High Temperature Superconductors

Ceramic materials are expected to be insulators -- certainly not superconductors, but that is just what Georg Bednorz and Alex Muller found when they studied the conductivity of a lanthanum-barium-copper oxide ceramic in 1986. Its critical temperature of 30 K was the highest which had been measured to date, but their discovery started a surge of activity which discovered superconducting behavior as high as 125 K.

Cuprate Superconductor Phases

Illustrative of the complexity of the high-temperature superconductor materials is this phase diagram which applies to the cuprate materials. At very low doping, they show the long range order of an antiferromagnet where the electron spins of neighboring domains are aligned antiparallel to each other.

Doping breaks up the antiferromagnetic order and they become insulators. Only with doping fraction between about 0.1 and 0.2 do they become superconductors.

YBa₂Cu₃O₇ Superconductor

The high temperature superconductors are ceramic materials with layers of copper-oxide spaced by layers containing barium and other atoms. The yttrium compound is somewhat unique in that it has a regular crystal structure while the lanthanum version is classified as a solid solution. The yttrium compound is often called the 1-2-3 superconductor because of the ratios of its constituents.