Physics

Hahaha, but to be fair, high Tc now typically refers to above the nitrogen boiling point of 77K, so 20K is a bit low even for them. I think the reference to 20K is in distinguishing between conventional vs unconventional superconductors--depending on if they can be explained by the BCS theory. When unconventional superconductor was first discovered, the highest known Tc for the conventional ones was about 20K. One could even say, therefore, calling it "high" is just experimentalists flexing to theorists.