An experimental and theoretical study of the electronic structure of ${\mathrm{K}}_2$${\mathrm{NiF}}_4$ is presented. Core-level and valence-band x-ray angle-integrated photoemission experiments are reported and the data analyzed through a configuration-interaction impurity-cluster model. The charge transfer and the Mott-Hubbard charge fluctuations are studied and the energies involved estimated. The effect of these mechanisms on the spectral structures is identified. The comparison of local-density-functional band calculations with angle-resolved photoemission experiments shows significant discrepancies. This demonstrates that electron-correlation effects are indeed important and one-electron band-structure calculations are inadequate to describe this compound.

We report Ca 2p electron-energy-loss spectra (EELS) of Ca silicides (${\mathrm{Ca}}_2$Si, CaSi, ${\mathrm{CaSi}}_2$) for various primary electron-beam energies (${\mathit{E}}_{\mathit{p}}$=565, 800, 1046, and 1500 eV). For fixed primary energy, the Ca 2p profiles are found to be very similar regardless of the site-specific environment. The high-energy excited EELS spectra bear a strong resemblance to the Ca 2p x-ray-absorption spectra (XAS) of Ca metal and ${\mathrm{CaSi}}_2$, while the low-primary-energy spectra are characterized by a strong transfer of spectral weight toward the threshold and an inversion of the 2${\mathit{p}}_{1/2}$ vs 2${\mathit{p}}_{3/2}$ branching ratio. Whatever the primary energy is, the EELS spectra are found to strongly deviate from Ca 2p spectra calculated in a pure bandlike approach (ground-state calculation). An insight into the role of screening effects is achieved via the calculation of the absorption profile of Ca metal excited by the presence of the 2p core hole. While the ground-state calculation accounts for the nonscreened system, the excited one describes a fully screened atom. We show that both the XAS and the high-energy EELS spectra reflect a configuration of partial screening of the 2p core hole. The strong transfer of spectral weight toward the threshold found in the low-primary-energy EELS spectra are interpreted in terms of a more complete screening due to the participation of the slower injected electron. Therefore, the degree of screening can be driven by varying the final-state energy of the injected electron.

We investigate the normal-state properties of high-${\mathit{T}}_{\mathit{c}}$ cuprates in terms of the t-t’-J model using a spin-rotation-invariant slave-boson technique. The second-neighbor hopping t’<0 is included in order to reproduce the Fermi surfaces of LSCO (${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$) and YBCO (${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$) type. The magnetic phase diagram of the t-t’-J model is derived within the saddle-point approximation, by taking into account also incommensurate spiral order. Compared to the pure t-J model, Néel order is stabilized in the low-doping region. In particular, for the case t’>0, which corresponds to the electron-doped systems, e.g., ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$ (NCCO), the antiferromagnetic correlations are strongly enhanced near half-filling. We include fluctuation corrections and calculate the dynamic (paramagnetic) spin susceptibility χ(q,ω) going beyond the random-phase approximation. The instability line of the paraphase obtained from χ(q) is in agreement with the saddle-point phase diagram. The wave-vector dependence of χ(q) reveals the commensurate (incommensurate) nature of spin fluctuations in YBCO (LSCO). Finally, the doping dependence of the Hall resistivity ${\mathit{R}}_{\mathit{H}}$(δ) is calculated, where the results agree surprisingly well with experiments on LSCO. For YBCO and NCCO, the t’ term suffices to give the correct dependence for ${\mathit{R}}_{\mathit{H}}$(δ).

We report on the behavior of the low-temperature transport properties and thermal expansion of the intermetallic compounds MNiSn (M=Ti,Zr,Hf) through the metal-insulator transition. In the low-conductivity samples [σ∼${10}^2$ (Ω cm${)}^{\mathrm{-}1}$] situated on the metallic side, the conductivity σ varies at T<10 K as σ≊${\mathrm{\sigma }}_0$+${\mathit{A}}_0$ √T , which may be ascribed to a Coulomb-interaction correction. For the same temperature interval an anomalous negative thermal expansion with negative values approximately proportional to the degree of interaction, evaluated from the conductivity, was observed. A discussion of the possible effect of electron-electron interaction on the electron thermal expansion based on the results of Altshuler and Aronov is also presented.

