P. Almeras

Photoemission Spectromicroscopy - a New Insight in the Chemistry of Snox Films for Gas Sensors

We used the novel experimental technique of photoemission spectromicroscopy to investigate the active area of SnO(x) thin films for gas sensors, deposited by dc sputtering. First, we analyzed the degree of oxidation and the homogeneity of as-deposited films in a high lateral resolution (30 mum) spectromicroscopy mode with an energy resolution of 0.4 eV. This led to a surprising discovery of a large amount of tin monoxide on the film surface. Then we studied the interaction of H-2 and H2O with the surface of polycrystalline SnO2 films used as sensitive layers in actual gas sensor devices. These results have been related to the resistivity changes of the corresponding devices: we found that such changes are primarily due to a reduction process in the film rather than to mere chemisorption.

Doping-induced change of superconducting gap anisotropy in Bi2Sr2Ca1Cu2O8+δ

High-resolution angle-resolved photoemission measurements were performed on single crystals of Bi2Sr2Ca1Cu2O8+δ with different oxygen stoichiometries. The data establish that the gap anisotropy (ratio of the gap along Γ-M to the gap along Γ-X) can be reversibly changed from ∼20:1 (optimal or underdoped) to ∼2:1 (overdoped). Differences in sample doping explain the conflicting reports on gap anisotropy in the literature. Possible effects of this change in gap anisotropy on the symmetry of the order parameter are discussed. There remains some ambiguity as to the relation between the order parameter and doping.

Pb0.4bi1.6sr2cacu2o8+X and Oxygen Stoichiometry - Structure, Resistivity, Fermi-Surface Topology, and Normal-State Properties

Keywords: ANGLE-RESOLVED-PHOTOEMISSION ; CU-O SUPERCONDUCTORS ; ELECTRONIC-STRUCTURE ; MODULATED STRUCTURE ; CONVERGENT-BEAM ; BAND-STRUCTURE ; BI2SR2CACU2O8 ; PB ; YBA2CU3O7-DELTA ; YBA2CU3O6.9 Note: Univ wisconsin,ctr appl superconduct,madison,wi 53706. ecole polytech fed lausanne,inst phys appl,ch-1015 lausanne,switzerland. Ma, ja, univ wisconsin,dept phys,1150 univ ave,madison,wi 53706.ISI Document Delivery No.: QT246 Reference LSE-ARTICLE-1995-022View record in Web of Science Record created on 2006-10-03, modified on 2017-05-12

Spatially Localized Energy Shifts in the Photoemission Spectroscopy of Insulators

Core‐level photoemission peaks taken on several insulators with spatial resolution of 30–300 μm rigidly shift in energy from place to place, most likely because of local charging effects. Such phenomena can affect the linewidth and the general lineshape of conventional, spatially integrated spectra even when a flood gun is used, which is one of the standard remedies against charging. This may require a critical revision of photoemission linewidths and lineshapes for many past experiments on insulators.

The Effect of Yttrium on the Ca and Sr Planes of Y-Doped Bi2sr2ca1cu2o8

Photoemission spectromicroscopy experiments on a series of Y-doped Bi2Sr2Ca1Cu2O8 single crystals show that the doping changes the valence of copper as required for the observed modification of the critical temperature. The doping, however, affects with substitutional reactions not only the CaO planes, but also the SrO planes.

Co-doped Bi2Sr2CaCu2Ox: Anderson localization and superconductivity

Abstract Cobalt-doping of Bi 2 Sr 2 CaCu 2 O x beyond a critical level of 1.5–1.6 at % produces both a large decrease in the superconducting transition critical temperature and all of the classic symptoms of Anderson localization. These include: a negative slope of the abplane resistivity vs temperature, no dispersing states near the Fermi level and a decrease of the photoemission signal in the same region. The correlation between Anderson localization and high-temperature superconductivity at low doping levels supports a non-local interaction for the paring mechanism, such as antiferromagnetic spin fluctuations.

Local Chemical-Composition of Hgbacacuo High-Temperature Superconductors

The microchemical composition and the electronic structure of the two superconductors HgBa2Ca2Cu3O8+δ (Hg‐1223) and HgBa2Ca1Cu2O6+δ (Hg‐1212) were analyzed with the technique of photoemission with high spatial resolution. This enabled primary spectral peaks to be separated from their satellites due to local charging induced by the primary photon beam. Each sample contains superconducting and impurity phases with inhomogeneous spatial distributions. The oxygenation‐induced increase in the critical temperature is related to a increase in the Cu‐O plane carrier concentration.

Chemical inhomogeneity and reactions of BiSrCaCuO surfaces detected by synchrotron imaging spectromicroscopy

A microscopic chemical analysis of BiSrCaCuO (BSCCO) 2212 single crystals was performed with an unprecedented combination of high (submicron) lateral resolution and high energy resolution (100 meV or better). These performances, achieved with imaging synchrotron spectromicroscopy, enabled us to detect in a very small fraction of the investigated specimens, morphological and chemical inhomogeneities. Specifically, we found deviations from stoichiometry in the surface Sr content which have a subtle effect on the chemical environment of at least one other element, Ca. The different chemical environment was found to influence surface chemical reactions.

Localization Effects in Transition-Metal Doped Bi2sr2cacu2o8+Y High-Temperature Superconductors

Doping Bi2Sr2Ca1Cu2O8+y with Co causes a superconductor-insulator transition. We study correlations between changes in the electrical resistivity ρab(T) and the electronic bandstructure using identical single crystalline samples. For undoped samples the resistivity is linear in temperature and has a vanishing residual resistivity. In angle resolved photoemission these samples show dispersing band-like states. Co-doping decreases Tc and causes and increase in the residual resistivity. Above a threshold Co-concentration the resistivity is metallic (dρab/dT>0) at room temperature, turns insulating below a characteristic temperature Tmin and becomes superconducting at even lower temperature. These changes in the resistivity correlate with the disappearance of the dispersing band-like states in angle resolved photoemission. We show that Anderson localization caused by the impurity potential of the doped Co-atoms provides a consistent explanation of all experimental features. The coexistance of insulating (dρab/dT <0) normal state behavior and superconductivity indicates that the superconducting ground state is formed out of spatially almost localized carriers.

7Ni‐O chemical interaction and the transition temperature of Ni‐doped Bi2Sr2Ca1Cu2O8

We find that even very low Ni doping levels of high‐quality Bi2Sr2Ca1Cu2O8 single crystals strongly affect the transition temperature Tc. We also observed that Tc is not related to the total Ni concentration, but only to that of Ni engaged in NiO‐type bonds. By controlling the temperature during crystal growth, one can modify the relative weight of Ni in NiO‐type bonds with respect to other configurations—and therefore Tc.