Showing papers in "Physical Review B in 1982"

Transition from metallic to tunneling regimes in superconducting microconstrictions: Excess current, charge imbalance, and supercurrent conversion

Abstract: We propose a simple theory for the $I\ensuremath{-}V$ curves of normal-superconducting microconstriction contacts which describes the crossover from metallic to tunnel junction behavior. The detailed calculations are performed within a generalized semiconductor model, with the use of the Bogoliubov equations to treat the transmission and reflection of particles at the $N\ensuremath{-}S$ interface. By including a barrier of arbitrary strength at the interface, we have computed a family of $I\ensuremath{-}V$ curves ranging from the tunnel junction to the metallic limit. Excess current, generated by Andreev reflection, is found to vary smoothly from $\frac{4\ensuremath{\Delta}}{3e{R}_{N}}$ in the metallic case to zero for the tunnel junction. Charge-imbalance generation, previously calculated only for tunnel barriers, has been recalculated for an arbitrary barrier strength, and detailed insight into the conversion of normal current to supercurrent at the interface is obtained. We emphasize that the calculated differential conductance offers a particularly direct experimental test of the predictions of the model.

Pseudopotentials that work: From H to Pu

Abstract: Recent developments have enabled pseudopotential methods to reproduce accurately the results of all-electron calculations for the self-consistent electronic structure of atoms, molecules, and solids. The properties of these potentials are discussed in the context of earlier approaches, and their numerous recent successful applications are summarized. While the generation of these pseudopotentials from all-electron atom calculations is straightforward in principle, detailed consideration of the differences in physics of various groups of atoms is necessary to achieve pseudopotentials with the most desirable attributes. One important attribute developed here is optimum transferability to various systems. Another is the ability to be fitted with a small set of analytic functions useful with a variety of wave-function representations. On the basis of these considerations, a consistent set of pseudopotentials has been developed for the entire Periodic Table. Relativistic effects are included in a way that enables the potentials to be used in nonrelativistic formulations. The scheme used to generate the numerical potentials, the fitting procedure, and the testing of the fit are discussed. Representative examples of potentials are shown that display attributes spanning the set. A complete tabulation of the fitted potentials is given along with a guide to its use.

Quantized Hall conductance, current carrying edge states, and the existence of extended states in a two-dimensional disordered potential

Abstract: When a conducting layer is placed in a strong perpendicular magnetic field, there exist current-carrying electron states which are localized within approximately a cyclotron radius of the sample boundary but are extended around the perimeter of the sample. It is shown that these quasi-one-dimensional states remain extended and carry a current even in the presence of a moderate amount of disorder. The role of the edge states in the quantized Hall conductance is discussed in the context of the general explanation of Laughlin. An extension of Laughlin's analysis is also used to investigate the existence of extended states in a weakly disordered two-dimensional system, when a strong magnetic field is present.

Nonlinear ionic pseudopotentials in spin-density-functional calculations

Abstract: A new method for generating and using first-principles pseudopotentials is developed to treat explicitly the nonlinear exchange and correlation interaction between the core and the valence charge densities. Compared to existing potentials, the new scheme leads to significant improvement in the transferability of the potential. In particular, the spin-polarized configurations are well described with a single potential. The need for separate spin-up and spin-down ionic pesudopotentials is, thus, eliminated. The method can easily be implemented with minimal increase in computational effort. Results for both atoms and solids are demonstrated.

