Ab initio electronic band structure study of III–VI layered semiconductors

We present a total energy study of the electronic properties of the rhombohedral γ-InSe, hexagonal ε-GaSe, and monoclinic GaTe layered compounds. The calculations have been done using the full potential linear augmented plane wave method, including spin-orbit interaction. The calculated valence bands of the three compounds compare well with angle resolved photoemission measurements and a discussion of the small discrepancies found has been given. The present calculations are also compared with recent and previous band structure calculations available in the literature for the three compounds. Finally, in order to improve the calculated band gap value we have used the recently proposed modified Becke-Johnson correction for the exchange-correlation potential.     

Calculation of the magnetization and total energy of vanadium as a function of lattice parameter

Self-consistent spin-polarized APW calculations have been performed to determine the energy band structure of metallic vanadium in an assumed ferromagnetic b.c.c. structure as a function of lattice parameter. The statistical exchange (‘Xα’) and muffin-tin approximations were used. At each lattice parameter for which a calculation was performed, the Xα cohesive energy, the pressure, and the magnetization were calculated. The calculated cohesive energy and pressure agree fairly well with experiment. The calculations also correctly predict the absence of a magnetic moment for vanadium at its equilibrium lattice constant. However, a nonmagnetic-to-magnetic transition is found to occur abruptly at a lattice constant which is about a factor of 1·25 larger than the equilibrium value, and which is in good qualitative agreement with the appearance of a local magnetic moment in certain vanadium alloys.     

The effect of vacancies on the electronic structure of Nbo

The crystal structure of NbO is considered as a NaCl structure with 25% ordered vacancies in each sublattice. Self-consistent energy band structure calculations using the augmented plane wave (APW) method have been carried out for a hypothetical NbO with the full NaCl structure and for the structure with the vacancies. We find that the introduction of the vacancies leads to (1) a lowering of the Nb-d like bands with respect to the O-p bands, (2) an almost completely filled “vacancy band”, and (3) a splitting and broadening of the O-p like band. These effects are shown to have a significant influence on both the theoretically calculated X-ray photoemission spectra (XPS) and the X-ray emission spectra (XES). All theoretical spectra obtained with the vacancy-structure are found to be in good agreement with experiments in contrast to those spectra calculated for hypothetical NbO in the NaCl structure.     

Electronic structure of LiGaS2

X-ray photoelectron spectroscopy (XPS) measurement has been performed to determine the valence band structure of LiGaS₂ crystals. The experimental measurement is compared with the electronic structure obtained from the density functional calculations. It is found that the Ga 3d states in the XPS spectrum are much higher than the calculated results. In order to eliminate this discrepancy, the LDA+U method is employed and reasonable agreement is achieved. Further calculations show that the difference of the linear and nonlinear optical coefficients between LDA and LDA+U calculations is negligibly small, indicating that the Ga 3d states are actually independent of the excited properties of LiGaS₂ crystals since they are located at a very deep position in the valence bands.     

Electronic structure of binary zirconium compounds: Cluster calculations using different molecular orbital methods

Cluster calculations of the electronic structure and charge distribution in the refractory compounds ZrX(X = C, N, O) and ZrO₂ have been performed using different molecular orbital methods; a semiempirical LCAO model based on the Mulliken-Wolfsberg-Helmholz approximation, and the Hartree-Fock-Slater model in both the Discrete Variational X_α and Multiple Scattering X_α versions. The main features of chemical bonding are discussed and illustrated by contour level diagrams of some bonding molecular orbitais of the valence band. Finally, the results are compared with X-ray emission and X-ray photoelectron spectra, and also with band structure calculations.     

