Electronic structures of the LaBO3 (B = Co, Fe, Al) perovskite oxides related to their catalysis

The discrete-variational (DV) X α molecular orbital method has been applied to investigate the bulk and surface electronic structures of perovskite oxides such as LaCoO₃, LaFeO₃ and LaAlO₃. The calculated XPS spectra for these oxides are in good agreement with the experimental ones and only the LaCoO₂ exhibited a rather high electron density of states near the Fermi level (E_F). The catalytic behavior of these oxides is discussed on the basis of their electronic structures; the marked catalysis by LaCoO₃ is associated with electron occupation of crystal field d states near E_F and with the buildup of surface charge so as to enhance the electron transfer between a surface cation and an interacting molecule.     

Regge cuts and large-|t| pp scattering

It is shown that recent measurements of the large-|t| pp elastic-scattering differential cross section at FNAL and CERN-ISR agree with earlier predictions based on the dominance of a pomeron pole(P) with αp(t) = 1.067 + 0.10 t, and a P ? P cut, out to the largest measured |t| values (12 GeV²). Some of the problems raised by this success are discussed.     

Raman scattering from 2H and 3R–NbS2

Raman scattering experiments have been carried out on 2H-NbS₂ crystals and 3R-NbS₂ crystals. The spectra obtained from both compounds have been found to contain all the Raman active modes predicted by group theory. A nearest-neighbour lattice dynamics model has also been used to analyze the spectra and qualitative agreement with the experimental results is obtained. The results are also discussed in terms of the modifications expected in the Raman spectra obtained from different polytypes of the same layered compound. Some discrepancies with previously reported results are found.     

y and ν distributions for neutral current reactions of the Weinberg-type

The y and ν distributions for inclusive neutrino and antineutrino reactions arising from a neutral current of the Weinberg-type are investigated in the framework of two quark parton models. While the ν distributions appear of little use at present, we show that by making a cut in the y variable, one can determine sin²θ_W reasonably accurately, independent of the cross section determination, even with the present narrow-band dichromatic neutrino beam at NAL.     

The AB−AC quasi-binary section of the order-disorder phase diagram

The AB?AC sections of the order-disorder phase diagram of a ternary alloy ordered with the L1_o binary superstructure are calculated according to the Kikuchi method in an approximation which enables four-body correlations in the atomic arrangement on the tetrahedron sites to be taken into account. The formation conditions of some ternary superstructures are considered. The change of the character of the order-disorder diagram versus the value and sign of the energy parameters, which reflect the ability of different component atomic pairs to order in the first coordination sphere, is discussed.     

Electronic structure and relative stabilities of L12 and D022 ordered structures occuring in transition metal alloys

It is shown that for transition metal alloys (1) the ordering energies can be determined accurately through the recursion method, (2) a generalized pertubation method, starting from the completely disordered state, is relevant to compute not only ordering energies but also differences in ordering energies i.e. relative stability energies which are smaller by an order of magnitude. As an example the results for the L1₂ and the D0₂₂ ordered structures are discussed.     

Thermoelectric power of quasi-one-dimensional Nb3X4 with X=S, Se andTe

The thermoelectric power of linear chain synthetic metals Nb₃X₄ (with X=S, Se and Te) was measured from 5 to 300 K. The thermopower is negative indicating a dominant transport by electrons. Common to three compounds, in lower temperature regions the thermopower rises linearly with temperature but soon saturates. With respect to Nb₃S₄ and Nb₃Se₄ we have found no special anomaly of the thermopower except for a little higher magnitude.With respect to Nb₃Te₄ anomalies in the thermopower vs temperature appear at about 80 and 20 K which are explained in terms of the charge- density-wave phase transition from the simultaneous measurement of the resistivity and the observation of the electron diffraction patterns.     

