Showing papers on "Electronic band structure published in 2003"

Band parameters for nitrogen-containing semiconductors

Abstract: We present a comprehensive and up-to-date compilation of band parameters for all of the nitrogen-containing III–V semiconductors that have been investigated to date. The two main classes are: (1) “conventional” nitrides (wurtzite and zinc-blende GaN, InN, and AlN, along with their alloys) and (2) “dilute” nitrides (zinc-blende ternaries and quaternaries in which a relatively small fraction of N is added to a host III–V material, e.g., GaAsN and GaInAsN). As in our more general review of III–V semiconductor band parameters [I. Vurgaftman et al., J. Appl. Phys. 89, 5815 (2001)], complete and consistent parameter sets are recommended on the basis of a thorough and critical review of the existing literature. We tabulate the direct and indirect energy gaps, spin-orbit and crystal-field splittings, alloy bowing parameters, electron and hole effective masses, deformation potentials, elastic constants, piezoelectric and spontaneous polarization coefficients, as well as heterostructure band offsets. Temperature an...

Variationally optimized atomic orbitals for large-scale electronic structures

Abstract: A simple and practical method for variationally optimizing numerical atomic orbitals used in density functional calculations is presented based on the force theorem. The derived equation provides the same procedure for the optimization of atomic orbitals as that for the geometry optimization. The optimized orbitals well reproduce convergent results calculated by a larger number of unoptimized orbitals. In addition, we demonstrate that the optimized orbitals significantly reduce the computational effort in the geometry optimization, while keeping a high degree of accuracy.

Tuning carbon nanotube band gaps with strain.

Abstract: We show that the band structure of a carbon nanotube (NT) can be dramatically altered by mechanical strain. We employ an atomic force microscope tip to simultaneously vary the NT strain and to electrostatically gate the tube. We show that strain can open a band gap in a metallic NT and modify the band gap in a semiconducting NT. Theoretical work predicts that band gap changes can range between � 100 meV per 1% stretch, depending on NT chirality, and our measurements are consistent with this predicted range.

Creation of effective magnetic fields in optical lattices: the Hofstadter butterfly for cold neutral atoms

Abstract: We investigate the dynamics of neutral atoms in a 2D optical lattice which traps two distinct internal states of the atoms in different columns. Two Raman lasers are used to coherently transfer atoms from one internal state to the other, thereby causing hopping between the different columns. By adjusting the laser parameters appropriately we can induce a non-vanishing phase of particles moving along a closed path on the lattice. This phase is proportional to the enclosed area and we thus simulate a magnetic flux through the lattice. This set-up is described by a Hamiltonian identical to the one for electrons on a lattice subject to a magnetic field and thus allows us to study this equivalent situation under very well defined controllable conditions. We consider the limiting case of huge magnetic fields—which is not experimentally accessible for electrons in metals—where a fractal band structure, the Hofstadter butterfly, characterizes the system.

The electronic structure and spectral properties of ZnO and its defects

Abstract: The electronic structure of ZnO and its defects, which include intrinsic point defects and their complexes, have been calculated using full-potential linear Muffin-tin orbital method. According to our calculation data, the positions of the defect state levels have been determined in the energy band of ZnO. Based on the results above, we analysis the mechanism of the absorption and emission spectra of ZnO and discuss the effects of the electronic structure of complete ZnO and its defects on the spectral properties.

Theoretical study of the electronic structure, chemical bonding and optical properties of KNbO3 in the paraelectric cubic phase

Abstract: The electronic energy band structure, density of states (DOS) and charge density contour of KNbO3 in the paraelectric cubic phase have been studied using the full-potential linearized augmented plane wave method within the generalized gradient approximation for exchange and correlation. The band structure shows an indirect (R–Γ) band gap. From the DOS analysis as well as charge density studies, we find that the bonding between K and NbO3 is mainly ionic while that between Nb and O is covalent. We have also reported results on the pressure variation of the energy gap of this compound and found that the band gap increases with increasing pressure. In order to understand the optical properties of the perovskite, the real and imaginary parts of the dielectric function, reflectivity, absorption coefficient, optical conductivity, electron energy-loss function, refractive index and extinction coefficient were calculated. The general profiles of the optical spectra were analysed and origins of the structures discussed.

