ANOMALOUS PEAK STRUCTURE OF SPECIFIC-HEAT AROUND T(C) OF BA2YCU3OY

Through a systematic study of the effects of oxygen stoichiometry on the specific heat of the Ba2YCu3Oy system, we have confirmed that a double-peak structure around T(c) is disclosed only for well-annealed samples in a narrow region near y = 6.92 and that the double-peak structure merges to a distinct single peak of mean-field type as the oxygen concentration is slightly decreased to y = 6.88-6.90. Evidence for such a double transition was also obtained in resistivity and magnetic susceptibility measurements. These results imply that each grain in polycrystalline samples containing more oxygen than y = 6.90 consists of two different superconducting portions with different T(c)'s, probably the portion of lower T(c)(approximately 89 K) surrounding the higher T(c)(approximately 92 K) portion. We propose that this difference of T(c) results from a different oxygen configuration around CuO chains, implying the important role of such a local arrangement of oxygen around CuO chains in determining the superconductive nature in the Ba2YCu3Oy system. In contrast, only a broad anomaly was observed in ortho-II phase samples which no longer possess such a CuO chain structure.