The incommensurate 5×3 structure, which is formed when silicon is deposited on copper(001) via a saturation exposure to silane at 420 K, has been examined with helium-atom scattering and scanning tunneling microscopy. The surface was found to be a two-dimensional copper-silicon alloy having large domains of a near perfect hexagonal structure with ${\mathrm{Cu}}_2$Si stoichiometry. The surface layer displays a rotation of one surface-lattice vector with respect to the substrate of 4.4°, and a high-order commensuration along the other surface-lattice vector, without rotation. The overlayer exhibited unequal densities of the four possible domains and it appears that the long-range order is influenced by the presence of steps on the copper(001) surface. The overall atomic density in the surface layer is 27.6% higher than on the unreconstructed surface. The driving force for the formation of the sheared hexagonal structure is believed to be the need to increase surface coordination and density together with a strongly directional in-plane bonding between Cu and Si atoms.

We propose a model that describes the diffusion-controlled aggregation exhibited by particles as they are deposited on a surface. The model, which incorporates deposition, particle and cluster diffusion, and aggregation, is inspired by recent thin-film-deposition experiments. We find that as randomly deposited particles diffuse and aggregate they configure themselves into a wide variety of fractal structures characterized by a length scale ${\mathit{L}}_1$. We introduce an exponent γ that tunes the way the diffusion coefficient changes with cluster size: if the values of γ are very large, only single particles can move, if they are smaller, all clusters can move. The introduction of cluster diffusion dramatically affects the dynamics of film growth. We compare our results with those of several recent experiments on two-dimensional nanostructures formed by diffusion-controlled aggregation on surfaces, and we propose several experimental tests of the model. We also investigate the spanning properties of this model and find another characteristic length scale ${\mathit{L}}_2$ (${\mathit{L}}_2$≫${\mathit{L}}_1$) above which the system behaves as a bond percolation network of the fractal structures each of length scale ${\mathit{L}}_1$. Below ${\mathit{L}}_2$, the system shows similarities with diffusion-limited aggregation. We find that ${\mathit{L}}_1$ scales as the ratio of the diffusion constant over the particle flux to the power 1/4, whereas ${\mathit{L}}_2$ scales with another exponent close to 0.9.

Spin-resolved photoemission studies of the interface formed by depositing silver on an Fe(001) substrate show that the silver d bands show evidence of hybridization with the substrate d bands through the presence of a spin polarization for the initial monolayer of silver. Subsequent layers of silver show no evidence of spin polarization in the d bands. Studies of the dispersion of the interface or quantum-well states associated with these thin films reveal that their effective mass is enhanced due to hybridization with the substrate. This has implications for theoretical models of the exchange coupling in the associated magnetic multilayers, where the bulk band structure is often assumed for the noble-metal intervening layer.

Femtosecond electron thermalization in metals was investigated using transient thermomodulation transmissivity and reflectivity. Studies were performed using a tunable multiple-wavelength femtosecond pump-probe technique in optically thin gold films in the low perturbation limit. An IR pump beam is used to heat the electron distribution and changes in electron temperature are measured with a visible probe beam at the d band to Fermi-surface transition. We show that the subpicosecond optical response of gold is dominated by delayed thermalization of the electron gas. This effect is particularly important far off the spectral peak of the reflectivity or transmissivity changes, permitting a direct and sensitive access to the internal thermalization of the electron gas. Using a simple rate-equation model, line-shape analysis of the transient reflectivity and transmissivity indicates a thermalization time of the order of 500 fs. At energies close to the Fermi surface, longer thermalization times ∼1–2 ps are observed. These results are in agreement with a more sophisticated model based on calculations of the electron-thermalization dynamics by numerical solutions of the Boltzmann equation. This model quantitatively describes the measured transient optical response during the full thermalization time of electron gas, of the order of 1.5 ps, and gives new insight into electron thermalization in metals.

A band-theory method is presented for the calculation of image states on a metal surface. It is modified from the film linearized augmented-plane-wave method for surface-electronic-structure calculations. A nonlocal exchange-correlation potential is incorporated to correct the long-range potential beyond the image plane, and a multiple-energy-parameter scheme is used to generate the basis in vacuum. A calculation on Ag(100) yields accurate binding energies, effective masses, and wave functions for the two lowest image states, which agree closely with the available experimental data.