Study of the L 23 edges in the 3 d transition metals and their oxides by electron-energy-loss spectroscopy with comparisons to theory

Abstract: Excitations of the $2p$ subshell in the $3d$ transition metals and their oxides have been studied by inelastic scattering of 75-keV electrons. The ${L}_{23}$ "white lines" which arise from dipole transitions to unoccupied $d$ states have been investigated in terms of their threshold energies, widths, and intensity ratios. Shifts in the ${L}_{3}$ threshold energy between the metal and oxide are different from the chemical shifts measured by x-ray photoemission spectroscopy and this suggests the importance of relaxation effects. Single-particle calculations for the ${L}_{3}$ spectra are also discussed. Measured ${L}_{3}$ linewidths are generally larger than those predicted by suitably broadened theory. A variation from the statistical ${L}_{3}$-to-${L}_{2}$ white-line intensity ratio of 2:1 has been observed across the $3d$ transition row, with values ranging between 0.8:1 for Ti to 5:1 for FeO. This behavior appears to be associated with the white lines since Cu with a filled $3d$ band exhibits the statistical results. It is suggested that the anomalous ratios may be explained by a breakdown of $j\ensuremath{-}j$ coupling caused by an exchange mechanism. Finally, the extended x-ray absorption fine-structure-type structure extending several hundred eV above the white lines is analyzed for Cr to provide the radial distribution function.

Exciton binding energy in quantum wells

Abstract: Variational calculations are presented of the ground exciton state in quantum wells. For the GaAs-GaAlAs system, the results obtained from a trial wave function not separable in spatial coordinates are shown to be valid throughout the entire well-thickness range, corresponding in the thin and thick limits to two- and three-dimensional situations, respectively. For the InAs-GaSb system, in which electrons and holes are present in spatially separated regions, the exciton binding is substantially reduced. In the limit of thin wells, the binding energy is only about one-fourth of the two-dimensional value.

THEORY OF STATIC STRUCTURAL PROPERTIES, CRYSTAL STABILITY, AND PHASE TRANSFORMATIONS: APPLICATION TO Si AND Ge

Abstract: We demonstrate that not only the static structural properties but also the crystal stability and pressure-induced phase transformations in solids can be accurately described employing an ab initio pseudopotential method within the local-density-functional formalism. With the use of atomic numbers of constituent elements and a subset of crystal structures as the only input information, the calculated structural properties of Si and Ge are in excellent agreement with experiment.

Direct measurement of gap state absorption in hydrogenated amorphous silicon by photothermal deflection spectroscopy

Abstract: We have measured the subgap otpical absorption of undoped, singly doped, and compensated hydrogenated amorphous silicon down to 0.6 eV using the sensitive technique of photothermal deflection spectroscopy. We show that this absorption is due to silicon dangling-bond defects located approx.1.4 eV below the conduction band. While doping also creates defects approx.1.4 eV below the conduction band, compensation removes them. The results suggest that for the undoped material the density-of-states maximum found in field-effect measurements is due to silicon dangling bonds.

Comparative theoretical study of the doping of conjugated polymers: Polarons in polyacetylene and polyparaphenylene

Abstract: Defect-state calculations on all-trans polyacetylene and polyparaphenylene have been performed in the framework of the adiabatic H\"uckel Hamiltonian with $\ensuremath{\sigma}$-bond compressibility. In polyacetylene, the study of the energetics of the separation of the radical (neutral defect) -ion (charged defect) pair induced upon doping indicates that the two defects tend to remain in close proximity, resulting in the formation of a polaron. The binding energy of the polaron is estimated to be about 0.05 eV with this model. Absorption spectra at low doping levels are shown to be compatible with polaron formation, thus demonstrating the nonuniqueness of the previously proposed soliton model in explaining these absorption data. At higher doping levels, interaction between polarons leads to the formation of charged solitons carrying no spin. In polyparaphenylene, defects are always correlated in pairs due to the absence of a degenerate ground state. At low doping, polarons with a binding energy estimated at 0.03 eV are formed on ionization of polyparaphenylene. The related deformation of the lattice is relatively sof, in agreement with crystallographic data on biphenyl anions, and extends over about five rings. Increasing the doping level leads to the formation of bipolarons (doubly charged defects) that require a stronger deformation of the lattice and carry no spin. The possibility of a conduction mechanism in polyparaphenylene involving motion of bipolarons is consistent with magnetic data indicative of very low Pauli susceptibility in the metallic regime of Sb${\mathrm{F}}_{5}$-doped polyparaphenylene.

