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.     

Variations of ionic binding energies and the distribution of charge carriers in orthorhombic La2−x Sr x CuO4

The significance of heterovalent, substitutional disorder for the distribution of charge carriers in La_2−x Sr _x CuO₄ has been investigated. Disorder is shown to cause strong variations of binding energies of the ions ranging to some eV for Sr contentsx=0.1. Balancing the energy for a hole transport, Cu³⁺+O²⁻→Cu²⁺+O⁻, and taking binding energy variations into account, the process is realized to become possible without consuming energy for a subset Θ for allx Cu³⁺ in one formula unit of La_2−x Sr _x CuO₄. The functions Θ(x) are presented for hole transports to apex and in-plane oxygens, respectively. The delocalization of charge carriers is interpreted to be caused by valency disorder on metal lattice sites.     

EPR of Fe3+ ion and symmetry of [AIF5·H2O]2? complex ion in (NH4)2AIF5·H2O single crystals

We used the spin-Hamiltonian method for the analysis of the electron paramagnetic resonance (EPR) spectrum of Fe³⁺ as a probe ion in (NH₄)₂AlF₅·H₂O single crystalline basic material. The theoretical expressions for the magnetic field (at which the fine structure transition lines appear) versus the angle between the magnetic field and the axis of symmetry of the magnetic complex are also given. These values were calculated by applying the perturbation theory to the second-order terms. From the experimental results (at 300 K and 9.21 GHz), the spin-Hamiltonian parameters were deduced:D=(668±10)·10⁻⁴ T,E=(−56±10)·10⁻⁴ T,a=(−54±10)·10⁻⁴ T,F=(30±10)·10⁻⁴ T. An isotropic superhyperfine structure was evidenced for the five fluorine ions. The obtained EPR data were used to determine the local symmetry of the Al³⁺ ion. A good agreement with X-ray diffraction measurements was found.     

Determination of absorption cross sections for oxygen molecules in the Schumann-Runge band region at high temperatures

We have measured the absorption cross sections of oxygen molecules in oxygen and in an oxygen-argon mixture heated by a shock wave, in the wavelength range 190–250 nm at temperatures of 1500–7000 K, for thermal equilibrium conditions behind the shock wave front. Analysis of the absorption cross sections obtained allowed us to select a data set that adequately describes the absorption characteristics of the electronic transition X³Σ _g ⁻ → B³Σ _u ⁻ for the oxygen molecule. In order to approximate the temperature dependence of these cross sections at a temperature of 1500–4500 K, we chose the function σ(λ, T) = σ₀(λ)(1 − exp (−θ/T)) exp (− n*θ/T) where θ₀ = 1.4·10⁻¹⁷, 1.4·10⁻¹⁷, 1.2·10^{− 17}, and 1.3·10⁻¹⁷ cm², n* = 3.1, 4.1, 5.6, and 7.47 for wavelengths 190, 210, 230, and 250 nm, respectively; θ = 2240 K is the characteristic temperature of the O₂ molecules. The approximation error was 19–25% and did not exceed the experimental error. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 13–17, January–February, 2006.     

Gemma experiment: Three years of the search for the neutrino magnetic moment

The result of the 3-year neutrino magnetic moment measurement at the Kalinin Nuclear Power Plant (KNPP) with the GEMMA spectrometer is presented. Antineutrino-electron scattering is investigated. A high-purity germanium detector of 1.5 kg placed at a distance of 13.9 m from the 3 GW_th reactor core is exposed to the antineutrino flux of 2.7 × 10¹³ cm⁻² s⁻¹. The scattered electron spectra taken in (5184 + 6798) and (1853 + 1021) h for the reactor ON and OFF periods are compared. The upper limit for the neutrino magnetic moment μ_v < 3.2 × 10⁻¹¹μ _B at 90% CL is derived from the data processing.     

Development of optical methods for simultaneous detection of I2, I−, IO 3 − , and I 3 − in liquid media in real Time

The absorption spectra of IO₃⁻ in liquid media are experimentally studied in the UV spectral range. The absorption cross sections are determined in the range 200–265 nm. The results are used to propose an optical method for the real-time simultaneous detection of molecular iodine and the I⁻, IO₃⁻ and I₃⁻¹ anions in liquid media. The measurement sensitivity for the above substances is sufficient for monitoring of the kinetics of physicochemical reactions that take place at various stages of processing of the spent nuclear fuel. This circumstance makes it possible to increase the efficiency and improve the ecological safety of the process.     