A systematic ab initio electronic-structure study of the late transition-metal–cerium compounds forming in the cubic Laves-phase structure, and of those cerium compounds (${\mathrm{CeRh}}_3$, ${\mathrm{CePd}}_3$, and ${\mathrm{CePt}}_3$) forming in the ${\mathrm{AuCu}}_3$ structure, is performed using the local-density approximation. Magnetic and cohesive properties are shown to be crucially dependent on the treatment of the cerium 4f electron. In all cases investigated (except for the palladium and platinum compounds) our calculations favor a picture with delocalized 4f electrons. Photoemission data for the strongly 4f-4d hybridized ${\mathrm{CeRh}}_3$ system is discussed and shown to have a substantial itinerant 4f character. Calculations for some fictitious ${\mathrm{AuCu}}_3$ compounds Ce${\mathit{A}}_3$, where A is a 4d element preceding Rh or a 5d element preceding Pt, are also presented. This is done in order to elucidate how the position of the cerium 4f band, relative to the Fermi level, changes when such a series of compounds is traversed. The hybridization between the cerium 4f states and the transition-metal d states is shown to vary in a systematic way, which, together with the filling of the transition-metal d band, can explain the position of the 4f band. The calculated 4f position is compared with data from inverse photoemission spectroscopy. Especially the anomalous position of the 4${\mathit{f}}^1$ peak, previously found in bremsstrahlung isochromat spectroscopy experiments for ${\mathrm{CeRh}}_3$, is found to fit into the calculated trend in a consistent manner.

We study, within the Hartree-Fock approximation, the relative stability of different classes of states for the single-band Hubbard model on a square lattice in the presence of a nearest-neighbor Coulomb repulsion V and away from half-filling. We consider (i) homogeneous metallic states (including spiral phases), (ii) states that exhibit phase separation into hole-rich and antiferromagnetic regions, and (iii) insulating (longitudinally polarized) spin-density-wave states with domain walls that trap holes. For V=0, the spiral states have the lowest energy for hole concentration δ>${\mathrm{\delta }}_{\mathit{s}}$. At ${\mathrm{\delta }}_{\mathit{s}}$∼0.25–0.3, there is a transition to domain walls for small to intermediate U/t. At larger U/t the transition is to a phase-separated state, with ${\mathrm{\delta }}_{\mathit{s}}$ decreasing slowly with increasing U/t. We find that even a small V destabilizes the domain walls relative to the spiral states; e.g., for V/t∼0.2, the domain-wall states are unstable for all U/t and all δ≥0.05. The effect of V on the phase-separated state is much less drastic; its region of stability relative to the spiral states also shrinks to smaller δ, but rather slowly. The last results are in agreement with the findings from a Schwinger-boson treatment of the t-J model.

Measurements of electrical resistivity, magnetic susceptibility, heat capacity, and powder neutron diffraction are presented on ${\mathrm{Sr}}_2$${\mathrm{RuO}}_{4+\mathrm{\delta }}$. Specimens with low-impurity levels allow magnetic susceptibility in the range 5<T<350 K to be measured; these measurements indicate a small effective magnetic moment of ${\mathrm{\mu }}_{\mathrm{eff}}$≊1${\mathrm{\mu }}_{\mathit{B}}$ per Ru ion. Electrical resistivity displays nonmetallic behavior in the range 1.2<T<300 K with ρ(295 K)=51 mΩ cm. A rather large linear heat-capacity coefficient of γ=45.6 mJ/mol ${\mathrm{K}}^2$ is reported; speculation is made regarding its origin. Rietveld refinement of powder-neutron-diffraction data provide insight into the physical properties and reveal interesting differences to isostructural ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. Comparisons are drawn to other transition-metal oxides.

