Showing papers on "Electronic band structure published in 1993"

Acoustic band structure of periodic elastic composites.

Abstract: We present the first full band-structure calculations for periodic, elastic composites. For transverse polarization of the vibrations we obtain a ``phononic'' band gap which extends throughout the Brillouin zone. A complete acoustic gap or a low density of states should have important consequences for the suppression of zero-point motion and for the localization of phonons, and may lead to improvements in transducers and in the creation of a vibrationless environment.

Electronic, optical, and structural properties of some wurtzite crystals.

Abstract: Using the first-principles orthogonalized linear-combination-of-atomic-orbitals method in the local-density approximation, the electronic structures and the linear-optical properties of ten wurtzite crystals, BeO, BN, SiC, AlN, GaN, InN, ZnO, ZnS, CdS, and CdSe are investigated. Results on band structures, density of states, effective masses, charge-density distributions, and effective charges are presented and compared. Optical properties of the ten wurtzite crystals up to a photon energy of 40 eV are calculated and the dielectric functions are resolved into components perpendicular and parallel to the z axis. The calculated results are compared with the available experimental data and other recent calculations. The structural properties of the wurtzite crystals are also studied by means of local-density total-energy calculations. It is shown that the calculated equilibrium volume and the bulk modulus are in good agreement with recent experimental data.

Electronic States of Carbon Nanotubes

Abstract: The π-electron states of carbon nanotubes (CN's) in magnetic fields are calculated in the effective-mass theory. A sensitive change of CN from metal to semiconductor depending on its structure is well reproduced. The band gap is inversely proportional to the tube diameter and exhibits a drastic change as a function of magnetic flux passing through the cylinder with period c h / e due to the Aharonov-Bohm effect. In a magnetic field perpendicular to the tube axis, the band-gap is reduced strongly and the energy spectra approach those of a graphite sheet.

First-principles theory of surface magnetocrystalline anisotropy and the diatomic-pair model.

Abstract: The state-tracking method proposed recently is employed for the first-principles determination of the magnetocrystalline-anisotropy (MCA) energy. A close relationship of the MCA energy to the band structure is found for transition-metal monolayers that show the change of sign of the MCA with respect to the band filling (atomic species) to be determined mainly by the spin-orbit coupling within the spin-down bands. The Fe monolayer with the \ensuremath{\pi}-bonding band as the highest occupied band exhibits positive MCA (easy axis along the layer normal). However, the effect of strain on the MCA of the Fe monolayer demonstrates the effect of the spin-orbit coupling between opposite-spin states. A model for the electronic origin of the magnetic anisotropy of this two-dimensional system is presented that explains the first-principles MCA results for iron and cobalt monolayers on the basis of the bonding character between two d atoms, the band broadening due to increase in coordination, and geometry (symmetry).

Quantum-well states and magnetic coupling between ferromagnets through a noble-metal layer.

Abstract: Using inverse photoemission and photoemission we find that the bulk bands become discretized in highly perfect layer structures, such as Cu on fcc Co(100), Cu on fcc Fe(100), Ag on bcc Fe(100), Au on bcc Fe (100), fcc Co on Cu(100), and bcc Fe on Au(100). The electronic structure is analyzed in the framework of quantum-well states consisting of bulk Bloch functions modulated by an envelope function. The wavelength of the envelope function is determined from the \ensuremath{\lambda}/2 interferometer fringes produced by the periodic appearance of quantum-well states with increasing film thickness. Using k conservation, one obtains an absolute measurement of the band dispersion for the s,p bands of Fe, Cu, Ag, and Au. Quantum-well states at the Fermi level are found to be closely connected with oscillatory magnetic coupling in superlattices. They are spin polarized, even in noble metals, due to the spin-dependent band structure of the confining ferromagnet. The oscillation period is half the wavelength of the envelope function. The corresponding wave vector is given by the Fermi wave vector and by the wave vector of the nearest s,p band edge via 2(${\mathit{k}}_{\mathrm{edge}}$-${\mathit{k}}_{\mathit{F}}$). This turns out to be equivalent to Ruderman-Kittel-Kasuya-Yosida theory.

