The evolution of core-level photoemission spectra has been studied during the initial stages of Cs and In overlayer growth on thin ${\mathrm{SiO}}_2$ layers on Si(111) substrates to probe the influence of the metal-${\mathrm{SiO}}_2$ interaction on the charge state of the resulting structure. The Si 2p soft-x-ray photoelectron spectra on the ${\mathrm{SiO}}_2$ and the Si substrate, which have been recorded simultaneously, show substantially different shifts to higher binding energy for the two components of the structure for Cs adsorption on the ${\mathrm{SiO}}_2$ surface, but only small uniform shifts for In overlayers. The Si 2p core-level spectra have been analyzed by simulating the spectra with a model that separates the Cs-covered ${\mathrm{SiO}}_2$ structure into thin layers with a constant dipole field gradient applied across them. Accordingly, the metal-induced Si 2p core-level shifts of the Si substrate are interpreted in terms of Si band-bending changes as a result of charge transport from the metal layer to the ${\mathrm{SiO}}_2$/Si interface. The differential Si 2p shifts, which lead to a larger separation of substrate and oxide peaks on cesiated ${\mathrm{SiO}}_2$/Si than on the pristine ${\mathrm{SiO}}_2$/Si structure, are discussed in terms of a Cs-induced dipole field across the ${\mathrm{SiO}}_2$ layer. It is proposed that the different behavior of Cs and In overlayers may be related to the different electronegativity and, consequently, to the different adsorbate dipoles of the two metals.

We present a theory to describe conductance fluctuations in ballistic metallic point contacts. The theory is based on interference of electron waves backscattered to the constriction of the device. Since the number of scattering events in a backscattered trajectory is small, the concept of local interference can be applied to make a first-principles calculation of the interference effect in the framework of wave optics. In the theory three different limits can be distinguished, depending on whether there is interference of remote trajectories, of near trajectories, or of a remote and a near trajectory. Interference of near trajectories leads to the estimate of Holweg et al. based on the Landauer formula, which for the amplitude of the fluctuations δG agrees with the experiments of these authors, but which cannot properly account for the characteristic magnetic-field scale ${\mathit{B}}_{\mathit{c}}$. Interference of remote trajectories, on the other hand, reproduces the result of Maslov et al., also based on the Landauer formula, which can explain ${\mathit{B}}_{\mathit{c}}$, but which predicts a much too small δG. Finally, a combination of near and remote trajectories leads to the proper description of the experimental data. The remote trajectory, which spreads over a region of the size of the elastic mean free path, controls ${\mathit{B}}_{\mathit{c}}$ of the fluctuations, while the near trajectory causes a substantial enhancement of δG above the value predicted by Maslov et al. Very good agreement with the experiments is obtained by taking into account enhanced scattering close to the constriction (a likely effect for the type of contacts studied), which gives an additional enhancement of δG. In this way, our theory successfully predicts both the value of δG and ${\mathit{B}}_{\mathit{c}}$ in a self-consistent way, lifting previous contradictions.

A method is presented for the determination of the deformation of the unit cell of a crystal consisting of atoms bound by a three-body potential energy such as that in the Keating model. This method is based on the idea that the potential energy depends on the unit-cell shape only through the metric tensor of the primitive lattice vectors. The method is applied to obtain the stable structure of strained layer superlattices (GaP${)}_1$(InP${)}_1$(111) grown on GaAs(001) substrates. The c axis perpendicular to the substrate surface is predicted to be inclined 0.7°.

The behavior of a sharp resonance in a resonant tunneling structure as it approaches and passes through the emitter threshold due to an applied voltage is analyzed using a resonance formalism. The transmission coefficient T(E) near resonance energy is written as a threshold factor times a Breit-Wigner formula. A typical example is used to show that the threshold factor and the resonance decay width are essential to reproduce the negative differential resistance region obtained from exact numerical calculations. For the valley contribution the resonance energy is below threshold and the crucial contribution comes from the part of the decay width that still lies above it.

