Electronic structure of the CuBS2 crystal

The band structure and spectra of the total and projected densities of states of a new crystal of the chalcopyrite family, namely, CuBS₂, have been calculated in terms of the density functional theory. It has been found that the crystal is a pseudo-direct-band-gap semiconductor, and the best theoretical estimate of the optical band gap is 3.44 eV. The upper valence band of the CuBS₂ crystal basically consists of the contributions from the p states of S atoms and the d states of Cu atoms. The crystal splitting is 0.2 eV. The bottom of the conduction band is basically formed by the sp states of boron and sulfur atoms with an admixture of the s states of copper atoms.     

Specific features of the electronic structure and spectral properties of NdNi5 − x Cu x compounds

The spectral properties of the intermetallic compounds NdNi_{5 ? x} Cu _x (x = 0, 1, 2) have been studied using optical ellipsometry in the wavelength range 0.22–16 μm. It has been established that substitution of copper atoms for nickel leads to noticeable changes in the optical absorption spectra, plasma frequencies, and relaxation frequencies of conduction electrons. Spin-polarized calculations of the electronic structure of these compounds have been performed in the local spin density approximation allowing for strong electron correlations (LSDA + U method) in the 4f shell of the rare-earth ion. The calculated electron densities of states have been used to interpret the experimental dispersion curves of optical conductivity in the interband light absorption region.     

Specific features of the behavior of the optical properties of TbNi<Subscript>5 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> intermetallic compounds

The optical properties of the TbNi_{5 ? x} Cu _x intermetallic compounds have been investigated in the spectral range 0.08–5.64 eV by the ellipsometric method. It is shown that substitution of nickel for copper atoms leads to a significant change in the frequency dependence of the optical conductivity; this change is related to modification of the electronic spectrum. The formation of a new interband absorption band has been revealed, whose intensity increases with an increase in the copper content. The concentration dependences of the plasma and relaxation frequencies of conduction electrons in the compounds under study are determined. Self-consistent calculation of the electronic structure of the TbNi₅ binary compound has been performed in the approximation of local electron spin density. The electron density of states for two spin projections and the optical conductivity of this compound have been calculated.     

Effect of doping by boron,carbon, and nitrogen atoms on the magnetic and photocatalytic properties of anatase

The effect of doping of titanium dioxide with the anatase structure by boron, carbon, and nitrogen atoms on the magnetic and optical properties and the electronic spectrum of this compound has been investigated using the ab initio tight-binding linear muffin-tin orbital (TB-LMTO) band-structure method in the local spin density approximation explicitly including Coulomb correlations (LSDA + U) in combination with the semiempirical extended Hückel theory (EHT) method. The LSDA + U calculations of the electronic structure, the imaginary part of the dielectric function, the total magnetic moments, and the magnetic moments at the impurity atoms have been carried out. The diagrams of the molecular orbitals of the clusters Ti₃ X (X = B, C, N) have been calculated and the pseudo-space images of the molecular orbitals of the clusters have been constructed. The effect of doping on the nature and origin of photocatalytic activity in the visible spectral range and the specific features of the generation of ferromagnetic interactions in doped anatase have been discussed based on the analysis of the obtained data. It has been shown that, in the sequence TiO_{2 ? y} N _y → TiO_{2 ? y} C _y → TiO_{2 ? y} B _y (y = 1/16), the photocatalytic activity can increase with the generation of electronic excitations with the participation of impurity bands. The calculated magnetic moments for boron and nitrogen atoms are equal to 1 μ_B, whereas the impurity carbon atoms are nonmagnetic.     

Structural and optical properties of the M@C59X cages (X=N,B and M=Li,Na)

Using B3LYP/6-31G* density functional level of theory, the structural and optical properties of the C₆₀ and M@C₅₉X cages have been investigated. Results indicate that the charge on C atoms and band gap of C₆₀ cage are changed dramatically with the substitution of one B or N atom at one of the C sites and the Li and Na atom encapsulations in the C₆₀ cage. The Mulliken analyses show that the charge is transferred completely between the alkali atoms and the C₅₉X cage. The substitutional and encapsulation doping (SED) reduce the optical gaps of the C₆₀ cage. Also, the oscillator strengths of the absorption peaks are dependent on dopant types.     