Identification of radiative transitions in highly porous silicon

Abstract: The authors report time-resolved photoluminescence spectroscopy of highly porous silicon. Their results show that the luminescence is due to localized quantum-confined excitons in undulating crystalline silicon wires. The resonantly excited photoluminescence spectrum exhibits satellite structure due to momentum-conserving phonons of crystalline silicon. This provides a clear signature of the crystalline-silicon electronic band structure. The spin states of the localized exciton are split by the electron-hole exchange interaction. This splitting is manifested both in the strong dependence of the luminescence lifetime on temperature, and as an energy gap in the resonantly excited photoluminescence spectrum. The experimental splitting is in good agreement with the value calculated for a localized exciton in crystalline silicon.

Electronic Structure of Materials

Abstract: The diatomic molecule from the finite to the infinite into two and three dimensions band gaps - origins and consequences s-p bonding - a case study in silicon free electron theory properties of free electron metals the transition metals structural stability of compounds introduction to modern quantitative theory where band theory breaks down set problems sample examination questions.

Quasiparticle band structure of AlN and GaN.

Abstract: The ab initio pseudopotential method within the local-density approximation and the quasiparticle approach have been used to investigate the electronic properties of AlN and GaN in the wurtzite and zinc-blende structures. The quasiparticle band-structure energies are calculated using a model dielectric matrix for the evaluation of the electron self-energy. For this calculation, good agreement with the experimental results for the minimum band gaps in the wurtzite structure is obtained. In the zinc-blende structure we predict that AlN will be an indirect (\ensuremath{\Gamma} to X) wide band-gap semiconductor (4.9 eV) and that GaN will have a direct gap of 3.1 eV at \ensuremath{\Gamma} in good agreement with recent absorption experiments on cubic GaN (3.2--3.3 eV). A discussion of the direct versus indirect gap as well as other differences in electronic structure between the wurtzite and zinc-blende phases is presented. Other properties of quasiparticle excitations are predicted in this work and remain to be confirmed by experiment.

Photonic band structure and defects in one and two dimensions

Abstract: We present an experimental and numerical study of electromagnetic wave propagation in one-dimensional (1D) and two-dimensional (2D) systems composed of periodic arrays of dielectric scatterers. We demonstrate that there are regions of frequency for which the waves are exponentially attenuated for all propagation directions. These regions correspond to band gaps in the calculated band structure, and such systems are termed photonic band-gap (PBG) structures. Removal of a single scatterer from a PBG structure produces a highly localized defect mode, for which the energy density decays exponentially away from the defect origin. Energy-density measurements of defect modes are presented. The experiments were conducted at 6–20 GHz, but we suggest that they may be scaled to infrared frequencies. Analytic and numerical solutions for the band structure and the defect states in 1D structures are derived. Applications of 2D PBG structures are briefly discussed.

First-principles calculation of the electronic structure of the wurtzite semiconductors ZnO and ZnS

Abstract: We report ab initio calculations of the lattice constants and the electronic band structure of the hexagonal wurtzite-structure semiconductors ZnO and ZnS. We employ the local-density approximation and solve the Kohn-Sham equations for nonlocal, separable, and norm-conserving pseudopotentials self-consistently. We use basis sets of localized Gaussian orbitals with s, p, d, and ${\mathit{s}}^{\mathrm{*}}$ symmetry. In particular, we investigate the influence of the Zn 3d electrons on the results for the lattice constants and the band structure. Results of calculations employing both ${\mathrm{Zn}}^{2+}$ and ${\mathrm{Zn}}^{12+}$ ionic pseudopotentials are presented and discussed. For ZnS, both the cubic zinc blende and the hexagonal wurtzite polytype have been studied. The calculated lattice constants are found to be in excellent agreement with experiment for both semiconductors when the d electrons are explicitly taken into account as valence electrons. The agreement of the calculated bands of ZnS with experimental data and with the results of a plane-wave calculation from the literature using about 6.000 plane waves for the cubic crystal is very good except for the absolute energy position of the d bands. For ZnO the calculated bands agree better with angle-resolved photoemission data when the ${\mathrm{Zn}}^{12+}$ pseudopotential is employed. The agreement, however, is still far from satisfactory and the calculated absolute position of the d bands is off, again. The discrepancies seem to be related to correlation effects in the narrow d bands. We find the Zn 3d electrons to strongly interact with the O 2p electrons in ZnO. According to our results, the p-d mixing in ZnO is about twice as large as in ZnS.

