Angle-resolved photoemission spectra of 2H-NbSe2

Angle-resolved photoemission measurements have been made on the metallic layer compound 2H-NbSe₂. From the results, E vs k_∥ dispersion curves have been obtained along two principal symmetry directions in the Brillouin zone. The experimental energy bands are found to be in good agreement with the results predicted by one-dimensional density of initial states along k_⊥.     

Investigating potential hard materials: the case of tetragonal TaMoN

The long search of new materials possessing exceptional properties, such as hardness continues to be a main challenge in materials science. Here we probe the ternary transition metals-nitride Ta-Mo-N system at ambient conditions by first-principles evolutionary technique in the hope of discovering new stable hard compound.A thermodynamically tetragonal stable phase Ta₂Mo₂N₂, space group P₄-nmm has been identified. Phonon dispersion and elastic constants calculations divulge the mechanical and dynamical stability of this compound. In addition, TaMoN compound shows -through electronic band structure and density of states calculations- metallic features. The hardness of Ta-Mo-N system is mainly due to the highly directional covalent Ta-N bonds.     

Intermediate dimensional character of charge transfer excitation modes in a two-leg cuprate ladder

Collective charge modes in the cuprate ladder compound Sr₁₄Cu₂₄O₄₁ are studied over the one-dimensional Brillouin zone using resonant inelastic X-ray scattering. Low energy (2-4 eV) spectral weight across the Mott gap is dominated by a broad, dispersive feature containing two distinct peaks that may be interpreted as independent modes. Details of low energy dispersion, intensity distribution across the Brillouin zone and peak composition fall between the characteristic spectra of quasi-1D (e.g. SrCuO₂) and -2D cuprates (e.g. Nd₂CuO₄). We demonstrate that dispersion and splitting between the two observed modes can be understood in a variant of the strong coupling limit (Hubbard-U?t) with a single band Hubbard model.     

Magnons and phonons in antiferromagnetic CeMg compound

The magnon dispersion curves in the antiferromagnetic phase and the acoustic phonon dispersion in the paramagnetic phase of CeMg compound have been investigated. The magnetic moment and the tetragonal distortion are measured versus temperature. Their variations are consistent with the occurrence of a highly magnetostrictive first-order transition at T_N = 19.5 K. Transverse magnetic excitations associated to transitions between two sublevels of the Г₈ crystal field ground state were measured along the main lines of the Brillouin zone. There is a large anisotropy of the dispersion depending whether the wavevector is parallel or perpendicular to the fourfold axis of the tetragonal cell. This dispersion can be well described by means of a dynamical susceptibility model in which only a few interactions occur.     

Lattice dynamics of II–VI,III–V compounds

The lattice vibrations of II–VI compound ZnSe and III–V compound InSb have been calculated in the frame work of Banerjee-Varshni's second neighbour ionic [SNI] model utilizing critical-point phonons as an input to determine the required seven parameters and the experimentally determined three elastic constants C₁₁, C₁₂, C₄₄ as restraints on the values of the parameters. A reinvestigation of the experimental elastic constants particularly C₁₁ and C₄₄ for ZnSe has been suggested. Results are presented for the dispersion curves along high symmetry directions. A reasonable agreement with the recently measured inelastic nutron scattering data is observed.     

Energy band structure and X-ray spectra of phenakite Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

The electronic structure of crystalline phenakite Be₂SiO₄ is investigated using x-ray emission spectroscopy (XES) (Be K _α XES, Si L _{2, 3} XES, O K _α XES) and x-ray absorption spectroscopy (XAS) (Be 1s XAS, Si 2p XAS, O 1s XAS). The energy band structure is calculated by the ab initio full-potential linearized augmented-plane-wave (FLAPW) method. The total and partial densities of states and the dispersion curves for the Be₂SiO₄ compound are presented. It is shown that the top of the valence band and the bottom of the conduction band of the Be₂SiO₄ compound are predominantly formed by the oxygen 2p states. According to the results obtained, the electron transition with the lowest energy supposedly can occur at the center of the Brillouin zone. The effective masses of electrons (0.5m _e ) and holes (3.0m _e ) for the Be₂SiO₄) compound are estimated.     

Ellipsometric study of the optical properties of Ca1 − x La x MnO3 single crystals (x = 0–0.2) under n-type doping

The dispersion of the real, ε₁(ω), and imaginary, ε₂(ω), parts of the complex permittivity of Ca_{1 ? x} La_xMnO₃ single crystals (x = 0, 0.05, 0.10, 0.12, 0.20) was studied at room temperature in the spectral region extending from 60 meV to 5 eV. It was found that substitution of lanthanum for calcium shifts the 3.1-eV absorption band in the optical-conductivity spectrum toward higher energies, with the spectral weight of the low-energy wing of the 2.2-eV band becoming redistributed to the band-gap region (E < 1.5 eV) of the starting CaMnO₃ compound. The specific features of optical-conductivity dispersion in the mid-IR region that occur under n-type doping were established. The frequency dependence of optical conductivity was shown to differ from the Drude behavior characteristic of metals. The optical-conductivity spectra of Ca_{1 ? x} La_xMnO₃ were compared with our earlier results on a series of hole-doped La_{1 ? x} Sr_xMnO₃ single crystal.     

