We discuss the results of 1/f noise measurements made on films of polycrystalline indium oxide and ZnO accumulation layers. In these systems, the amount of static disorder can be readily and reversibly changed by fine-tuning the stoichiometry, which gives one a convenient and unique method to study the sensitivity of the noise to changes in some transport parameters. We present detailed experimental evidence that rules out classical percolation phenomena as an explantation for the high noise level observed in these materials. We elaborate on a qualitative model given by Cohen et al. that ascribes the noise characteristics of these systems to an impending metal-insulator transition and dwell on some of its implications.

A photoemission feature is observed in the O-2s–O-2p band gap of both UHV-cleaved single-crystal ${\mathrm{V}}_2$${\mathrm{O}}_5$(001) and UHV-fractured single-crystal ${\mathrm{TiO}}_2$. There are three possible origins of this feature: (1) the ground states of ${\mathrm{V}}_2$${\mathrm{O}}_5$ and ${\mathrm{TiO}}_2$ may contain electron-energy levels with binding energies that correspond to the observed feature; (2) many-body effects may result in electrons that are photoexcited to energies other than ħ${\mathrm{\omega }}_0$ above their ground-state energies; or (3) the band-gap feature may result from the inelastic energy losses suffered by the electrons after photoexcitation. Band-structure and energy-level calculations performed by others eliminate possibility (1). We have performed photoemission, resonant photoemission, and electron-energy-loss measurements in order to discuss the remaining two possibilities. By removing the inelastic backgrounds from the photoemission spectra of ${\mathrm{V}}_2$${\mathrm{O}}_5$(001) and ${\mathrm{TiO}}_2$ using experimentally measured electron-energy-loss spectra, we have shown that the band-gap feature is at least partially due to the inelastic energy losses suffered by the photoexcited O 2p electrons during transport through the crystal.

The x-ray fluorescence and absorption of highly oriented pyrolytic graphite have been measured using monochromatic synchrotron radiation. The spectra can be separated into contributions from π- and σ-band components by measuring at different angles of incidence and at different emission angles. The shape of the x-ray fluorescence spectra varies dramatically with excitation energy near the C K edge. This dependence on excitation energy can be interpreted within a resonant-inelastic-scattering formalism. The results are compared with previously published band-structure calculations and photoemission results, and demonstrate the potential for using x-ray fluorescence to obtain symmetry-resolved band information.

The electron spectral function is calculated for a model including electron-phonon coupling to Einstein phonons. The spectrum is studied as a function of the electronic bandwidth and the energy ${\mathrm{\varepsilon }}_{\mathit{k}}$ of the level from which the electron is removed. A cumulant expansion is used for the time-dependent Green’s function, and the second- and fourth-order cumulants are studied. This approach is demonstrated to give accurate results for an exactly solvable two-level model with two electronic levels coupling to local phonons. For a one-band, infinite, three-dimensional model the cumulant expansion gives one satellite in the large-bandwidth limit. As the bandwidth is reduced, the spectrum calculated with the fourth-order cumulant develops multiple satellites, if ${\mathrm{\varepsilon }}_{\mathit{k}}$ is close to the Fermi energy ${\mathit{E}}_{\mathit{F}}$, and as the bandwidth becomes small, results similar to the two-level model are obtained. If ${\mathrm{\varepsilon }}_{\mathit{k}}$ is more than a phonon energy below ${\mathit{E}}_{\mathit{F}}$, the spectrum instead shows a very broad peak, due to the decay of the hole into a hole closer to ${\mathit{E}}_{\mathit{F}}$ and a phonon. If the spin degeneracy of the electrons is taken into account, the broadening due to the decay of a hole into a hole closer to ${\mathit{E}}_{\mathit{F}}$ and an electron-hole pair becomes important, even if ${\mathrm{\varepsilon }}_{\mathit{k}}$ is closer to ${\mathit{E}}_{\mathit{F}}$ than the phonon energy. The validity of Migdal’s theorem for ${\mathit{A}}_3$${\mathrm{C}}_{60}$ (A=K,Rb) is discussed. The intersubband electron-phonon coupling is appreciable for ${\mathit{A}}_3$${\mathrm{C}}_{60}$, and it may be argued that the effective bandwidth is large. It is shown that Migdal’s theorem is, nevertheless, not valid for ${\mathit{A}}_3$${\mathrm{C}}_{60}$.