We present a self-consistent field theory and numerical calculations for a set of temperatures and electron densities for the interband absorption coefficient and the frequency-dependent refractive index for an array of isolated quantum wires. The peaks in the absorption coefficient correspond to interband transitions, resulting in the resonant absorption of light. The peaks in the refractive index are slightly redshifted from those in the absorption coefficient because the former involve both the real and imaginary parts of the response function, whereas the latter is directly proportional to the imaginary part of the response and inversely proportional to the refractive index. There is interference between the real and imaginary parts when the derivative of the absorption coefficient is taken with respect to frequency, which greatly modifies the line shape in the absorption spectrum. The oscillations in the derivative spectrum are due to the quantization of the energy levels related to the in-plane confining potential for such reduced dimensional systems. There are appreciable changes in the absorption spectrum when the electron density or temperature is increased. One interband transition peak is suppressed in the high electron density limit and the thermal depopulation effect on the electron subbands can be easily seen when the temperature is high. We also find that the exciton coupling weakens the shoulder features in the absorption spectrum. This paper is relevant to optical characterization techniques.

We present an ab initio molecular dynamics study of the structure and dynamics of a close-packed monolayer of Pb on the Ge(111) surface, with coverage FTHETA=4/3, at different temperatures. The room-temperature (√3 × √3 ) structure is characterized by large in-plane fluctuations of the Pb adatoms, and agrees well with recent x-ray standing wave data. At T∼800 K we observe a (1×1) disordered structure showing in-plane diffusion of the Pb atoms. Disordering is confined to the plane of the overlayer. A strong correlation with the solid Ge substrate is present, leading to preferential residence sites and diffusion paths of the Pb atoms on the surface. The calculated local density of states for the high-temperature phase is found to agree with recent scanning tunnel microscope observations, which show a simply Pb-terminated (1×1) surface with an apparent coverage FTHETA=1.

We present improved wave functions for the ground state, Laughlin quasihole, and quasiparticle excitations of the fractional quantum Hall effect. These depend explicitly on the effective strength of Coulomb interaction and reproduce Laughlin’s original result in the limit of no Coulomb interaction.

The distribution function of the local density of states is computed exactly for the Wigner-Dyson ensemble of random Hamiltonians. In the absence of time-reversal symmetry, precise agreement is obtained with the supersymmetry theory by Efetov and Prigodin of the NMR line shape in disordered metal particles. Upon breaking time-reversal symmetry, the variance of the Knight shift in the smallest particles is reduced by a universal factor of 2/3.

The elementary excitation spectra of quasi-one-dimensional quantum wires is calculated and compared with inelastic light scattering experiments corresponding to recently fabricated modulation-doped quantum-well wires. Self-consistent (Hartree) potentials, wave functions, and subband structure are used as the starting point for the calculation of the collective and single-particle excitations of the system, within the random-phase-approximation. Excellent quantitative agreement is found between theory and experiment, lending strong support to the existence of a distribution of negatively charged centers close to the heterojunction interface.

A theoretical investigation is presented of the properties of shallow donor impurities in a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As superlattice in the presence of a magnetic field directed along the growth axis. The energy levels of several of the lowest donor states have been obtained as functions of (1) the well width, (2) the donor position, and (3) the magnetic-field strength. The calculation is based on a variational approach in which we use a trial wave function with two variational parameters. This wave function shows exponential behavior in sufficiently small magnetic fields and Gaussian behavior in sufficiently large magnetic fields. The magnetopolaron effect on these donor energies is studied within second-order perturbation theory where a formal summation over all electron states is performed. The effect of band nonparabolicity is also included in order to correctly explain magneto-optical experimental results at high magnetic fields. Our results are in very good agreement with the available experimental data in the whole magnetic-field range and for weakly and strongly coupled superlattices.

We compare measured energy shifts of photoluminescence lines in Si/${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ quantum wells as a function of excitation power with theoretical calculations to conclude that the band alignment of the heterointerface is type II. Experimentally, we study molecular-beam epitaxy-grown fully strained Si/${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ single quantum wells with Ge fractions from 10% to 36%. These show significant blueshifts of the luminescence at increasing excitation density. Theoretically, we calculate self-consistently transition energy changes taking into account band bending due to the photoinduced charge carriers. Only for type-II band alignment are the experimental and theoretical results compatible.

