New bulk high-Tc superconductors of Pr_(1-x)Ca_xBa_2Cu_3O_(7-delta) prepared at high pressure

摘要

A new series of bulk superconductors, Pr_(1-x)Ca_xBa_2Cu_3O_(7-delta) (0.4<=x<=0.6), were prepared under a high pressure of 5 GPa at 1000 deg C. The structure and superconduction properties of samples are sensitive to the oxygen environments. The samples synthesized under a high oxygen-pressure environment with orthorhombic structure show the highest 52 K Tc in all ingredients. But the samples synthesized in a reducing environment with tetragonal structure exhibit optimal Tc as high as 98 K. Our results unambiguously demonstrate the valence state of Pr is a mixed sate greater than 3+. We conclude hole depletion is the primary cause of the suppression of superconductivity by Pr in PrBa_2Cu_3O_7, eliminating the hypothesis based upon pair-breaking.