The temperature dependences of conductivity and magnetoconductance (MC) have been measured, both parallel and perpendicular to the chain axis, in oriented polyacetylene films doped with iodine. The evolution of anisotropy in conductivity as a function of doping time has been studied. Hydrostatic pressure increases the conductivity for pressures up to 4 kbar; at higher pressure the conductivity gradually decreases. The reduced activation-energy function [W=-T[d(lnρ)/dT] is temperature independent in the intermediate temperature region (T≊180–60 K) suggesting that the material is just on the metallic side of the critical regime of the disorder-induced metal-insulator transition. However, at 8 kbar, W(T) exhibits a positive temperature coefficient from 180 to 1.2 K, implying that the system is more metallic due to enhanced interchain transport. The ${\mathit{T}}^{1/2}$ dependence of conductivity and the enhanced negative MC, at very low temperatures, indicates that the role of electron-electron interactions is significant. MC measurements have been carried out in all the five different combinations of current with respect to the chain axis and field directions. The sign of MC is positive (negative) when the field is perpendicular (parallel) to the chain axis. The anisotropy in MC is attributed to the anisotropic diffusion coefficient. This is shown by the enhanced positive contribution to the MC at 8 kbar, when the field (current) is parallel (parallel) to the chain axis.

At 1.2 K, when both the magnetic field and current are perpendicular to the chain axis, the field-induced crossover from positive to negative MC shows the subtle interplay of weak localization and electron-electron interaction contributions to MC. The inelastic scattering lengths, at 1.2 K, in parallel and perpendicular directions to the chain axis are found to be 1163 and 210 Å, respectively. The ${\mathit{T}}^{\mathrm{-}3/4}$ dependence of the inelastic scattering length at low temperatures is consistent with the ${\mathit{T}}^{3/4}$ dependence of conductivity. This indicates that the inelastic electron-electron scattering known to be of importance in disordered metals is the dominant scattering mechanism in iodine-doped oriented polyacetylene.

An experimental study of the local electronic structure of Ce-doped yttria (2% and 5% at.) was performed using photoelectron and x-ray-absorption spectroscopies (XPS and XAS). The local atomic arrangement in the solid solutions is well represented by an eightfold oxygen coordination around Ce atoms, with a Ce-O distance shorter than in ${\mathrm{CeO}}_2$ (2.28 Å as Y-O instead of 2.34 Å). The x-ray-absorption spectra measured at the Ce ${\mathit{L}}_3$ edge and the Ce 3d photoelectron lines are characteristic of the ${\mathrm{CeO}}_8$ environment, as in ${\mathrm{CeO}}_2$. However, the changes observed in the excitation spectra of the solid solutions are characteristic of an increase of the charge-transfer energy, due to the shortening of the Ce-O distance. The oxygen K-edge spectra evidence the existence of extended unoccupied f-character states, characteristic of tetravalent Ce.

A many-body theory is developed for a multiorbital tight-binding model including intra-atomic interactions. Thereby, local three-body correlations are taken into account. The theory is applied to the 3d states of ferromagnetic nickel. The multiplet structures of three-particle states are fully taken into account. Their inclusion leads to some states close to the bottom of the 3d bands, thereby strongly influencing the quasiparticle states. The self-energy correction is found to narrow the bandwidth and the exchange splitting as compared with their mean-field values, in good agreement with photoemission experiments. Satellite peaks are found in the spectral function around 6 eV below the Fermi level. Their shapes are quite different for the majority- and minority-spin parts, a feature which has been pointed out before, in particular in connection with finite cluster calculations.