New types of short-range orientational ordering in ortho-para hydrogen monolayers

Orientational ordering in monolayers of solid hydrogen is discussed in view of recent experimental findings in NMR studies of (ortho)_c-(para)_1−c-hydrogen mixtures on boron nitride substrate reported for ortho-H₂ concentrations 0.35≤c≤0.92 and temperatures 0.14≤T≤1.80 K. Analysis of the temperature-concentration behavior for the observed NMR frequency splitting is given on the basis of a two-dimension (J=1)_c-(J=0)_1−c-rotor model for which the fundamental parameters, quadrupolar coupling constant Γ₀=0.50±0.03 K and the crystal-field amplitude V₀=0.70±0.10 K, are derived from experiment. The two distinct para-rotational (PR) short-range ordered structures driven by positive and negative crystalline fields are described in terms of the local alignment and orientation of the polar principal axis. It is shown that they are rather different from the ferromagnetic-type ordering suggested earlier by Harris and Berlinsky as a unique PR structure. The common quadrupolar glass and a new hindered rotor phases found below the 2D site percolation threshold c_p=0.72 are also discussed.     

Band structure for some Nb3X compounds

The tight binding method is used to calculate band structure for a number of Nb₃X compounds. The 4d band of the niobium, as well as the s-p bands of the X-atoms, are considered. The transfer and overlap integrals are calculated using tabulated atomic orbitals. The crystal field parameters (CFP) are estimated more carefully, taking into account deviations from a muffin-tin potential. We employ here the independent band approximation in which no interband interaction is considered. This approximation has been proved successful for the V₃Ga case, and it is estimated here that interband interaction may shift some levels up or down by 50 mRy or so, but the overall band-structure does not change. The Fermi level is found to lie in the neighbourhood of three peaks; one belongs to the δ₂(d_xy) band and the two others belong to the π(d_xz, d_yz) band.     

Force field of A(I)B(III)−S2 chalcopyrite compounds at zero wave vector

Published experimental frequencies are used to calculate Urey-Bradley force fields and ionic charges of some ABS₂ chalcopyrite compounds. The values obtained for the A?S and B?S force constants as well as the ionic charges are discussed.     

Radiative muon capture for N = Z nuclei

The commonly used elementary hamiltonian of radiative muon capture has been confirmed, while the alternative Hwang-Primakoff approach is shown not to be gauge invariant.In the inclusive process on N = Z nuclei, the closure approximation is avoided by using a realistic nuclear excitation spectrum.The study is exemplified by a detailed application to ⁴⁰Ca. Predictions are given for the high-energy photon spectrum, circular polarization and asymmetry with respect to the muon polarization for various values of the pseudoscalar coupling constant g_p. A semi-quantitative agreement is found with the data on the spectrum; more precise experiments are necessary to determine g_p.     

Bounded perturbations of the P(ϕ)2 theory

As a perturbation to the P(?)₂ theory we consider interaction densities of the form V(?(x)), where ?(x) is a scalar hermitian boson field and V(α) is a bounded real continuous function. It is proved that the asymptotic fields exist and are equal to the asymptotic fields of the P(?)₂ theory. The connection with non-polynomial theories of rational type is indicated. Furthermore the consequences of a bounded perturbation for the S-matrix and the spectral properties are discussed.     

SIMPLIFIED DISPERSION RELATIONSHIPS FOR IN-VACUO PIPES

The dispersion characteristics of a pipe offer a way to gain physical insight into its dynamic behaviour. Whilst these can be found in the literature they are generally calculated by numerically solving the characteristic equation. In this paper, a simplified characteristic equation of an in vacuo pipe is presented and from this analytical expressions for the wavenumbers for the circumferential modes below the ring frequency are derived. It is shown that before waves cut on and propagate, they change from being decaying standing waves at low frequencies to being nearfield waves. A simplified expression is also determined for the cut on frequencies of the n?2 circumferential modes. Simulations are presented to validate the results against some established theories of pipe vibration.     

Kinetic study of non-isothermal crystallization of BixSe100−x chalcogenide glasses

Results of differential thermal analysis (DTA) under non-isothermal conditions on four glasses of Bi_xSe_100−x (x=5, 10, 15 and 25 at%) are reported and discussed. The glass transition temperatures (T_g), the onset crystallization temperatures (T_c) and the peak temperatures of crystallization (T_p) were found to be dependent on the compositions and the heating rates. From the dependence on the heating rates of (T_g) and (T_p), the activation energy for glass transition, E_g, and the activation energy for crystallization, E_c, are calculated and their composition dependence is discussed. The crystalline phases resulting from DTA and scanning electron microscopy (SEM) have been identified using X-ray diffraction. According to the Avrami exponent (n), the results show a one-dimensional growth for the composition Bi₅Se₉₅ and two-dimensional growth for the compositions Bi₁₀Se₉₀, Bi₁₅Se₈₅ and Bi₂₅Se₇₅. The kinetic parameters determined have made it possible to discuss the glass-forming ability.     

