Investigation of the magnetic properties of the Fe2-xCrxCaO4 system

Conclusions Due to the fact that the iron ions in two neighbouring elementary cells along the c-axis are in symmetry positions in relation to the oxygen anion (through which the interaction with other iron ions is achieved), the stronger 120° and 130° interactions could be considered as interactions not by zigzag-formed chains, but by the folded planes with c-axis as a generant, i.e., the quasi-twodimensional magnetic properties are evinced in Cr_xFe_2–xCaO₄.     

On the Material Invariant Formulation of Maxwell’s Displacement Current

Maxwell accounted for the apparent elastic behavior of the electromagnetic field by augmenting Ampere’s law with the so-called displacement current, in much the same way that he treated the viscoelasticity of gases. Maxwell’s original constitutive relations for both electrodynamics and fluid dynamics were not material invariant. In the theory of viscoelastic fluids, the situation was later corrected by Oldroyd, who introduced the upper-convective derivative. Assuming that the electromagnetic field should follow the general requirements for a material field, we show that if the upper convected derivative is used in place of the partial time derivative in the displacement current term, Maxwell’s electrodynamics becomes material invariant. Note, that the material invariance of Faraday’s law is automatically established if the Lorentz force is admitted as an integral part of the model. The new formulation ensures that the equation for conservation of charge is also material invariant in vacuo. The viscoelastic medium whose apparent manifestation are the known phenomena of electrodynamics is called here the metacontinuum.     

Propagation of an attosecond X-ray pulse in an atomic-scale waveguide

The guiding of an attosecond (as) X-ray pulse in a two-dimensional atomic structure is analyzed. The results obtained from a simple model show that an atomic-scale waveguide which consists of a barium monoxide lattice where five rows of atoms have been removed can successfully guide pulses with duration 20 as and wavelength 10 Å. This is attributed to an improved total external reflection in the discrete atomic structure. When the carrier frequency is higher than the plasma frequency, the guide causes minor distortions in the pulse profile. For lower frequencies, the X-ray pulse is significantly affected by the guide dispersion. These results indicate that nano- and sub-nanostructures can be designed for transportation and processing of coherent attosecond-duration electromagnetic pulses, which is important for the newly emerging field of X-ray photonics.     

The drop size in membrane emulsification determined from the balance of capillary and hydrodynamic forces

Here, we investigate experimentally and theoretically the factors that determine the size of the emulsion droplets produced by membrane emulsification in "batch regime" (without applied crossflow). Hydrophilic glass membranes of pore diameters between 1 and 10 mum have been used to obtain oil-in-water emulsions. The working surfactant concentrations are high enough to prevent drop coalescence. Under such conditions, the size of the formed drops does not depend on the surfactant type and concentration, on the interfacial tension, or on the increase of viscosity of the inner (oil) phase. The drops are monodisperse when the working transmembrane pressure is slightly above the critical pressure for drop breakup. At higher pressures, the size distribution becomes bimodal: a superposition of a "normal" peak of monodisperse drops and an "anomalous" peak of polydisperse drops is observed. The theoretical model assumes that, at the moment of breakup, the hydrodynamic ejection force acting on the drop is equal to the critical capillary force that corresponds to the stability-instability transition in the drop shape. The derived equations are applied to predict the mean size of the obtained drops in regimes of constant flow rate and constant transmembrane pressure. Agreement between theory and experiment is established for the latter regime, which corresponds to our experimental conditions. The transition from unimodal to bimodal drop size distribution upon increase of the transmembrane pressure can be interpreted in terms of the transition from "dripping" to "jetting" mechanisms of drop detachment.     

Dynamic correlations with time-dependent quantum Monte Carlo

In this paper, we solve quantum many-body problem by propagating ensembles of trajectories and guiding waves in physical space. We introduce the "effective potential" correction within the recently proposed time-dependent quantum Monte Carlo methodology to incorporate the nonlocal quantum correlation effects between the electrons. The associated correlation length is calculated by adaptive kernel density estimation over the walker distribution. The general formalism is developed and tested on one-dimensional helium atom in laser field of different intensities and carrier frequencies. Good agreement with exact results for the atomic ionization is obtained.     