First-principles calculation of the energy of compressed calcium

The energy of a calcium crystal with a simple cubic lattice as a function of the ratio (t/U) between two internal parameters of the Hubbard model has been calculated using the Hubbard model for the s bands, equations of motion, and direct algebraic method. The electronic spectra have been calculated for the 4s band of the crystal in two principal symmetry directions of the first Brillouin zone. The calculations have been performed at temperatures T ₁ = 0 K and T ₂ = 1000 K. All calculations have been carried out for different interaction energies U of s electrons, one angle, and their different concentrations n in the range 0 ≤ n ≤ 2. The calculations have demonstrated that the dependences of the energy and electronic spectra in this compressed state are very smooth. The occupation of the Ca 4s band is in good agreement with the results of the pioneering calculations of compressed Ca (and a number of other metals), which were carried out by Gandel’man and his colleagues in the Wigner-Seitz spherical cell approximation. It has been shown that the performed analysis accurately reproduces the data obtained on the superconductivity in terms of the Bardeen-Cooper-Schrieffer theory if the 4s band is half-occupied.     

Location of direct transitions on the Λ line of the band structure of the noble metals using angle resolved photoelectron spectroscopy (ARUPS)

The triangulation method of angle-resolved photoemission for the absolute localization of direct transitions in k-space has been used on (110) and (111) faces in the ΓKLUX or (1$\text{1}$0) mirror plane in order to determine some points of the electronic bulk band structure of Cu, Ag and Au along the Λ line. It is found that the experimental points for the occupied bands and the unoccupied bands are in good agreement with recent ground state band structure calculations based on the local density functional theory.     

Reduction in the electronic band gap of titanium oxide nanotubes

The structural and electronic properties of individual titanium oxide nanotubes have been studied using both empirical and ab initio calculations. Two different types of titanium oxide nanotubes (A-nanotube and B-nanotube) have been constructed and energy-minimized by molecular mechanics calculations. We found that the A-nanotubes are energetically more favorable than the B-nanotubes. The electronic band structure of the titanium oxide nanotubes was also calculated with respect to the tubule diameter and the tubule type using the ab initio method. The band gap of the A-nanotube was reduced by up to 60% as the tubule diameter decreases from 1.2 nm to 0.5 nm.     

Band structure and optical spectra of RbNH4SO4 crystals

First-principal density functional theory (DFT) calculations of the band structure, density of states and dielectric functions ε(E) of the rubidium ammonium sulfate (RAS) crystal, RbNH₄SO₄, in the orthorhombic phase Pnma have been carried out using the CASTEP code. Valence electron bands of the crystal are flat in k-space, that responds to the relatively great effective mass, m*?5m_e. The top valence band of the crystal has been found to be the most flat, what might be an evidence of a weak chemical bonding of the sulfate complexes (SO₄) in the crystal and therefore for the predisposition to structural instability and phase transitions. The characteristic feature is that two top valence bands are originated almost entirely from p-electrons of oxygen. The bottom part of the conduction band is formed mainly by the hydrogen atoms, the higher parts of this band—by a mixed set of chemical elements and orbital moments. The calculated refractive indices in the range of crystal's transparency agree satisfactorily with the experiment considering that the infrared absorption is not taken into account in calculations.     

Consistency tests of Ampcalculator and chiral amplitudes in SU(3) Chiral Perturbation Theory: A tutorial-based approach

The positive-parity yrast bands of ^{79, 81, 83, 85, 87, 89}Y isotopes have been studied using the projected shell model (PSM). Nuclear-structure properties like yrast spectra, transition energies, band diagrams, kinetic moment of inertia, rotational frequencies and reduced transition probabilities (B(M1) and B(E2) are calculated. The results obtained from the PSM calculations are also compared with the available experimental as well as theoretical data and, in general, a reasonable agreement is obtained between them. Calculations in the present work also predict that these isotopes have multi-quasiparticle structure.     

Electronic structure and optical properties of anatase doped with bismuth and carbon

The calculations of the electronic structure of pure anatase and the anatase doped with carbon and/or bismuth have been carried out using the ab initio tight-binding linear muffin-tin orbital (TB-LMTO) method in the local spin density approximation with the inclusion of single-site Coulomb correlations (LSDA + U). The dielectric function, absorption coefficient, and refractive index have been calculated in the random phase approximation. It has been found that, upon doping, narrow bands of carbon and bismuth impurity states are formed in the band gap. The calculations of the optical absorption coefficient have demonstrated that the C,Bi-doping can lead to the absorption in the visible region and an enhancement of the absorption in the near-ultraviolet region. Therefore, the C,Bi-doping can increase the photocatalytic activity on the surface of doped anatase.     