Three-dimensional band structure and bandlike mobility in oligoacene single crystals: A theoretical investigation

Abstract: Quantum-chemical calculations coupled with a tight binding band model are used to study the charge carrier mobilities in oligoacene crystals. The transfer integrals for all nonzero interactions in four crystalline oligoacenes (naphthalene, anthracene, tetracene, and pentacene) were calculated, and then used to construct the excess electron and hole band structures of all four oligoacene crystals in the tight binding approximation. From these band structures, thermal-averaged velocity–velocity tensors in the constant-free-time and the constant-free-path approximations for all four materials were calculated at temperatures ranging from 2 to 500 K. The bandwidths for these oligoacenes were found to be of the order of 0.1–0.5 eV. Furthermore, comparison of the thermal-averaged velocity–velocity tensors with the experimental mobility data indicates that the simple band model is applicable for temperatures only up to about 150 K. A small-polaron band model is also considered, but the exponential band narrowing effect is found to be incompatible to experimental power law results.

Photonic band structures solved by a plane-wave-based transfer-matrix method.

Abstract: Transfer-matrix methods adopting a plane-wave basis have been routinely used to calculate the scattering of electromagnetic waves by general multilayer gratings and photonic crystal slabs. In this paper we show that this technique, when combined with Bloch's theorem, can be extended to solve the photonic band structure for 2D and 3D photonic crystal structures. Three different eigensolution schemes to solve the traditional band diagrams along high-symmetry lines in the first Brillouin zone of the crystal are discussed. Optimal rules for the Fourier expansion over the dielectric function and electromagnetic fields with discontinuities occurring at the boundary of different material domains have been employed to accelerate the convergence of numerical computation. Application of this method to an important class of 3D layer-by-layer photonic crystals reveals the superior convergency of this different approach over the conventional plane-wave expansion method.

Band structure and fundamental optical transitions in wurtzite AlN

Abstract: With a recently developed unique deep ultraviolet picoseconds time-resolved photoluminescence (PL) spectroscopy system and improved growth technique, we are able to determine the detailed band structure near the Γ point of wurtzite (WZ) AlN with a direct band gap of 6.12 eV. Combined with first-principles band structure calculations we show that the fundamental optical properties of AlN differ drastically from that of GaN and other WZ semiconductors. The discrepancy in energy band gap values of AlN obtained previously by different methods is explained in terms of the optical selection rules in AlN and is confirmed by measurement of the polarization dependence of the excitonic PL spectra.

Photophysical properties and photocatalytic activities under visible light irradiation of silver vanadates

Abstract: α-AgVO3, β-AgVO3, Ag4V2O7 and Ag3VO4 prepared by precipitation and solid-state reactions showed intense absorption bands in the visible light region due to band gap transitions Comparison of a diffuse reflectance spectrum of α-NaVO3 with that of α-AgVO3 with a diopside-type structure revealed that a band gap (25 eV) of α-AgVO3 was 06 eV smaller than that (31 eV) of α-NaVO3 The electronic band structure study using a plane-wave based density functional method indicated that the decrease in the band gap of α-AgVO3 was due to Ag 4d orbitals which formed a valence band at a more negative level than O 2p orbitals Among α-AgVO3, β-AgVO3, Ag4V2O7 and Ag3VO4, only Ag3VO4 showed a photocatalytic activity for O2 evolution from an aqueous silver nitrate solution under visible light irradiation Holes photogenerated in Ag3VO4 can migrate to the reaction sites on the surface more easily than those of other silver vanadates, because the content of silver forming a valence band is large It resulted in that holes photogenerated in Ag3VO4 are able to oxidize H2O to form O2

Theory of optical processes in semiconductors : bulk and microstructures

Abstract: 1 INTRODUCTION 2 CLASSICAL THEORY OF OPTICAL PROCESSES 3 PHOTONS 4 ELECTRON BAND STRUCTURE AND ITS MODIFICATIONS 5 INTERBAND AND IMPURITY ABSORPTIONS 6 EXCITONIC ABSORPTION 7 ABSORPTION AND REFRACTION IN AN ELECTRIC FIELD 8 INTERBAND MAGNETO-OPTICAL EFFECTS 9 FREE CARRIER PROCESSES 10 RECOMBINATION PROCESSES 11 INTRODUCTION TO TWO-DIMENSIONAL SYSTEMS 12 OPTICAL PROCESSES IN QUANTUM WELLS 13 EXCITONS AND IMPURITIES IN QUANTUM WELLS 14 OPTICAL PROCESSES IN QUANTUM WIRES AND DOTS 15 SUPERLATTICES 16 STRAINED LAYERS 17 EFFECTS OF ELECTRIC FIELD ON LOW DIMENSIONAL SYSTEMS