Quasiparticle band-structure calculations for C, Si, Ge, GaAs, and SiC using Gaussian-orbital basis sets.

Abstract: We report state-of-the-art first-principles calculations of the quasiparticle energies of prototype homopolar and heteropolar covalent semiconductors described in terms of the electron self-energy operator. The wave functions are calculated within density-functional theory using the local-density approximation and employing nonlocal, norm-conserving pseudopotentials. The self-energy operator is evaluated in the GW approximation. Employing the plasmon-pole approximation for the frequency dependence of the dielectric matrix ${\mathrm{\ensuremath{\epsilon}}}_{\mathbf{G},\mathbf{G}\ensuremath{'}}$(q,\ensuremath{\omega}), its static part is fully calculated within the random-phase approximation (RPA) as well as by using a number of different models. All calculations are carried out employing localized Gaussian orbital basis sets. This will turn out to be very useful for detailed studies of the quasiparticle properties of more complex systems such as bulk defects including lattice relaxation and reconstructed surfaces with large unit cells or interfaces, which are otherwise computationally too demanding. Using an s,p,d,s* basis set of 40 Gaussian orbitals for Si, for example, yields already convergent results in excellent agreement with the results of a 350-plane-wave calculation in the corresponding plane-wave representation. Most of our results for Si, diamond, Ge, and GaAs are in very good agreement with experimental data and with available plane-wave GW calculations. To our knowledge, our results for SiC are the first quasiparticle energies reported so far for this important material of high current technological interest. Also in this case we find very good agreement with the available experimental data except for E(${\mathrm{L}}_{1\mathrm{c}}$). We believe that this deviation may be attributed to experimental uncertainties. In particular, we discuss and scrutinize the applicability of six different models for the static dielectric matrix ${\mathrm{\ensuremath{\epsilon}}}_{\mathbf{G},\mathbf{G}\ensuremath{'}}$(q,0) in the GW approximation ranging from the simple Hartree-Fock expression over diagonal models to nondiagonal models that take the local fields within the inhomogeneous electronic charge density into account. Some of the nondiagonal models are shown to yield results in very good agreement with the full RPA results.

Electrostatic sample-tip interactions in the scanning tunneling microscope.

Abstract: Local surface photovoltage (SPV) measurements were used to measure how the electric field of a scanning tunneling microscope tip perturbs the electronic band structure at Si(001), Si(111)-(7\ifmmode\times\else\texttimes\fi{}7), and H-terminated Si(111) surfaces. The results demonstrate that tip-induced band bending is important under typical STM conditions even on surfaces whose surface Fermi levels are nominally ``pinned.'' Spatially resolved measurements of band bending as a function of sample bias show that atomic-scale contrast in SPV images can result from local variations in the ability of the surface states under the tip to screen external electric fields.

Quantum mechanics of electrons in crystals with graded composition.

Abstract: : We construct the effective Hamiltonian describing the motion of electrons in compositionally graded crystals which is valid throughout a given energy band and part way into the gaps. The effective Hamiltonian, constructed from the band structures of uniform crystals, also includes the effects of a slowly varying applied scalar potential U(r). Near the edges of a nondegenerate band, this effective Hamiltonian reduces to an effective mass Hamiltonian with position dependent mass (one of several forms previously appearing in the literature): H sub eff = 1/2 pi(1/m*(r)) sub ij pj + Epsilon(r) + U(r), where Epsilon(r) is the energy of the band edge as a function of position. The analogous effective mass Hamiltonian for degenerate bands is also derived. Next, we examine more general states-not restricted to the vicinity of a band edge in crystals with composition and applied potential variation in one direction. We obtain a WKB-type solution for the envelope functions, as well as the appropriate turning point connection rules.

Atomic structure and doping of microtubules.

Abstract: The atomic and electronic structures of microtubules of graphitic carbon have been investigated by ab initio molecular dynamics. Fully optimized atomic structures of two microtubules, a reflection symmetric and a chiral, are found to differ little from their ideal, graphite-derived geometries. Both are semiconductors with direct band gaps at the \ensuremath{\Gamma} point and along the \ensuremath{\Delta} direction, respectively. It is shown by direct calculations in large supercells that substitutional N and B dope microtubules n and p type.

