Luminescence in germania-silica fibers in a 1-2 µm region.

We analyze the origins of the luminescence in germania-silica fibers with high germanium concentration (about 30 mol.% GeO2) in the region of 1-2 µm with a laser pump at the wavelength 532 nm. We show that such fibers demonstrate the high level of luminescence which unlikely allows the observation of photon triplets, generated in a third-order spontaneous parametric downconversion process in such fibers. The only efficient approach to the luminescence reduction is the hydrogen saturation of fiber samples; however, even in this case, the level of residual luminescence is still too high for three-photon registration.

[Period-tripling in Multiscale Physical and Biological Events].

A recent paper by S.J. Puetz et al. (Chaos, Solitons -& Fractals, v. 62-63, p. 55, 2014) described a fundamental period-tripled model. It involves periods of different astronomical (quasars, Sun), geophysical (geomagnetic, climatic, volcanic) and some biological processes. This work contains statistics for sixteen pairs of a period-tripled sequence. These periods range from -50 years to 1.5 billion years and no signs of the timescale limitations are found. We believe that the universal scope of the fundamental period-tripled model can be used for the development of new methodology of research data analysis: the main idea is that the spectrum of the periods of the studied event should be tested for the similarity with the spectrum of fundamental period-tripling pattern (because of the fundamental nature of the period-tripled model). Using this method, in this study we complement an already described period-tripled model with periods of human memory performance ranging from one minute to one month also adding seven relevant periods/frequencies of the period-tripled model in the range of human hearing. We make a conclusion that these characteristic frequencies may form the basis for music and singing phenomena. The new methodology is particularly appropriate for being applied in medicine and engineering.

Isolated attosecond pulses from laser-driven synchrotron radiation.

A quantitative theory of attosecond pulse generation in relativistically driven overdense plasma slabs is presented based on an explicit analysis of synchrotron-type electron trajectories. The subcycle, field-controlled release, and subsequent nanometer-scale acceleration of relativistic electron bunches under the combined action of the laser and ionic potentials give rise to coherent radiation with a high-frequency cutoff, intensity, and radiation pattern explained in terms of the basic laws of synchrotron radiation. The emerging radiation is confined to time intervals much shorter than the half-cycle of the driver field. This intuitive approach will be instrumental in analyzing and optimizing few-cycle-laser-driven relativistic sources of intense isolated extreme ultraviolet and x-ray pulses.

Electrically controlled polarized photoluminescence of CdSe/ZnS nanorods embedded in a liquid crystal template.

A novel homogeneous composite material, consisting of luminescent CdSe/ZnS quantum nanorods, embedded in the nematic liquid crystal 5CB, has been prepared. Liquid crystal cells and free-standing stretched polymer films incorporating this composite material were characterized using polarized micro-photoluminescence and electro-optical measurements under an applied electric field. A liquid crystal induced, macroscopic orientation of the nanorods in a thin layer of the material has been demonstrated. A conventional liquid crystal cell, filled with this composite, exhibits 40% modulation of the nanorod's photoluminescence intensity when subjected to an external electric field. These results indicate that quantum nanorods may have practical applications in photonic devices.

Electrode screening by ionic liquids.

In this work we are concerned with the short-range screening provided by the ionic liquid dimethylimidazolium chloride near a charged wall. We study the free energy profiles (or potentials of mean force) for charged and neutral solutes as a function of distance from a charged wall. Four different wall charge densities are used in addition to a wall with zero charge. The highest magnitude of the charge densities is ±1 e nm(-2) which is close to the maximum limit of charge densities accessible in experiments, while the intermediate charges ±0.5 e nm(-2) are in the range of densities typically used in most of the experimental studies. Positively and negatively charged solutes of approximately the size of a BF ion and a Cl(-) ion are used as probes. We find that the ionic liquid provides excellent electrostatic screening at a distance of 1-2 nm. The free energy profiles show minima which are due to layering in the ionic liquid near the electrodes. This indicates that the solute ions tend to displace ionic liquid ions in the layers when approaching the electrode. The important role of non-electrostatic forces is demonstrated by the oscillations in the free energy profiles of uncharged solutes as a function of distance from the wall.

A superionic state in nano-porous double-layer capacitors: insights from Monte Carlo simulations.

Recently observed anomalous properties of ionic-liquid-based nanoporous supercapacitors [C. Largot et al., J. Am. Chem. Soc., 2008, 130, 2730-2731] have attracted much attention. Here we present Monte Carlo simulations of a model ionic liquid in slit-like metallic nanopores. We show that exponential screening of the electrostatic interactions of ions inside a pore, as well as the image-charge attraction of ions to the pore surface, lead to the 'anomalous' increase of the capacitance with decreasing the pore width. The simulation results are in good agreement with the experimental data. The capacitance as a function of voltage is almost constant for low voltages and vanishes above a certain threshold voltage. For very narrow pores, these two regions are separated by a peak. With increase of the pore size the peak turns into a bump and disappears for wide pores. This effect, related to a specific character of the voltage-induced filling of nanopores with counterions at high densities, is yet to be verified experimentally.

