We have prepared and identified as a single phase the high-temperature superconducting compound in the chemical system Y-Ba-Cu-O, an orthorhombic, distorted, oxygen-deficient perovskite of stoichiometry Ba2YCu309q (6=2.1). Samples exhibit zero resistance at 91 K, with a transition width of 1.5 K. The Meissner eAect attains a value of 76% of the independently measured diamagnetic susceptibility. We estimate parameters that characterize this superconductor, e.g. , @=3-5 mJ (mole Cu) K 2. The critical current density at 77 K and H =0 exceeds 1100 A/cm .
We have studied the effect of isotopic substitution on the superconducting Tc in the 90-K superconductors Ba2YCu307 and Ba2EuCu307 by replacing ^^O with the heavier isotope '^O. Samples with approximately 75% of the '^O replaced by '^O were prepared by gas-phase ion exchange. In these samples the phonon frequencies, measured by Raman spectroscopy, are reduced by the expected ~4%. The transition temperatures, however, are found to change by less than 0.2%. This change in Tc is much less than that expected for strongly coupled phonon-mediated superconductivity.
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