In 1911, Kamerlingh Onnes discovered that the resistivity of Hg becomes zero at 4.19 K, which is the phenomenon known as superconductivity. Since then, many kinds of superconductors have been discovered by extensive efforts. Superconductors show i) perfect conductivity and ii) perfect diamagnetism below superconducting transition temperatures T_c. In 1957, J. Bardeen, L.N. Cooper, and J.R. Schrieffer (BCS) shed light on the elucidation of the mechanism of the superconductivity. Their theory was that superconductivity was mediated by a coupling between electrons and phonons, making various underlying questions clear.

After 1986, superconductors in copper-oxides with CuO₂ planes were reported. The mother compounds of the superconductors are antiferromagnetic insulators, where the BCS theory cannot explain the mechanism of the superconductivity. The T_c's in cuprates are extremely higher than those in usual superconductors, so that they are called high-T_c cuprate superconductors. The mechanism has not been clear, although more than 20 years of extensive researches all over the world.

In this presentation, I would like to briefly introduce the character of cuprate superconductors and one of most advanced experimental results in our laboratory.

1911年にH. Kamerlingh OnnesによりHgでの超伝導が発見されて以来、様々な金属・合金で超伝導体が発見されてきた。すべての超伝導体は、超伝導転移温度T_c以下で i) 電気抵抗ゼロおよび ii) 完全反磁性を示し、その微視的なメカニズムは、1957年にBardeen、Cooper、SchriefferによるBCS理論により説明することができた。

その後1986年以降、CuO₂面を有する銅酸化物において超伝導が発見された。これらは反強磁性絶縁体を母物質とし、その超伝導発現機構はBCS理論では説明できない。また従来の超伝導体にくらべT_cが非常に高いことから「高温超伝導体」とよばれ、これまで20年以上ものあいだ世界中で精力的な研究が行われてきたが、いまだ発現機構は解明されていない。

今回の発表では高温超伝導体の特徴から最先端の結果までを、我々のグループの結果を含め簡単に紹介したい。