Side chain dynamics and alternative hydrogen bonding in the mechanism of protein thermostabilization.

To elucidate the mechanism of protein thermostabilization, the thermodynamic properties of small monomeric proteins from mesophilic and thermophilic organisms have been analyzed. Molecular dynamics simulations were employed in the study of dynamic features of charged and polar side chains of amino acid residues. The basic conclusion has been made: surface charged and polar side chains with high conformational mobility can form alternative hydrogen bonded (H-bonded) donor-acceptor pairs. The correlation between the quantitative content of alternative H-bonds per residue and the temperature of maximal thermostability of proteins has been found. The proposed mechanism of protein thermostabilization suggests continuous disruption of the primary H-bonds and formation of alternative ones, which maintain constant the enthalpy value in the native state and prevent a rapid increase of the conformational entropy with the rising temperature. The analysis of the results show that the more residues located in the N- and C-terminal regions and in the extended loops that are capable of forming alternative longer-range H-bonded pairs, the higher the protein thermostability.

Molecular dynamics simulation of the behaviour of water in nano-confined ionic liquid-water mixtures.

This work describes the behaviour of water molecules in 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid under nanoconfinement, between graphene sheets. By means of molecular dynamics simulations, the adsorption of water molecules at the graphene surface is studied. A depletion of water molecules in the vicinity of the neutral and negatively charged graphene surfaces, and their adsorption at the positively charged surface are observed in line with the preferential hydration of the ionic liquid anions. The findings are appropriately described using a two-level statistical model. The confinement effect on the structure and dynamics of the mixtures is thoroughly analyzed using the density and the potential of mean force profiles, as well as by the vibrational densities of the states of water molecules near the graphene surface. The orientation of water molecules and the water-induced structural transitions in the layer closest to the graphene surface are also discussed.

Restructuring of the electrical double layer in ionic liquids upon charging.

We have investigated the electrical double layer (EDL) structure at an interface between ionic liquid (IL) and charged surface using molecular dynamics simulations. We show that for three different models of ILs the EDL restructuring, driven by surface charging, can be rationalized by the use of two parameters--renormalized surface charge (κ) and charge excess in the interfacial layers (λ). Analysis of the relationship between the λ and κ parameters provides new insights into mechanisms of over-screening and charge-driven structural transitions in the EDL in ionic liquids. We show that the restructuring of the EDL upon charging in all three studied systems has two characteristic regimes: (1) transition from the bulk-like (κ(Ion) = 0) to the multilayer structure (κ(Ion) ≈ 0.5) through the formation of an ionic bilayer of counter- and co-ions; and (2) transition from the multilayer (κ(Ion) ≈ 0.5) to the crowded (κ(Ion) > 1) structure through the formation of a monolayer of counter-ions at κ(Ion) = 1.

Wavelet treatment of radial distribution functions of solutes.

Discrete wavelets are applied to parametrize the radial distribution functions of hydrated ions and hydrophobic solutes. The data on radial distribution functions are derived from the integral equation theory and neutron scattering experiment. The Coifman and the discrete Meyer basis sets are used for the wavelet approximation. The quality of the approximation is verified by calculations of the solvation energy, the coordination number, and the change in chemical potential of solutes.

Wavelet treatment of the intrachain correlation functions of homopolymers in dilute solutions.

Discrete wavelets are applied to the parametrization of the intrachain two-point correlation functions of homopolymers in dilute solutions obtained from Monte Carlo simulations. Several orthogonal and biorthogonal basis sets have been investigated for use in the truncated wavelet approximation. The quality of the approximation has been assessed by calculation of the scaling exponents obtained from the des Cloizeaux ansatz for the correlation functions of homopolymers with different connectivities in a good solvent. The resulting exponents are in better agreement with those from recent renormalization group calculations as compared to the data without the wavelet denoising. We also discuss how the wavelet treatment improves the quality of data for correlation functions from simulations of homopolymers at varied solvent conditions and of heteropolymers.

[Comparison of forms of "unfit histograms" by a wavelet transform method]. / Sravnenie formy "nesostoiatel'nykh gistogramm" s pomoshch'iu veivlet-analiza.

Two methods checking changes in time of histogram patterns were investigated. These methods are relied on discrete wavelet transform. The result is that in the some cases these methods can more effectively to examine latency in the behavior of histograms form constructed by on overlapping short segments of experimental data.

[The study of autowave mechanisms of electrocardiogram variability during high frequency arrhythmias: mathematical modeling data]. / Issledovanie avtovolnovykh mekhanizmov variabel'nosti élektrokardiogramm vo vremia vysokochastotnykh aritmii: rezul'tat matematicheskogo modelirovaniia.

