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.     

Deposition-evaporation stochastic systems in two and higher dimensions

We study the stochastic dynamics of deposition-evaporation cooperative processes of dimers, trimers, etc., in two- and higher-dimensional lattices. The dimer system in bipartite lattices allows for an exact solution of dynamic correlations and scaling functions by means of a quantum spin equivalence. Autocorrelations exhibit a diffusive asymptotic kinetics and crossovers of different dynamic regimes in highly anisotropic lattices. Monte Carlo simulations combined with finite-size scaling arguments support the validity of the diffusive picture in more general situations. Steady-state coverages and diffusion constants are obtained using mean-field approaches, spin wave calculations, and random walk analyses in nearly jammed configurations.     

Echo planar imaging of perfluorocarbons

Emulsions of perfluorotributylamine (FTBA) and perflubron were evaluated for their utility in ¹⁹F echo planar imaging. Fluorine images of the emulsions were obtained in a phantom and two mice that had been predosed. Both agents, but particularly perflubron, show potential for fluorine echo planar studies because of the long spin-spin relaxation times of the CF₃ resonances. High resolution thin slice images obtained in as little as 26.6 ms are presented.     

Plant polysaccharides I. Polysaccharides ofLagochilus and their biological activity

Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, fax (3712) 89 14 75. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 313–314, March–April, 1995. Original article submitted November 7, 1994.     

Photosensitized formation of singlet oxygen by vitamins of the B group

Presented at the XV IUPAC Symposium on Photochemistry, Prague, July 17–22, 1994.     

Groups of Exponent Eight are Different

The free groups of exponent eight are shown to differ from thoseof sufficiently large exponent in the nature of their finitesubgroups.     

Amplification of radiation due to the four-photon parametric process in potassium vapor

The results of the experimental investigation of the radiation amplification due to the stimulated four-photon parametric process in potassium vapor are reported. Two cells with active medium (an oscillatoramplifier system) are used in the experiments. The dominating role of IR radiation 6S → 5P (3.66 µm) in the amplification of violet radiation 5P → 4S (0.4 µm) generated due to the parametric process at two-photon excitation 4S → 6S is shown. This role is conditioned by optically pumped stimulated emission (OPSE). Optical conditions for the amplification of a seed radiation are determined in accordance with their intensity and the buffer gas (He) pressure. The problem of the image conversion from the IR to a visible one with quantum efficiency above 100% is discussed.     

Logarithmic corrections in a quantization rule. The polaron spectrum

A nonlinear integrodifferential equation that arises in polaron theory is considered. The integral nonlinearity is given by a convolution with the Coulomb potential. Radially symmetric solutions are sought. In the semiclassical limit, an equation for the self-consistent potential is found and studied. The potential has a logarithmic singularity at the origin, and also a turning point at 1. The phase shifts at these points are determined. The quantization rule that takes into account the logarithmic corrections gives a simple asymptotic formula for the polaron spectrum. Global semiclassical solutions of the original nonlinear equation are constructed.Moscow Institute of Electronic Engineering; Moscow Power Institute. Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 97, No. 1, pp. 78–93, October, 1993.     

Invariants of chaotic Hamiltonian systems

The invariants of chaotic bounded Hamiltonian systems and their relation to the solutions of the first variational equations of the equations of motion are studied. We show that these invariants are characterized by the fact that they either lose the property of differentiability as functions on phase space or that a certain formal power series defined in terms of the derivatives of the invariants has zero radius of convergence. For a specific example, we show that the former possibility appears to apply.     

Numerical Study on Population Inversion and Lasing Conditions in an Optically Thick Recombining Helium Plasma

Lasing conditions for He I in a cold recombining helium plasma are numerically discussed including optical thicknesses. A collisional radiative model is used to calculate population densities. We discuss the effect of self absorptions by the background residual gas on the lasing conditions quantitatively. The optical thickness is essential in the calculation and affects the population inversions between level pairs of the singlet state drastically, while those of the triplet state are not affected very much. In the optically thin plasma, the laser lower levels of the singlet state depopulate mainly through radiative transitions. On the other hand, they depopulate mainly through electron collisional deexcitations in the optically thick plasma. Results of the calculations are verified by the spectroscopic observations of an arc-heated magnetically trapped expanding plasma jet generator which we developed. The requirement for the background pressure in which the lasing is possible is discussed quantitatively.