Structural and doping effects in the half-metallic double perovskite A 2 CrWO 6 (A=Sr, Ba, and Ca)

Abstract: The structural, transport, magnetic, and optical properties of the double perovskite A2CrWO6 with A = Sr,Ba,Ca have been studied. By varying the alkaline earth ion on the A site, the influence of steric effects on the Curie temperature TC and the saturation magnetization has been determined. A maximum TC = 458 K was found for Sr2CrWO6 having an almost undistorted perovskite structure with a tolerance factor f~1. For Ca2CrWO6 and Ba2CrWO6 structural changes result in a strong reduction of TC. Our study strongly suggests that for the double perovskites in general an optimum TC is achieved only for f~1, that is, for an undistorted perovskite structure. Electron doping in Sr2CrWO6 by a partial substitution of Sr2+ by La3+ was found to reduce both TC and the saturation magnetization Ms. The reduction of Ms could be attributed both to band structure effects and the Cr/W antisites induced by doping. Band structure calculations for Sr2CrWO6 predict an energy gap in the spin-up band, but a finite density of states for the spin-down band. The predictions of the band structure calculation are consistent with our optical measurements. Our experimental results support the presence of a kinetic energy driven mechanism in A2CrWO6, where ferromagnetism is stabilized by a hybridization of states of the nonmagnetic W site positioned in between the high spin Cr sites.

Diluted II-VI Oxide Semiconductors with Multiple Band Gaps

Abstract: We report the realization of a new mult-band-gap semiconductor. Zn(1-y)Mn(y)OxTe1-x alloys have been synthesized using the combination of oxygen ion implantation and pulsed laser melting. Incorporation of small quantities of isovalent oxygen leads to the formation of a narrow, oxygen-derived band of extended states located within the band gap of the Zn(1-y)Mn(y)Te host. When only 1.3% of Te atoms are replaced with oxygen in a Zn0.88Mn0.12Te crystal the resulting band structure consists of two direct band gaps with interband transitions at approximately 1.77 and 2.7 eV. This remarkable modification of the band structure is well described by the band anticrossing model. With multiple band gaps that fall within the solar energy spectrum, Zn(1-y)Mn(y)OxTe1-x is a material perfectly satisfying the conditions for single-junction photovoltaics with the potential for power conversion efficiencies surpassing 50%.

A coherent three-dimensional Fermi surface in a high-transition-temperature superconductor

Abstract: All conventional metals are known to possess a three-dimensional Fermi surface, which is the locus in reciprocal space of the long-lived electronic excitations that govern their electronic properties at low temperatures. These excitations should have well-defined momenta with components in all three dimensions. The high-transition-temperature (high-T(c)) copper oxide superconductors have unusual, highly two-dimensional properties above the superconducting transition. This, coupled with a lack of unambiguous evidence for a three-dimensional Fermi surface, has led to many new and exotic models for the underlying electronic ground state. Here we report the observation of polar angular magnetoresistance oscillations in the overdoped superconductor Tl2Ba2CuO6+delta in high magnetic fields, which firmly establishes the existence of a coherent three-dimensional Fermi surface. Analysis of the oscillations reveals that at certain symmetry points, however, this surface is strictly two-dimensional. This striking form of the Fermi surface topography, long-predicted by electronic band structure calculations, provides a natural explanation for a wide range of anisotropic properties both in the normal and superconducting states. Our data reveal that, despite their extreme electrical anisotropy, the high-T(c) materials at high doping levels can be understood within a framework of conventional three-dimensional metal physics.

Superconductivity phase diagram of Na(x)CoO2*1.3H2O.

Abstract: The microscopic origin of superconductivity in the high-transition-temperature (high-Tc) copper oxides remains the subject of active inquiry; several of their electronic characteristics are well established as universal to all the known materials, forming the experimental foundation that all theories must address. The most fundamental of those characteristics, for both the copper oxides and other superconductors, is the dependence of the superconducting Tc on the degree of electronic band filling. The recent report of superconductivity1 near 4 K in the layered sodium cobalt oxyhydrate, Na0.35CoO2·1.3H2O, is of interest owing to both its triangular cobalt–oxygen lattice and its generally analogous chemical and structural relationships to the copper oxide superconductors. Here we show that the superconducting Tc of this compound displays the same kind of behaviour on chemical doping that is observed in the high-Tc copper oxides. Specifically, the optimal superconducting Tc occurs in a narrow range of sodium concentrations (and therefore electron concentrations) and decreases for both underdoped and overdoped materials, as observed in the phase diagram of the copper oxide superconductors. The analogy is not perfect, however, suggesting that NaxCoO2·1.3H2O, with its triangular lattice geometry and special magnetic characteristics, may provide insights into systems where coupled charge and spin dynamics play an essential role in leading to superconductivity.