Effective stopping-power charges of swift ions in condensed matter

Abstract: The effective charge of energetic ions as it pertains to the stopping power of solids is calculated in a dielectric-response approximation. The density distribution of $N$ electrons bound in an ion of atomic number ${Z}_{1}$ is given by a variational statistical approximation. The effective charge $Z_{1}^{}{}_{}{}^{*}e$ is always larger than the ionic charge ${Q}_{1}=({Z}_{1}\ensuremath{-}N)e$, because of close collisions. A comprehensive low-velocity formula predicts $Z_{1}^{}{}_{}{}^{*}e$ for given $Q$ as a function of the ratio between the ion size and the mean electron spacing in the medium. At high velocities one obtains a partition rule of stopping powers for the effective charge of ionic projectiles. The results are compared with new precision stopping-power measurements on C, Al, and Au with $_{7}\mathrm{N}$ ions.

Theoretical investigations of superlattice band structure in the envelope-function approximation

Abstract: We extend our previous investigations on the band structure of superlattices by applying the envelope-function approximation to four distinct problems. We calculate the band structure of HgTe-CdTe superlattices and show that these materials can be either semiconducting or zero-gap semiconductors, i.e., behave exactly like the ternary ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}$ Te random alloys. We analyze the superlattice dispersion relations in the layer planes (Landau superlattice subbands) and we compare the longitudinal and transverse effective masses of semiconducting InAs-GaSb superlattices. We calculate the general equation for the bound states due to aperiodic layers, taking account of the band structure of the host materials. We finally derive the dispersion relations of polytype ($\mathrm{ABC}$ or $\mathrm{ABCD}$) superlattices.

Vacancies and carbon impurities in α - iron: Electron irradiation

Abstract: Positron-lifetime measurements are reported on electron-irradiated high-purity $\ensuremath{\alpha}$-iron and on iron doped with controlled amounts of carbon impurities. We show that in pure iron monovacancies are mobile at stage III around 220 K causing vacancy clustering into small three-dimensional agglomerates. These clusters anneal out between 500 and 600 K. In carbon-doped iron a competing mechanism is the formation of a highly asymmetric carbon-vacancy pair at 200 K. These pairs dissociate around 490 K, resulting in another release of free vacancies in the lattice. Controversial aspects on vacancy properties in earlier investigations are discussed.

Growth of spontaneous periodic surface structures on solids during laser illumination

Abstract: Spontaneous periodic surface structures, or ripples, are frequently observed after illumination of metals, semiconductors, and dielectrics by intense laser pulses. We develop a theory which predicts the observed spacing, polarization, and growth properties of these ripples. In this model, one or several Fourier components of a random surface disturbance scatter light from the incident beam very nearly along the surface. The interference of this diffracted optical wave with the incident beam then gives rise to optical interference fringes which can reinforce the initial disturbance. Sinusoidal corrugations on either metallic or molten surfaces seem to provide strong positive feedback for ripple growth, whereas sinusoidal gratings in temperature, electron-hole density, or dielectric constant seem much less well correlated with observations.

Lattice-dynamical model for graphite

Abstract: The lattice dynamics of pristine graphite is presented with the use of a Born---von K\'arm\'an model. With the consideration of interactions to fourth neighbor both intraplane and interplane, good agreement is simultaneously obtained with ir, Raman, and inelastic neutron scattering measurements of lattice modes and with the measured elastic constants. The second-order Raman spectrum is also calculated and compared with experiment.