The quasi-one-dimensional conductor ${\mathrm{BaVS}}_3$ exhibits successive phase transitions at ∼240 K (linear chains to zigzag chains), ∼70 K (metallic to semiconducting), and ∼35 K (paramagnetic to antiferromagnetic). We have made high-resolution ultraviolet-photoemission-spectroscopy (UPS), x-ray-photoemission-spectroscopy, electrical-resistivity, and magnetic-susceptibility measurements on ${\mathrm{BaVS}}_3$. UPS spectra near the Fermi level of the metallic phase exhibit a power-law dependence on the electron binding energy, indicating that conduction electrons in ${\mathrm{BaVS}}_3$ behave as a Luttinger liquid. The power-law exponent is large (<∼1), indicating that electron-electron interaction is long ranged and possibly that electron-phonon interaction is also important. The spectra exhibit gradual changes with temperature. In particular, a semiconducting gap starts to open well above the metal-to-semiconductor transition temperature and fully develops below it. We propose that the gradual orthorhombic distortion of the Jahn-Teller type below ∼240 K lowers one of the d levels, ${\mathit{d}}_{\mathit{x}\mathit{y}}$, and that below ∼70 K electrons are fully transferred to the ${\mathit{d}}_{\mathit{x}\mathit{y}}$ band. The ${\mathit{d}}_{\mathit{x}\mathit{y}}$ band then becomes half-filled, resulting in the opening of a Mott-Hubbard gap.

We have investigated the effects of ${\mathrm{C}}_{60}$ molecular orientation on the electronic structure of fcc ${\mathrm{C}}_{60}$ solids by performing a band-structure calculation with self-consistent mixed-basis all-electron and full-potential approach within the local-density approximation. The degeneracies at special k points, the dispersion, and the widths of bands and the location of the valence-band maximum and the conduction-band minimum have been found to be sensitive to the orientation of the fullerenes in fcc lattices.

The room-temperature reflectance of a well-characterized series of samples (x-ray, neutron activation and thermigravimetric analyses, resistivity, magnetization) in the ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{TiO}}_3$ system has been measured between 50 and 40 000 ${\mathrm{cm}}^{\mathrm{-}1}$ on samples ranging from the antiferromagnetic insulating (${\mathrm{LaTiO}}_3$) to the metallic (${\mathrm{La}}_{0.88}$${\mathrm{TiO}}_3$) part of the phase diagram. The electronic portion of the low-frequency optical conductivity increases with frequency at the lowest frequencies, similar to several barely metallic systems. This non-Drude behavior can be modeled as the sum of the two low-frequency oscillators, a Drude contribution that increases systematically with doping and a broad midinfrared continuum. The midinfrared band, which may be associated with transitions across the Hubbard gap, persists in highly doped samples in agreement with theoretical predictions. If one assumes a single low-frequency component with frequency-dependent scattering rate, one finds a negative mass enhancement below 150 ${\mathrm{cm}}^{\mathrm{-}1}$ in metallic samples close to the metal-insulator phase boundary.

We present a method for calculating dielectric matrices of periodic systems. Unlike the conventional method, which uses a plane-wave basis, the present method employs a product basis, which, in the linear-muffin-tin-orbital formalism, consists of products of orbitals. The method can be used for any system, including sp as well as narrow band systems. We demonstrate the applicability of our method by calculating the energy-loss spectra of Ni and Si, including local-field effects that require the full dielectric matrix. Good agreement with experiment is found. The small number of basis functions makes the method suitable for self-energy calculations within the GW approximation, without making the so-called plasmon-pole approximation for the dielectric matrix.

Several improvements of the tetrahedron method for Brillouin-zone integrations are presented. (1) A translational grid of k points and tetrahedra is suggested that renders the results for insulators identical to those obtained with special-point methods with the same number of k points. (2) A simple correction formula goes beyond the linear approximation of matrix elements within the tetrahedra and also improves the results for metals significantly. For a required accuracy this reduces the number of k points by orders of magnitude. (3) Irreducible k points and tetrahedra are selected by a fully automated procedure, requiring as input only the space-group operations. (4) The integration is formulated as a weighted sum over irreducible k points with integration weights calculated using the tetrahedron method once for a given band structure. This allows an efficient use of the tetrahedron method also in plane-wave-based electronic-structure methods.

