Comparative XANES study on the two electron-doped high-Tc superconductor systems, (Sr,La)CuO2 and (Nd,Ce)2CuO4 (original) (raw)
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Physical review, 1990
Detailed analyses of Cu K-edge x-ray-absorption near-edge structure (XANES) of electron-doped and hole-doped Cu-based oxides with several Ce/Sr substitution rates, Nd2 "Ce"Cu04 y and La2 "Sr"Cu04 have revealed that each Ce/Sr substitution has an effect on-4-5 Cu sites and that an additional electron or hole supplied with the substitution is distributed mainly to-4-5 [CuO, ] units, yielding "impurity (localized) states. " When x =0.15 the "impurity states" amount to-60-75 % of the whole [Cu02] units; the interaction and superposition among the localized states are caused to produce bands like Fermi-liquid states. The substitution causes substantial and a little change in the XANES spectra of Nd-Ce-Cu-0 and La-Sr-Cu-O, respectively. This means that the electron doping takes place mainly at the Cu site of [CuOz] in Nd-Ce-Cu-0 and that the hole doping in La-Sr-Cu-0 takes place mainly at the oxygen site. In the "unreduced" Nd-Ce-Cu-0 samples no monovalent species but rather divalent species with relatively large 3d ' contributions are found. In the "reduced" and superconducting Nd-Ce-Cu-0 sample (x =0. 15), Cu20-like monovalent species are found but probably correspond to segregated impurity phases. The t:ssential effect of the reduction to induce the superconductivity in Nd-Ce-Cu-0 (x-0.15) has not yet been revealed.
Physica C-superconductivity and Its Applications, 1995
The local atomic and electronic structure of cerium substitutional defects in Ndl.soCeo.2oCuO4+ 8 was investigated by means of X-ray absorption spectroscopy (EXAFS and XANES of Ce L m edge), as a function of the oxygen partial pressure. It has been found that in all of the experimental conditions investigated (1, 10 -3 and 10 -6 atm), the Ce formal oxidation state should be regarded as IV. This is explained in terms of the bond configuration in this material. The consequences for the electron counting in the CuO 2 planes are discussed. As is shown by EXAFS, the increased charge on the 4e site in the T' structure mandates a local lattice distortion around cerium: the co-ordination cuboid collapses along the (001) direction. Consequently, the Ce-O bond length is shorter than the Nd-O bond length. 0921-4534/95/$09.50
Physical Review B, 1998
Polarization-dependent x-ray-absorption spectroscopy at the Cu L 3 and Ce M 4,5 edges has been performed on epitaxial films of Nd 1.85 Ce 0.15 CuO 4Ϫ␦ vs defect concentration induced by He ϩ ion irradiation. The Cu L 3 edge exhibits an increase in the integral intensity of both the white line in Eʈab (ϳ15%) and the first peak in Eʈc (ϳ20%) after superconductivity suppression. At the same time, partial filling of Ce 4 f orbitals (ϳ16%) takes place, though the shape of Ce M 4,5 edges remains the same as for the formally tetravalent Ce ion. The mechanism of superconductivity suppression in Nd 1.85 Ce 0.15 CuO 4Ϫ␦ under He ϩ ion irradiation is discussed. The Cu L 3 -edge measurements vs Ce doping indicate that the major part (ϳ70%) of the excess electrons fill Cu 3d x 2 Ϫy 2 states in Nd 1.85 Ce 0.15 CuO 4Ϫ␦ . The remaining part extends beyond the CuO 2 superconducting plane forming ''impurity'' states that seem to be localized within a few unit cells.