We have studied the effects of random-hopping matrix elements and random potentials on the properties of the one-dimensional Hubbard model. Using a quantum Monte Carlo technique, disorder-averaged static spin- and charge-density susceptiblities have been evaluated for various strengths of the disorder. Results for the spin susceptibility at wave number q=2${\mathit{k}}_{\mathit{F}}$ indicate that this quantity, which is the fastest diverging susceptibility of the pure system, diverges as T→0 also when there is randomness in the hopping matrix elements, but not in the presence of random potentials. Both types of disorder cause a divergence of the uniform magnetic susceptibility. However, for random potentials a finite critical strength of the disorder appears to be required. At half-filling the transition from Mott (gapped) to Anderson (gapless) insulating behavior has been studied. A critical disorder strength is needed to destroy the gap, in agreement with Ma’s renormalization group calculations.

We calculate the direct and inverse photoemission spectra of 3d transition metals with fcc or bcc structure. The dynamics of the d electrons is described by an extended Hubbard model including five canonical d bands in the one-particle operator ${\mathit{H}}_0$ and all relevant on-site Coulomb and exchange matrix elements in the interaction Hamiltonian ${\mathit{H}}_1$. For the ground state a quantum-chemical ansatz is made taking local spin and density correlations into account. The retarded Green’s functions of the d electrons are evaluated by using the projection technique of Mori and Zwanzig. Thereby the dynamics of the additional particle is projected onto local spin and density excitations in analogy to the ground-state calculation and treated exactly within that restricted operator space. In the case of Ni the correct satellite position and a reasonable reduction factor of the bandwidth are obtained by using an experimentally determined parameter set. The numerical calculations for our model Hamiltonian also predict multiplet structures in the photoemission spectra of Co and Fe.

The pertinence of four-order gradient corrections to the exchange-only energy functional ${\mathit{E}}_{\mathit{x}}$[n] for producing accurate exchange potentials ${\mathit{v}}_{\mathit{x}}$(r) is examined utilizing the exact ${\mathit{v}}_{\mathit{x}}$(r)’s obtained from the optimized-potential model (OPM) for spherical atoms and jellium spheres (whose inner regions have slowly varying densities for which the gradient expansion should be valid). It is found that the fourth-order contributions containing ${\mathrm{\nabla }}^2$n represent important ingredients of ${\mathit{E}}_{\mathit{x}}$[n] which should be included in gradient-based nonlocal extensions of the local-density approximation. In accordance with this observation the ${\mathit{v}}_{\mathit{x}}$(r)’s resulting from the fourth-order gradient expansion are closer to the exact ${\mathit{v}}_{\mathit{x}}^{\mathrm{OPM}}$(r)’s than those from generalized gradient approximations which do not contain ${\mathrm{\nabla }}^2$n contributions.

The electronic structure and the optical properties of an excitonic n-string which is a bound state of n Frenkel excitons (n=2,3,. . .) are theoretically clarified. The binding mechanism may be attributed to the electrostatic dipolar interaction among the Frenkel excitons, which is in contrast to the formation mechanism of the otherwise analogous excitonic molecules in a system of Wannier excitons; it makes it possible to form bound states of more than two Frenkel excitons. The effects of the n-string states on the electronic structure and optical properties are studied. The two-photon absorption and the differential transmission spectra are shown to depend strongly on the direction of the induced static dipole moment, which determines whether the interaction between excitons is attractive or repulsive.

