Copper oxide superconductors

Abstract

The copper oxides (cuprates) stand out in their high Tc values, 135 K, and are only family that achieved Tc exceeding 100 K. Thanks to the enormous research efforts over 28 years, understanding of the cuprates has remarkably advanced, though some important issues remain under hot debate. A consensus which has been achieved is that primarily repulsive electron-electron interactions lead to superconductivity with such high Tc as well as to unconventional d-wave pairing. Superconductivity emerges upon doping into the Mott insulating CuO2 planes. The temperature-doping phase diagram of the cuprates is extremely complex with several orders or incipient orders showing up. Among the known orders, the pseudogap order encompasses a wide doping-temperature region and appears to be home to the superconducting order and its fluctuation at very high temperatures. In contrast to these generic features, Tc values of the cuprates are strongly material dependent. A few parameters, such as disorder and multilayer, have been identified that control the wide-spread Tc. Tc is actually enhanced in some cuprates by reducing disorder in the crystal, and there is a possibility of Tc enhancement to 150 K or even higher by optimizing multilayer structure.