We study a model of fermions interacting with a gauge field and calculate gauge-invariant two-particle Green’s functions or response functions. The leading singular contributions from the self-energy correction are found to be canceled by those from the vertex correction for small q and Ω. As a result, the remaining contributions are not singular enough to change the leading-order results of the random-phase approximation. It is also shown that the gauge-field propagator is not renormalized up to two-loop order. We examine the resulting gauge-invariant two-particle Green’s functions for small q and Ω, but for all ratios of Ω/${\mathit{v}}_{\mathit{F}}$q, and we conclude that they can be described by Fermi-liquid forms without a diverging effective mass.

We extend to the periodic Anderson model (PAM) the diagrammatic expansion in cumulants that was employed by Hubbard to study his model of a narrow band of strongly correlated electrons. The PAM is a lattice of localized and strongly correlated electrons with spin one-half and without orbital degeneracy, hybridized with a wide band of uncorrelated conduction electrons. We have extended the model by considering localized electronic states with an arbitrary scheme of energy levels: this extension would be useful to study intermediate valence compounds of Eu or Tm with the present formalism. We give the rules for the diagrammatic calculation of the grand canonical potential and of the Green’s functions for the general model: only connected diagrams appear in those calculations and the lattice sums are unrestricted. To generate the cumulant averages it was necessary to employ external fields ξ that are Grassman variables. We have found a simple way to extend the diagrammatic rules to the ξ≠0 case. The absence of excluded site restrictions, that leads to complicated excluded volume problems in other treatments, and the existence of linked cluster expansions, are features of the cumulant expansion. As an application of the present method, we have calculated the occupation numbers of localized and conduction electrons for the PAM in the limit of infinite Coulomb repulsion (U→∞).

An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.

Using strong-coupling perturbation theory for the Hubbard model, explicit expressions are obtained for the integrated weights, energy positions, and widths of the upper and lower Hubbard bands separately, both for the optical and photoemission spectrum. In one dimension all expressions can be explicitly evaluated using the large-U Bethe-ansatz wave function. The k→-dependent moments for the one-particle spectrum are compared with the closely related two-pole ansatz. The strong momentum dependence of the spectra demonstrates the importance of the k→-dependent correlation functions in the spectral moments, which are often neglected. In order to estimate corrections due to the neglect of higher-order terms (in t/U), all results are compared with numerical-diagonalization data for intermediate-U values. The sum-rule results show a rapid spectral-weight transfer from the high- to the low-energy regime upon doping, similar to what is observed experimentally in one-particle and optical spectra of Cu (high-${\mathit{T}}_{\mathit{c}}$) and Ni compounds. Such fast weight redistributions as a function of the carrier density are a natural consequence of the strong correlations in these materials. The redistribution of intensity in the optical conductivity is connected with the dilution of the spin system by the added holes. For the one-particle spectrum the decrease of weight in the upper Hubbard band away from half filling can be understood by state counting at U=∞, and the effect is strongly enhanced for small doping by both the first- and second-order terms in t/U. Altogether, the one-particle and optical spectra show the importance of the three-site hopping term. Therefore the t-J model does not represent well the spectral properties of the Hubbard model at large U. The local spin order is essential and determines both the k→ dependence of the one-particle spectrum and the intensities in the optical spectrum.

The Fermi surface (FS) of the alkali metals (Li, Na, K, Rb, and Cs) has been studied using a full-potential linear muffin-tin orbital (FPLMTO) method together with both the local-density approximation (LDA) and the generalized gradient approximation of Perdew and Wang (PW91). We demonstrate that FPLMTO calculations based on the LDA improve the FS of K, Rb, and Cs compared to LMTO calculations within the atomic-sphere approximation (ASA). Most importantly, our FPLMTO-LDA calculations reproduce all essential features of the FS of Cs, whereas the previous LMTO-ASA LDA calculations erroneously resulted in a FS that is open at the N point. The gradient-corrected FPLMTO calculations further improve the FS for K, Rb, and Cs and give results in good agreement with experiment. For Li and Na the FPLMTO calculations slightly worsen the FS, both when using the LDA and PW91.