An approximate ab initio electronic dynamics technique has been developed to describe the weak interactions between molecules within hydrogen bonded systems. This technique is developed to concentrate on the intermolecular binding, since in thses sytems the intramolecular interactions are relatively unchanged. These weak intermolecular interactions are described using a linear combination of atomic orbitals formalism, and consideration of the expansion of the overlap S between molecules up to order ${\mathit{S}}^2$. The method is especially designed to perform large-scale molecular-dynamics simulations of interacting closed-shell molecules explicitly including the microscopic quantum effects. As an application of the theory, several molecular-dynamical quenched simulations of the ${\mathrm{H}}_2$O dimer and trimer systems have been performed. To demonstrate the method’s efficiency, a long 5-ps (25 000 steps) simulation of a ${\mathrm{H}}_2$O trimer system was carried out. Comparisons with other first-principles calculations and experimental data show that the theory presented here suitably describes the dimer and trimer systems. The method is transferable to other molecular systems, with its main advantage being its computational simplicity.

Starting from an exact recursive type solution for linearly coupled Schrödinger equations, we formulate analytic wave functions for the time-independent scattering problem and the time-dependent process. This enables us to handle much larger-scale scattering problems than before. It is especially important for solid-structure analysis by the diffraction method. Furthermore, the relationship between the recursive method and the Wentzel, Kramers, and Brillouin approximation is also clarified.

A Kohn-Sham approach is presented for analyzing the many-body properties of LCAO Hamiltonians. The total electronic energy of the system is shown to be a function of the different orbital occupancies. Then an exchange-correlation potential is introduced for each orbital, taking into account extra-atomic and intra-atomic many-body effects. Using this potential, the total energy can be obtained by calculating self-consistently the orbital occupancies, avoiding the use of a local representation as is done in the conventional LDA calculations. The method is applied to the calculation of the chemisorption energy and the charge transfer for the deposition of Na on Al(100), and the interaction of H with the GaAs(110) surface. Hydrogen is shown to passivate GaAs(110) surfaces for a monolayer deposition.

The formation energies of substitutional transition-metal alloys are examined by several means. First, two types of direct total-energy calculations are considered, namely, (i) the local-density approximation (LDA), and (ii) a tight-binding (TB) approximation thereof. Second, these directly calculated total energies are used to construct two Ising-like cluster expansions that, if sufficiently accurate, could be used to construct the full statistical mechanics of transition-metal alloys. These are (a) the Connolly-Williams (CW) method, and (b) direct configurational averaging (DCA). Finally, the ability of these two cluster expansions [(a) and (b)] to fit and predict a large number of the underlying directly calculated [(i) and (ii)] total energies is tested, by the average prediction error χ. These tests are performed for a large number of Pd-V alloys, and also, to a more limited extent, for the Pd-Rh, Pd-Ti, and Pt-V systems. We find for Pd-V that (i) direct TB calculations show significant overbinding (too-negative formation energies) relative to the LDA, with average error of χ=112 meV/atom (a typical formation energy of ${\mathrm{Pd}}_{0.50}$${\mathrm{V}}_{0.50}$ is ∼-250 meV/atom); (ii) the CW cluster expansion mimics quite well the results of the respective direct calculations, whether LDA (χ=19 meV/atom) or TB (χ=19 meV/atom); (iii) the DCA cluster expansions provides a less accurate depiction of the TB energies on which it is based (χ=65 meV/atom); (iv) the prediction errors for the equimolar random alloys are significantly larger using the DCA than using the CW method. In light of (i) above, it appears that the tight-binding model needs to be refined before it can be used systematically for (either DCA or CW) cluster expansions.

