摘要
Ⅰ. INTRODUCTIONSince the discovery of oxide superconductor YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> with transition temperature above liquid nitrogen one year ago, a tremendous amount of work has been done on oxide superconductors in thousands of laboratories in the world. It has already been shown that yttrium could be replaced by other rare-earth elements