Systematics of multilayer adsorption phenomena on attractive substrates

Abstract: This paper presents a systematic classification of multilayer-adsorption phenomena on attractive substrates, with emphasis on the buildup of thick films. The approach is based on statistical mechanics and includes adsorption-desorption effects and the interrelation of bulk and surface behavior. The surface phase diagram depends qualitatively on the relative strengths and ranges of adatom-adatom and adatom-substrate attractions. When the adatom-substrate attraction dominates (strong substrate), the film builds up uniformly, as the bulk adatom density increases, and the excess surface density diverges at coexistence (complete wetting). The buildup proceeds via an infinite sequence of discrete layer transitions (layering) at low temperatures (below the roughening temperature ${T}_{R}$ and smoothly at higher temperatures, as originally noted by de Oliveira and Griffiths. Substrates of intermediate strength are characterized by a wetting temperature ${T}_{W}$ above which wetting at coexistence is approached. The relative values of ${T}_{W}$ and ${T}_{R}$ define three subregions: When ${T}_{W}l{T}_{R}$ layering occurs, with an infinite sequence of transitions between ${T}_{W}$ and ${T}_{R}$ when ${T}_{R}\ensuremath{\lesssim}{T}_{W}$, layer transitions have coalesced into a single thick-film\char22{}thin-film transition (prewetting); when ${T}_{R}\ensuremath{\ll}{T}_{W}$, prewetting may disappear, leaving only a critical-wetting transition on the coexistence axis. For still weaker substrates, wetting is incomplete at all temperatures; however, a variety of drying phenomena may occur on the high-density side of bulk coexistence. Specific calculations are given for a lattice-gas model at $T=0$ and in the mean-field approximation. Conclusions are informed, in addition, by certain exact results and symmetries. The last section includes a critical discussion of the relation of the lattice-gas model to the real world and a brief review of relevant experimental data.

Grazing-incidence diffraction and the distorted-wave approximation for the study of surfaces

Abstract: A method is developed for calculating the scattering of a beam of x rays striking the plane surface of a crystal at an angle below the critical angle for total external reflection The low penetration under this condition offers the possibility of determining the special structure of the surface layers, as has been pointed out by Marra, Eisenberger, and Cho A distorted-wave approach is developed whereby the crystal is first taken to be a homogeneous dielectric slab for the purpose of calculating the distorted wave The distorted wave is considered to illuminate the actual crystal, from which a scattering pattern can then be calculated The effects of absorption can be taken into account; in some cases absorption may offer the possibility of observing near-surface structures at angles of incidence larger than the critical angle Such illumination may also be useful for performing fluorescence analysis of the near-surface layers to determine their impurity content Synchrotron sources offer new opportunities for measurements of these kinds Thermal neutrons may also be used in place of x rays and offer unique opportunities for studying surface magnetization and, through inelastic scattering, surface phonons and, conceivably, surface magnons

Measurement of the density of gap states in hydrogenated amorphous silicon by space charge spectroscopy

Abstract: We report a comprehensive study of the dynamic response of junction space-charge layers in undoped and P${\mathrm{H}}_{3}$-doped $a$-Si:H films grown by the rf-glow-discharge technique. By using the numerical analysis methods discussed in the adjoining theory paper, we are able consistently to interpret a variety of transient response and ac admittance measurements in terms of a bulk density of gap states $g(E)$ which is characteristic of each sample. While the general shape of $g(E)$ seems to be a characteristic property of $a$-Si:H, the overall concentration of gap states depends on growth conditions and doping. The density of states at approximately midgap is observed to vary between values as low as about 2\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ e${\mathrm{V}}^{\ensuremath{-}1}$ in undoped films and as high as 1\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ e${\mathrm{V}}^{\ensuremath{-}1}$ in some P${\mathrm{H}}_{3}$-doped films. The general shape of our $g(E)$ is dominated by a deep minimum (${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ e${\mathrm{V}}^{\ensuremath{-}1}$) between 0.3 and 0.6 eV from the conduction band and a broad shoulder of states extending from the valence band up to midgap. The significant difference between this type of bulk $g(E)$ and previous models for the density of states in $a$-Si:H may be explained by the effects of states at or near the surface of the films which strongly influence the previous types of measurements. We discuss recent transport and optical measurements and show that they provide strong support for our density of states as opposed to previous models for $g(E)$.