By small-amplitude perturbation theory and by a computer-simulation approach we have studied the incoherent scattering of electromagnetic waves from a randomly rough, dielectric film deposited on a planar, perfectly conducting surface. The thickness of the dielectric film is such that in the absence of the roughness the scattering system supports two guided waves. As a consequence, each multiply scattered wave is now degenerate. The coherent interference of each of these degenerate waves with the waves obtained from them by time reversal produces two satellite peaks in the angular dependence of the intensity of the incoherent component of the scattered field, in addition to the enhanced backscattering peak. These satellite peaks occur at scattering angles ${\mathrm{\theta }}_{\mathit{s}}$ that are related to the angle of incidence ${\mathrm{\theta }}_0$ by sin${\mathrm{\theta }}_{\mathit{s}}$=-sin${\mathrm{\theta }}_0$±(c/ω)[${\mathit{q}}_1$(ω)-${\mathit{q}}_2$(ω)], where ${\mathit{q}}_1$(ω) and ${\mathit{q}}_2$(ω) are the wave numbers of the two guided waves supported by the scattering system at the frequency ω of the incident field. These peaks are present in the results of both the perturbation and simulation calculations. They are shown to be multiple-scattering effects, and not a single-scattering phenomenon.

We present theoretical investigations of various hydrocarbon species adsorbed on hydrogenated flat (100), flat (111), and stepped (100) surfaces of diamond. We use ab initio density-functional molecular-dynamics simulations and a dynamical quenching minimization algorithm to calculate adsorption energies and minimum-energy configurations of different binding configurations. The onefold adsorption energies of the hydrocarbon fragments on all the surfaces were found to be in the order ${\mathit{E}}_{\mathrm{C}2\mathrm{}}$H>${\mathit{E}}_{\mathrm{CH}2\mathrm{}}$>${\mathit{E}}_{\mathrm{CH}3\mathrm{}}$>${\mathit{E}}_{\mathrm{C}2\mathrm{}}$${\mathrm{H}}_2$. C$ sub 2 roman H sub 2— is predicted to have stable twofold binding sites on both the terrace site and near step edges of the diamond (100) substrate. Adsorption on the flat (111) surface is found to be weaker compared to binding on the flat and stepped (100) substrates. We found several adsorption configurations where adsorption energies on the near step edges are different from those on the flat terrace. We studied local surface relaxations due to the adsorbed molecule. The binding of the hydrocarbon admolecule in the presence of an adsorbate is investigated. In general, we found weaker binding for molecules adsorbed on adjacent surface radical sites. Preliminary results on hydrocarbon adsorption at finite temperature are discussed.

Ground-state energies and on-site density-density correlations are calculated for the one-dimensional Hubbard model using a linear combination of the Hubbard projection operators. The mean-field coefficients in the resulting linearized equations of motion (EOM) depend on both one-particle static expectation values as well as static two-particle correlations. To test the model, the one-particle expectation values are determined self-consistently while using Lanczos-determined values for the two-particle correlation terms. Ground-state energies and on-site density-density correlations are then compared as a function of U to the corresponding Lanczos values on a 12-site Hubbard chain for 1/2 and 5/12 fillings. To further demonstrate the validity of the technique, the static correlation functions are also calculated using a similar EOM approach, which ignores the effective vertex corrections for this problem, and compares those results as well for a 1/2-filled chain. These results show marked improvement over standard mean-field techniques.

We present a one-dimensional model describing the interaction of spin-1/2 impurities with band electrons of a paramagnetic host metal. The model naturally describes the creation and annihilation of electron pairs localized at impurities, and the interaction between localized and band electrons. Using the Bethe-ansatz solution the thermodynamic properties of the system are calculated.

We present an investigation of the sinh-cosh (SC) interaction model with twisted boundary conditions. We argue that, when unlike particles repel, the SC model may be usefully viewed as a Heisenberg-Ising fluid with moving Heisenberg-Ising spins. We derive the Luttinger-liquid relation for the stiffness and the susceptibility, both from conformal arguments, and directly from the integral equations. Finally, we investigate the opening and closing of the ground-state gaps for both SC and Heisenberg-Ising models, as the interaction strength is varied.

According to the semiclassical interpretation of resonant-magnetotunneling-spectroscopy experiments, a plot of the resonant-tunneling-diode (current) peak bias versus transverse magnetic field should give (after appropriate scaling of the axes) the zero-field E(${\mathbf{k}}_{\mathrm{\parallel }}$) dispersion relations for the quantum well of the device. Here we derive a general criterion for the validity of this interpretation. We find that, in most devices, for carriers tunneling a distance ${\mathit{z}}_0$ into a quantum well of width w, so long as the ratio λ=w/${\mathit{z}}_0$ is sufficiently small the semiclassical interpretation is justified, the error being at worst of order λ, but usually only of order ${\lambda }^2$.