We study the convergence and the stability of fictitious dynamical methods for electrons. first, we show that a particular damped second-order dynamics has a much faster rate of convergence to the ground state than first-order steepest-descent algorithms while retaining their numerical cost per time step. Our damped dynamics has efficiency comparable to that of conjugate gradient methods in typical electronic minimization problems. Then, we analyze the factors that limit the size of the integration time step in approaches based on plane-wave expansions. The maximum allowed time step is dictated by the highest frequency components of the fictitious electronic dynamics. These can result either from the larger wave vector components of the kinetic energy or from the small wave vector components of the Coulomb potential giving rise to the so called charge sloshing problem. We show how to eliminate large wave vector instabilities by adopting a preconditioning scheme in the context of Car-Parrinello ab initio molecular-dynamics simulations of the ionic motion. We also show how to solve the charge sloshing problem when this is present. We substantiate our theoretical analysis with numerical tests on a number of different silicon and carbon systems having both insulating and metallic character.

We report Hall-effect measurements on single crystals of antiferromagnetic pyrite-type ${\mathrm{NiS}}_2$ (${\mathit{T}}_{\mathit{N}}$=40 K), in which the resistivity appears to show a metal-insulator transition at T≊100 K. Our transport measurements demonstrate that this transition is not intrinsic to the bulk: At low temperatures the transport is dominated by a metalliclike conduction at the surface, with carrier density ${\mathit{n}}_{\mathit{s}}$≊5×${10}^{14}$ ${\mathrm{cm}}^{\mathrm{-}2}$, and mobility ${\mathrm{\mu }}_{\mathit{s}}$≊1.3 ${\mathrm{cm}}^2$/Vs at T=0. For T>100 K the sample conductance is dominated by the bulk, which behaves like a doped semiconductor; the acceptorlike impurity states have binding energy ${\mathit{E}}_{\mathit{B}}$=80 meV. The valence-band mobility is μ≊0.002 ${\mathrm{cm}}^2$/Vs.

The orbital Kondo effect is treated in a model where, additional to the conduction band, there are localized orbitals close to the Fermi energy. If the hopping between the conduction band and the localized heavy orbitals depends on the occupation of the atomic orbitals in the conduction band, then orbital Kondo correlation occurs. The noncommutative nature of the coupling required for the Kondo effect is formally due to the form factors associated with the assisted hopping, which in the momentum representation depends on the momenta of the conduction electrons involved. The leading logarithmic vertex corrections are due to the local Coulomb interaction between the electrons on the heavy orbital and in the conduction band. The renormalized vertex functions are obtained as a solution of a closed set of differential equations and they show power behavior. The amplitude of large renormalization is determined by an infrared cutoff due to finite energy and dispersion of the heavy particles. The enhanced assisted hopping rate results in mass enhancement and attractive interaction in the conduction band. The superconductivity transition temperature calculated is largest for the intermediate mass enhancement, ${\mathit{m}}^{\mathrm{*}}$/m≊2–3. For larger mass enhancement the small one-particle weight (Z) in the Green’s function reduces the transition temperature, which may be characteristic for other models as well. The theory is developed for different one-dimensional and square-lattice models, but the applicability is not limited to them. In the one-dimensional case charge- and spin-density susceptibilities are also discussed. Good candidates for the heavy orbital are f bands in the heavy fermionic systems and nonbonding oxygen orbitals in high-temperature superconductors and different flatbands in the quasi-one-dimensional organic conductors.

The dispersions of the ${\mathrm{\Gamma }}_6^{\mathit{c}}$ conduction band and the ${\mathrm{\Gamma }}_8^{\mathit{v}}$ heavy-hole band in InP are measured with 1-meV accuracy over a range of wave vectors using directionally averaged hot-electron luminescence spectroscopy. The k⋅p method is used to relate wave vectors to the measured energies, and to investigate the sensitivity of these dispersions to the k⋅p parameters in general and to the Luttinger parameters ${\gamma }_1$, ${\gamma }_2$, and ${\gamma }_3$ in particular. It is found that not one but many different Luttinger parameter triplets are consistent with the hot-electron luminescence data, and several works from the literature are reviewed in this new context.