Influence of elastic and inelastic scattering on energy spectra of electrons emitted from solids

Abstract: We have investigated theoretically the spectrum of electrons emitted from a solid in the case of a primary excitation function corresponding to a narrow line as in Auger and photoelectron spectroscopy. We concentrate on the influence of elastic and inelastic electron scattering on the shape and intensity of the spectrum in the vicinity of the primary peak. A near-peak as well as an off-peak region is identified; the critical energy loss separating the two regions is shown to depend on the relative significance of angular deflection and inelastic scattering. A rigorous formula has been derived that allows the extraction of the primary spectrum from a measured electron current if the normalized energy-loss function is available. The physical significance of the escape depth extracted from experiments with overlayers is analyzed. We find that it is not justified in general to identify the escape depth measured by means of overlayers with the inelastic mean free path.

Relaxational dynamics of the Edwards-Anderson model and the mean-field theory of spin-glasses

Abstract: Langevin equations for the relaxation of spin fluctuations in a soft-spin version of the Edwards-Anderson model are used as a starting point for the study of the dynamic and static properties of spin-glasses. An exact uniform Lagrangian for the average dynamic correlation and response functions is derived for arbitrary range of random exchange, using a functional-integral method proposed by De Dominicis. The properties of the Lagrangian are studied in the mean-field limit which is realized by considering an infinite-ranged random exchange. In this limit, the dynamics are represented by a stochastic equation of motion of a single spin with self-consistent (bare) propagator and Gaussian noise. The low-frequency and the static properties of this equation are studied both above and below ${T}_{c}$. Approaching ${T}_{c}$ from above, spin fluctuations slow down with a relaxation time proportional to ${|T\ensuremath{-}{T}_{c}|}^{\ensuremath{-}1}$ whereas at ${T}_{c}$ the damping function vanishes as ${\ensuremath{\omega}}^{\frac{1}{2}}$. We derive a criterion for dynamic stability below ${T}_{c}$. It is shown that a stable solution necessarily violates the fluctuation-dissipation theorem below ${T}_{c}$. Consequently, the spin-glass order parameters are the time-persistent terms which appear in both the spin correlations and the local response. This is shown to invalidate the treatment of the spin-glass order parameters as purely static quantities. Instead, one has to specify the manner in which they relax in a finite system, along time scales which diverge in the thermodynamic limit. We show that the finite-time correlations decay algebraically with time as ${t}^{\ensuremath{-}\ensuremath{ u}}$ at all temperatures below ${T}_{c}$, with a temperature-dependent exponent $\ensuremath{ u}$. Near ${T}_{c}$, $\ensuremath{ u}$ is given (in the Ising case) as $\ensuremath{ u}(T)\ensuremath{\sim}\frac{1}{2}\ensuremath{-}{\ensuremath{\pi}}^{\ensuremath{-}1}(\frac{1\ensuremath{-}T}{{T}_{c}})+\ensuremath{\sigma}{(\frac{1\ensuremath{-}T}{{T}_{c}})}^{2}$. A tentative calculation of $\ensuremath{ u}$ at $T=0$ K is presented. We briefly discuss the physical origin of the violation of the fluctuation-dissipation theorem.

Spontaneous dimerization in the S = 1 2 Heisenberg antiferromagnetic chain with competing interactions

Abstract: Spontaneous dimerization is found in the $S=\frac{1}{2}$ isotropic Heisenberg antiferromagnetic chain with competing nearest- and next-nearest-neighbor exchange, $\frac{{J}_{2}}{{J}_{1}}\ensuremath{\gtrsim}\frac{1}{6}$, and results from the same umklapp processes that lead to the N\'eel state when easy-axis exchange anisotropy is present. Spontaneous and externally induced dimerizations are contrasted.