Self-guiding in two-dimensional photonic crystals.

Abstract: Dielectric periodic media can possess a complex photonic band structure with allowed bands displaying strong dispersion and anisotropy. We show that for some frequencies the form of iso-frequency contours mimics the form of the first Brillouin zone of the crystal. A wide angular range of flat dispersion exists for such frequencies. The regions of iso-frequency contours with near-zero curvature cancel out diffraction of the light beam, leading to a self-guided beam.

Electronic Band Structure and Photocatalytic Activity of Ln2Ti2O7 (Ln = La, Pr, Nd)

Abstract: Photocatalytic activity in the water splitting of Ln2Ti2O7 (Ln = La, Pr, Nd) with a layered structure was highly dependent on their electronic band structure. The conduction band of La2Ti2O7 consisted mainly of Ti 3d and La 5d, whereas the valence band consisted mainly of O 2p and Ti 3d. The empty La 4f level was found to be located ca. 3.6 eV above the bottom of the conduction band from both the electronic band-structure calculation and the XPS measurement. The occupied and unoccupied Ln 4f level in Ln2Ti2O7 was shifted to lower energy as the number of 4f electrons increased. This shift of the Ln 4f band made it possible for the band-gap energy of both Pr2Ti2O7 and Nd2Ti2O7 to be decreased. For Nd2Ti2O7, the unoccupied Nd 4f level located between the conduction band and the valence band was found to be detrimental to photocatalytic activity in water splitting because it could act as an electron-trapping site.

Ratio problem in single carbon nanotube fluorescence spectroscopy

Abstract: The electronic band gaps measured in fluorescence spectroscopy on individual single wall carbon nanotubes isolated within micelles show significant deviations from the predictions of one electron band theory. We resolve this problem by developing a theory of the electron-hole interaction in the photoexcited states. The one-dimensional character and tubular structure introduce a novel relaxation pathway for carriers photoexcited above the fundamental band edge. Analytic expression for the energies and line shapes of higher subband excitons are derived, and a comparison with experiment is used to extract the value of the screened electron-hole interaction.

First-principles study of optical properties of barium titanate

Abstract: The optical properties of perovskite barium titanate in the core-level spectra are investigated by the first principles under scissor approximation. There are nine peaks at the curve of the imaginary part of dielectric function. The optical spectra are assigned to interband contribution from O 2p valence bands to Ti 3d conduction bands in the low-energy region and outer core electron excitation (core level excitation) from near valence band semicore levels Ba 5p and O 2s to conduction band in the high-energy region. In contrast to the calculated results by the tight-binding linear muffin-tin orbitals method, our results are in better agreement with the experimental results.

Density functional theory investigation of the geometric and spintronic structure of h -BN/Ni(111) in view of photoemission and STM experiments

Abstract: The $(1\ifmmode\times\else\texttimes\fi{}1)$ commensurate layer of hexagonal boron nitride on nickel is investigated by density functional theory calculations. The full-potential linear-augmented-plane-wave method is used to obtain total energies for different structural models, the spin-resolved band structure, and local density of states (LDOS) above the surface. The calculations confirm the accepted structure model of a corrugated layer with nitrogen placed at on-top sites. From the remaining two possibilities for boron the one with boron on fcc hollow sites is energetically slightly favored, but the energy difference to the structure with boron on the hcp hollow site is smaller than the thermal energy during h-BN synthesis. The spin-resolved electronic (spintronic) band structure indicates h-BN to be an insulator. The calculated band structure of the $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ bands agrees well with photoemission data. The polarization-induced charge transfer from h-BN to Ni reduces the magnetic moment in the Ni top layer below the bulk value. The LDOS in front of the surface indicates distinct atomic sites in scanning tunneling microscopy (STM) images which are complemented with STM images. From the distance dependence of the LDOS for Ni(111) and h-BN/Ni(111) the apparent height of heteronuclear atomic steps may be derived. From its spin and energy dependence we propose tunneling experiments with strong magnetic contrast.