155Gd and 161Dy Mössbauer study of R1+εFe4B4 alloys (R = Gd,Dy)

Tetragonal R_1+εFe₄B₄ alloys with R = Gd and Dy have been investigated by Mössbauer spectroscopy, using the ¹⁵⁵Gd and ¹⁶¹Dy resonances, respectively. The Gd quadrupolar interaction e²qQ = 12.65(5)mm/s is the largest observed to date in metallic compounds of Gd. For Dy this interaction is e²qQ = 74(2) mm/s, a value rather small for a Dy compound. Both results imply a strong lattice contribution to the electric field gradient. A crystal-field term A⁰₂ = -2450(50) K/a²_o is inferred. Our data are consistent with a point-charge calculation, provided charges of opposite signs are assumed for Fe and B atoms. Hyperfine parameters show some dispersion, reflecting the quasi incommensurate nature of the R and Fe+B sublattices in the R_1+εFe₄B₄ structure.     

Dispersion characteristic of hexagonal and square lattice chalcogenide As2Se3 glass photonic crystal fiber

In this paper, we report a chalcogenide As₂Se₃ glass photonic crystal fiber (PCF) for dispersion compensating application. We have used the improved fully vectorial effective index method (IFVEIM) for comparing the dispersion properties (negative and zero dispersion) and effective area in hexagonal and square lattice of As₂Se₃ glass PCF using different wavelength windows. It has been demonstrated that due to their negative dispersion parameter and negative dispersion slope in wavelength range 1.2-2.5 μm, both lattice structures of As₂Se₃ glass PCFs, with pitch (Λ = 2 μm), can be used as dispersion compensating fibers. Further, design parameters have been obtained to achieve zero dispersion in these fibers. It is also shown that As₂Se₃ glass PCF provides much higher negative dispersion compared to silica PCF of the same structure, in wavelength range 1.25-1.6 μm and hence such PCF have high potential to be used as a dispersion compensating fiber in optical communication systems.     

A new dioxime ligand and its trinuclear copper(II) complex: Synthesis, characterization and optical properties

The synthesis and characterization of a new dioxime ligand, (H₂L), (III) and its trinuclear copper(II) complex, [Cu₃L₂(H₂O)₂](ClO₄)₂, (V) is described. IR spectra show that the ligand acts in a tetradentate manner and coordinates N₄ donor groups of H₂L to copper(II) ion. The structure of the metal chelate is clarified with the help of elemental analysis, magnetic moment, conductometric and spectroscopic measurements. The optical absorption studies reveal that the transition is direct with band gap energy values are calculated. The optical constants such as refractive index and dielectric constant of the compound were determined. The refractive index dispersion curves of the films obey the single-oscillator model and oscillator parameters. Optical dispersion parameters E₀ and E_d developed by Wemple-DiDomenico were calculated.     

SOFC-anodes,proof for a finite-length type Gerischer impedance?

The impedance of a symmetric cell with Ni/Ti-doped YSZ cermet anodes was measured as function of ambient (P_H2, P_H2O) and temperature. The impedances showed identical shapes with a minor dispersive contribution in the high frequency region and a dominating dispersion down to 0.01 Hz. The characteristic shape of this dispersion was clearly in between a finite-length Warburg (FLW) and a Gerischer impedance. Analysis of the dispersion in the Bode representation, after subtraction of the high frequency contribution, showed a clear relation with the Gerischer impedance. Assuming a finite-length constraint led to quite reasonable modelling of the data. The parameter set of this finite-length Gerischer showed a consistent dependence on P_H2, P_H2O. A qualitative interpretation of this modified Gerischer is presented.     

Determination des angles d'obliquite d'un cristal liquide en phase nematique au voisinage d'une surface

Using several laser sources the dispersion of the quadrupole nonlinear tensor d⁽²⁾(CaCO₃) has been determined in the ultraviolet and visible range 265 – 592 nm. The measured dependence is in a good agreement with the predicted dispersion d⁽²⁾(CaCO₃) ~ λ^-1.     

Evolution of the band structure of quasiparticles with doping in copper oxides on the basis of a generalized tight-binding method

Two methods for stabilizing the two-hole ³ B _1g state as the ground state instead of the Zhang-Rice singlet are determined on the basis of an orthogonal cellular basis for a realistic multiband pd model of a CuO₂ layer and the dispersion relations for the valence band top in undoped and doped cases are calculated. In the undoped case, aside from the valence band, qualitatively corresponding to the experimental ARPES data for Sr₂CuO₂Cl₂ and the results obtained on the basis of the t-t′-J model, the calculations give a zero-dispersion virtual level at the valence band top itself. Because of the zero amplitude of transitions forming the virtual level the response corresponding to it is absent in the spectral density function. In consequence, the experimental ARPES data do not reproduce its presence in this antiferromagnetic undoped dielectric. A calculation of the doped case showed that the virtual level transforms into an impurity-type band and acquires dispersion on account of the nonzero occupation number of the two-hole states and therefore should be detected in ARPES experiments as a high-energy peak in the spectral density. The computed dispersion dependence for the valence band top is identical to the dispersion obtained by the Monte Carlo method, and the ARPES data for optimally doped Bi₂Sr₂CaCu₂O_8+δ samples. The data obtained also make it possible to explain the presence of an energy pseudogap at the symmetric X point of the Brillouin band of HTSC compounds.     