Covalent effects in the effective-medium theory of chemical binding: Hydrogen heats of solution in the 3 d metals

Abstract: The effective-medium-theory estimate of the binding energy of an atom to a host (another atom, molecule, or solid) from the host electron density, and the binding energy of the atom in a homogeneous electron gas has been extended to include covalent effects. The covalent term involves the difference in the one-electron energies between the atom in the host and the atom in homogeneous electron-gas systems. The scheme is tested for helium impurities in several transition metals and for hydrogen impurities in all the metals from potassium to copper. For helium all available binding energies are well reproduced and all the observed trends in the hydrogen heat of solution can be accounted for.

Scaling for first-order phase transitions in thermodynamic and finite systems

Abstract: Scaling behavior for first-order phase transitions can be derived alternatively but consistently from renormalization-group, phenomenological, or finite-size considerations. A general analysis of densities at a renormalization-group fixed point demonstrates that if the coexistence of $p$ distinct phases is possible, then $p$ distinct eigenvalue exponents must equal the spatial dimensionality. This basic eigenvalue (or scaling) exponent condition can also be derived phenomenologically by various arguments not depending on detailed renormalization-group considerations. A scaling description of first-order phase transitions is presented and extended to finite systems with linear dimensions $L$, leading to a rounding proportional to ${L}^{\ensuremath{-}d}$, response-function maxima varying as ${L}^{d}\ensuremath{\propto}N$, and boundary-condition-dependent shifts which may be as large as $\ensuremath{\sim}{L}^{\ensuremath{-}1}$.

Electron-hole-pair quenching of excited states near a metal

Abstract: We present a calculation of the nonradiative damping of a dipole outside a metal surface. The calculation uses a realistic surface potential for the metal conduction electrons in contrast to most earlier studies. A simple experiment is suggested to test the theoretical predictions.

Monte Carlo Simulations of One-dimensional Fermion Systems

Abstract: We discuss a new method to perform numerical simulations of one-dimensional systems with fermion and boson degrees of freedom. The method is based on a direct-space, imaginary-time representation of the fermion field. It is fast so that systems having up to 100 sites can easily be simulated. In addition, the method provides an intuitive physical "picture" of the ground state of a one-dimensional many-body system. We discuss in detail how to implement the method and how to compute various physical quantities. In particular, we show how to extend the method to study averages of off-diagonal quantities in an occupation-number representation. To assess the accuracy of our procedure, we apply it to free fermions in one dimension and compare with exact results. We then study a model of spinless interacting fermions and obtain the expected phase structure and behavior of correlation functions. We also consider the extended Hubbard model at various points in its phase diagram and study the behavior of spin-density, charge-density, and pairing correlation functions. We then study the Gross-Neveu model and show how the behavior depends on the number of fermion flavors. Finally, we consider an electron-phonon model and study its behavior both in the one-particle polaron sector and in the half-filled-band case. Along the way we show pictures of the ground-state configurations that give physical insight into the properties of the systems, like charge-density-wave, spin-density-wave, and superconducting states, "fractional charges," and solitons. We conclude by comparing our method with other methods and discuss the possibility of extending it to higher dimensions.