Measuring photoionization cross-sections of excited atomic states

A new method of photoionization cross-section measurement based on the observation of saturation in the ion yield as a function of radiation intensity has been proposed. The photoionization cross-sections for the 6²P_1/2 and 6²P_3/2 levels of Rb atoms of the fundamental and second harmonics of ruby laser radiation are measured, with a tunable pulsed dye laser used for excitation. The following values of cross-sections are obtained: (1.7±0.4)·10⁻¹⁷ cm² and (1.5±0.4)·10⁻¹⁷ cm² for the levels 6²P_3/2 and 6²P_1/2, respectively, ionized by radiation of v₂=14403 cm⁻¹, and (1.9±0.5)·10⁻¹⁷ cm² for the 6²P_3/2 level ionized by v₂=28806 cm⁻¹ radiation.     

The nonlinear optical property and photoinduced anisotropy of?a?novel azobenzene-containing fluorinated polyimide

A novel azobenzene-containing fluorinated polyimide was synthesized. The nonlinear optical property and photoinduced birefringence of a polyimide thin film were investigated. Large third-order nonlinear refraction (n ₂=−4.49×10⁻¹¹ cm²/W) was observed in the polyimide thin film by carrying out Z-scan measurement. The polyimide thin film exhibited larger nonlinear refraction than that of a mono-azo dye doped PMMA thin film (n ₂=−1.63×10⁻¹² cm²/W). The photoinduced birefringence of the polyimide thin film (▵ n∼10⁻²) under different pump intensities was investigated; it was much larger than that of the mono-azo dye doped PMMA thin film (▵ n∼10⁻³). Moreover, the time constants for birefringence growth and relaxation processes were determined.     

Electrical transport properties of CuWO4

The temperature dependence of the electrical conductivity, thermoelectric power and dielectric constant of the antiferromagnetic CuWO₄ have been studied in the temperature range 300–1000 K. The conductivity results can be summarised by the equations σ_I=6.31 × 10⁻³ exp (−0.29 eV/kT) ohm⁻¹ cm⁻¹ in the temperature range 300–600 K and σ_II=3.16 × 10⁵ exp (−1.48 eV/kT) ohm⁻¹ cm⁻¹ between 600 K and 1000 K. The thermoelectric power can be expressed byθ=[− 1.25 (10³/T) + 3.9] mV/K. Initially dielectric constant increases slowly but for high temperatures its increase is fast.     

The effect of Ca2+ in Li0.24+xLa0.59−xCaxTiO3-LaPO4 system

Li_0.2375+xLa_0.5875−xCa_xTiO₃-LaPO₄ fast ionic conductor was synthesized by high temperature solid-state reaction. A.C. impedance measurements show that the compositions of system have better conductivities in low doping content of Ca²⁺, but the conductivity goes down from 1.039·10⁻⁴ Scm⁻¹ to 1.173·10⁻⁵ Scm⁻¹ with increasing the content of Ca²⁺. The activation energy is about 20 kJ·mol⁻¹ in the temperature range of 25–400 °C. X-ray powder diffraction shows that the doping Ca²⁺ would not affect the structure of compositions in the system. The main phase is β-Li_0.24+xLa_0.59−xCa_xTiO₃ perovskite solid solution, LaPO₄ as a second phase also can be found. With the increasing content of Ca²⁺ (x>0.05), another phase CaTiO₃ appears obviously. IR measurement also indicates that the structure of compositions in the system would not be affected by Ca²⁺ doping. The decomposition voltage of Li_0.24+xLa_0.59−xCa_xTiO₃-LaPO₄ keeps on 1.6 V.     