Dielectric dispersion formula for ferroelectrics

A unified dielectric dispersion formula for order-disorder, extended order-disorder, such as KD₂PO₄ and displacive ferroelectrics is proposed. The dispersion formula has two parameters c and d which measure deviation from order-disorder and displacive types, respectively.     

Composition-tuned refractive index and oscillator parameters in TlGaxIn1−xS2 layered mixed crystals (0≤x≤1)

The optical properties of TlGa_xIn_1?xS₂ mixed crystals have been studied through transmission and reflection measurements in the wavelength range 400–1100 nm. These measurements allowed determination of the spectral dependence of the refractive index for all compositions of the mixed crystals studied. The dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-effective-oscillator model. The compositional dependences of the refractive index dispersion parameters (oscillator energy, dispersion energy and zero-frequency refractive index) were revealed.     

Structure,optical spectroscopy and dispersion parameters of ZnGa2Se4 thin films at different annealing temperatures

Thin films of ZnGa₂Se₄ were deposited by thermal evaporation method of pre-synthesized ingot material onto highly cleaned microscopic glass substrates. The chemical composition of the investigated compound thin film form was determined by means of energy-dispersive X-ray spectroscopy. X-ray diffraction XRD analysis revealed that the powder compound is polycrystalline and the as-deposited and the annealed films at T_a = 623 and 673 K have amorphous phase, while that annealed at T_a = 700 K is polycrystalline with a single phase of a defective chalcopyrite structure similar to that of the synthesized material. The unit-cell lattice parameters were determined and compared with the reported data. Also, the crystallite size L, the dislocation density δ and the main internal strain ε were calculated. Analyses of the AFM images confirm the nanostructure of the prepared annealed film at 700 K. The refractive index n and the film thickness d were determined from optical transmittance data using Swanepoel's method. It was found that the refractive index dispersion data obeys the single oscillator model from which the dispersion parameters were determined. The electric susceptibility of free carriers and the carrier concentration to the effective mass ratio were determined according to the model of Spitzer and Fan. The analysis of the optical absorption revealed both the indirect and direct energy gaps. The indirect optical gaps are presented in the amorphous films (as-deposited, annealed at 623 and 673 K), while the direct energy gap characterized the polycrystalline film at 700 K. Graphical representations of ε₁, ε₂, tan δ, ? Im[1/ε*] and ? Im[(1/ε* + 1)] are also presented. ZnGa₂Se₄ is a good candidate for optoelectronic and solar cell devices.     

Birefringence of CuGa(S1−x Sex)2 single crystals

The magnitude and dispersion of birefringence of single crystals of CuGa(S_1?x Se_x)₂ solid solutions is studied in the spectral region of 0.5–2.5 μ at T=300 K. The effect of the substitution of selenium for sulfur on special features of birefringence dispersion is analyzed within the framework of the single-oscillator model.     

Angle-resolved photoemission of the c(2 × 2) and c(3 × 1) oxygen overlayers on Fe(110)

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the band structure of the c(2×2) and (3×1) oxygen overlayers on Fe(110). The symmetries of the O-2p-derived states at the center of the surface Brillouin zone $\text{(}\text{Γ}\text{)}$ were identified using polarized light. At $\text{Γ}$ the p_xp_y- and p_z-derived levels are at about 5.5 and 7.0 eV below the Fermi level, respectively, for both ordered overlayers. The p-states of the c(2×2)-O structure show very little dispersion (?0.1 eV) with $\text{k}{}_{\mathrm{}}$. On the other hand, the c(3×1)-O overlayer exhibits considerable dispersion of ～1.6 eV. The essential features of the measured dispersion are reproduced well by the dispersion predicted in a qualitative way for an isolated c(3×1) oxygen monolayer.     

Investigation of dispersion characteristic in MI- and MII-type single mode optical fibers

Dispersion characteristic of MI and MII type single mode optical fibers is analytically investigated. For this purpose modal analysis of these fibers to obtain possible wave vectors for given system parameters are done. Then using numerical evaluation of the presented analytical relations, chromatic and waveguide dispersions are calculated. The effects of geometrical and optical parameters of the fibers on dispersion characteristics are investigated. In this analysis, we show that with increase of Δ (optical parameter) for MI structure the slope of dispersion curve is decreased and the case is reversed for MII structure. Also, with rising of Q (geometric parameter) for MI structure the slope of dispersion curve is decreased and the situation is reversed for MII structure. Finally, we show that with boosting of R₂ for MII structure the slope of dispersion is increased. As a final result, our simulations show that small values for optical parameters are better in MII structure for multi-channel optical communications. In MI structure to obtain small dispersion slope, Q can be increased that is easy for fabrication in practice. Finally, Q and R₂ are suitable parameters for control of dispersion in the proposed structures.