Augusto Marcelli

L2,3 XANES of the High Tc Superconductor YBa2Cu3O~7 with Variable Oxygen Content

Abstract The copper L2,3 soft x-ray absorption spectra of the single phase high temperature (Tc=91 K) superconductor YBa2Cu3O≈7 have been measured by using synchrotron radiation at LURE. The spectra show at the absorption threshold a white line at 931.1 eV due to Cu 2p 3d10 final states (the underline indicates the hole) for the Cu 3d9 initial state, and a broad high energy shoulder with the maximum at 933 eV assigned to Cu 2p 3d10 L and to Cu 2p 3d10 L 4s1 final states ( L indicates a hole in the oxygen 2p derived valence band), i.e. to the initial states 3d9 L . The intensity of this last feature has been found to decrease with decreasing oxygen content x in YBa2Cu3O6.5+x. No evidence of Cu 2p 3d9 final states has been found. This experiment gives direct evidence that additional oxygen x does not give Cu3+ ions with Cu 3d8 configuration but gives 3d9 L electronic configuration (i.e. Cu2+O- pairs) and no gap is observed between the 3d9 (Cu+) and 3d10 L configuration (i.e. Cu+O- pairs). Therefore in the metallic phase which exhibits superconductivity a continous set of itinerant states 3d9 L and 3d10 L is observed.

Optimum inhomogeneity of local lattice distortions in La2CuO4+y

Electronic functionalities in materials from silicon to transition metal oxides are, to a large extent, controlled by defects and their relative arrangement. Outstanding examples are the oxides of copper, where defect order is correlated with their high superconducting transition temperatures. The oxygen defect order can be highly inhomogeneous, even in optimal superconducting samples, which raises the question of the nature of the sample regions where the order does not exist but which nonetheless form the “glue” binding the ordered regions together. Here we use scanning X-ray microdiffraction (with a beam 300 nm in diameter) to show that for La2CuO4+y, the glue regions contain incommensurate modulated local lattice distortions, whose spatial extent is most pronounced for the best superconducting samples. For an underdoped single crystal with mobile oxygen interstitials in the spacer La2O2+y layers intercalated between the CuO2 layers, the incommensurate modulated local lattice distortions form droplets anticorrelated with the ordered oxygen interstitials, and whose spatial extent is most pronounced for the best superconducting samples. In this simplest of high temperature superconductors, there are therefore not one, but two networks of ordered defects which can be tuned to achieve optimal superconductivity. For a given stoichiometry, the highest transition temperature is obtained when both the ordered oxygen and lattice defects form fractal patterns, as opposed to appearing in isolated spots. We speculate that the relationship between material complexity and superconducting transition temperature Tc is actually underpinned by a fundamental relation between Tc and the distribution of ordered defect networks supported by the materials.

Evidence of 3d9-ligand hole states in the superconductor La1.85Sr0.15CuO4 from L3 X-ray absorption spectroscopy

The Cu L3 X-ray absorption spectra of La2−xSrxCuO4−y (x=0, 0.15 and 0.88) have been measured with high resolution. For comparison the L3 X-ray absorption spectrum of the formally trivalent Cu compound La2Li0.5Cu0.5O4 is reported. The semiconductor La2CuO4, with Cu formal valence v=2, shows a single white line due to the 2p3d10 final state arising from the Cu ed9 initial state. The superconducting sample with x=0.15 (Tc=37 K) shows the additional 2p3d10L final states arising from the initial states 3d9L (where L is a hole in the oxygen derived band, ligand hole). The characteristic feature of the superconducting material is that there is no energy gap between the final states 2p3d10 and 2p3d10L. In the semiconducting compound, formed by increasing Sr concentration (x=0.88), a red shift of 0.2 eV of the 2ped10 final state is enough to create a gap between the two states. The conductivity in these ceramic materials is assigned to the itinerant 3d9L states (probed by the 2p 3d10L final state) which indicate the presence of oxygen hole carriers. These results support a microscopic mechanism for high Tc superconductivity due to pairing of oxygen holes coupled with localized Cu 3d9 localized states i.e. of two itinerant 3d9L states.

REDUCTION AND SORPTION OF CHROMIUM BY Fe(II)-BEARING PHYLLOSILICATES: CHEMICAL TREATMENTS AND X-RAY ABSORPTION SPECTROSCOPY (XAS) STUDIES

The reduction of hexavalent chromium species in aqueous solutions by interaction with Fe(II)-bearing solid surfaces was studied using a 0.96 × l0−3 M Cr(VI) solution and iron-rich clays with different Fe(II)/Fe(III) ratios, layer charge, and exchange properties, i.e., chlorite, corrensite, and montmorillonite. Experimental studies demonstrated that Fe(II)-bearing phyllosilicates reduce aqueous Cr(VI) ions at acidic pH. Chlorite and corrensite, owing to the high Fe(II)/Fe(III) ratio, are electrochemically reactive, as rapid Cr(VI) reduction indicated. In contrast, montmorillonite showed minimum to nil reactivity towards Cr(VI). Furthermore, corrensite, which is high in both Fe(II)/Fe(III) ratio and exchange capacity, adsorbs the greatest amount of chromium.X-ray absorption spectroscopy at Al, Mg, Fe, and Cr K-edges was used to investigate the adsorbed chromium species. The montmorillonite sample, unaffected by treatment with Cr(VI) solution, displays no change at any investigated edge. Edge shape and energy also do not change for the Mg and Al spectra in corrensite, and changes are minor in chlorite. By contrast, the Fe K-edge changes both in chlorite and corrensite, and indicates an increase of Fe(III) in treated samples at the expense of pre-existing Fe(II). Cr K-edge spectra show that chlorite and corrensite sorb Cr(III), which implies its reduction from Cr(VI) in the interacting solution.