The influence of the interaction between electrons on the Aharonov-Bohm effect is investigated in the framework of the Hubbard model. The repulsion between electrons associated with strong correlation is compared with the case of attraction such as U-center pairing. We apply the Bethe-ansatz method and exact numerical diagonalization to the Hubbard Hamiltonian. It is shown that the quasi-half-quantum-flux periodicity occurs for any nonzero values of U for two electrons. For a large number of sites, or strong U, the quasiperiodicity becomes an exact half-quantum flux periodicity. The character of the state created on the ring is different in both cases. In U-center pairing the electrons are bound in pairs located on the same site. For strong correlations (large positive U) the electrons tend to be as far from each other as possible. We show by numerical solution of the Bethe-ansatz equations that for three electrons the flux peiodicity of the ground-state energy is equal to 1/3. The one-third periodicity may occur even for small values of the ratio U/t, for the very dilute system. It is shown analytically using the Bethe-ansatz equations for N electrons that for dilute systems with arbitrary value of the Hubbard repulsion U, the fractional Aharonov-Bohm effect occurs with period ${\mathit{f}}_{\mathit{T}}$=1/N in units of the elementary flux quantum. Such a period occurs when the value Nt/LU is small, where L is the number of sites. Parity effects disappear in this fractional regime. We discuss possible experiments to detect fractional Aharonov-Bohm effects.

Electronic structures of R${\mathrm{Ni}}_2$ (R=Y, Ce, Pr, and Nd) are investigated systematically by using photoemission spectroscopy (PES) and compared with those of R${\mathrm{Co}}_2$. Double-peak structures are observed in the R 4f PES spectra, in which the spectra weight near ${\mathit{E}}_{\mathit{F}}$ decreases from Ce to Pr and Nd, suggesting a decrease in the R 4f–Ni 3d hybridization from Ce to Pr and Nd. For the same R, the spectral weight near ${\mathit{E}}_{\mathit{F}}$ in R${\mathrm{Ni}}_2$ is smaller than that in R${\mathrm{Co}}_2$, implying a smaller R 4f hybridization with Ni 3d electrons in R${\mathrm{Ni}}_2$ than with Co 3d electrons in R${\mathrm{Co}}_2$. These trends are consistent with those in the calculated hybridization matrix elements for ${\mathit{RM}}_2$ (M=Co and Ni). A pronounced Ni 3d satellite is observed, which arises from Ni 3d Coulomb correlation effects. The Ni 3d partial spectral weight distribution in R${\mathrm{Ni}}_2$ exhibits important discrepancies from the calculated Ni 3d angular momentum projected local density of states (PLDOS), such as peak positions and line shapes, indicating substantial Ni 3d correlation effects. These discrepancies are qualitatively similar to, and quantitatively larger than, those for R${\mathrm{Co}}_2$. Quasiparticle spectral densities are calculated by using the Hubbard Hamiltonian and including Co and Ni 3d electron correlation effects, which yield a better agreement with PES spectra than the local-density approximation PLDOS’s.

A calculational method based on Fourier expansion is applied to a study of the strong-field magneto-transport of a uniform free-electron metal, inside which is embedded a simple cubic arrary of identical spheres or cylinders, which have a different conductivity tensor. When the magnetic field is strong enough, the magnetoresistance exhibits very strong variations with the direction of the field. This can be understood in some of the cases by simple physical considerations about the distortions induced in the current flow by the inclusions. The strong dependence on the field direction is qualitatively, and sometimes even quantitatively, similar to what is observed in some metallic crystals which have a noncompact Fermi surface.

With the aid of functional-integral technique, we show that the strongly correlated electron system described by the Hubbard model at finite temperature is approximately equivalent to the dynamically disordered electron system. For larger Coulomb interaction U, we show that it has localized states in the middle region of the band. Thus, the system exhibits a continuous metal-insulator transition, in which the activation energy for conduction is continuous, caused purely by the electron-electron interaction near half filling.