Band structure and semiconducting properties of FeSi.

Abstract: The results of linear augmented-plane-wave band calculations for cubic FeSi, carried out in the local-density approximation, predict a small (-0.11 eV) indirect semiconductor gap which agrees well with empirical estimates (-0.13 eV). The origin of this gap, which occurs within the Fe (3d) manifold, can be traced to a pseudogap that is present in the reference rocksalt phase that underlies the lower-symmetry FeSi structure. The relationship between the FeSi band structure and several types of many-body correlation mechanisms that have been advanced to explain its anomalous temperature-dependent magnetic properties is discussed

Calculation of optical excitations in cubic semiconductors. I. Electronic structure and linear response.

Abstract: The electronic structures and the linear optical dielectric functions of 18 cubic semiconductors are studied by the first-principles orthogonalized linear-combination-of-atomic-orbitals (OLCAO) method in the local-density approximation. The crystals studied include the group-IV semiconductors C, Si, and Ge; the III-V compounds AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb; and the II-VI semiconductors ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe. Results are presented for the band structures, for the density of states, and for the real and imaginary parts of the linear dielectric functions for photon energies up to 12 eV. The results are compared with other existing calculations and experimental data. Some interesting correlations and trends among the 18 semiconductors studied are pointed out, and possible problems with the optical excitation calculation are discussed. These results provide the groundwork for the calculation of nonlinear optical properties on these crystals using the full band-structure approach in the two papers to follow. It is argued that optical excitations in semiconductors can be efficiently carried out using the OLCAO method without resorting to empirical methods or model studies. The present calculation gives band gaps larger than the well-converged result of first-principles pseudopotential calculations. The consequence of this difference on the optical properties is discussed.

Electronic and structural properties of GaN by the full-potential linear muffin-tin orbitals method: The role of the d electrons.

Abstract: The structural and electronic properties of cubic GaN are studied within the local-density approximation by the full-potential linear muffin-tin orbitals method. The Ga 3d electrons are treated as band states, and no shape approximation is made to the potential and charge density. The influence of d electrons on the band structure, charge density, and bonding properties is analyzed. Due to the energy resonance of Ga 3d states with nitrogen 2s states, the cation d bands are not inert, and features unusual for a III-V compound are found in the lower part of the valence band and in the valence charge density and density of states. To clarify the influence of the d states on the cohesive properties, additional full- and frozen-overlapped-core calculations were performed for GaN, cubic ZnS, GaAs, and Si. The results show, in addition to the known importance of core-valence exchange-correlation nonlinearity, that an explicit description of closed-shell interaction has a noticeable effect on the cohesive properties of GaN. Since its band structure and cohesive properties are sensitive to a proper treatment of the cation d bands, GaN appears to be somewhat exceptional among the III-V compounds and reminiscent of II-VI materials.

Electronic structure of a stepped graphite surface

Abstract: A first-principles band calculation and theoretical simulation of scanning tunneling microscopy and spectroscopy (STM and STS) are performed on a stepped surface of graphite. The calculated band structure and the density of states are similar to those of fullerene tubules. Unlike in fullerene tubules, however, a localized state on the step appears at the Fermi level due to the existence of an edge in the graphite sheet. The calculated STM image shows a triangular lattice structure similar to that of the bulk graphite. However, the positions of the peaks of the tunneling current do not coincide with the atomic sites and shift gradually as they diverge from the step. The calculated STS spectra at a location near the step show a strong peak reflecting the localized state.

Theoretical investigation of random Si-C alloys.

Abstract: We have performed a first-principles investigation of the microscopic properties of random crystalline Si-C alloys. An ab initio tight-binding molecular-dynamics method is used to determine the microscopic atomic structure of the alloys. For small to moderate concentrations of C in Si, we find that the electronic structure shows a decrease of the band gap from that of pure Si. This result is unexpected since both ordered SiC and pure carbon (diamond) have much larger band gaps than Si. Plane-wave calculations were also done on ordered structures to further check this result and to determine the effeats of ordering. For the atomic structure, it is found that there are two different types of Si-A bonds. The first type is the Si-C bond near 1.7 A\r{} as in bulk SiC, and the second type is a much shorter 1.65-A\r{} bond for a carbon atom in a near-planar ${\mathit{sp}}^{2}$ configuration with its Si neighbors.