The influence of thermal relaxation on the oscillatory properties of two-gradient convection in a vertical slot

We study the effects of the Maxwell–Cattaneo (MC) law of heat conduction on the flow of a Newtonian fluid in a vertical slot subject to both vertical and horizontal temperature gradients. Working in one spatial dimension (1D), we employ a spectral expansion involving Rayleigh’s beam functions as the basis set, which are especially well-suited to the fourth order boundary value problem (b.v.p.) considered here, and the stability of the resulting dynamical system for the Galerkin coefficients is investigated. It is shown that the absolute value of the (negative) real parts of the eigenvalues are reduced, while the absolute values of the imaginary parts are somewhat increased, under the MC law. This means that the presence of the time derivative of the heat flux increases the order of the system, thus leading to more oscillatory regimes in comparison with the usual Fourier case. Moreover, no eigenvalues with positive real parts were found, which means that in this particular situation, the inclusion of thermal relaxation does not lead to destabilization of the motion.     

Computational insight into protein circular dichroism: detailed analysis of contributions of individual chromophores in TEM-1 ��-lactamase

Circular dichroism (CD) is an extremely powerful method in dynamically developed areas such as proteomics and drug design. However, it is characterized with a low signal resolution, and therefore it is difficult to assign the signals to specific chromophores. In this study, we demonstrate a systematic computational strategy for revealing the contributions of all individual chromophores to complex near-UV CD spectra of proteins. The methodology not only reveals the individual chromophores contributions without any structural perturbation, but also makes mechanistic insight into physical mechanisms possible. We have applied our strategy to a TEM-1 ??-lactamase from E. coli??an enzyme of crucial importance to bacterial resistance to ??-lactam antibiotics. We analyzed the free enzyme structure, two acyl-enzyme structures and the structure of the transition state analog, thus simulating delicate but very important conformational changes that could take place during enzyme catalysis and binding. Such analysis also accounts for the important effects of the electrostatic environment that could be altered during experiments. We revealed in silico (without structural manipulations) that the strongest contribution in the near-UV CD is due to W210. The individual contribution of each aromatic chromophore was very dynamic with respect to structural changes and electrostatic effects. In contrast, the disulfide group contribution is relatively resistant to such structural dynamics but was dramatically influenced by alterations in the electrostatic environment.     

Synthesis and electrophilic cyclization reactions of diphenyl 3‐methylhexa‐ 1,3,4‐trien‐3‐yl phosphine oxide

Diphenyl 3‐methylhexa‐1,3,4‐trien‐3‐yl phosphine oxide can be readily prepared via an atom‐economical 2,3‐sigmatropic rearrangement of the mediated alkenynyl phosphinite formed in situ by a reaction of 2‐methylhex‐5‐en‐3‐yn‐2‐ol with diphenylchlorophosphine. Electrophilic cyclization reactions of prepared 1‐vinylallenyl phosphine oxide were investigated as it was established that the reactions proceeded with formation of heterocyclic compounds with participation of the allenic and/or 1,3‐dienic part of the vinylallenic system with neighboring group participation of the phosphoryl and/or vinylic group. © 2012 Wiley Periodicals, Inc. Heteroatom Chem 23:345–351, 2012; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.21023     

Reshaping of attosecond x-ray pulses in thin crystals

A method for reshaping and control of the duration of attosecond x-ray pulses in thin crystals is proposed. The finite width of the reflection and transmission curves around the Bragg angle allows one to engineer Fabry-Perot-type filters for the generation of a large variety of attosecond pulse shapes. The method considered here can be used to manipulate attosecond pulses produced by high-harmonic generation and also for shorter wavelengths for attosecond pulses from x-ray free-electron lasers. X-ray pulses with controllable amplitude and phase may find useful applications in the newly emerging area of attosecond time-resolved spectroscopy.