Studying the possibility of deep laser cooling of <Superscript>24</Superscript>Mg atoms in an optical lattice: Two-level quantum model

The possible deep laser cooling of ²⁴Mg atoms in a deep optical lattice in the presence of an additional pumping field resonant to the narrow 3s3s¹S₀ → 3s3p³P₁ (λ = 457 nm) optical transition is studied. Two quantum models of the laser cooling of atoms in the optical trap are compared. One is based on the direct numerical solution to the kinetic quantum equation for an atomic density matrix; it considers both optical pumping and quantum recoil effects during interaction between the atoms and field photons. The second, simplified model is based on decomposing the states of the atoms over the levels of vibration in the optical trap and analyzing the evolution of these states. The comparison allows derivation of optical field parameters (pumping field intensity and detuning) that ensure cooling of the atoms to minimal energies. The conditions for fast laser cooling in an optical trap are found.     

Optical spectroscopy and electronic structure of the Er5Ge3 compound

The results of the study of the optical properties and electronic structure of the Er₅Ge₃ compound have been presented. In the wavelength range of 0.22–15 μm (0.083–5.64 eV), the optical constants have been measured, and the spectral and electronic characteristics have been determined. The spin-polarization calculations of the band spectrum have been performed in the local electron spin density approximation (LSDA) with a correction for strong correlations in the 4f shell of the rare-earth atom (LSDA + U method). The main features of the experimental dispersion dependence of the optical conductivity in the region of quantum light absorption have been interpreted based on the results of calculations of the electron density of states.     

Structural and optical properties of lithium borobismuthate glasses

Glasses with compositions 25Li₂O-(75−x)Bi₂O₃-x B₂O₃, with 0?x?30 mol%, have been prepared using the melt quenching technique. The density and the molar volume have been determined. IR spectroscopy is used as a structural probe of the nearest neighbor environment in the glass network. The optical transmittance and reflectance spectrum of the glasses have been recorded in the wavelength range 400-1100 nm. The values of the optical band gap E_g^opt for indirect transition and refractive index have been determined for 0?x?30 mol%. The average electronic polarizability of the oxide ion α_o²⁻ and the optical basicity have been estimated from the calculated values of the refractive indices. Variations in the different physical parameters such as the density, molar volume, optical band gap, refractive index, average electronic polarizability of the oxide ion and optical basicity with B₂O₃ content have been analyzed and discussed in terms of the changes in the glass structure.     

Transient optical absorption and luminescence in <Emphasis Type="Italic">A</Emphasis>Pb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> (<Emphasis Type="Italic">A</Emphasis> = K,Rb) crystals

This paper reports on a study of transient optical absorption and pulsed cathodoluminescence in APb₂Cl₅ (A = K, Rb) in the visible and ultraviolet spectral regions. The measurements performed by absorption optical spectroscopy with nanosecond time resolution showed the transient optical absorption of APb₂Cl₅ to derive from optical transitions in hole centers, and that the optical density relaxation kinetics is mediated by interdefect tunneling recombination in complementary pairs which involves Frenkel defects on the cation sublattice and self-trapped carriers. The slow components in the transient optical absorption decay kinetics, with characteristic times ranging from a few ms to seconds, have been assigned to diffusion-mediated annihilation of interstitial atoms with alkali metal vacancies. The mechanisms underlying creation and relaxation of the short-lived Frenkel defects on the cation sublattice and self-trapped carriers have been analyzed.     

Theoretical analysis of structure, thermodynamic properties, and optical properties of Ge-doped amorphous SiO2

Different stable geometric configurations of Ge doped amorphous SiO₂ (a-SiO₂) system, originating from one, two, or three Si atoms in various places of the a-SiO₂ substituted by Ge atoms randomly have been investigated using interatomic potentials in this work. The most stable structures have been identified and corresponding evolutional rules obtained. The structural growth pattern for Ge-doped a-SiO₂ system is that Ge atoms tend to spread far away from each other and keep away from the center. Furthermore, the thermodynamic properties including speci?c heat, Debye temperature, vibrational entropy, and so on are calculated from the structure with 16 Si atoms of the constructed a-SiO₂ cell replaced by Ge atoms and with the biggest Ge-Ge distance. It can be seen that entropy of Ge doped system with larger specific heat is higher than that of the pure system with smaller specific heat. At last, optical properties including optical absorption spectrum and electron energy loss function of nGe-doped a-SiO₂ (n=0-3, 8) system is also obtained.     

