Browsing by Author "Chmaissem, O."

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    High-Tc superconducting cuprates, (Ce,Y)sO2s-2Sr2(Cu2.75Mo0.25)O6+d: Tc-increase with apical Cu-O decrease at constant Cu-O planar distance
    (2014) Marezio, M.; Chmaissem, O.; Bougerol, C.; Karppinen, M.; Yamauchi, H.; Geballe, T.H.; Department of Chemistry
    Evidence for high-Tc cuprate superconductivity is found in a region of the phase diagram where non-superconducting Fermi liquid metals are expected. Cu valences estimated independently from both XANES measurements and bond valence sum calculations are greater than 2.25 and are in close agreement with each other for structures of the homologous series given in the title with s = 1, 2, 3, 4 and 5. Two questions arise from the present perspective: 1) Is all the action in the CuO2 layers? 2) Is there superconductivity beyond the usual dome? The record short apical oxygen distance found in the homologous series especially in the s = 1 member, at odds with the current theory, suggests the possibility of a new pairing mechanism. The apical Cu-O distance in the s = 1 member decreases upon oxygenation from 2.29 to 2.15 dot A while the Cu valence increases to 2.45 dot A.
  • Thumbnail Image
    Item
    Overdoped cuprates with high-temperature superconducting transitions
    (AIP Publishing, 2013) Marezio, M.; Chmaissem, O.; Bougerol, C.; Karppinen, Maarit; Yamauchi, H.; Geballe, T. H.; Kemian laitos; Department of Chemistry; Kemian tekniikan korkeakoulu; School of Chemical Technology
    Evidence for high-Tc cuprate superconductivity is found in a region of the phase diagram where non-superconducting Fermi liquid metals are expected. Cu valences estimated independently from both x-ray absorption near-edge spectroscopy and bond valence sum measurements are >2.3, and are in close agreement with each other for structures in the homologous series (Cu 0.75 Mo 0.25)Sr2(Y,Ce)sCu2O5+2s+δ with s = 1, 2, 3, and 4. The record short apical oxygen distance, at odds with current theory, suggests the possibility of a new pairing mechanism. The possibility that the charge reservoir layers are able to screen long range coulomb interactions and thus enhance Tc is discussed.
  • Thumbnail Image
    Item
    Superconductivity and oxygen ordering correlations in the homologous series of (Cu, Mo)Sr2(Ce, Y)sCu2O5+2s+[delta]
    (American Physical Society (APS), 2010) Chmaissem, O.; Grigoraviciute, I.; Yamauchi, H.; Karppinen, Maarit; Marezio, M.; Kemian laitos; Department of Chemistry; Kemian tekniikan korkeakoulu; School of Chemical Technology
    A detailed study of the structure-property relationship is reported for the first four members of the high-Tc superconducting homologous series of (Cu,Mo)Sr2(Ce,Y)sCu2O5+2s+δ [(Cu,Mo)−12s2]. In this series, the adjacent CuO2 planes are separated by a single Y-cation layer for s=1 and a fluorite-type (Ce,Y)−[O2−(Ce,Y)]s−1 layer block for s≥2. Even though this series may be considered a conventional homologous series from the chemical point of view, we emphasize that the structures are different from those of the Tl-, Hg-, Bi-, etc.,-based series by the fact that the inserted fluorite-type blocks are insulating. We show the formation of the higher s members via intercalation of additional Ce-O2 layer(s) into the crystal lattices of the lower members of the series. Neutron powder-diffraction data demonstrate that the Ce/Y ratio is not constant at the different (Ce,Y) layers in the fluorite-structured block and that the innermost (Ce,Y) layer(s) are significantly Ce rich compared with the outer ones. Two independent crystallographic sites are identified for the extra oxygen atoms in the basal (Cu0.75Mo0.25)O1+δ plane with site fractional occupancies that strongly correlate with the properties of the material. A short-range ordered structure is proposed for the (Cu0.75Mo0.25)O1+δ layers that could explain both the superconducting properties of the materials and the enhanced Tc for the first member of the series.
  • Thumbnail Image
    Item
    Superconductivity and oxygen ordering correlations in the homologous series of (Cu,Mo)Sr2(Ce,Y)sCu2O5+2s+δ
    (2010) Chmaissem, O.; Grigoraviciute, I.; Yamauchi, H.; Karppinen, M.; Marezio, M.; Department of Chemistry
    A detailed study of the structure-property relationship is reported for the first four members of the high-Tc superconducting homologous series of (Cu,Mo)Sr2(Ce,Y)sCu2O5+2s+δ [(Cu,Mo)−12s2]. In this series, the adjacent CuO2 planes are separated by a single Y-cation layer for s=1 and a fluorite-type (Ce,Y)−[O2−(Ce,Y)]s−1 layer block for s≥2. Even though this series may be considered a conventional homologous series from the chemical point of view, we emphasize that the structures are different from those of the Tl-, Hg-, Bi-, etc.,-based series by the fact that the inserted fluorite-type blocks are insulating. We show the formation of the higher s members via intercalation of additional Ce-O2 layer(s) into the crystal lattices of the lower members of the series. Neutron powder-diffraction data demonstrate that the Ce/Y ratio is not constant at the different (Ce,Y) layers in the fluorite-structured block and that the innermost (Ce,Y) layer(s) are significantly Ce rich compared with the outer ones. Two independent crystallographic sites are identified for the extra oxygen atoms in the basal (Cu0.75Mo0.25)O1+δ plane with site fractional occupancies that strongly correlate with the properties of the material. A short-range ordered structure is proposed for the (Cu0.75Mo0.25)O1+δ layers that could explain both the superconducting properties of the materials and the enhanced Tc for the first member of the series.