We show that the dynamics of a dissipative quantum particle in a constant electric field can be solved exactly by applying the Feynman-Vernon influence-functional theory. We derive the drift velocity and its autocorrelation function as well as the fluctuation-dissipation theorem. For such a simple linear dissipative system, it is interesting to find that the drift velocity always depends on the field linearly, and the fluctuation is not influenced by the field. We also find, for the Ohmic dissipation, a ${\mathit{t}}^{\mathrm{-}2}$ power law long-time tail for the drift-velocity autocorrelation function at zero temperature.

Absolute measurements of the photoabsorption coefficient of Mg, Al, and Si from 25 eV up to the ${\mathit{L}}_3$ absorption edge are presented. Transmission measurements were performed on free-standing thin films using a laser-produced plasma source. The surface oxide is corrected for by taking the ratio of the absorption for different film thicknesses. The values so obtained are, in general, lower than have been reported in the literature. Structure below the edge is observed for Al and Si. Despite the fact that the absorption below the ${\mathit{L}}_3$ edge is due to the valence or conduction electrons, the magnitude of the absorption coefficient for the solid is much higher than is predicted by a simple Drude model and is close to that expected from calculations for the free atom.

From first-principles density-functional calculations of the charge distribution the crystal-field (CF) parameters for 4f states of Er and Dy impurities in Ag and Au have been evaluated. The calculations are based on an optimized linear combination of atomic orbitals scheme, where the local-density approximation (LDA) is used for the conduction-electron states, while the localized rare-earth 4f states are treated as ‘‘open core shell.’’ As the 4f localization cannot be properly described within LDA, a self-interaction correction for the 4f states is included. In this way, any artificial constraints on the 4f charge density employed in earlier first-principles CF calculations are avoided. The calculated CF parameters agree well with recent neutron scattering data.

Dispersed oxygen in Ge leads to the formation of ${\mathrm{Ge}}_2$O ‘‘quasimolecules’’ whose asymmetric stretching mode (${\nu }_3$), exhibits a remarkable, temperature-dependent fine structure when examined under the ultrahigh resolution of a Fourier-transform spectrometer. The large number of sharp lines observed arises from the coupling of ${\nu }_3$ with the low-energy symmetric bending ${\nu }_2$ mode; the thermal population of the ${\nu }_2$ levels, together with the shifts associated with the isotopes of Ge in their natural abundances, underlie its fine structure and temperature dependence. By incorporating oxygen in monoisotopic ${}_{}{}^{70}{}_{}{}^{}\mathrm{Ge}$, ${}_{}{}^{73}{}_{}{}^{}\mathrm{Ge}$, ${}_{}{}^{74}{}_{}{}^{}\mathrm{Ge}$, or ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}$, the excitation spectrum of the ${\nu }_2$+${\nu }_3$ coupling is selectively simplified. From the temperature dependence of the intensities of the lines in the fine structure, the energies of the ${\nu }_2$ levels are deduced. The ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ counterpart of the ${\nu }_2$+${\nu }_3$ coupling excitation has been observed and a consistent interpretation of the ${\nu }_2$ levels as well as the shifts of the ${\nu }_2$+${\nu }_3$ coupling excitations associated with any isotopic combination of oxygen and Ge is obtained by using an interaction mass m’=23.1 amu and a Ge-O-Ge bond angle 2α=111°.

We calculate the lattice properties and electronic structure of cobalt disilicide using an ab initio total-energy method with optimized pseudopotentials. Special attention is paid to the development of a soft robust pseudopotential for the 3d element Co. The calculated band structure of ${\mathrm{CoSi}}_2$ agrees well with the results of all-electron studies and the equilibrium lattice constant, bulk modulus, and zone-center phonon frequencies obtained are in good agreement with experimental data.