Enlightening the stable ferromagnetic phase of SrAO3(A= Cr,Fe and Co) compounds using spin polarized quantum mechanical approach

This is an investigation of the atomic structure, opto-electronic and magnetic properties of SrAO₃ (A = Cr, Fe and Co) compounds using first principles method. The ferromagnetic behavior is found the most stable phase for SAO. The calculated Goldschmidt's tolerance factor values predict that the studied compounds have a stable structure. Moreover, the calculated formation energy shows that SrAO₃compounds are thermodynamically stable. The calculated density of states shows that the present compounds are metal and the direction of the magnetic moments of SrCrO₃ is anti parallel to its spin. The charge density contours display a mixture of the covalent and ionic bonds between the content atoms of SrAO₃ compounds. The optical parameters are calculated using the dielectric function real and imaginary parts. From the electronic and optical properties results, beneficial industrial applications can be expected for the present compounds.     

Effect of Bi2O3 content on physical and optical properties of 15Li2O-15K2O-xBi2O3-(65−x) B2O3: 5V2O5 glass system

Multi-component bismuth borate glasses doped with vanadium ions 15Li₂O-15K₂O-xBi₂O₃-(65−x) B₂O₃: 5V₂O_5, (x=3, 5, 7, 10, 12 and 15) have been prepared using conventional melt quench technique. Characterization of the prepared glasses has been done using X-ray diffraction, differential scanning calorimetry and density measurements. The effect of Bi₂O₃ content on the optical properties of the present glass system is studied from the optical absorption spectra recorded in the wavelength range 200-800 nm. The fundamental absorption edge has been identified from the optical absorption spectra. The values of optical band gap for indirect allowed transitions have been determined using available theories. The origin of the Urbach energy is associated with the phonon-assisted indirect transitions. The density and molar volume studies indicate that Bi₂O₃ in these glasses is acting partly as network modifier and partly as network former. The variations in the optical band gap energies, density and molar volume with Bi₂O₃ content have been discussed in terms of changes in the glass structure. Values of the theoretical optical basicity, average crosslink density and the average electronic polarizability are also reported.     

Electronic structure of the TbMn<Subscript>0.33</Subscript>Ge<Subscript>2</Subscript> compound: Band calculation and optical experiment

The results of the investigation of the electronic structure and optical properties of the TbMn_0.33Ge₂ compound have been presented. The spin-polarized calculations of the band spectrum have been performed within the framework of the local spin density approximation (LSDA) with a correction for strong correlations in the 4f shell of the rare-earth ion (the LSDA + U method). The optical constants have been measured using the ellipsometric method and a number of spectral and electronic characteristics of the compound under investigation have been determined over a wide range of wavelengths. The interband part of the experimental dependence of the optical conductivity has been interpreted using the results of the calculation of the electron density of states.     

Ab-initio study of the electromagnetic response and polarizability properties of carbon chains

We present the most complete set of calculations to date of the ground state electronic properties and of the optical/UV response function of linear carbon chains C_N, using ab-initio methods based on local density and on time-dependent local density approximations (LDA and TDLDA). Making use of the associated transition densities and wavefunctions we are able to provide microscopic insight into the collectivity of the corresponding plasmon spectrum in terms of correlated particle–hole excitations. From this analysis it is found that the (one-dimensional 1-D) delocalization of π (valence) electrons is responsible for the conspicuous values of the static dipole polarizability and of the high value of the exponent describing its dependence with the number of carbon atoms. Within this framework the electronic structure and linear response function of a carbon ring is also calculated. Although many properties of this function are similar to that associated with linear chains of the same number of atoms, the corresponding polarizabilities differ appreciably, providing a reliable method to distinguish between linear and close structures. The first principle results of the properties of linear carbon chains are compared with both theoretical and experimental results available in the literature, and constitute the basis for a systematic study of these 1-D sp-bonded systems, which have been found to be involved in such seemingly disparate phenomena as fullerene growth mechanism and diffuse interstellar bands.     

