d-Symmetry superconductivity as a consequence of valence-bond type correlations

It is shown that d _x ²−y ² symmetry of superconducting order due to valence bond (VB) type correlations is possible. The VB correlations are compatible with antiferromagnetic (AF) spin order. For the two-dimensional Hubbard model with arbitrary doping, the variational method of local unitary transformations is used to construct explicitly a uniform state with VB structure. The d-channel attraction of holes is a consequence of the modulation of hops by the populations of centers accompanying VB formation, and the parameters of the modulation are determined variationally. The increase in the density of states at the Fermi level accompanying AF splitting of the band, which is absent in the paramagnetic state, is important for the gap width. The gap width and its ratio to T _c are of the order of 2Δ≃0.1t and 2Δ/kT _c≃4–4.5 with U/t≃8. The agreement between the phase diagram found and experiment is discussed. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 350–355 (10 March 1998)     

Types of Fermi surface segments of cuprate stripe phases

The mean field method was used to study zones and Fermi surfaces of the stripe phases of the Hubbard model, which describes the electronic properties of cuprates. It was shown for structures with stripes parallel to bonds that the Fermi surface consisted of open surface segments and closed surfaces around hole pockets. Segments of the first type originate from quasi-one-dimensional zones of states localized on domain walls. The properties of zones in comparison with the zones observed in angle-allowed photoemission spectra and the possible influence of the corresponding two types of carriers on the dependence of the Hall constant on temperature and doping are discussed. In view of the observed magnetic quantum oscillations, the search for electron pockets was undertaken. Their formation was shown to be possible in two-layer (but not one-layer) models.     

Dehydration of Ethanol to Ethylene on Ring- and Trilobe-Shaped Catalysts

The indicators of ethanol to ethylene catalytic dehydration process on trilobe- and ring-shaped samples of an alumina catalyst were compared at the fixed parameters: thermal agent temperature and catalyst load. Experiments were performed in a flow-through reactor of a laboratory setup and in a tubular reactor of a pilot installation. The use of the less active ring-shaped catalyst ensures higher values of ethanol conversion, ethylene yield, and catalyst performance and significantly lower hydraulic resistance, compared to the more active trilobe-shaped catalyst. This is caused by lower intensity of the heat absorption on the less active catalyst and, correspondingly, to the higher temperature in the ring bed due to higher bed porosity.     

Numerical simulation of the resonant forbidden bragg reflection in germanium

??Forbidden?? reflections are observed in synchrotron radiation diffraction when the energy of incident radiation is close to the absorption edges in crystals. In the present paper, a new method for calculating the intensity of thermal-motion-induced forbidden reflections is proposed. It includes two steps: simulation of the temporary atomic displacements with the help of the ab initio molecular dynamics followed by quantum-mechanical calculations of the resonant scattering amplitude for various configurations. This technique is applied to the calculation of the 600 reflection thermal behavior in Ge and gives an adequate quantitative fitting of experimental data. The proposed simulation method of the thermal-motion-induced forbidden reflections is suitable for any crystal structure and promises to explain many results obtained up to now in synchrotrons.     

Kinetics of β-picoline oxidation to nicotinic acid over vanadia-titania catalyst. 3. The oxidation of nicotinic acid

The oxidation of nicotinic acid (NA) over binary vanadia-titania catalysts with different contents of residual sulfate ions (0.2 and 2.8 wt. %) was studied at 250–300°C in the feed reaction mixture of the composition NA: O₂: H₂O = 0.01–0.17: 5–20: 0–20 (vol. %), with nitrogen as a balance. The kinetic parameters of the process were determined. It was shown that even a ten-fold increase in the content of residual sulfate ions in the samples has no significant effect on the kinetics of the NA oxidation.     

Photochemical transformations of chalcone-podands into cyclobutane-containing benzocrown ethers

A method for the synthesis of cyclobutane-containing benzocrown ethers by the template photocycloaddition of the corresponding chalcone-podands in solutions was developed. It was shown by X-ray diffraction analysis, ¹H and ¹³C NMR, and UV spectroscopy that the intramolecular [2π+2π] photocycloaddition (PCA) is stereoselective with the predominant formation of structures of the β- and γ-truxinic type. The PCA rate was shown to depend on the size of the oxyethylene spacer and stereoorientation of the chalcone fragments in the podands. The selectivity of the PCA process can be controlled by a change in the concentration of the template (alkaline metal ions) and variation of the wavelength range of irradiation.     

Catalytic effect of nanosized metal oxides on the Hantzsch reaction

The effect of nanosized copper and aluminum oxides, which have a higher sorption capacity than that of bulk samples, on the Hantzsch reaction was studied. The adsorption of starting benzaldehydes and ethyl acetoacetate on the surface of copper and aluminum nanooxides resulted in the activation of these molecules and accelerated the Hantzsch reaction. In addition, considerable activation of ammonia and intermediates (chalcone and enamine) on the surface of aluminum nanooxide facilitated an increase in the rate and selectivity of the process. The experimental results were used to develop a one-pot method for the preparation of nifedipine and nitrendipine.     

Determination of the position of impurity atoms in yttrium-aluminum garnet by means of forbidden reflections

The possibility of determining the position of terbium impurity atoms in yttrium-aluminum garnet lattice is analyzed. It is shown that in the presence of the impurity in both aluminum and yttrium positions the tensor factors of the nearest yttrium atoms change. To determine the position of impurity atoms in the yttrium-aluminum garnet unit cell, measurements should be carried out at energies near the L-absorption edges of terbium.