We use self-consistent solutions to Schrödinger’s and Poisson’s equations to calculate several observable characteristics of wide parabolic quantum wells in a perpendicular magnetic field. We calculate the dc magnetoconductivity, optical absorption, and acceptor photoluminescence of electrons in these wells. Spin effects are included in both the bare interaction with the magnetic field and the many-body interactions through a local-spin-density exchange-correlation potential. We find several observable signatures of spin effects, enhanced by many-body interactions, in both the magnetotransport and the optical properties of these wells. These include disappearance of integer quantum Hall effect minima in the resistivity, spin polarization, magnetic depopulation of states, and nonlinear Landau fan diagrams. We relate details of these observable phenomena to their microscopic origins in these systems.

The dynamics of spin-boson systems at very low temperatures has been studied using a real-time path-integral simulation technique, which combines a stochastic Monte Carlo sampling over the quantum fluctuations with an exact treatment of the quasiclassical degrees of freedom. To a large degree, this special technique circumvents the dynamical sign problem and allows the dynamics to be studied directly up to long real times in a numerically exact manner. This method has been applied to two important problems: (1) crossover from nonadiabatic to adiabatic behavior in electron-transfer reactions, (2) the zero-temperature dynamics in the antiferromagnetic Kondo region 1/2<K<1, where K is Kondo’s parameter.

The Sham-Kohn theory of an inhomogeneous electron gas is applied to the calculation of the subband structure of p-type accumulation and inversion layers on the Si [110] surface. Exchange and correlation effects are shown to be as important as for the corresponding n-type systems. Subband differences are increased considerably. The dependence of the exchange-correlation potential on the valence band coupling has been investigated, thus being applicable to a large class of inhomogeneous semiconductor systems. Time dependent density-functional calculations of response functions have been performed. By the solution of the Bethe-Salpeter equation, depolarization and excitonic shifts of intersubband plasmons, as well as the properties of intrasubband plasmons, are calculated and compared with experiments.

p-type accumulation and inversion layers at a [110] Si surface in large external magnetic fields are investigated in the Sham-Kohn approximation. We consider the magnetic field dependence of the self-consistent potential for up to two occupied subbands. The transversal resistivity is calculated in SCBA and compared with the density of states in the lowest order cimulant approximation. Some features of the Shubnikov–de Haas effect, i.e., the beating pattern for the case of one occupied subband and the second oscillation, which appears when one further subband is populated, could be reproduced quantitatively. The dependence of the node in the beating pattern on an externally applied uniaxial strain is explained qualitatively.

The plane-wave pseudopotential (PWPP) method is used to perform density-functional-theory (DFT) calculations for two grain boundaries in aluminum studied previously using the embedded-atom method (EAM) and high-resolution transmission electron microscopy: (1) a Σ=11 (11¯3)129° symmetric tilt boundary and (2) a Σ=3 (112) twin boundary. Results are presented for the grain-boundary energies and the electron-density distributions in the boundary regions. The DFT results confirm that the EAM provides a good description of structural properties. However, the DFT grain-boundary energies are a factor of 2 larger than the EAM values, consistent with discrepancies found previously for (111) stacking faults in aluminum. In addition, two sources of instability are identified in the Teter-Payne-Allan preconditioned conjugate-gradients algorithm [Phys. Rev. B 40, 12 255 (1989)] used to perform the PWPP calculations. Simple techniques are proposed that eliminate these instabilities and their utility is demonstrated for these grain-boundary calculations.

By means of a Green-function, volume-integral-equation approach we study numerically the scattering of a surface plasmon polariton at a planar vacuum-metal interface by dielectric and metallic defects that are either embedded in the metal substrate or are situated in the vacuum region on the substrate. We calculate the transmission and reflection coefficients for the surface plasmon polariton, as well as the efficiency of its conversion into volume electromagnetic waves in the vacuum propagating away from the surface. We also compute the near field in the vicinity of the surface defect.

