Self-injection length in La0.7Ca0.3MnO3-YBa2Cu3O7-δ ferromagnet-superconductor multilayer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La_0.7Ca_0.3MnO₃ (LCMO) and YBa₂Cu₃O_7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO₃ (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 μm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, I _c, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a J _c of ∼ 2 × 10⁶ A/cm², the LCMO side showed about half the value for the same thickness (1800 ?). The difference in J _c indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of J _c at two different thicknesses (1800 ? and 1500 ?) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 ?. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO₃ deposited by PLD on YBCO did not show any evidence of self-injection.     

Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

La_0.67Ba_0.33MnO₃ (LBMO) thin film is deposited on a 36.7°C SrTiO₃ bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.     

On the dynamical coupling between the dipolar and tunneling system of CH3-groups in solids at high temperatures

For methyl groups a dynamical coupling has been predicted between the dipolar and tunnelling systems, also at high temperatures where the observable tunnelling frequency at thermal equilibrium ω_t is zero. This is experimentally confirmed by observed non-exponential dipolar relaxation for ω₀²τ_c² ? 1.     

Molar extinction coefficients of some carbohydrates in aqueous solutions

Molar extinction coefficients of some carbohydrates viz. l-arabinose (C₅H₁₀O₅), d-glucose (C₆H₁₂O₆), d-mannose (C₆H₁₂O₆), d-galactose (C₆H₁₂O₆), d(-) fructose (C₆H₁₂O₆) and maltose (C₁₂H₂₄O₁₂) in aqueous solutions have been determined at 81, 356, 511, 662, 1173 and 1332 keV by gamma ray transmission method in a narrow beam good geometry set-up. These coefficients have been found to depend upon the photon energy following a 4-parameter polynomial. These extinction coefficients for different sugars having the same molecular formula have same values varying within experimental uncertainty. Within concentration ranges studied, Beer-Lambert law is obeyed very well.     

ESR of carbomethoxycyanobenzyl radicals. π- and σ-spin delocalization into the phenylring

The ESR spectra of a series of carbomethoxycyanobenzyl radicals have been examined at temperatures between 40° and 200°C. Anomalous spin-density distribution in the phenylring is observed with the sterically hindered orthomethyl substituted species. This is interpreted in terms of π- and σ-spin delocalization into the phenylring with the aid of INDO SCF MO calculations.     

Enantioselective hydrogenation of alkenes with iridium-PHOX catalysts: a kinetic study of anion effects

In the asymmetric hydrogenation of unfunctionalized olefins with cationic iridium-PHOX catalysts, the reaction kinetics and, as a consequence, catalyst activity and productivity depend heavily on the counterion. A strong decrease in the reaction rate is observed in the series [Al[OC(CF3)3]4]- >BArF- >[B(C6F5)4]- >PF6- >BF4- >CF3SO3-. With the first two anions, high rates, turnover frequencies (TOF >5000 h(-1) at 4 degrees C), and turnover numbers (TONs) of 2000-5000 are routinely achieved. The hexafluorophosphate salt reacts with lower rates, although they are still respectable; however, this salt suffers from deactivation during the reaction and extreme water-sensitivity, especially at low catalyst loading. Triflate and tetrafluoroborate almost completely inhibit the catalyst. In contrast to the hexafluorophosphate salt, catalysts with [Al[OC(CF3)3]4]-, BArF-, and [B(C6F5)4]- as counterions do not lose activity during the reaction and remain active, even after all the substrate has been consumed. In addition they are much less sensitive to moisture and, in general, rigorous exclusion of water and oxygen is not necessary. A first-order rate dependence on the hydrogen pressure was determined for the BArF- and the PF6- salts. At low catalyst loading, the rate dependence on catalyst concentration was also first order. The rate dependence on the alkene concentration was strikingly different for the two salts. While the reaction rate observed for the BArF- salt slightly decreased with increasing alkene concentration (rate order -0.2), a rate order of approximately 1 was determined for the corresponding hexafluorophosphate at low alkene concentrations.