A simple model describing the electrical activation of moderate doses of silicon implanted into GaAs is presented. It involves only the defects ${\mathrm{Si}}_{\mathit{i}}^{+}$, ${\mathrm{Si}}_{\mathrm{Ga}}^{+}$, ${\mathrm{Si}}_{\mathrm{As}}^{\mathrm{-}}$, and EL2. Besides fitting available activation data (carrier concentration versus silicon concentration), it also fits existing data describing the variation of carrier concentration with substrate EL2 concentration.

The luminescence quenching of Er in crystalline Si at temperatures between 77 and 300 K is investigated. Samples were prepared by solid-phase epitaxy of Er-implanted amorphous Si layers with or without O codoping. After epitaxial regrowth at 620 °C, thermal annealing at 900 °C for 30 sec was performed in order to eliminate residual defects in the regrown layer and electrically and optically activate the Er ions. Measurements of photoluminescence intensity and time decay were performed as a function of temperature and pump power. By increasing the temperature from 77 K to room temperature the luminescence intensity decreases by ∼ three orders of magnitude in the Er-doped sample without O codoping, but only by a factor of 30 in the O-doped sample. In this sample room-temperature photo-luminescence and electroluminescence have been observed. Time-decay curves show a fast initial decay (∼100 μsec) followed by a slow decay (∼1 msec), with the relative intensity of these two components depending on temperature, pump power, and O codoping. The decay curves can be fitted by a sum of two exponential functions revealing the existence, in both samples, of two different classes of optically active Er sites. The concentration of excitable sites belonging to the slow-decaying class is similar for the samples with or without O codoping and rapidly decreases when temperature is increased. At temperatures above 150 K the Er luminescence is dominated by the fast-decaying centers the concentration of which is greatly increased by the presence of O. It is found that in the absence of oxygen room-temperature luminescence is hampered by the limited amount of excitable Er ions. In contrast, in O-doped samples the nonradiative decay of excited Er is the main quenching mechanism. The main factors determining the temperature quenching of Er luminescence and the crucial role of oxygen are discussed.

We report measurements of the temperature dependence of the ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ nuclear quadrupole resonance frequency ${\nu }_{\mathit{Q}}$, the Knight shift K, and the spin-lattice relaxation rate 1/${\mathit{T}}_1$ in the small-gap semiconductor ${\mathrm{Ce}}_3$${\mathrm{Bi}}_4$${\mathrm{Pt}}_3$ between 1.8 and 300 K. Corresponding measurements also are reported for the nonmagnetic metallic isomorph ${\mathrm{La}}_3$${\mathrm{Bi}}_4$${\mathrm{Pt}}_3$. The ${\nu }_{\mathit{Q}}$ data in the Ce compound show a characteristic departure from metallic-to-insulating behavior when the sample is cooled below ${\mathit{T}}_{\mathit{M}}$=80 K, the temperature of the susceptibility maximum, attributable to a loss of low-frequency vibrational modes in the insulating state. The Knight shift has both isotropic and axial components; this anisotropy originates from the presence of Ce via a transferred hyperfine coupling between Ce 4f and conduction electrons. An s-f exchange constant ≥0.4 eV is found, consistent with hybridization in other rare-earth intermetallic compounds. A change in the scaling between the susceptibility and both the isotropic and axial Knight shifts at temperature ${\mathit{T}}_{\mathit{M}}$ provides evidence that hybridization between the Ce 4f orbitals and the conduction electrons is responsible for the gap structure. The temperature dependence of the 1/${\mathit{T}}_1$ data is consistent with a model electronic density of states possessing a temperature-independent gap δ of 180 K and a bandwidth of the order of 1600 K. The temperature dependence of 1/${\mathit{T}}_1$ can also be fit well with a temperature-dependent gap with δ(0) also ≊180 K.