Generation of 3rd and 5th harmonics in a thin superconducting film by temperature oscillations and isothermal nonlinear current response

The generation of harmonics of the voltage response is considered when an AC current is applied through a superconducting film above T_c. It is shown that almost at all temperatures the mechanism of the temperature oscillations created by the AC current and the temperature dependence of the resistance dominates over the isothermal nonlinear electric conductivity. Only in a narrow critical region close to T_c the latter is essential for the generation of the harmonics. A detailed investigation of harmonics generation provides an accurate method for measuring the thermal boundary conductance between the film and the insulating substrate. The critical behaviour of the third harmonic will give a new method for the determination of the lifetime of metastable Cooper pairs above T_c. The comparison of the calculated fifth harmonics of the voltage with the experiment is proposed as an important test for the applicability of the employed theoretical models. Received 8 September 2001     

Induced bromination of aromatic hydrocarbons with alkali metal bromides in the presence of oxidants

The features of induced bromination of aromatic hydrocarbons in the NaBr(KBr)-HX-H₂O₂(NaOCl) system were studied.     

Characteristics of radionuclide contamination of different zones of Semipalatinsk Nuclear Test Site “Opytnoe pole”

Data on the spatial distribution of radionuclides (²⁴¹Am, ²³⁹Pu, ¹³⁷Cs and ¹⁵²Eu) formed during nuclear explosions of different types near P2 SNTS test site are presented. Radionuclide contamination induced by the explosions varies in the concentrations of individual radionuclides, their proportions and species. Examination of the variations is a crucial task to plan remediation activities as well as those aimed at decrease of radiation risk for population and prevention of repeated contamination. Concentrations of ²⁴¹Am and ^239+240Pu that are the most toxic radionuclides in the area lie in hundred thousands of Bqkg^?1. The most contaminated areas are classified by the radionuclide concentration, ratio and form present in soil.     

Microwave promoted methylation of plant polysaccharides

O-Methylation is of outstanding importance in structural polysaccharide chemistry. A novel method for the methylation of polysaccharides using microwave (MW) irradiation is described. Seed gum from Cyamopsis tetragonolobus (Guar) was fully methylated with dimethyl sulphate and sodium hydroxide using 100% microwave power for 4 min in 68% yield. The completely methylated seed gum thus obtained was hydrolyzed by 70% formic acid followed by 0.5N H₂SO₄ under full microwave power for 1.16 and 1.66 min, respectively. The partially methylated monosaccharides were separated and identified.     

A diode-pumped passively Q-switched Nd:GdVO4 laser with a GaAs saturable absorption

We have demonstrated a passively Q-switched operation of Nd:GdVO₄ laser in which a GaAs crystal is used as the saturable absorber for the first time as far as we know. A maximum average output power of 1.64 W was obtained at an incident pump power of 12 W, the corresponding optical conversion efficiency and peak power were 13.7% and 116.8 W, respectively. The maximum peak energy obtained in the experiment by 50% transmission couple was 19 μJ.     

Runge–Kutta interpolants for high precision computations

Runge–Kutta (RK) pairs furnish approximations of the solution of an initial value problem at discrete points in the interval of integration. Many techniques for enriching these methods with continuous approximations have been proposed. Here we construct C ¹ continuous, eighth and ninth order interpolation methods for a recently appeared RK pair of orders 9(8). These interpolants share a very small leading truncation error making them suitable for use at quadruple precision, i.e. 32–33 decimal digits of accuracy. Extended numerical results justify our effort.     

Size of the interpolated cation and hole carrier density: two key parameters for the optimisation of colossal magnetoresistive properties of Pr-based manganites

The exploration of the magnetic and transport properties of four series of manganese perovskites, Pr_0.7Ca_0.34−xA_xMnO_3−δ (A=Sr, Ba), Pr_0.7−xLa_xCa_0.3 MnO_3−δ and Pr_0.66Ca_0.34−x Sr_xMnO_3−δ has allowed four phases with colossal magnetoresistive (CMR) properties to be isolated: Pr_0.7Ca_0.25Sr_0.025MnO_3−δ and Pr_0.66Ca_0.26Sr_0.08MnO_3−δ that exhibit a variation of resistance of 2.5. 10⁷% and 10⁹% at μ₀ H=5 T for T=88 K and 50 K respectively, Pr_0.58La_0.12Ca_0.3 MnO_3−δ that exhibits a variation of 6.10⁶% for μ₀ H=5 T at T=80 K and Pr_0.7Ba_0.025Ca_0.275MnO_3−δ for which a resistance variation of 5.10⁹%, at T=50 K, for μ₀ H=5 T is evidenced. for each compound of this series except the barium phase, one observes that the temperature T_max, which corresponds to the resistance maximum on the R(T) curves in zero magnetic field, increases dramatically as the mean size of the interpolated cations increases, and that the CMR effect correlatively decreases dramatically. The comparison of the two series Pr_0.7Ca_0.3−xSr_xMnO_3−δ and Pr_0.66Ca_0.34−xSr_xMnO_3−δ shows also the crucial role of the hole carrier density: for a same mean ionic radius of the interpolated cation T_max is decreased of about 50 K by introducing 0.034 hole per Mn mole.