Stability and electronic structure of the (1×1) SrTiO 3 (110) polar surfaces by first-principles calculations

Abstract: The electronic and atomic structures of several $(1\ifmmode\times\else\texttimes\fi{}1)$ terminations of the (110) polar orientation of the ${\mathrm{SrTiO}}_{3}$ surface are systematically studied by first-principles calculations. The electronic structure of the two stoichiometric SrTiO and ${\mathrm{O}}_{2}$ terminations are characterized by marked differences with respect to the bulk, as a consequence of the polarity compensation. In the former, the Fermi level is located at the bottom of the conduction band, while in the latter the formation of a peroxo bond between the two surface oxygens results in a small-gap insulating surface with states in the gap of the bulk projected band structure. We also consider three nonstoichiometric terminations with TiO, Sr, and O compositions, respectively, in the outermost atomic layer, which automatically allows the surface to be free from any macroscopic polarization. They are all insulating. The cleavage and surface energies of the five terminations are computed and compared, taking into account the influence of the chemical environment as a function of the relative richness in O and Sr. From our calculations, it appears that some (110) faces can even compete with the ${\mathrm{TiO}}_{2}$ and SrO terminations of the (100) cleavage surface: in particular, the (110)-TiO termination is stable in Sr-poor conditions, the (110)-Sr one in simultaneously O- and Sr-rich environments. The available experimental data are compared to the outcomes of our calculations and discussed.

Optical properties and electronic structure of rock-salt ZnO under pressure

Abstract: This letter reports on the pressure dependence of the optical absorption edge of ZnO in the rock-salt phase, up to 20 GPa. Both vapor-phase monocrystals and pulsed-laser-deposition thin films on mica have been investigated. Rock-salt ZnO is shown to be an indirect semiconductor with a band gap of 2.45±0.15 eV, whose pressure coefficient is very small. At higher photon energies, a direct transition is observed (4.6 eV at 10 GPa), with a positive pressure coefficient (around 40±3 meV/GPa between 5 and 19 GPa). These results are interpreted on the basis of first-principles electronic band structure calculations.

Lattice solitons in Bose-Einstein condensates

Abstract: We systematically study the properties of lattice solitons in Bose-Einstein condensates with either attractive or repulsive atom interactions. This is done, by exactly solving the mean-field Gross-Pitaevskii equation in the presence of a periodic potential. We find new families of lattice soliton solutions that are characterized by the position of the energy eigenvalue within the associated band structure. These include lattice solitons in condensates with either attractive or repulsive atom interactions that exist in finite or semi-infinite gaps, as well as nonlinear modes that exhibit atomic population cutoffs.

Theory for elastic wave scattering by a two-dimensional periodical array of cylinders: An ideal approach for band-structure calculations

Abstract: We extend the multiple-scattering theory to elastic waves propagating in two-dimensional periodical composites. The formalism for the band-structure calculation is presented by taking into account the full vector character of the elastic waves. Besides the efficiency in conventional band structure calculation for elastic waves, the approach shows special advantages in handling the systems with mixing solid and fluid components, for which the conventional plane-wave approach fails. As the applications of the formalism, we calculate the band structure for systems with lead cylinders in epoxy matrix, steel cylinders in water, and water-filled cylindrical holes in steel matrix.

Band structure, elementary excitations, and stability of a Bose-Einstein condensate in a periodic potential

Abstract: We investigate the band structure of a Bose-Einstein condensate in a one-dimensional periodic potential by calculating stationary solutions of the Gross-Pitaevskii equation, which have the form of Bloch waves. We demonstrate that loops (``swallow tails'') in the band structure occur both at the Brillouin zone boundary and at the center of the zone, and they are therefore a generic feature. A physical interpretation of the swallow tails in terms of periodic solitons is given. The linear stability of the solutions is investigated as a function of the strength of the mean-field interaction, the magnitude of the periodic potential, and the wave vector of the condensate. The regions of energetic and dynamical stability are identified by considering the behavior of the Gross-Pitaevskii energy functional for small deviations of the condensate wave function from a stationary state. It is also shown how for long-wavelength disturbances the stability criteria may be obtained within a hydrodynamic approach.

Electronic Response and Bandstructure Modulation of Carbon Nanotubes in a Transverse Electrical Field

Abstract: The electronic properties of carbon nanotubes (NTs) in a uniform transverse field are investigated within a single orbital tight-binding (TB) model. For doped nanotubes, the dielectric function is found to depend not only on symmetry of the tube, but also on radius and Fermi level position. Band gap opening/closing is predicted for zigzag tubes, while it is found that armchair tubes always remain metallic, which is explained by the symmetry in their configuration. The bandstructures for both types are considerably modified when the field strength is large enough to mix neighboring subbands.