Resistivity and Carrier Mobility of the SmBa2 Cu3 O6+x Superconductor with Different Oxygen Doping Levels

Abstract DC conductivity measurements between 15 and 300 K are reported for SmBa2 Cu3O6+x samples with different oxygen doping amounts (x) produced by annealing under appropriate high temperature and oxygen pressure conditions and quenching. Samples with x≥0.5 are superconductors: Tc ~60 K at x=0.7, Tc >80 K at ; x=0.9. The transition from superconduction to non-superconduction corresponds to the tetragonal to orthorhombic structural tran-sition and to the transition from semiconducting to metallic temperature dependence of the resistivity. Oxygen doping causes a sudden increase of hole mobility near x=0.5. Below this threshold, the be-havior of the carrier mobility is in agreement with an Anderson localization

Early stages of solid state reactions: insights from micro-XRD and XAS

The mechanism of a solid state reaction in its early stages can be explored by investigating the time evolution of a model reactive system made of a thin layer of one reagent deposited onto a single crystal slab of the other reagent. Insights can be retrieved by comparing results at both local and long length scales obtained with films of different thicknesses and deposited onto different crystal orientations. In particular, reaction between ZnO and Al2O3has been chosen, as the spinel-forming reactions have been and still remain a model experimental system for investigating solid state reactions and because in the ZnO/Al2O3phase diagram, spinel is the only stable compound. The reaction initial steps have been investigated by using synchrotron X-ray diffraction, atomic force microscopy and X-ray absorption spectroscopy at the Zn-K edge starting from zincite films deposited onto (110)-, (012)-, (001)-oriented corundum single crystals [1,2]. The reaction eventually yields ZnAl2O4spinel but via a complex mechanism involving side and intermediate non-equilibrium compounds that do not appear in the equilibrium phase diagram of the pseudo-binary system. Spinel, when occurs, is polycrystalline at the end but initially forms with a few preferred orientations. Intermediate phases form before and in parallel with the growth of the spinel. Their number, composition, structure and kinetic role strongly depend on substrate orientation and film thickness. A more detailed understanding of the reactivity can be inferred by comparing EXAFS results to those of grazing incidence diffraction experiments of the films deposited on the (001) face of Al2O3and heat-treated at 10000C for different lengths of time. Information on the structure of the intermediate phases is given and results are discussed by comparing different films thickness to clarify the role of interfacial free energy and crystallographic orientation

Local structural properties of (Mn,Fe)Nb2O6 from Mössbauer and X-ray absorption spectroscopy

The MnNb2O6–FeNb2O6 solid solution has been investigated by Fe–K- and Mn–K-edge X-ray absorption (XANES and EXAFS), and Mössbauer spectroscopy. The first-shell M—O bond lengths deduced from EXAFS show a fairly small compositional dependence. A degree of static disorder, which increases with increasing manganese content, is clearly seen by the loss of correlation for the next-neighbour (NN) interaction. Hyperfine parameters from Mössbauer spectra are consistent with variations in the average environment, as recorded by X-ray data. Line broadening of the Mössbauer spectra provides evidence for next-neighbour effects and is consistent with there being no significant clustering of Fe or Mn within the samples. There appear to be differences in the way the columbite structure accommodates Fe2+ and Mn2+ ions. In ferrocolumbite all the Fe octahedra are close to being identical, while there are local structural heterogeneities at a longer length scale, presumably in ordering the precise topology of polyhedra immediately adjacent to the octahedron. By contrast, the manganocolumbite seems to have some diversity in the precise coordination at the MnO6 octahedra, but a greater uniformity in how the adjacent polyhedra are configured around them

Dewetting of Pt Nanoparticles Boosts Electrocatalytic Hydrogen Evolution Due to Electronic Metal-Support Interaction

