Nonlinear transmission coefficient of ytterbium-holmium fiber at the wavelength 978 nm

An experimental and theoretical investigation of the nonlinear transmission coefficient of ytterbium-holmium codoped silica fiber (YHF) at continuous-wave 978-nm pumping is reported. An analysis of the fiber absorption and luminescence spectra under 978-nm pumping reveals a variety of processes participating in the mixed (Yb³⁺, Ho³⁺) system: the energy transfer Yb³⁺ → Ho³⁺, the up-conversion and excited-state absorption in Ho³⁺, and the luminescence quenching due to the presence of Yb³⁺-Yb³⁺ ion pairs. These processes are shown to notably affect the transmission coefficient of YHF, which is reflected in a pronounced modification of its dependences on pump power, fiber length, and dopant concentrations, in comparison with the case of purely ytterbium-doped fiber (YF). A modeling of the experimentally measured dependences of the YHF transmission coefficient on pump power and fiber length allows us to obtain the coefficients addressing the energy transfer process Yb³⁺ → Ho³⁺ in YHF. A comparison of YHF and YF at the same pumping conditions reveals that, in YHF, a considerable part of the pump power absorbed in the Yb³⁺ subsystem is transferred into the Ho³⁺ subsystem; this results in an effective population of its ⁵ I ₇ state and makes YHF promising for 2-μm lasing and amplifying.     

Computer study of the temperature dependence of physical properties of noncrystalline silicon nanoparticles

Variations in the structure and kinetic properties of vitreous and amorphous Si₄₀₀ nanoparticles upon heating from 300 to 1700 K are studied by molecular dynamics. The nanoparticle density increases with temperature and approaches the density of bulk solid silicon. A transition from a unimodal to a bimodal distribution of bond lengths is observed upon heating. This transition is more pronounced in the case of the vitreous nanoparticle. The average bond length in the amorphous nanoparticle is, as a rule, larger than that in the vitreous one, and the average number of bonds per atom is lower than that in the vitreous nanoparticle for nearly all studied temperatures. Negative values of the excess potential energy correspond to middle concentric layers of nanoparticles. Liquid layers form in the surface region of nanoparticles in the vicinity of the melting transition. A kinetic test indicating the beginning of nanoparticle melting is formulated.     

Specifics of the formation of alloys and their hydrides in Ti-Zr-H system

Studies of the combustion processes in the Ti-H, Zr-H, and Ti-Zr-H systems made it possible to develop a principally new method for producing refractory metal alloys through the compaction of a titanium hydride-zirconium hydride powder mixture followed by dehydrogenation. The procedure is briefly described. Experimental data on the formation of titanium-zirconium alloys with different structures, including an ω-phase alloy obtained at atmospheric pressure, are discussed. The experimental results clearly show that the structure of the alloy depends on the composition of the initial charge and the hydrogen content in the hydrides used. The interaction of the alloys obtained with hydrogen under conditions of self-propagating high-temperature synthesis yielded Ti_0.9Zr_0.1H_1.87 (Ti₉ZrH_18.7), Ti_0.67Zr_0.33H_1.81 (Ti₂ZrH_5.42), Ti_0.5Zr_0.5H_1.37 (TiZrH_2.74), and Ti_0.3Zr_0.7H_1.96 (TiZr_2.3H_6.53) hydrides with face-centered cubic (fcc) and body-centered tetragonal (bct) structures. The removal of hydrogen from these hydrides (by annealing in vacuum at 700–1050°C) results in the recovery of the initial α and ω phases.     

The performance of the jet identification and reconstruction in heavy ions collisions with CMS detector

A jet reconstruction algorithm is developed for events with a high particle density in the calorimetric system. The performance of the reconstruction of hard QCD jets with initial parton energies 50–300 GeV is studied in central Pb–Pb collisions with a modified cone jet finder which includes an algorithm for event-by-event background subtraction. The heavy ion background is simulated using the HIJING Monte Carlo generator with . Results on the achieved jet reconstruction efficiency, purity, energy and spatial resolution are presented.     