Collective and two-particle excitations in78Se

Excited states in⁷⁸Se have been studied up to spin (12)? at about 5.8 MeV in the⁷⁶Ge(α, 2n) reaction using in-beamγ-ray spectroscopy. Mean lifetimes could be determined for 27 of the 33 levels observed by applying Doppler shift and pulsed-beam timing methods. According to theB(E2) values most of the levels have been grouped into collective bands. Irregularities in the level spacings of the yrast band above spin 6? are interpreted to be due to the interaction of the ground state band withg _9/2 two-proton and two-neutron excitations. The mutual mixing of these configurations is reflected by strongM 1 transitions between the mixed states. The interaction strengths between the configurations involved have been estimated from three-band mixing calculations.     

Photonic band structure and effect of ε and μ on the reflectivity of one-dimensional magnetic photonic crystal structure

In this paper, we study the photonic band structure and reflection properties in one-dimensional magnetic photonic crystals (MPCs). Investigation of dispersion characteristics shows that in the case of MPCs, photonic band gaps arise due to the contrast in the wave impedance, not due to the contrast in the refractive index, while contrast in the refractive index of the two layers decides the position and number of the band gaps. We also study the effect of permittivity and permeability on reflection bands, which shows that the structure that has larger values of magnetic permeability (μ) than dielectric permittivity (ε) have wider TM-reflection bands, whereas the structure for which ε is greater than μ has wider TE-reflection bands. But the gap to mid-gap frequency ratio for TM-reflection bands is larger than TE-reflection bands. Thus, magnetic permeability has greater impact on the reflectivity of MPCs than dielectric permittivity. Finally, the analysis of the omni-reflectance in MPCs has also been studied.     

Temperature dependence of photonic crystals based on thermoresponsive magnetic fluids

The influence mechanisms of temperature on the band gap properties of the magnetic fluids based photonic crystals are elaborated. A method has been developed to obtain the temperature-dependent structure information (A_sol/A) from the existing experimental data and then two critical parameters, i.e. the structure ratio (d/a) and the refractive index contrast (Δn) of the magnetic fluids photonic crystals are deduced for band diagram calculations. The temperature-dependent band gaps are gained for z-even and z-odd modes. Band diagram calculations display that the mid frequencies and positions of the existing forbidden bands are not very sensitive to the temperature, while the number of the forbidden bands at certain strengths of magnetic field may change with the temperature variation. The results presented in this work give a guideline for designing the potential photonic devices based on the temperature characteristics of the magnetic fluids based photonic crystals and are helpful for improving their quality.     

Structure of the doubly odd nucleus 180Ta: Description of 23 bands

The structure of the doubly-odd nucleus ¹⁸⁰Ta has been studied by γ–γ coincidence measurements with a DC beam at 52 and 57 MeV and time-correlated γ–γ coincidence measurements with a pulsed beam at 55 MeV via the ¹⁷⁶Yb(¹¹B, α3n)¹⁸⁰Ta reaction. In all measurements, γ-rays were detected in coincidence with charged particles. In the time-correlated γ–γ coincidence measurements with a pulsed ¹¹B beam, three rotational bands and one octupole vibrational band have been identified above the I^π=15⁻ T_1/2=30 μs isomer. The configuration of three bands built on 8⁺ states has been discussed by means of three-band mixing calculations. BCS calculations with blocking have been used in support of configuration assignment of four- and six-quasiparticle structures. Totally, 19 rotational bands, one β-, one γ- and two octupole-vibrational bands, plus one intrinsic state have been identified with two-, four- and six-quasiparticle configurations. The K values of these bands range from 0 to 19. The K-forbidden transition rates are discussed on the basis of mixing between states with widely different K-values. The BBCS calculations predict a K^π=22⁻ isomer not identified experimentally in this nor in previous works.A search for specific intermediate states which could explain the transformation from K^π=9⁻ to 1⁺ during the astrophysical s- and r- processes was negative.     

