d-symmetry superconductivity due to valence bond correlations

It is shown that d-symmetry superconductivity due to valence bond correlations is possible. Valence bond correlations are compatible with antiferromagnetic spin order. In order to explictly construct a homogeneous state with the valence bond structure in the two-dimensional Hubbard model for an arbitrary doping, we have used the variational method based on unitary local transformation. Attraction between holes in the d-channel is due to modulation of hopping by the site population in course of the valence bond formation, and corresponding parameters have been calculated variationally. An important factor for the gap width is the increase in the density of states on the Fermi level due to antiferromagnetic splitting of the band. The gap width and its ratio to the T _c are 2Δ≃0.1t and 2Δ/kT _c≃4.5−4 for U/t≃8. The correspondence between the theoretical phase diagram and experimental data is discussed. The dependence of T _c on the doping δ=|n−1| and the Fermi surface shape are highly sensitive to the weak interaction t′ leading to diagonal hoppings. In the case of t′>0 and p-doping, the peak on the curve of T _c(δ) occurs at the doping δ _opt, when the energy of the flattest part of the lower Hubbard subband crosses the Fermi level at k∼(π,0). In underdoped samples with δ<δ _opt, the anisotropic pseudogap in the normal state corresponds to the energy difference |E(π,0)−μ| between this part of the spectrum and the Fermi level. Zh. éksp. Teor. Fiz. 114, 985–1005 (September 1998)     

Absolute intensity and phase of the resonant X-ray scattering from a germanium crystal

The 222 and 600 reflections near the germanium absorption K edge were studied on the Kurchatov synchrotron radiation source. The energy spectrum of the 222 reflection is caused by the interference of the weak nonresonant and purely resonant contributions to the tensor atomic factor, whereas the 600 reflection is purely resonant. The energy dependence of the magnitude and phase of the resonant contribution to the scattering amplitude was determined from a change in the interference pattern. The numerical simulation of the energy spectra of reflections with the inclusion of the dipole-quadrupole and thermally induced contributions shows that the latter is dominant at room temperature. Original Russian Text ? E.Kh. Mukhamedzhanov, M.M. Borisov, A.N. Morkovin, A.A. Antonenko, A.P. Oreshko, E.N. Ovchinnikova, V.E. Dmitrienko, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 12, pp. 896–900.     

Preparation and Properties of Polymers Derived from Epoxy Resins and Oligochelatotitanophenylenesiloxanes

Oligochelatotitanophenylenesiloxanes of various structures were suggested as curing agents for epoxy resins. The compound formulas and curing conditions were chosen so as to ensure the maximal degree of cross-linking and hence the best physicomechanical properties of the polymers.     

4-Hydroxyproline containing podands as new chiralcatalysts for the asymmetric Biginelli reaction

The novel C₂-symmetric chiral organocatalysts of (2S,4R)-hydroxyproline containing podand type have been synthesized and evaluated in the asymmetric Biginelli reaction. Combination of polyether spacer with (2S,4R)-hydroxyproline allowed one to enhance the effect of the latter on enantioselectivity of the Biginelli reaction by a factor of 5. The best results have been achieved for salts of (2S,4R)-hydroxyproline containing podands with trifluoroacetic acid.

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Enzymatic Lysis of Living Microbial Cells: A Universal Approach to Calculating the Rate of Cell Lysis in Turbidimetric Measurements

The data of the turbidimetric measurement for the enzymatic lysis of various living bacterial cells are analyzed. A method for the correct recalculation of the turbidimetric data (–ΔA/Δt) into absolute values of the change in the concentration of living cells (–ΔCFU/Δt) is proposed. The dimensionless efficiency of cell lysis–(1/CFU₀) · ΔCFU/Δt for various bacterial cells is calculated to correctly compare the efficiency of the action of different bacteriolytic factors on various bacterial cells.     

Catalytic dehydration of ethanol into ethylene in a tubular reactor of the pilot installation on alumina catalysts with varied grain size

Synthesis of ethylene on trefoil and cylindrical experimental acid-modified aluminum oxide samples was studied under an ethanol (94%) gas load of 920–2200 h^–1 and heat-carrier temperature of 400–440°C. In the conditions of a 98% ethanol conversion, the higher activity of the trefoil made it possible to reduce the height of the bed and its hydraulic resistance and, accordingly, raise the specific catalyst throughput for ethylene. Compared with industrial aluminum oxide, the throughput of 1 g of the catalyst for ethylene on experimental samples is higher by 2.5–6.5 kg yr^–1, and the specific expenditure of ethanol is lower by 0.22–0.23 kg kg^–1. The endothermic process in a tubular reactor is characterized by a high parametric sensitivity of the average integral temperature along the catalyst bed, with the average temperature being higher on the less active catalyst. Thus, the higher average temperature can compensate for the lower activity of the catalyst without additional increase in the contact duration and(or) heat-carrier temperature.     

Numerical Models of the Vertical Turbulent Exchange in Stably Stratified Water Body: II. Results of Numerical Experiments

On the basis of the developed models [1], numerical modeling of the problem on penetration of a turbulent layer of mixed liquid in linearly stratified medium under constant shear stress is performed. The calculation results are in satisfactory agreement with the known experimental data and show a significant effect of anisotropy of the flow on its main characteristics.     

The Fermi surface reconstruction in stripe phases of cuprates

Mean-field study of the stripe structures is conducted for a hole-doped Hubbard model. For bond-directed stripes, the Fermi surface consists of segments of an open surface and the boundaries of the hole pockets which appear in the diagonal region of momenta under certain conditions. Segments of the first type are due to one-dimensional bands of states localized on the domain walls. The relation of bands to the doping and temperature dependences of the Hall constant is discussed. In connection with the observation of quantum magnetic oscillations, a systematic search for the electron pockets has been carried out. It is shown that the formation of such pockets in bilayer models is quite possible.