High-frequency cardiac arrhythmias are very dangerous, as they quite often lead to sudden death. These high-frequency arrhythmias are frequently produced by rotating autowaves. In the given work, the dynamics of a rotating three-dimensional excitation scroll wave and the influence of this dynamics on the variability of model electrocardiograms (ECGs) were simulated with the use of the Aliev--Panfilov model for both homogeneous and heterogeneous excitable media. Model ECGs were obtained by summing up local membrane potentials, while ECG variability was estimated numerically through the normalized variability analysis. In the homogeneous medium, the stability of the scroll wave to its filament perturbations was shown to be dependent both on the excitability of the medium and tension of the filament, while in the heterogeneous medium, the scroll was shown to be unstable. It was shown that the scroll wave dynamics affects essentially the variability of the model ECGs, and the ECG variability increases as the excitation threshold value grows. It was found that, at some parameters of the excitable medium, the variability of ECGs in the homogeneous medium is higher than in the heterogeneous medium.

[A possible relationship between the scatter in results of measurements of 239Pu alpha-activity and quasiperiodic fluctuations in a geophysical medium]. / O vozmozhnoi sviazi razbrosa rezul'tatov izmenenii al'fa-aktivnosti obraztsov 239Pu s kvaziperiodicheskimi kolebaniiami v geofizicheskoi srede.

A correlation between quasiperiodic variations in 239Pu alpha-activity and geophysical periods was revealed.

[Regular changes in histogram forms in physical measurements and mathematical modeling]. / O zakonomernykh izmeneniiakh form gistogramm, poluchennykh pri fizicheskikh izmereniiakh i matematicheskom modelirovanii.

A study of macroscopic fluctuations for objects separated by large distances confirmed the conclusion drawn earlier that, if the objects being measured are in different time zones, the increase in the probability of occurrence of histograms of similar form corresponds to the difference in the local time at the points of measurement. It was also found that, upon realization of pseudo-random sequences of numbers in mathematical generators, sequences of histograms very similar to those in real physical series can be realized. This suggests the presence of previously unknown regularities, both physical and mathematical, in sequences traditionally considered as absolutely random.

[Correlation of fine structures of distributions of amplitudes of a photomultiplier dark current fluctuations with the Earth rotations about its axis]. / Korreliatsiia tonkoi struktury raspredeleniia amplitud fluktuatsii temnovogo toka fotoumnozhitelei s vrashcheniem Zemli vokrug svoei osi.

The fine structures of distributions of photomultiplier dark current fluctuations measured in two laboratories 2000 km distant from other: in the international Institute of Biophysics (Neuss, Germany) and in the Moscow State University (Moscow, Russia) were compared. It is shown that similar forms of appropriate histograms are apparently more often realized at both locations at the same local time. This confirms the previous conclusion that the fine structure of distributions correlates with rotation of the Earth about its axis.

Heterogeneity of the local structure in sub- and supercritical ammonia: a Voronoi polyhedra analysis.

We report results of molecular dynamics simulations and detailed analysis of the local structure of sub- and supercritical ammonia in the range of temperature between 250 and 500 K along the 135 bar isobar. This analysis is based on the behavior of distributions of metric and topological properties of the Voronoi polyhedra (VP). We show that by increasing the temperature, the volume, surface, and face area distributions of the Voronoi polyhedra as well as the vacancy radius distribution broaden, particularly near the temperature T(α), where the calculated thermal expansion coefficient has its maximum. Furthermore, the rate of increase of the corresponding mean values and fluctuations increases drastically when approaching T(α). This behavior clearly indicates that the local structure, as described by the VP, becomes increasingly heterogeneous upon approaching this temperature. This heterogeneous distribution of ammonia molecules is traced back to the increase of the large voids with increasing temperature, and is also clearly seen in the behavior of the fluctuation of the local density, as measured by the VP. More interestingly, the maximum in the heterogeneity coincides with the maximum of the fluctuation in the density of the VP.

Anisotropically and high entanglement of biphoton states generated in spontaneous parametric down-conversion.

We show both theoretically and experimentally that biphoton wave packets generated via spontaneous parametric down-conversion can be strongly anisotropic and highly entangled. The conditions under which these effects exist are found and discussed.

Observation of nonspreading wave packets in an imaginary potential.

We propose and experimentally demonstrate a method to prepare a nonspreading atomic wave packet. Our technique relies on a spatially modulated absorption constantly chiseling away from an initially broad de Broglie wave. The resulting contraction is balanced by dispersion due to Heisenberg's uncertainty principle. This quantum evolution results in the formation of a nonspreading wave packet of Gaussian form with a spatially quadratic phase. Experimentally, we confirm these predictions by observing the evolution of the momentum distribution. Moreover, by employing interferometric techniques, we measure the predicted quadratic phase across the wave packet. Nonspreading wave packets of this kind also exist in two space dimensions and we can control their amplitude and phase using optical elements.

Wavelet treatment of structure and thermodynamics of simple liquids.

A new algorithm is developed to solve integral equations for simple liquids. The algorithm is based on the discrete wavelet transform of radial distribution functions. The Coifman 2 basis set is employed for the wavelet treatment. To solve integral equations we have applied the combined scheme in which the coarse part of the solution is calculated by wavelets, while the fine part by the direct iterations. Tests on the PY and HNC approximations have indicated that the proposed procedure is more effective than the conventional method based on the hybrid algorithm. Possibilities for application of the method to molecular liquids and mixed quantum-classical systems are discussed.