First-principles study of electronic structure and magnetism of cubic Al1−xErxN using the LSDA+U approach

First-principles calculations, by means of the full-potential augmented plane wave method using the LSDA+U approach (local spin density approximation with Hubbard-U corrections), have been carried out for the electronic structure of the Al_0.75Er_0.25N. The LSDA+U method is applied to the rare-earth 4? states. We have investigated the electronic and magnetic properties.The Al_0.75Er_0.25N is shown to be a semiconductor, where the filled ? states are located in the valence bands and the empty ones above the conduction band edge. The magnetic interaction of the rare-earth ion with the host states at the valence and conduction band edges has been investigated and discussed.     

Band structure of surface states and resonances for clean Cu(1 1 0) surface

We have used the layer KKR method to calculate the Shockley and Rydberg surface states and resonances for Cu(1 1 0) for a given model of the surface potentials. This method has not been used before to predict all of the surface band structure for the energy range from the bottom of the conduction band to ∼7 eV above the vacuum level. The previous methods that used only local electron interactions in ab initio calculations could not produce the Rydberg surface barrier bands while those relying on nearly-free-electron parameterisation of bands could not deal with d-bands.     

On the nature of the “free electron pair” on phosphorus in aromatic phosphorus compounds: The photoelectron spectrum of 2-phosphanaphthalene

The electronic structure of the 2-phosphanaphthalene system has been studied by photoelectron spectroscopy. The assignment of bands in the PE spectra of 2-phosphanaphthalene and 3-methyl-2-phosphanaphthalene has been performed with the aid of two useful generalisations, one pertaining to the energies of the the π ionizations, and the other to the phosphorus n ionization, as well as by calculations using the extended CNDO/S method. The n ionization of the 2-phosphanaphthalenes can be observed as a well separated single band. From the breadth of this n band the “lone pair” is seen to have considerable bonding character resulting from orbital mixing with other σ-orbitals of the molecule. This appears to be the first evidence of appreciable delocalization of ldlone pair” phosphorus electrons in aromatic molecules, which, judging from the breadth of the bands, is almost as large as with pyramidal phosphorus atoms.     

Measurements and calculations of self-broadening coefficients of lines belonging to the v2 band of H216O

The self-broadening coefficients of 150 lines belonging to the v₂ band of H₂¹⁶O between 1770 and 2250 cm^-1 have been measured using Fourier transform spectra (resolution ≈ 0.005 cm^-1). The four different methods which have been used to deduce the self-broadening coefficients from experiment are described in detail. The estimated average uncertainty is about 15% and varies from 7 to 30%, depending on the method used and on the line involved. Two theoretical calculations, one based on the Anderson-Tsao-Curnutte method and the other on the recent method proposed by Davies, have been performed, retaining only the dipole-dipole interaction. For some lines of the v₂ band and for some pure rotation lines, calculations based on other formalisms have also been performed. For all of these calculations, we have used accurate spectroscopic data: precise energy levels, realistic wavefunctions, and a complete dipole-moment operator expansion in order to compute the transition probabilities. As compared to the previously calculated values of the pioneering work of Benedict and Kaplan, where the Anderson-Tsao-Curnutte method was used, our calculations show improvements by about 14% in the agreement between measured and calculated self-broadening coefficients.     

Microscopic study of low-lying collective bands in77Kr

The structure of the collective bands in⁷⁷Kr is investigated within our deformed shell model (DSM) based on Hartree-Fock states. The different levels are classified into collective bands on the basis of their B(E2) values. The calculatedK = 5/2⁺ ground band agrees reasonably well with the experiment. An attempt has been made to study the structure of the 3-quasiparticle band based on large J state in this nucleus. The calculated collective bands, the B(E2), and B(M1) values are compared with available experimental data. The nature of alignments in the low-lying bands is also analyzed.