The degree of spatial localization of the Fe 3d levels in the quasi-two-dimensional correlated insulator iron phosphorus trisulphide (${\mathrm{FePS}}_3$) is probed using resonant and angle-resolved photoemission in the range hν=40∼65 eV. Our results indicate that the 3d levels in this compound are not as localized as assumed by an existing ionic model. Spectra can be adequately fitted using an atomic multiplet theory for the 3${\mathit{d}}^{\mathit{n}\mathrm{-}1}$ final state modified by the presence of an octahedral ligand field. However, a relatively low Racah exchange parameter B=0.075 eV is deduced and is interpreted in terms of greater hybridization with neighboring sulfur atoms than in iron oxides and halides. Resonant photoemission spectra collected near the iron M edge yielded excitation spectra composed of three prominent peaks approximately characterized by Fano line shapes. Systematic variation of the Fano asymmetry parameter q through the 3d multiplet is attributed to differential hybridization of various Fe 3d levels with sulfur 3d orbitals. Finally, we report angle-resolved photoemission results in the mirror symmetry planes, which indicate that some components of the 3d multiplet are substantially delocalized by band formation parallel to the layers. We summarize and discuss these results in terms of the covalency of this material.

The structure of deuterated n-butane [${\mathrm{CD}}_3$(${\mathrm{CD}}_2$${)}_2$${\mathrm{CD}}_3$] adsorbed on the (0001) surfaces of an exfoliated graphite substrate has been investigated by elastic neutron diffraction. The aim of this study was to elucidate the effect of steric properties, particularly the rodlike shape of the molecule, on both the monolayer and multilayer film structure. Our principal findings are as follows: (1) The solid monolayer has a rectangular unit cell commensurate with the graphite lattice in one direction (2 √3 ×∞). Profile analysis of the monolayer diffraction pattern shows an improved fit with two molecules in the unit cell arranged in a herringbone (HB) pattern rather than the rectangular-centered structure originally proposed. The HB structure also yields a lower potential energy calculated for a monolayer cluster. (2) The growth mode of the film is ‘‘quasiepitaxial’’ at low temperature consisting of the following steps: (a) adsorption of the crystalline monolayer, (b) adsorption of a disordered second layer and compression of the monolayer, (c) onset of preferentially oriented bulk growth with continued adsorption of disordered material, (d) bilayer crystallization, and (e) reentrant growth of preferentially oriented bulk particles and disordered material. (3) A Rietveld profile analysis of the diffraction pattern obtained at a coverage of six layers is consistent with the (100) plane of the bulk butane monoclinic phase parallel to the graphite surface. (4) For coverages of two to three layers, there is evidence of a wetting transition at ∼133 K, about 2 K below the bulk melting point.

Both the monolayer and bulk Bragg peaks disappear and are replaced by a diffraction pattern characteristic of a liquid film with a high degree of short-range order. It is suggested that the butane molecule’s steric properties are responsible for the incomplete wetting of the film as well as for the preferential orientation of the bulk phase. Our quasiepitaxial growth model for butane is similar to one proposed previously for nitrogen films on graphite. After comparing the structure and growth of the two films, we suggest that it may apply to a number of other physisorbed films composed of rod-shaped molecules having a larger aspect ratio than butane.

The behavior of quantum corrections to magnetoconductivity associated with the effect of weak electron localization is studied on thin Bi films (≃100 Å thick) in a temperature range of 0.3–10 K with the effect of electron overheating being realized. The temperature dependences of the time ${\mathrm{\tau }}_{\mathit{e}\mathrm{-}\mathrm{ph}}$ of electron-phonon energy relaxation are determined. The weakening of the electron-phonon interaction predicted for the case of the inequality ${\mathit{q}}_{\mathrm{ph}}$l<1 that yields a so-called ‘‘dirty limit’’ (${\mathit{q}}_{\mathrm{ph}}$ is the thermal phonon wave vector, l is the electron mean free path) is observed. Two ways are used to realize the dirty limit: an artificial decrease in the mean free path l and a lowering of the temperature below the characteristic one, ${\mathit{T}}_2$=ħs/kl (s is the phonon velocity).