We present the results of a low-intensity femtosecond pump and probe study of hydrogenated amorphous silicon (a-Si:H) and its germanium alloys (a-Si:Ge:H) in the probe spectral regions of 0.52–0.81 and 1.19–1.48 eV. We find that the picosecond photoinduced change in the energy and time dependence of the complex dielectric constant is consistent with a model for the injected carrier relaxation which combines extended-to-tail and tail-to-tail thermal carrier transitions, and which uses an optical probe sensitivity to carriers in the localized tail states. The measurements on a-Si:H are in quantitative agreement with our model over the entire probe spectral range and from subpicosecond to nanosecond probe delay times; however, we find only qualitative agreement between the model and the pump-probe results of two series of a-Si:Ge:H alloys prepared under different conditions. Fits to the subpicosecond data of both the a-Si:H and a-Si:Ge:H samples indicate that the carriers are initially trapped into the tail states with a characteristic attempt frequency at least eight times higher than the rate at which they are subsequently trapped into deeper tail states.

Optical detection of electron-nuclear double resonance (ODENDOR) is used to investigate the 1.1-eV photoluminescence of a phosphorus-antisite-related defect in as-grown p-type GaP. We establish that the observed ODENDOR spectra arise from the ${\mathit{M}}_{\mathit{S}}$=0 state of a spin S=1 excited-state system in which there are no first-order magnetic hyperfine contributions. Nevertheless, including higher-order effects via matrix diagonalization, hyperfine interactions are extracted for the central P and several shells of both P and Ga neighbors, confirming that the defect has a ${\mathrm{P}}_{\mathrm{Ga}}$-${\mathit{Y}}_{\mathrm{P}}$ structure, where ${\mathit{Y}}_{\mathrm{P}}$ denotes an impurity or vacancy at a nearest-neighbor P site. In the case of the Ga neighbors, first-order electric quadrupole interactions are present in the ${\mathit{M}}_{\mathit{S}}$=0 manifold and serve to distinguish the three neighbor shells. The excited-state wave function of the defect is highly localized, essentially all of it accounted for within the first-nearest P shell. No ODENDOR signals are observed that can be attributed to ${\mathit{Y}}_{\mathrm{P}}$, and its identity remains undetermined.

The local structure of ${\mathrm{C}}_{\mathrm{As}}$ acceptors in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As has been investigated by studying the nondegenerate localized vibrational modes of H-${\mathrm{C}}_{\mathrm{As}}$ pairs with ${\mathit{A}}_1$ symmetry, rather than those of isolated ${\mathit{C}}_{\mathrm{As}}$ impurities. Infrared absorption and Raman scattering measurements have been made on ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As: ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ epilayers that (a) had been exposed to a radio-frequency hydrogen (deuterium) plasma or (b) contained hydrogen incorporated during growth. Arguments are advanced that indicate that the H(D) atom should occupy a bond-centered site between ${\mathrm{C}}_{\mathrm{As}}$ and Ga atoms rather than between ${\mathrm{C}}_{\mathrm{As}}$ and Al atoms at low temperatures. An ab initio local-density-functional calculation indicates that the energy is then lowered by 0.24 eV. This analysis has led to the assignment of five antisymmetric stretch modes and five symmetric (X) modes to H-${\mathrm{C}}_{\mathrm{As}}$ pairs at sites where the carbon atom that was originally unpaired had zero, one, two, three, or four Al nearest neighbors.

We present an ellipsometric study of ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$Se films with 0≤x<0.1 grown on GaAs substrates. We attribute the significant distortions of the interference patterns at energies between 1.6 and 1.8 eV to absorption due to localized d-state transitions between the crystal-field levels of ${\mathrm{Co}}^{2+}$ ions on Zn sites. There is also evidence of an absorption feature near 4.3 eV within the ${\mathit{E}}_0$+${\mathrm{\Delta }}_0$-${\mathit{E}}_1$ band-gap region. We propose that this structure has the characteristics of a charge-transfer-type p-to-d transition. A similar absorption band also seems to be present in ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$Se system.