the system size, and ${\mathit{l}}_0$ is of the order of the magnetic length. Thus it is unlikely that the Josephson effect can be observed for weakly coupled 2DEG subsystems, despite the strong analogy between the FQHE in the Chern-Simons formulation and two-dimensional superconductivity.

Using x-ray-photoelectron spectroscopy and Auger-electron spectroscopy, we have resolved surface, bulk, and interface Ca and F core-level emission in thin films (3–8 triple layers) of ${\mathrm{CaF}}_2$ and ${\mathrm{SrF}}_2$ on Si(111). We confirmed these assignments using x-ray-photoelectron diffraction (XPD) and surface modification. XPD was also used to identify the growth modes of the films as being either laminar or layer plus islands; in the latter case we have resolved buried and uncovered interface F and Ca/Sr emission. We compare the observed energy differences between surface, bulk, and interface emission to theoretical estimates of the extra-atomic contributions to emission energies. We find excellent agreement considering only the Madelung (electrostatic) potentials for the initial-state contribution and polarization response for the final-state contribution, including the effect of tetragonal strain. Small discrepancies for emission from metal atoms bonded to the Si substrate are interpreted in terms of chemical shifts.

When ${\mathrm{C}}_{60}$ is in contact with Ag the metal donates electronic charge into the lowest unoccupied molecular orbital of the ${\mathrm{C}}_{60}$ molecules at the interface, resulting in a monomolecular metallic layer of ${\mathrm{C}}_{60}$. We find no evidence for the formation of a bulk ${\mathrm{Ag}}_{\mathit{x}}$${\mathrm{C}}_{60}$ compound.

The structure of the nitrogen-adsorbed NbC(111) surface has been investigated in ultrahigh vacuum by impact-collision ion-scattering spectroscopy (ICISS), reflection high-energy electron diffraction (RHEED), and by computer simulation. At room temperature, nitrogen is dissociatively adsorbed on the surface, which shows a diffuse RHEED pattern of (√3 × √3 )R30° structure at low coverage and (1×1) structure at saturation coverage. When the nitrogen-saturated surface is annealed at 950 °C for a few seconds, the RHEED pattern changes into a sharp (2×2) pattern. It has been found that, in both cases, a nitrogen atom goes into the threefold-hollow site on the second-layer carbon atom without changing the structure of the substrate surface. The height of a nitrogen atom from the first layer is 1.14 Å and the N-Nb distance is 2.15 Å, which is close to the sum of each of the ionic radii. The coverage of the (√3 × √3 )R30° and (2×2) structure are estimated at 0.33 and 0.5 ML, respectively. Judging from the coverage, the apparent (2×2) structure is considered to be the (2×1) structure with three equivalent domains.

The structureless self-consistent jellium model for simple metal clusters is corrected by reintroducing the ionic structure via second-order pseudopotential perturbation theory. It is found that this formulation is enough to bring this approximate treatment into a quantitative agreement with fully fledged ab initio Car-Parrinello structure optimization. Furthermore, the ionic perturbation leads to a substantial improvement in the description of the optical response.

We have measured adsorption isotherms for ${\mathrm{N}}_2$ films on BN for temperatures between 51 and 75 K and coverages in the first layer. Data measured below 65.8 K display two features: a substep and (at slightly higher coverages) a kink in the slope of the isotherm. Isotherms measured at and above 65.8 K have no features present in them. The isotherm and isothermal compressibility data for ${\mathrm{N}}_2$ on BN are compared with very similar corresponding data for Kr, CO, and ${\mathrm{N}}_2$ on graphite; this comparison is used to propose an interpretation for the observed features of ${\mathrm{N}}_2$ on BN. Our results for ${\mathrm{N}}_2$ are also contrasted with data for Kr on BN; the differences observed between these two systems are discussed.

We report the joint studies of experimental and theoretical surface band structures of Nb(001). Angle-resolved photoelectron spectroscopy was used to determine surface-state dispersions along three high-symmetry axes Γ¯M¯, Γ¯X¯, and M¯X¯ in the surface Brillouin zone. Ten surface bands have been identified. The experimental data are compared to self-consistent pseudopotential calculations for the 11-layer Nb(001) slabs that are either bulk terminated or fully relaxed (with a 12% contraction for the first interlayer spacing). The band calculations for a 12% surface-contracted slab are in better agreement with the experimental results than those for a bulk-terminated slab, except for a surface resonance near the Fermi level, which is related to the spin-orbit interaction. The charge profiles for all surface states or resonances have been calculated. Surface contraction effects on the charge-density distribution and the energy position of surface states and resonances will also be discussed.

The dispersions of the sp bands in Ag-Au(111) superlattices are calculated. The calculation employs the nearly free-electron approximation for the wave functions within the Ag and Au layers. A one-band model for the superlattice band structure is developed and applied to the present system. This results in a simple analytic formula for the band dispersions of the superlattice geometry. The calculated band dispersions are in good agreement with experimental results determined previously by photoemission spectroscopy.