X-ray absorption studies of atomic and electronic structures of Nd2−xMxCuO4 (M = Ce and Th)
Physica B: Condensed Matter, 1990
Cu K and Ce L,-edge X-ray absorption near-edge structure (XANES) and extended fine structure (EXAFS) are employed to study the electronic and atomic structures of Ce and Th doped Nd,CuO, electron superconductors. A particular effort is made to identify possible monovalent copper impurity phases that can affect the interpretation of spectroscopic data in terms of the intrinsic electronic property of the doped materials. For samples with no impurity phase detectable in either EXAFS or X-ray diffraction, our XANES data show that excess electrons are introduced to the Cu site by Ce and Th doping. In support of electron doping, Ce is demonstrated to have a valence greater than 3+, similar to that in CeO,
The effect of cerium doping and oxygen treatment of Nd2−xCexCuO4Hall effect measurements
Physica C-superconductivity and Its Applications, 1991
We have measured the temperature dependent Hall coefficient RH and its variation with oxygen treatment and cerium concentration x in polycrystalline Nd2_xCexCuO r for x= 0.15, 0.17 and 0.18 (y~ 4). We find that the heat treatment of Nd2_xCexCuOy in a reducing atmosphere delocalizes the doped charge carriers, transforming Nd2_xCe~CuOy from a semiconductor into a superconducting metal. The apparent superconducting fraction in strongly reduced Nd2_xCexCuOy is a maximum for a particular cerium doping concentration Xoptim~. For X=)Coptimal, we find a negative Hall coefficient that saturates at low temperature, while for higher cerium doping concentrations, we find a positive Hall coefficient that continues to increase with decreasing temperature. The temperature dependence of the Hall coefficient at higher than optimal x appears to be dominated by the non-superconducting fraction. Contrary to some recent suggestions, the temperature dependence of the Hall coefficient is not evidence against the existence of n-type superconductivity in the "electron-doped" copper oxides. 0921-4534/91/$03.50©
The role of oxygen in the electron-doped superconducting system Nd2−x Ce x CuO4
Hyperfine Interactions, 1994
M6ssbauer effect studies suggest the presence of extraneous oxygen in the whole family of title compounds. At certain concentrations of Ce, oxygen is rather tenaciously bound. Such oxygen causes superparamagnetism in the magnetically ordered material, and also brings about the formation of spin clusters in the metallic material.
The effect of cerium doping and oxygen treatment of Nd2−xCexCuO4
Physica C: Superconductivity, 1991
We have measured the temperature dependent Hall coefficient RH and its variation with oxygen treatment and cerium concentration x in polycrystalline Nd2_xCexCuO r for x= 0.15, 0.17 and 0.18 (y~ 4). We find that the heat treatment of Nd2_xCexCuOy in a reducing atmosphere delocalizes the doped charge carriers, transforming Nd2_xCe~CuOy from a semiconductor into a superconducting metal. The apparent superconducting fraction in strongly reduced Nd2_xCexCuOy is a maximum for a particular cerium doping concentration Xoptim~. For X=)Coptimal, we find a negative Hall coefficient that saturates at low temperature, while for higher cerium doping concentrations, we find a positive Hall coefficient that continues to increase with decreasing temperature. The temperature dependence of the Hall coefficient at higher than optimal x appears to be dominated by the non-superconducting fraction. Contrary to some recent suggestions, the temperature dependence of the Hall coefficient is not evidence against the existence of n-type superconductivity in the "electron-doped" copper oxides.
Le Journal de Physique IV, 1997
Polarisation-dependent x-ray absorption spectroscopy at the Cu L3 and Ce M4,5 edges have been performed on the epitaxial films Nd1.85Ce0,15C~04-~ vs. defect concentration induced by ~e + ion irradiation. The Cu 5 edge exhibits increasing of integral intensity of both the white line in Ellab and the first peak in Ellc by about 15% and 30% , respectively, after suppression of superconductivity. The partial filling of Ce 4f orbitals (-15%) takes place although the Ce ions remain formally tetravalent, ~e +~. Thus, XAS experiments give a clear evidence that the excess electrons leave Cu 3d1O~-y2 orbital and become simultaneously localised in the Ce-O(2) chains. The symmetry of the fiee states and the mechanism of superconductivity suppression in Ndl.8jCeo, 15Cu04g under ~e + ion irradiation are discussed.
European Physical Journal B, 2004
The local structure around the Ce substitutional defects has been investigated by Ce and Nd K-edge EXAFS on Nd2−xCexCuO 4±δ samples with different dopant concentrations (x = 0.05, 0.10, 0.15 and 0.20) and oxygen amounts. The lattice distortion around the Ce • Nd can be described as a shrinkage of the oxygen cuboid along the z crystallographic direction. Moreover, the CuO4 planes become corrugated. The lattice distortion is well localized and is largely independent of temperature and Ce and oxygen content. EXAFS measurements made at the Nd-K edge indicate that the local chemical environment of Nd is not distorted. The difference electron density maps, obtained from low temperature single crystal X-ray diffraction data, revealed three strong positive residuals which have been ascribed to cerium (at 0, 0, z ∼ = 0.41) and to two non equivalent oxygen apical positions O3 (at 0, 0, z ∼ = 0.19) and O4 (at 0, 0 z ∼ = 0.24). The electron population of O4 site, which is closer to cerium (dCe−O4 = 1.96Å), is 3−4 times the O3 one.