Superconductivity - Classification

Classification

There is not just one criterion to classify superconductors. The most common are

• By their response to a magnetic field: they can be Type I, meaning they have a single critical field, above which all superconductivity is lost; or they can be Type II, meaning they have two critical fields, between which they allow partial penetration of the magnetic field.
• By the theory to explain them: they can be conventional (if they are explained by the BCS theory or its derivatives) or unconventional (if not).
• By their critical temperature: they can be high temperature (generally considered if they reach the superconducting state by just cooling them with liquid nitrogen, that is, if T_c > 77 K), or low temperature (generally if they need other techniques to be cooled under their critical temperature).
• By material: they can be chemical elements (as mercury or lead), alloys (as niobium-titanium or germanium-niobium or niobium nitride), ceramics (as YBCO or the magnesium diboride), or organic superconductors (as fullerenes or carbon nanotubes, though these examples technically might be included among the chemical elements as they are composed entirely of carbon).