DICSI station at KCSR and NT: Determination of optimal requirements to the formation of an SR beam using cylindrical x-ray optical zoom lenses

To modernize the DICSI station, new hardware and software have been created according to the conditions of the operating storage ring “Sibir’-2” beam channel. The recording system has been successfully tested on the operating channel of the storage ring VEPP-3 (Budker Institute of Nuclear Physics Siberian Branch, Russian Academy of Sciences, Novosibirsk) using the time-resolved “diffraction cinema” technique in studying real biological objects in the static and dynamic modes of data accumulation. At the DICSI station, structural changes in medical and biological objects are permanently investigated. The specific structure dimensions of these objects are in the nanometric range.     

Ferroelectric phase transitions in ultrathin films of BaTiO3

We present molecular dynamics simulations of a realistic model of an ultrathin film of BaTiO3 sandwiched between short-circuited electrodes to determine and understand effects of film thickness, epitaxial strain, and the nature of electrodes on its ferroelectric phase transitions as a function of temperature. We determine a full epitaxial strain-temperature phase diagram in the presence of perfect electrodes. Even with the vanishing depolarization field, we find that ferroelectric phase transitions to states with in-plane and out-of-plane components of polarization exhibit dependence on thickness; it arises from the interactions of local dipoles with their electrostatic images in the presence of electrodes. Secondly, in the presence of relatively bad metal electrodes which only partly compensate the surface charges and depolarization field, a qualitatively different phase with stripelike domains is stabilized at low temperature.     

On the existence of ion-acoustic solitary waves in a gravitating dusty plasma having charge fluctuation

Theoretical investigation on the propagation of ion-acoustic waves in an unmagnetized self-gravitating plasma has been made for the existence of solitary waves using the reductive perturbation method. It is observed that nonlinear excitations follow a coupled third-order partial differential equation which is slightly different from the usual case of coupled Korteweg-de Vries (K-dV) system. It appears that the system so deduced is a two-component generalization of the previous one derived by Paul et al. (1999) in which it was shown that ion-acoustic solitary waves can not exist in such system.     

Stochastic model of tilting mode in nuclear fission

A model of induced nuclear fission was developed with consideration of thermodynamically fluctuating orientation degree of freedom (tilting) of deformed nuclei. This model was applied to analysis of the experimental angular anisotropy of fission fragments in the ¹⁶O + ²³²Th, ²³⁸U, ²⁴⁸Cm, ²⁰⁸Pb, ²⁰⁹Bi; ¹²C + ²³⁶U; ¹⁹F + ²⁰⁸Pb; and ¹¹B + ²³⁷Np reactions. Information on the equilibrating time of the tilting mode was obtained. The text was submitted by the authors in English.     

Entropy production as a regularization factor in solving an inverse diffusion problem

Inverse diffusion problems allow the diffusion coefficients to be determined from experimental concentration profiles. Solutions of the inverse diffusion problem are unstable toward perturbations of the initial concentration profiles. By the example of inverse diffusion problem solution for a model binary system it is demonstrated that the use of the production entropy to minimize the discrepancy functional stabilizes the solution even for significant perturbations of the initial concentration profile. The choice of the entropy production for the smoothing component of the discrepancy functional is physically adequate to the formulated problem, and the entropy production can be used to solve inverse problems of heat and mass transfer. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 79–83, June, 2006.     

Algorithm for nonlinear digital filtering of harmonic filling of phase-shift keyed signals

In this work, we propose a method for nonlinear digital filtering of phase-shift keyed signals with different carrier frequencies on the basis of separating the phase discontinuities of the input signals. The method allows one to avoid compensation for unknown frequency shift in the problems of determining the time delay of signals in the case of multichannel propagation. The efficiency of the proposed method is shown for the signals which are short informative packages with the phase-shift keying against the noise background in the presence of the Doppler effect. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 8, pp. 704–711, August 2006.