Structural determination of titanium-oxide nanoparticles by x-ray absorption spectroscopy

To understand and improve the applications of titanium-oxide nanoparticles, it is extremely important to perform a detailed investigation of the surface and the interior structural properties of nanocrystalline materials, such as rutile and anatase with diameter of few nanometers. Here, x-ray absorption spectroscopy has been used to identify the local Ti environment and characterize the related electronic structure. We combine experimental results at the Ti K edge in both bulk and nanocrystal samples to determine the lattice distortion via the characteristic pre-edge features and the variation in the multiple-scattering region of the x-ray absorption near-edge structure spectra. The correlation between peak intensities and surface-to-volume ratio of nanoparticles is also discussed.

Optical performances of SINBAD, the Synchrotron INfrared Beamline At DAΦNE

SINBAD (Synchrotron Infrared Beamline At DAphiNE) is the first Italian synchrotron radiation beamline operating in the infrared range. It collects the radiation emitted by DANE, an electron-positron collider designed to work at 0.51 GeV with a beam current I> 1 A. The actual performances of the beamline, in terms of brilliance gain with respect to blackbodies and polarization properties, are presented and discussed. Finally, the stability of the SINBAD source, a critical issue for Fourier-transform infrared spectroscopy, is discussed.

Biological applications of synchrotron radiation infrared spectromicroscopy

Extremely brilliant infrared (IR) beams provided by synchrotron radiation sources are now routinely used in many facilities with available commercial spectrometers coupled to IR microscopes. Using these intense non-thermal sources, a brilliance two or three order of magnitude higher than a conventional source is achievable through small pinholes (<10 μm) with a high signal to-noise ratio. IR spectroscopy is a powerful technique to investigate biological systems and offers many new imaging opportunities. The field of infrared biological imaging covers a wide range of fundamental issues and applied researches such as cell imaging or tissue imaging. Molecular maps with a spatial resolution down to the diffraction limit may be now obtained with a synchrotron radiation IR source also on thick samples. Moreover, changes of the protein structure are detectable in an IR spectrum and cellular molecular markers can be identified and used to recognize a pathological status of a tissue. Molecular structure and functions are strongly correlated and this aspect is particularly relevant for imaging. We will show that the brilliance of synchrotron radiation IR sources may enhance the sensitivity of a molecular signal obtained from small biosamples, e.g., a single cell, containing extremely small amounts of organic matter. We will also show that SR IR sources allow to study chemical composition and to identify the distribution of organic molecules in cells at submicron resolution is possible with a high signal-to-noise ratio. Moreover, the recent availability of two-dimensional IR detectors promises to push forward imaging capabilities in the time domain. Indeed, with a high current synchrotron radiation facility and a Focal Plane Array the chemical imaging of individual cells can be obtained in a few minutes. Within this framework important results are expected in the next years using synchrotron radiation and Free Electron Laser (FEL) sources for spectro-microscopy and spectral-imaging, alone or in combination with Scanning Near-field Optical Microscopy methods to study the molecular composition and dynamic changes in samples of biomedical interest at micrometric and submicrometric scales, respectively.

Evaporation of Ethanol and Ethanol-Water Mixtures Studied by Time-Resolved Infrared Spectroscopy

The knowledge of the physics and the chemistry behind the evaporation of solvents is very important for the development of several technologies, especially in the fabrication of thin films from liquid phase and the organization of nanostructures by evaporation-induced self-assembly. Ethanol, in particular, is one of the most common solvents in sol-gel and evaporation-induced self-assembly processing of thin films, and a detailed understanding of its role during these processes is of fundamental importance. Rapid scan time-resolved infrared spectroscopy has been applied to study in situ the evaporation of ethanol and ethanol-water droplets on a ZnSe substrate. Whereas the evaporation rate of ethanol remains constant during the process, water is adsorbed by the ethanol droplet from the external environment and evaporates in three stages that are characterized by different evaporation rates. The adsorption and evaporation process of water in an ethanol droplet has been observed to follow a complex behavior: due to this reason, it has been analyzed by two-dimensional infrared correlation. Three different components in the water bending band have been resolved.

Infrared properties of chemical-vapor deposition polycrystalline diamond windows

Low-resolution transmittance and reflectance spectra of high-quality chemical-vapor deposition (CVD) diamond windows were measured in the infrared in the 2.5-500-mum wavelength range (20-4000 cm(-1)). High-resolution measurements on a window with nearly parallel surfaces show well defined interference fringes at low frequencies. By standard procedures the optical constants n and k of CVD diamond were determined, for the first time to the authors knowledge, in the far-infrared region. It is shown that a window with a large wedge angle, close to 1 degrees , does not produce appreciable interference fringes. Modeling of these results confirms that interference fringes can be avoided by use of properly wedged CVD diamond windows. This result is of considerable relevance to the use of CVD diamond windows in spectroscopic applications for which fringe suppression is a major requirement.

Octahedral versus tetrahedral coordination of Al in synthetic micas determined by XANES

Abstract We used the JUMBO monochromator at SSRL to measure the Al K-edge X-ray absorption spectra of synthetic micas having variable Al content and occupancy, from 0 to 2/4 in the octahedral M positions, and 0 to ⅔ in the tetrahedral T positions. The measured Al K edges differ markedly, but the differences may have a common explanation: (1) Micas containing ⅓ Al in M or ¼ Al in T have K edges that differ in the energy and intensity of the first two features, which are related to interaction of Al with its first-shell nearest neighbors (O and OH or F). They are nearly identical to the K edges of reference minerals such as albite (tetrahedral Al only) or grossular (octahedral Al only). (2) Micas containing Al in both M and T have K edges that can be interpreted as a weighed combination of the simple edges.