The positions of empty lattices (EL’s) are defined as the positions obtained by shifting the real lattices by ${\mathit{a}}_0$/2 along the [100] direction and, in order to study how the crystal potentials of semiconductors with small packing fraction should be represented within the framework of a muffin-tin (MT) approximation, the MT potentials are constructed on the EL sites. Furthermore, in order to take into account the effect of a nonspherical potential due to the site symmetry of ${\mathit{T}}_{\mathit{d}}$, the nonspherical potential whose angular momentum l=3 is built up. The electronic structures of the common semiconductors Si, Ge, GaAs, ZnSe, InSb, and CdTe are calculated by a method in which the nonspherical potential and the MT potential on the EL site are introduced into the orthogonalized-plane-wave method within the framework of the MT approximation based on the self-consistent-field atomic-structure calculations. It is shown that the symmetry of the potential consisting of MT potentials on real atoms and EL sites coincides with that of the nonspherical potential, which is closely related to ${\mathit{sp}}^3$ hybrid orbitals, and that the calculations including both the nonspherical potentials and the MT potentials on the EL sites predict fairly well the experimental facts rather than the calculations using only the MT potentials on the real atoms.

The optical properties of cubic and hexagonal GaN thin films, grown by electron-cyclotron resonance microwave plasma-assisted molecular-beam epitaxy on silicon and sapphire substrates, respectively, have been studied at photon energies up to 25 eV with conventional and synchrotron-radiation spectroscopic ellipsometry. The fundamental gaps of the two polytypes are located at different energies, namely at 3.25 and 3.43 eV for cubic and hexagonal GaN. Analysis of the dielectric function of the two phases in the region 4.5–9.5 eV with appropriate models yields the energy location and broadening of the observed critical points. These critical points are assigned to specific points in the zinc-blende and wurtzite Brillouin zones, respectively, making use of the latest published band-structure studies and a comparison is made between the corresponding results for GaN, GaAs, and GaP. Measurements in the temperature range from 80 to 650 K provide the temperature dependence of these parameters. The features observed in the reflectivity spectra of hexagonal GaN are discussed in relation to other works. Kramers-Kronig analysis of the reflectivity between 0 and 33 eV of the hexagonal polytype verifies the existence of a broad feature centered at 14 eV. Finally, average properties, such as the effective ir dielectric constant and the effective number of valence electrons per atom are calculated for the two polytypes and compared to GaAs and GaP.

We report fine structure in the luminescence of a bound multiexciton complex in CdTe. Data obtained from temperature-dependent, power-dependent, and time-resolved photoluminescence spectroscopy are all consistent with the interpretation that the complex is comprised of a biexciton bound to a neutral acceptor. The fine structure of the biexciton luminescence transition consists of a doublet that is due to a splitting in the bound-exciton final state. These data conclusively demonstrate the existence of a bound multiexciton complex in a direct-gap semiconductor.

Nominally undoped zinc-blende polytype 3C-SiC single crystals have been studied by conventional electron-spin resonance (ESR), by the magnetic circular dichroism (MCD) absorption technique, and by optically detected ESR via the MCD (MCD-ESR). Apart from the dominant ESR signal ascribed to residual boron acceptors, an anisotropic spectrum with many lines is observed near liquid-He temperatures. It collapses into a hyperfine octet for H∥[111]. This spectrum is identified as the anisotropic ${}_{}{}^2{}_{}{}^{}$E ground-state resonance of ${\mathrm{V}}^{4+}$(3${\mathit{d}}^1$) on the cubic Si site in 3C-SiC. Above 15 K this spectrum disappears and an isotropic hyperfine octet appears. It is interpreted as the motionally averaged spectrum of the anisotropic ${\mathrm{V}}^{4+}$ spectrum. The data are analyzed in terms of Ham’s theory of the dynamic Jahn-Teller effect for a ${}_{}{}^2{}_{}{}^{}$E state. A sharp optical line at 6681 ${\mathrm{cm}}^{\mathrm{-}1}$ has been found in MCD absorption. On this line the MCD-ESR of ${\mathrm{V}}^{4+}$ is observed, which identifies the line as being related to ${\mathrm{V}}^{4+}$. It is interpreted as the zero-phonon line of the ${\mathrm{V}}^{4+}$ ${}_{}{}^2{}_{}{}^{}$E${\to }^2$${\mathit{T}}_2$ crystal-field transition.

