H. Yamauchi

Precision synthesis of infinite-layer (Sr,La)CuO2 superconductors

We report a new synthesis route to prepare high-quality samples of the infinite-layer (Sr1−xLax)CuO2 superconductor system. This route includes a reductive annealing of the Sr–La–Cu–O precursor prior to high-pressure synthesis of the infinite-layer phase. Careful optimization of synthesis parameters allows us to obtain single-phase (Sr1−xLax)CuO2 samples within the wide composition range of 0≤x≤0.125. It is revealed that the value of Tc(≈43 K) is nearly independent of x, whereas the superconductivity volume fraction exhibits a bell-shaped behavior with x, peaking at x≈0.10 with a record high value of 86%.

Synthesis and Chemical Characterization of Pb2Ba2EuCu3Oz Samples

Solid state synthesis of the Pb-2213 phase with a simultaneous replacement of Sr by Ba and Y by Eu was found to yield single phase Pb2Ba2EuCu3Oz, samples. The oxygen content of the as-synthesized (final annealing in argon) sample (z = 8.16) was analyzed by a coulometric titration technique. An oxygen-rich sample corresponding to z = 9.79 was obtained by annealing the as-synthesized material in oxygen. The amount of the incorporated oxygen was conveniently controlled and determined by performing the oxygen-annealing in a thermobalance. The excess oxygen increased all the three lattice parameters and promoted an orthorhombic-to-tetragonal phase transition. The expansion of the Pb-2213 cell was seen in the 151Eu Mossbauer spectra as a decrease in the isomer shift. Even in the spectrum of the argon-annealed sample no traces of divalent europium were found, but the observed values of the Mossbauer parameters were typical of the Eu site in high T c cuprates.

High-Tc Superconducting Cuprates, (Ce,Y)sO2s-2Sr2(Cu2.75Mo0.25)O6+δ: Tc-increase with apical Cu-O decrease at constant Cu-O planar distance

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 while the Cu valence increases to 2.45 .