1,111 research outputs found
Superconductivity in CoSr2(Y1-xCax)Cu2O7+d
The roles of aliovalent Ca(II)-for-Y(III) substitution and
high-pressure-oxygen annealing in the process of "superconducterizing" the
Co-based layered copper oxide, CoSr2(Y1-xCax)Cu2O7+d (Co-1212), were
investigated. The as-air-synthesized samples up to x = 0.4 were found
essentially oxygen stoichiometric (-0.03 <= d <= 0.00). These samples, however,
were not superconductive, suggesting that the holes created by the
divalent-for-trivalent cation substitution are trapped on Co in the charge
reservoir. Ultra-high-pressure heat treatment carried out at 5 GPa and 500C for
30 min in the presence of Ag2O2 as an excess oxygen source induced bulk
superconductivity in these samples. The highest Tc was obtained for the
high-oxygen-pressure treated x = 0.3 sample at ~40 K.Comment: 16 pages, 6 figures, submitted to Solid State Communication
Oxygen non-stoichiometry in Ru-1212 and Ru-1222 magnetosuperconductors
Here we report the results of thermogravimetric (TG) analysis on the oxygen
non-stoichiometry of RuSr2GdCu2O8 (Ru-1212) and
RuSr2(Gd0.75Ce0.25)2Cu2O10(Ru-1222) samples. With TG annealings carried out in
O2 and Ar atmospheres it was found that the oxygen content in Ru-1212 remains
less affected upon various annealings, while for Ru-1222 wider-range
oxygen-content tuning is possible. When heated in H2/Ar atmosphere the both
phases release oxygen upon breaking down to mixtures of metals (Ru and Cu) and
binary oxides (CeO2, Gd2O3, and SrO) in two distinct steps around 300 and 450
oC. This reductive decomposition reaction carried out in a thermobalance was
utilized in precise oxygen content determination for these phases. It was found
that the 100-atm O2-annealed Ru-1212 sample was nearly stoichiometric, while
the similarly treated Ru-1222 sample was clearly oxygen deficient. X-ray
absorption near-edge (XANES) spectroscopy was applied to estimate the valence
of Ru in the samples. In spite of the fact that the Ru-1212 phase was shown to
possess less oxygen-deficient RuO2 layer, the valence of Ru as probed with
XANES was found to be lower in Ru-1212 than that in Ru-1222.Comment: 11 pages text, 4 pages Figs. To ISS 2002 YOKOHAMA for PHYSICA
Enhanced thermoelectric properties of NaxCoO2 whisker crystals
Single-crystalline whiskers of thermoelectric cobalt oxide, NaxCoO2, have
been grown by an unconventional method from potassium-containing compositions,
and their transport properties, and chemical compositions were determined. The
growth mechanism was analyzed and interpreted by means of thermogravimetric
analysis. At 300 K, electrical resistivity r and thermoelectric power S of the
whisker are 102 u ohm cm and 83 uV/K, respectively. The power-factor, S2/r,
thus is enhanced up to ~ 68 uW/K2 cm.Comment: 8 pages, 4 figures, to appear in Appl. Phys. Let
Experimental results and analysis from the 11 T Nb3Sn DS dipole
FNAL and CERN are developing a 5.5-m-long twin-aperture Nb3Sn dipole suitable
for installation in the LHC. A 2-m-long single-aperture demonstrator dipole
with 60 mm bore, a nominal field of 11 T at the LHC nominal current of 11.85 kA
and 20% margin has been developed and tested. This paper presents the results
of quench protection analysis and protection heater study for the Nb3Sn
demonstrator dipole. Extrapolations of the results for long magnet and
operation in LHC are also presented.Comment: 10 pages, Contribution to WAMSDO 2013: Workshop on Accelerator
Magnet, Superconductor, Design and Optimization; 15 - 16 Jan 2013, CERN,
Geneva, Switzerlan
Iron and molybdenum valences in double-perovskite (Sr,Nd)2FeMoO6: electron-doping effect
Double perovskite, (Sr1-xNdx)2FeMoO6, was doped with electrons through
partial substitution of divalent Sr by trivalent Nd (0 < x < 0.2). The Fe
valence and the degree of B-site order were probed by 57Fe Mossbauer
spectroscopy. Replacing Sr by Nd increased the fraction of Fe and Mo atoms
occupying wrong sites, i.e. antisite disorder. It had very little effect on the
Fe valence: a small but visible increase in the isomer shift was seen for the
mixed-valent FeII/III atoms occupying the right site indicating a slight
movement towards divalency of these atoms, which was more than counterbalanced
by the increase in the fraction of antisite Fe atoms with III valence state. It
is therefore argued that the bulk of the electron doping is received by
antisite Mo atoms, which - being surrounded by six MoV/VI atoms - prefer the
lower IV/V valence state. Thus under Nd substitution, the charge-neutrality
requirement inflicts a lattice disorder such that low-valent MoIV/V can exist.Comment: 15 pages, 6 figures, to appear in Solid State Commu
Overdoped Cuprates With High Temperature Superconducting Transitions
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 structure
(XANES) and bond valence sum (BVS) measurements are > 2.3 for structures in the
homologous series (Cu0.75Mo0.25)Sr2(Y,Ce)sCu2O5+2s+{\delta} with s = 1, 2, 3,
and 4. The s = 1 member, (Cu0.75Mo0.25)Sr2YCu2O7+{\delta}, 0 \leq {\delta} \leq
0.5, is structurally related to YBa2Cu3O7 in which 25% of the basal Cu cations
[i.e. those in the chain layer] are replaced by Mo, and the Ba cations are
replaced by Sr. After oxidation under high pressure the s = 1 member becomes
superconducting with Tc = 88K. The Cu valence is estimated to be ~2.5, well
beyond the ~2.3 value for which other High-Tc cuprates are considered to be
overdoped Fermi liquids. The increase in valence is attributed to the
additional 0.5 oxygen ions added per chain upon oxidation. The record short
apical oxygen distance, at odds with current theory, suggests the possibility
of a new pairing mechanism but further experiments are urgently needed to
obtain more direct evidence. From the structural point of view the members with
s \geq 2 are considered to be equivalent to single-layer cuprates. All have Tc
~ 56 K which is significantly higher than expected because they also have
higher than expected Cu valences. The XANES-determined valences normalized to
give values in the CuO2 layers are 2.24, 2.25, and 2.26 for s = 2, 3, and 4,
while the BVS values determined for the valence in the CuO2 layer alone are
2.31-2.34 for the s = 2 and 3 members. No evidence for periodic ordering has
been detected by electron diffraction and high resolution imaging studies. The
possibility that the charge reservoir layers are able to screen long range
coulomb interactions and thus enhance Tc is discussed
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