Solid-state dewetting is the heat-induced agglomeration of thin metal films into defined nanoparticles (NPs). Dewetted Pt nanoparticles are investigated on F-doped SnO2 (FTO) substrates as model binder-free electrodes for the hydrogen evolution reaction (HER). Dewetting of Pt films into particles exposes the FTO substrate and the metal/support (Pt-FTO) contact line. Despite the decrease in Pt electrochemical surface area (ECSA) upon dewetting, dewetted NPs show a &gt;3-fold increase in ECSA-normalized HER activity compared to as-deposited nanocrystalline Pt films. Electrodes designed with dewetted Pt NPs of different sizes show that the HER activity does not only correlate with the ECSA but also increases with increasing the Pt-FTO contact line length. The smaller the NPs, the larger the Pt-FTO contact line, and the higher the activity. This effect is ascribed to electronic metal-support interaction (EMSI), due to electron transfer from FTO to Pt. It is proposed that EMSI effects alter the electronic structure of Pt sites near the Pt-FTO contact line, facilitating the H2 evolution kinetics. When NPs are a few nm-sized, a large mass fraction of Pt is affected by EMSI, resulting in a further increase of HER activity compared to NPs ≥10 nm despite the lower ECSA.</p

Dark Matter Substructure in Galactic Halos

We use numerical simulations to examine the substructure within galactic and cluster mass halos that form within a hierarchical universe. Clusters are easily reproduced with a steep mass spectrum of thousands of substructure clumps that closely matches observations. However, the survival of dark matter substructure also occurs on galactic scales, leading to the remarkable result that galaxy halos appear as scaled versions of galaxy clusters. The model predicts that the virialised extent of the Milky Way's halo should contain about 500 satellites with circular velocities larger than Draco and Ursa-Minor i.e. bound masses > 10^8Mo and tidally limited sizes > kpc. The substructure clumps are on orbits that take a large fraction of them through the stellar disk leading to significant resonant and impulsive heating. Their abundance and singular density profiles has important implications for the existence of old thin disks, cold stellar streams, gravitational lensing and indirect/direct detection experiments.Comment: Astrophysical Journal Letters. 4 pages, latex. Simulation images and movies at http://star-www.dur.ac.uk:80/~moore

Dependence of the Ce(III)/Ce(IV) ratio on intracellular localization in ceria nanoparticles internalized by human cells

CeO2 nanoparticles (CNPs) have been investigated as promising antioxidant agents with significant activity in the therapy of diseases involving free radicals or oxidative stress. However, the exact mechanism responsible for CNP activity has not been completely elucidated. In particular, in situ evidence of modification of the oxidative state of CNPs in human cells and their evolution during cell internalization and subsequent intracellular distribution has never been presented. In this study we investigated modification of the Ce(iii)/Ce(iv) ratio following internalization in human cells by X-ray absorption near edge spectroscopy (XANES). From this analysis on cell pellets, we observed that CNPs incubated for 24 h showed a significant increase in Ce(iii). By coupling on individual cells synchrotron micro-X-ray fluorescence (μXRF) with micro-XANES (μXANES) we demonstrated that the Ce(iii)/Ce(iv) ratio is also dependent on CNP intracellular localization. The regions with the highest CNP concentrations, suggested to be endolysosomes by transmission electron microscopy, were characterized by Ce atoms in the Ce(iv) oxidation state, while a higher Ce(iii) content was observed in regions surrounding these areas. These observations suggest that the interaction of CNPs with cells involves a complex mechanism in which different cellular areas play different roles

Metastable Ni(I)-TiO _2-x Photocatalysts: Self-Amplifying H₂ Evolution from Plain Water without Noble Metal Co-Catalyst and Sacrificial Agent

Decoration of semiconductor photocatalysts with cocatalysts is generally done by a step-by-step assembly process. Here, we describe the self-assembling and self-activating nature of a photocatalytic system that forms under illumination of reduced anatase TiO2 nanoparticles in an aqueous Ni2+ solution. UV illumination creates in situ a Ni+/TiO2/Ti3+ photocatalyst that self-activates and, over time, produces H-2 at a higher rate. In situ X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy show that key to self-assembly and self-activation is the light-induced formation of defects in the semiconductor, which enables the formation of monovalent nickel (Ni+) surface states. Metallic nickel states, i.e., Ni-0, do not form under the dark (resting state) or under illumination (active state). Once the catalyst is assembled, the Ni+ surface states act as electron relay for electron transfer to form H-2 from water, in the absence of sacrificial species or noble metal cocatalysts.Web of Science14548261322612