Optical anisotropy of the (100) surfaces in AlxGa1−x As ternary compounds

The reflectance anisotropy spectra of the clean (100) surfaces of the Al_xGa_1?x As ternary compounds at aluminum concentrations 0≤x≤0.5 have been measured and thoroughly studied. In the spectral range from 1.6 to 3.5 eV, the signal caused by the optical transitions in the arsenic dimers dominates in the spectra of the clean arsenic-terminated GaAs surfaces. For the ternary compounds, an increase in the aluminum concentration brings about the broadening of this signal and its shift toward the low-energy range. This is explained by the appearance of additional signals associated with the optical transitions in the nonequivalent arsenic dimers, in which a part of the Ga atoms in the bulklike bonds is replaced by the Al atoms. An increase in the number of the substituted gallium atoms leads to a decrease in the energy of optical transition in the dimer. The fundamental optical transition energies are determined for the nonequivalent dimers.     

Effect of the optical orientation of spin angular momenta of sodium atoms in the 32 S 1/2 state and metastable helium atoms in the 23 S 1 state on the Na-He gas-discharge plasma conduction

The observation of the effect of mutual orientation of spin angular momenta of sodium atoms in the 3² S _1/2 state and helium atoms in the 2³ S ₁ state on the electrical conduction of a Na-He gas-discharge plasma is reported. In the experiments, a simultaneous optical orientation of atoms by the optical emission of sodium and helium lamps was carried out. The influence of the mutual orientation on the conduction was observed as a change in the high-frequency voltage across electrodes of a gas-discharge camera caused by the destruction of orientation of sodium atoms. The effect is explained by the dependence of Penning ionization in the interaction of sodium and helium atoms on the mutual orientation of spin angular momenta of partners.     

Optical spectroscopy and electronic structure of compounds HoNi5 − x Al x (x = 0, 1, 2)

The optical properties of the compounds HoNi_{5 ? x} Al _x (x = 0, 1, 2) have been investigated using the ellipsometric method in the wavelength range from 0.22 to 16 μm. The electronic structure of these intermetallic compounds has been calculated in the local electron-spin density approximation with the correction for strong electronic interactions in the 4f shell of the holmium ions. The experimental dispersion dependences of optical conductivity in the region of interband light absorption have been interpreted based on the results of the calculation of the electron density of states. The plasma and relaxation frequencies of electrons have been determined.     

Ab initio calculations study of the electronic, optical and thermodynamic properties of NaMgH3, for hydrogen storage

The structural stability, electronic structure, optical and thermodynamic properties of NaMgH₃ have been investigated using the density functional theory. Good agreement is obtained for the bulk crystal structure using both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy. It is found from the electronic density of states (DOS) that the valence band is dominated by the hydrogen atoms while the conduction band is dominated by Na and Mg empty states. Also, the DOS reveals that NaMgH₃ is a large gap insulator with direct band gap 3.4 eV. We have investigated the optical response of NaMgH₃ in partial band to band contributions and the theoretical optical spectrum is presented and discussed in this study. Optical response calculation suggests that the imaginary part of dielectric function spectra is assigned to be the interband transition. The formation energy for NaMgH₃ is investigated along different reaction pathways. We compare and discuss our result with the measured and calculated enthalpies of formation found in the literature.     

Influence of MgO on structure and optical properties of alumino-lithium-phosphate glasses

MgO doped lithium alumino phosphate glasses (PLA: P₂O₅+Li₂O+Al₂O₃+MgO) were prepared by melt quenching technique. Raman spectra display three significant peaks at 698, 1164 and 1383 cm⁻¹ attributed to: symmetric stretching vibrations of the bridging oxygen (BO) in the P–O–P chains, symmetric stretching vibrations of the PO₂ groups, and the asymmetric vibrations vas(PO₂) of the non-bridging oxygen (NBO) atoms, respectively. Also, the density, molar volumes and ion concentration have been discussed and correlated with the structural changes within the glassy matrix. Some optical constants such as refractive index and dispersion parameters (E_o: single-oscillator energy and E_d: dispersive energy) of the glasses were determined. Finally, the values of the optical band gap for direct and indirect allowed transitions have been determined from the absorption edge studies. It is deduced that the values of E_opt increase with increasing MgO content. It was assigned to structural changes induced from the formation of non-bridging oxygen. The Urbach energy (ΔE) was found to decrease from 0.578 to 0.339 eV with increasing MgO content from 0.5 to 2 mol.     

Optical properties and electronic structure of US

The optical properties of uranium sulphide single crystals have been determined for the first time. An excellent agreement is found between the structure in the optical spectrum and the results of a self-consistent cellular multiple scattering calculation. The results evidence that the 5? electrons form a resonance state at E_F and that the 5??6d coupling produces a dip in the 6d density of states near E_F which is responsible for many peculiar properties of US.