ESR studies on calcite encrustation on Fili neotectonic fault, Greece

Electron spin resonance (ESR) study was done on calcite encrustation on Fili neotectonic fault surface, Greece. Normally such calcite encrustations on fault surfaces are not observed. Significantly, the ESR study has detected the presence of nitrate NO₃²⁻ radical in this calcite encrustation, havingg _⊥=2.0063±0.0001 and hyperfine coupling constantA _⊥=3.44 mT, the second such detection of nitrate NO₃²⁻ radical following a sample from Scott Glacier, Antarctica. From isochronal thermal annealing measurement the NO₃²⁻ radical was found to be quite stable, only fully annealed at 475°C. This study also shows that the ESR, as a tool, can be suitably applied to date the age of formation of the calcite encrustation with SO₃⁻ as an ESR dating signal by additive γ-ray irradiation. A preliminary estimation indicates the age of formation of calcite precipitation at Fili fault, Greece to be about 5600 years.     

Highly Selective Fluorescent Recognition of Pyrophosphate in Water by a New Chemosensing Ensemble

A new chemosensing ensemble that displays sensitive and selective fluorescent recognition of pyrophosphate in water at pH 7.4 has been developed. The ensemble is constructed by a copper complex (receptor) and eosin Y (indicator), the constructed ensemble is capable of highly selectively discriminate pyrophosphate from other common existing anions such as CH₃COO⁻, HSO₄⁻, NO₃⁻, H₂PO₄⁻, HPO₄²⁻, PO₄³⁻, NCS⁻, I⁻, Cl⁻, Br⁻, F⁻as well as some structurally similar carboxylates such as citrate, tartrate, oxalate, malonate, succinate and glutarate.     

Critical current in silver clad Y-Ba-Cu-O superconducting wires

Silver clad wires of highT _c superconductor Y₁Ba₂Cu₃O_7−x have been fabricated through the powder metallurgy technique. The reacted wires show a midpointT _c of 84K. A critical current density of 26·4 A cm⁻² (77K, 0T) is obtained in these wires. The wires, however, turn complete normal only at a current density of 280 A cm⁻². The reasons for low critical current density obtained in these wires are discussed.     

Small-size resonant photoacoustic cell with reduced window background for laser detection of gases

A photoacoustic resonant cell with the inner volume is ∼0.5 cm³ is presented. The shape of the cell cavity is chosen such as to minimize a background signal arising due to the absorption of laser beam in the cell windows. The experimental setup, the measurement procedure, and the design of the cell are described. The results of detecting ammonia in a nitrogen flow using the R(30) oscillating line of a CO₂ laser are represented. The minimal detectable absorption achieved in experiment is ∼3.2 × 10⁻⁸ cm⁻¹ W Hz^−1/2.     

Polymer compatibility, phase separation and high ambient conductivity in low — dimensional polymer electrolyte blends with lithium salts

AC and DC conductivities of complexes of Li salts with the amphiphilic helical polyether poly[2,5,8,11,14-pentaoxapentadecamethylene(5-hexadecyloxy-1,3-phenylene)] (I) (Type A complexes) and blends of I with copolymers of poly(tetramethylene oxide) oligomer coupled with either — (CH₂-)- (polymer IIC1) or — (CH₂)₁₂ — (IIC12) (Type C complexes) are reported. Whereas Type A complexes give reversible AC impedance plots log σ vs. 1/T plots which are ca. 10⁻⁷ S cm⁻¹ at ambient, the Type C blends rise to ca. 10⁻³ S cm⁻¹ at 100 °C and on cooling to ambient maintain this high level. In Type C systems with IIC12 this transformation is stable and permanent. Optical microscopy reveals phase separation of extensive well-organised lamellae of the Type A phase from the Type C blend following heating. Polymer II resides in thin layers in the interlamellar spaces serving to transfer ions between them. DC data at ambient temperatures for Li | I : II : Li salt | Li cells indicate conductivities 10⁻³ to 10⁻² S cm⁻¹ over extended periods (24 hours). Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Third neighbour hyperfine field shift in a diluteNiAl alloy

In the binary Ni_98.5Al_1.5 alloy we observed well resolved satellite hyperfine fields at¹¹¹Cd nuclear probes. The hyperfine shifts derivated are −4.0(3); −8.5(3) and ⃛14.0(3)% relative to the pure nickel value. From the relative intensity of the four components as well as from the quadrupole interaction data we identify the satellites as impurity-probe configurations containing zero, one, two or three Al impurities in the third neighbour shell only. The third neighbour contribution to the hyperfine field at Cd in nickel equals ΔH ₃= +4.5kG/μ _B-atom, allowing an estimate ΔH ₁≃ −17.8 kG/μ _B-atom for the first shell contribution, confirmed by a similar experiment on Ni_98.5Pd_1.5.     

