7.5 Discovery of High Temperature Superconductivity

In 1986, 75 years after the discovery of superconductivity, George Bednorz and Karl Müller at IBM, Zurich demonstrated superconductivity in a perovskite structured lanthanum based cuprate oxide which showed a T_C of 35 K for which the inventors also won Physical Noble prize in 1987. This was a remarkable discovery as it later allowed chemical substitution in perovskite cuprates to push the transition temperatures well beyond the liquid nitrogen temperature (77 K) which is a much cheaper and easily accessible medium as compared to liquid helium.

This was realized by replacement of La by Y to give rise to YBa₂Cu₃O_7-x(YBCO) which showed aT_C of  ~92 K as first shown by Wu and his students at University of Alabama, Huntsville in 1987. The materials show highest T_C when the materials are slightly oxygen deficient i.e. when x = 0.15. Superconductivity disappears at x ≈ 0.6, when structure of YBCO changes from orthorhombic to tetragonal.

Subsequently many other oxides such as thallium and mercury based oxide compounds showed even higher transition temperatures and these are usually called as type-II superconductors.

A list of important oxide superconductors is shown below with their structures and transition temperatures. If you recall, some of these structures were shown in Module 1. As you can see that critical temperature is dependent very strongly on how the chemical substitutions are made into the parent structure.
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