critical temperature
/ KRIT-ih-kul TEM-per-uh-chur /
Water has a clear switching point: above zero degrees it is liquid, below it is ice, and the change happens at one definite temperature. Superconductors have their own switching point, but instead of freezing or melting, the material flips between an ordinary resistive metal and a perfect zero-resistance state. Cross that line and the change is just as sharp.
The critical temperature, written Tc, is the temperature below which a material becomes superconducting. Above it, electrons jostle and scatter as in any metal; the instant the material is cooled below Tc, Cooper pairs form and lock into their shared collective state, and resistance collapses. The value is a fixed property of each material — about 4 kelvin for mercury, 7 for lead, and over 90 for some copper-oxide compounds — set by how strongly the electrons can be made to pair.
This matters because Tc is the single number that decides how hard a superconductor is to use: a higher Tc means cheaper, easier cooling and broader applications. Be careful not to read Tc as a fixed cliff regardless of conditions — a strong magnetic field or a large current lowers the effective transition temperature, so a material that superconducts when idle may fail under load even though it is plenty cold.
The copper-oxide compound YBCO superconducts below about 92 kelvin — above the boiling point of liquid nitrogen at 77 kelvin — so it can be cooled with a cheap, widely available coolant instead of expensive liquid helium.
Crossing the 77 K liquid-nitrogen line was a turning point: above it, cooling becomes cheap.
The phrase 'high-temperature' superconductor is misleading to everyday ears: a Tc of 90 kelvin is still about minus 183 degrees Celsius, colder than anywhere on Earth's surface. It is 'high' only compared with the few-kelvin Tc of classic superconductors.