Study of the temperature and magnetic-field dependence of the negative magnetoresistance in the Mott variable-range-hopping regime is made on n-type ${\mathrm{CuInSe}}_2$. The relative magnetoresistance Δρ/ρ(0) is found to be proportional to ${\mathit{T}}^{\mathrm{-}\mathrm{\alpha }}$${\mathit{B}}^2$ in the low-field region below about 0.35 T. Below 4 K, where ${\mathit{R}}_{\mathrm{hop}}$/ξ≥1.5, it is observed that α=3/4. This confirms, to our knowledge for the first time, the theories based on quantum interferences for the negative magnetoresistance in the variable-range-hopping regime. However, α=1.22 above 4 K where ${\mathit{R}}_{\mathrm{hop}}$/ξ<1.5. This is very similar to the values reported for other semiconductors. This leads us to suggest that for the correct application of the theories it is necessary that phase coherence should be maintained over a time scale larger than the hopping time and that the hopping length should be greater than the localization length.

We analyzed the physical properties of hydrogenated amorphous silicon (a-Si:H) samples grown by plasma-enhanced chemical vapor deposition, by means of infrared spectroscopy, mass-density, and optical measurements. We applied the usual infrared spectroscopic techniques to evaluate the amount of incorporated hydrogen and to investigate the influence of the Si-H bonding configurations on the optical and electronic properties of a-Si:H obtained by this widely used deposition process, and with parameters so chosen as to obtain films having state of the art characteristics. We carried out an exhaustive study of the different infrared absorption peaks, generally ascribed to different bonding configurations of H in Si, with a view to distinguishing the different contributions of SiH, ${\mathrm{SiH}}_2$ (${\mathrm{SiH}}_2$${)}_{\mathit{n}}$ to the spectral response of the material and to relating them to deposition temperature. The correlation of this infrared spectral analysis with mass-density measurements has evidenced that, for hydrogen content below 10 at. %, isolated SiH and ${\mathrm{SiH}}_2$ bonds occur, whereas, with higher hydrogen concentrations, the formation of (${\mathrm{SiH}}_2$${)}_{\mathit{n}}$ chains gives rise to empty cavities where hydrogen is preferentially incorporated. In addition, optical measurements show that samples deposited at temperatures below 170 °C exhibit energy gaps and defect densities, which are quite different to those expected if only the effects of SiH groups are considered.

The electronic structure of semiconducting diamond is calculated by the scalar-relativistic linear muffin-tin-orbital method within the local-density approximation. Information about matrix elements, effective masses, and Luttinger parameters is extracted by comparison with k⋅p calculations. An extended 16×16 k⋅p calculation is performed using the parameters above as input so as to obtain the detailed band structure of the higher valence and lower conduction band states around the Γ point in the (110) direction.

The truncation of the infinite hierarchy of equations of motion is discussed for optically excited semiconductors. We derive a complete set of equations of motion, which are valid up to third order in the excitation field amplitude. To illustrate the results the induced four-wave-mixing signals for linearly polarized laser fields are computed by numerically integrating the ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ semiconductor Bloch equations. As the dominating nonlinearity, a strong excitation-induced dephasing effect is obtained in the ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ regime.