Unconventional Characteristics of REBCO Containing CuO Double Chain and Similar Defect Structures

Crystal structure of RE247 (RE₂Ba₄Cu₇O_15-<i>_δ</i>) can be regarded as alternate stacking of RE123 and RE124 along the <i>c</i>-axis direction. There are three copper sites in RE247, which are at CuO₂ plane, CuO_1-<i>_δ</i> chain and CuO double chain. Among them, CuO_1-<i>_δ</i> chain has large oxygen nonstoichiometry, <i>δ</i> = 0~1, as in the case of RE123. Superconductivity with <i>T</i>_c above 90 K can be observed for Y247 with <i>δ</i> ~ 0 , while it does not disappear even in largely oxygen deficient composition, <i>δ</i> ~1, because the mean valence of copper is always higher than 2. <i>T</i>_c of RE247 monotonically decreases with an increase in <i>δ</i> except for Pr247. Pr247 shows 20 K class bulk superconductivity at <i>δ</i> ~0.6 where CuO₂ plane is insulating state due to hole trapping by Pr³⁺. Therefore, the superconductivity of Pr247 is considered to be originated in CuO double chain. In addition, reentrant superconductivity by changing oxygen composition from <i>δ</i> ~ 0 to 0.7 was observed in partially Y doped Pr247, where superconductivity by CuO₂ plane is observed at <i>δ</i> ~ 0 and that by CuO double chain appeared at <i>δ</i> &gt; 0.4. (Pr,Y)247 also shows bulk superconductivity at <i>δ</i> ~0.5, however, its flux pinning properties are very poor reflecting very long interlayer distance between superconducting layer more than 2 nm. On the other hand, stacking faults similar to CuO double chain can be introduced in RE123 crystals by three new methods. For RE123 thin films, CuO double chain-like stacking faults was found to be introduced and their density and distribution were controlled by post annealing conditions, such as temperature, holding time and partial pressure of vapor. The defect introduced Y123 films show largely improved <i>J</i>_c in low field region. Similarly, CuO double chain-like stacking faults was introduced in silver coated RE123 films by oxygen annealing. Although the mechanism of defect generation due to diffusion of silver into the RE123 films is still unclear, some defect introduced films show improved <i>J</i>_c-<i>B</i> characteristics. High energy electron irradiation is one of the effective method to improve critical current properties of RE123 crystals. TEM observation revealed that CuO double chain-like short stacking faults generated in the crystal. Role of CuO double chain and similar defects on superconductivity and enhanced critical current properties of RE123 crystal will be discussed.