Electronic structure and its dependence on local order for H/Si(111)-(1×1) surfaces

Abstract: The valence and core level spectra of chemically prepared, ideally H-terminated Si(111) surfaces are characterized by remarkably sharp features. The valence band levels and their dispersion are well described by first-principles calculations using a quasiparticle self-energy approach within the GW approximation. From the ${\mathrm{Si}}_{2\mathit{p}}$ spectra, an upper limit of 35\ifmmode\pm\else\textpm\fi{}10 meV is derived from the core hole lifetime broadening, a value substantially lower than previously measured.

Photoelectron spectroscopic study of the valence and core-level electronic structure of BaTiO3.

Abstract: We present valence and core-level photoemission measurements from vacuum-fractured, single-crystal barium titanate. These results resolve contradictory measurements in the literature which have employed other methods of sample surface preparation. The valence-shell electronic structure is compared with previously published results of band structure and cluster calculations. Resonant photoemission is used to probe the covalent coupling between titanium and oxygen in the cubic and tetragonal phases of this ionic compound. Photoelectron spectra of the Ti 2p and O 1s core levels reveal the valence of these two ions to be ${\mathrm{TiO}}_{2}$-like. Valence, core, satellite, and Auger transitions are also assigned and tabulated.

Unified approach to the electronic structure of strained Si/Ge superlattices.

Abstract: A tight-binding model in the three-center representation, with an orthogonal ${\mathit{sp}}^{3}$ set of orbitals and interactions up to third neighbor, is introduced. This model gives a good description of bulk Si and Ge and reproduces known results for their band structures, including the lowest conduction band, their density of states, effective masses, deformation potentials, and dielectric function. Also, this is an efficient model as far as computer time is concerned; therefore, it is most appropriate for application to superlattices (SL's). In particular, it is used to study the electronic properties of some strained Si/Ge superlattices. Their band structure, confinement of superlattice states, transition probabilities, effective masses, and spin splittings were investigated and the influence of the strain and superlattice periodicity was studied. It was found that under specific conditions of growth, some SL's can be direct-gap materials. Finally, the comparison with experimental results shows that the present model is a realistic one and can be used to describe the electronic properties of the strained Si/Ge SL's and clarify many of the points that are under debate.

Experimental and theoretical studies of the F+H2 transition state region via photoelectron spectroscopy of FH−2

Abstract: The transition state region of the F+H2 reaction is studied by photoelectron spectroscopy of FH2−. The photoelectron spectra consist of overlapping electronic bands with different angular distributions. The ground state band shows partially resolved features which differ depending on whether the anion is made from normal or para hydrogen. This dependence on the anion nuclear spin statistics implies that these features are due to progressions in bending levels of the neutral FH2 complex. In order to confirm this, and to determine the sensitivity of the photoelectron spectrum to the bend potential near the F+H2 transition state, three‐dimensional simulations of the FH2− photoelectron spectrum were performed assuming various potential energy surfaces for the F+H2 reaction. We found that the London–Eyring–Polanyi–Sato surface proposed by Takayanagi and Sato gave better agreement than either the T5a or 5SEC surfaces. From the higher energy band, we can extract information on the F+H2 excited electronic states,...

Calculated quasiparticle band gap of beta -C3N4.

Abstract: The quasiparticle electronic band structure of C[sub 3]N[sub 4] carbon nitride in the [beta]-Si[sub 3]N[sub 4] structure is calculated using the [ital GW] approximation. The indirect band gap is predicted to be 6.4[plus minus]0.5 eV and the minimum direct gap is found to be at [Gamma] with a value of 6.75 eV. A plane-wave local-density-approximation band structure is also presented.

Band structure effects of transport properties in icosahedral quasicrystals.

Abstract: Transport properties and optical conductivity are calculated for the crystalline approximant AlMn alloy. The number of electrons at the Fermi energy is very small and, based on the band structure, the anomalously small dc conductivity and temperature dependent thermoelectric power are explained. A model calculation for the two-dimensional Penrose lattice with random phason strain shows that it becomes more conductive than the perfect Penrose lattice. We propose the mechanism of the interband transition by phason randomness and inelastic scattering for the origin of the anomalous conductivity in real quasicrystals