Raman studies of benzene-derived graphite fibers

Abstract: Carbon fibers prepared from thermal decomposition of benzene at \ensuremath{\sim} 1100\ifmmode^\circ\else\textdegree\fi{}C are studied by Raman spectroscopy as a function of heat-treatment temperature. The structural ordering at each heat-treatment temperature is monitored by observation of both the Raman-allowed ${E}_{2{g}_{2}}$ mode at 1580 ${\mathrm{cm}}^{\ensuremath{-}1}$ and the disorder-induced lines at \ensuremath{\sim} 1360 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the first-order spectra and at 2730 and 2970 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the second-order spectra. Raman and resistivity results indicate three characteristic heat-treatment temperatures relevant to the establishment of in-plane and interplanar ordering. Using fibers heat treated to the maximum available temperature of 2900\ifmmode^\circ\else\textdegree\fi{}C, Raman spectroscopy shows that single-staged fibers can be prepared by acceptor intercalation, in agreement with direct Debye-Scherrer x-ray measurements. Resistivity measurements on pristine fibers previously heat treated to 2900\ifmmode^\circ\else\textdegree\fi{}C show a metallic temperature dependence with $\ensuremath{\rho}=70$ \ensuremath{\mu}\ensuremath{\Omega} cm at 300 K and a residual resistance ratio of 1.5. Upon intercalation, a resistivity $\ensuremath{\rho}=7$ \ensuremath{\mu}\ensuremath{\Omega} cm at 300 K and a residual resistance ratio of 5 is achieved. Raman-spectroscopy and temperature-dependent resistivity measurements demonstrate that the benzene-derived fibers exhibit the highest degree of ordering achieved in fibers and provide an attractive host material for intercalation.

Lifetime of a quasiparticle in a two-dimensional electron gas

Abstract: We have investigated the inelastic Coulomb lifetime ${\ensuremath{\tau}}_{\mathrm{ee}}$ of a quasiparticle near to the Fermi surface of a two-dimensional electron gas. Within a perturbative approach based upon the random-phase approximation, we find that at low temperature $\frac{1}{{\ensuremath{\tau}}_{\mathrm{ee}}}$ behaves like ${T}^{2}\mathrm{ln}T$. Furthermore at small quasiparticle excitation energy, the leading contribution to $\frac{1}{{\ensuremath{\tau}}_{\mathrm{ee}}}$ is inversely proportional to the electronic density and does not depend upon the electric charge. Although the plasmon frequency goes to zero at long wavelength, plasmon emission contributes to the quasiparticle decay only when the quasiparticle excitation energy exceeds a certain threshold. The threshold becomes a small fraction of the Fermi energy in the high-density limit.

Oxygen K near-edge fine structure: An electron-energy-loss investigation with comparisons to new theory for selected 3 d Transition-metal oxides

Abstract: Excitations of the oxygen $1s$ subshell in selected $3d$ transition-metal oxides have been studied by inelastic scattering of 75-keV electrons. Striking variations in the near-edge fine structure are reported and an interpretation is developed based on an empirical molecular orbital energy-level model. We compare our observed fine structure with that evinced in the metal $K$ and ${L}_{3}$ edges in these same oxides. While the molecular-orbital model seems adequate for interpreting the spectra of Ti${\mathrm{O}}_{2}$, it fails for at least some of the oxides studied. For example, in the case of NiO, a self-consistent Hartree-Fock computation for the oxygen $1s$ excitation spectrum gives results showing that the near-edge structure is not adequately described by the unoccupied density of states of the solid before core-hole excitation. Instead, the initial spectral peaks are shown to be core excitons. However, for Ti${\mathrm{O}}_{2}$, a tight-binding extended H\"uckel calculation neglecting the core hole yields a density of states that displays peaks in good agreement with the experimental data. Speculations on the origin of the difference between the spectra of NiO and Ti${\mathrm{O}}_{2}$ are offered.

Electronic properties of graphite: A unified theoretical study

Abstract: We have calculated the electronic structure of three-dimensional graphite using the modified first-principles Korringa-Kohn-Rostoker technique developed for and applied to the intercalation compound Li${\mathrm{C}}_{6}$. Whereas previous calculations of the electronic band structure of graphite provide explanations either for moderate- to high-energy excitations or for low-energy and Fermi-surface properties, we find excellent agreement between our results and experiments in both regimes. Our analysis of the band structure is based on a comparison with experiments of predicted optical transitions, values for the Slonczewski-Weiss-McClure parameters which we obtain from a fit to our bands, and Fermi-surface properties. We also present a density of states for our band structure and several constant-energy surfaces. Our discussion includes a comparison with other theoretical work.