Electron-hole liquid and excitonic molecules in quasi-two-dimensional SiGe layers of Si/SiGe/Si heterostructures

The electron-hole liquid (EHL) in SiGe layers of Si/Si_{1 − x} Ge _x /Si quantum-confinement heterostructures is discovered. It is composed of quasi-two-dimensional holes in the quantum well formed by the SiGe layer and quasi-three-dimensional electrons, which occupy a wider region of space centered on this layer. The densities of electrons and holes in the EHL are determined to be p ₀ ≈ 8.5 × 10¹¹ cm⁻² and n ₀ ≈ 4.8 × 10¹⁸ cm⁻³, respectively. It is demonstrated that the gas phase consists of excitons and excitonic molecules. The conditions on the band parameters of the structure under which the formation of the EHL of this kind and biexcitons is possible are formulated.     

2p3/2?13x?1?3x?13d?1 X-ray satellites spectra in the Lα1 region of 4d transition elements

The X-ray satellite spectra arising due to 2p _3/2 ⁻¹3x ⁻¹−3x ⁻¹3d ⁻¹ (x ≡ s, p, d) transition array, in elements with Z = 40 to 48, have been calculated, using available Hartree-Fock-Slater (HFS) data on 1s ⁻¹−2p ⁻¹ 3x ⁻ and 2p _3/2 ⁻¹−3x ⁻¹,3x ⁻¹ Auger transition energies. The relative intensities of all the possible transitions have been estimated by considering cross — sections for the Auger transitions simultaneous to a hole creation and then distributing statistically the total cross sections for initial two hole states 2p _3/2 ⁻¹−3x ⁻¹ amongst various allowed transitions from these initial states to 3x ⁻¹ 3d ⁻¹ final states by Coster-Kronig (CK) and shake off processes. In both these processes initial single hole creation is the prime phenomenon. Each transition has been assumed to give rise to a Gaussian line and the overall spectrum has been computed as the sum of these Gaussian curves. The calculated spectra have been compared with the measured satellite energies in Lα₁ spectra. Their intense peaks have been identified as the observed satellite lines. The peaks in the theoretical satellite spectra were identified as the experimentally reported satellites α₃, α₄ and α₅, which lie on the high-energy side of the Lα₁ dipole line.     

Measurements of the temperature and water vapor concentration in a hot zone by tunable diode laser absorption spectrometry

A tunable diode laser absorption spectroscopy (TDLAS) technique and appropriate instrumentation was developed for the measurement of temperature and water vapor concentrations in heated gases. The technique is based on the detection of the spectra of H₂O absorption lines with different energies of low levels. The following absorption lines of H₂O were used: 7189.344 cm⁻¹ (E″=142 cm⁻¹), 7189.541 cm⁻¹ (E″=1255 cm⁻¹), 7189.715 cm⁻¹ (E″=2005 cm⁻¹). Spectra were recorded using fast frequency scanning of a single distributed feedback (DFB) laser. A unique differential scheme for the recording of the absorption spectra was developed. An optimal technique for fitting the experimental spectra was developed.     

EPR spectra of γ-irradiated hydrated barium dithionate single crystals

EPR spectra of barium dithionate hydrate single crystals γ-irradiated at low (80 Gy) and high (10 kGy) doses are studied. Four lines, the strongest of which is due to the SO₃⁻ radical, are observed in the EPR spectrum of the low-irradiated samples. Another line seems to belong to SO₂⁻. The strong line and weak lines with hyperfine structure and lines for pairs of closely spaced SO₃⁻ centers with a strong angular dependence are observed at high irradiation doses. The main values of the SO₃⁻ hyperfine coupling tensor and the dipole-dipole coupling constants of the SO₃⁻ pairs are determined. A quantum-chemical calculation of the electronic structure of isolated SO₃⁻ and SO₂⁻ radicals is performed. Values of the g tensors and hyperfine couplings are calculated. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 18–22, January–February, 2008.