Radiative transfer: Gaussian quadrature formulas for integrals with the weight functions exp(-|x−t|) and En(|x−t|)

One to six point Gaussian quadrature formulas have been computed for the roots and weight coefficients of the integrals, from zero to infinity, of the integrands f(x) exp(-|x?t|) and f(x)E_n(|x?t|). These are then tabulated as functions of t. Analytic formulas are given explicitly for one and two point quadratures for any n. The asymptotic behavior of the roots and weight coefficients is discussed. Estimates of errors for the quadratures are also given.     

The complete O(ɛ2) reggeon field theory scaling law dσel/dt and its applications at collider energies

We give the complete asymptotic reggeon field theory (RFT) scaling law for the elastic cross section in O(?²), extending previous results to include the 3-pomeron cut. This provides the most detailed realistic test of this theory. Numerically we find the encouraging result that the small-t SPS collider data for dσ_el/dt are in agreement with these new asymptotic RFT predictions. However, ISR data are not compatible with this theory, either in s or t dependence. Previous positive ISR results were largely due to the existence of a j = 1 fixed cut which is not present here. Our results, coupled with the observation that critical RFT fits to σ_tot require large non-leading terms, imply that a lower bound for the RFT asymptotic scale in single renormalized propagator processes (σ_tot,dσ_el/dt) lies around SPS-collider energies. The asymptotic scale for multi-propagator processes (the rapidity plateau, ab→aX reactions, multiplicity distributions etc.) is expected to be well beyond the SPS collider. A possible global RFT picture is them that perturbation theory (including finite rapidity scale effects due to the excitation of new quantum numbers) is applicable through ISR and up to the SPS collider, consistent with recent observations. The critical RFT can become the relevant diffractive theory for σ_tot and dσ_el/dt at larger t at and above the SPS collider. Predictions for dσ_el/dt at larger t at the SPS collider are given in this paper and we urge experimentalists to test them.     

The scattering effects of the redistribution function RIII(x′, n′; x, n) on spectral line formation

The effects of frequency and angular redistribution on line formation are examined in static isothermal atmospheres for scattering described by the redistribution function R_III(x′, n′; x, n). For an optically thin atmosphere, it is found that the emergent line intensities obtained using R_III are essentially the same as R_I and R_II. In the case of a semi-infinite atmosphere, the emergent line intensities obtained using R_III, whether angle-averaged or angle-dependent, did not differ substantially from the complete redistribution results.     

Nonlinear optical susceptibilities χ of nitridosilicate powders

Nitridosilicates are of interest as novel nonlinear materials due to their extraordinary chemical and thermal stability. Unfortunately, large nitridosilicate single crystals are presently not available for the investigation of their nonlinear optical properties. The first experiments are presented in which an averaged nonlinear susceptibility χ⁽²⁾ for several nitridosilicates is studied by using two different powder techniques, the Kurtz Perry method and the SHEW method (Second Harmonic Wave generated by an Evanescent Wave). We observe nonlinearities of the new materials which are comparable to those of LiIO₃. The highest averaged M_eff=(χ_eff⁽²⁾)²/4n_2ωn_ω² values found are ∼0.9 pm²/V². The refractive indices of the materials are determined to be between n=2 and 3.     

Electronic structure of cubic ErxGa1−xN using the LSDA+U approach

Electronic structure calculations were performed for substitutional erbium rare-earth impurity in cubic GaN using density-functional theory calculations within the LSDA+U approach (local spin-density approximation with Hubbard-U corrections). The LSDA+U method is applied to the rare-earth 4f states. The Er_xGa_1−xN is found to be a semiconductor, where the filled f-states are located in the valence bands and the empty ones above the conduction band edge. The filled and empty f-states are also shown to shift downwards and upwards in the valence and conduction bands, respectively, with increase in the U potentials.