We investigate the electronic structure of three-dimensional quantum dots with two electrons in high magnetic fields and find that the combined effect of electron-electron interaction and hybrid-magnetoelectric quantization governs the whole energy spectrum. As a result, the spin transitions in the ground state of a quantum-dot disk can be quenched in the three-dimensional dot by relaxing the vertical confinement, or can be reentrant by tilting the magnetic field. We demonstrate that such behavior clearly appears in heat capacity and magnetization at sufficiently low temperatures.

Valence-band structures and hole effective masses for strained-layer quantum-well laser structures were calculated, using ${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As/InGaAsP material system as an example. A 4×4 strain Hamiltonian with heavy-hole and light-hole band mixing was used in the calculation. Systematic numerical results have been presented for a large range of material parameters and quantum-well widths.

The hydrodynamic description of two-dimensional plasma nonlocality in a magnetic field [as represented by the dynamic nonlocal longitudinal dielectric function of interacting two-dimensional (2D) electrons] is seen to be inappropriate for direct application in the determination of the magnetic-field dependence of shielding in the static limit. We find that this shortcoming is remedied by inversion of the full 2D hydrodynamic dielectric tensor in magnetic field and selection of its longitudinal element for static shielding.

The effects of interlayer Coulomb interactions on two component [${\mathit{m}}_{\mathrm{\uparrow }}$${\mathit{m}}_{\mathrm{\downarrow }}$0] fractional quantum Hall states are investigated. Modeling the density fluctuations as harmonic coupled magnetorotons, we compute the dispersion of the density waves in a coupled double-layer system with no electron tunneling and find that the magnetoroton gap in the [330], [550], and [350] states collapses at interlayer spacings of d/l≃0.76, 2.67, and 1.55, respectively, in the limit of very thin layers.

The eigenenergies and the dipole excitation spectrum of a single hole in a quantum dot is calculated as a function of a magnetic field. To describe a realistic dot based on an ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs heterostructure we use a triangular subband potential and a parabolic lateral confinement. The bulk valence band structure is considered by using the Luttinger Hamiltonian. In the absence of band mixing, the parabolic confinement would lead to only two dipole transitions, with the well known resonance frequencies previously established for dots containing electrons. The strong coupling of light- and heavy-hole states leads to a substantial shift of these resonance frequencies and introduces new features at transition energies comparable to the heavy-hole–light-hole subband separation.

We examine the influence of band mixing of heavy-hole and light-hole states on the heavy-hole low-field mobility along the semiconductor superlattice growth direction. We find that the first heavy-hole miniband width changes its sign with increasing wave-vector component in the layer plane. The effect results from the interference between heavy-hole and light-hole tunneling amplitudes. We demonstrate that the band mixing can drastically alter the temperature dependence of the hole mobility, as well as its dependence on barrier thickness and hole concentration.

We present a microscopic theory and experimental results concerning resistance resonance in two tunneling coupled quantum wells with different mobilities. The shape of the resonance appears to be sensitive to the small angle scattering rate on remote impurities and to the electron-electron scattering rate. This allows the extraction of scattering parameters directly from the transport measurements. The negative resonance in a Hall coefficient is predicted and observed.

Based on a tight-binding Friedel model for the density of d-electron states, we investigate the critical size for the metal-nonmetal transition in transition-metal clusters. An analytic equation for the critical size has been introduced for the cluster with close-packed structure. At the critical point (about 30–50 atoms), the density of states of the cluster near the Fermi level exceeds 1/${\mathit{k}}_{\mathit{B}}$T and the discrete energy levels form a quasicontinuous band. The classical conducting-sphere-droplet model becomes applicable for describing the size-dependent ionization potential of those clusters larger than the critical size. Our estimation of the critical size is supported by various experimental evidence such as the threshold ionization potential, the scanning tunneling microscopy, and the core-level energy shifts.

By using scanning tunneling microscopy (STM), a molecular structure was found to coexist in a (BEDT-TTF${)}_2$Cu(NCS${)}_2$ crystal grown at room temperature, in addition to the well-known κ phase with metal-superconductor phase transition at ∼10 K. The obtained two-dimensional structure of the phase was close to that of the α-(BEDT-TTF${)}_2$Cu(NCS${)}_2$ crystal previously reported as a charge-transfer salt with metal-insulator transition at ∼200 K. The temperature dependence of I-V (tunneling current–bias voltage) curves measured by STM showed both phase transition properties. A superstructure with a 2×2 unit cell formed by modulated charge density on the BEDT-TTF molecules was also observed for the phase, which was formed by uniting two BEDT-TTF molecules.