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Superconductors can be classified in accordance with several criteria that depend on physical properties, current understanding, and the expense of cooling them or their material.
Type II superconductors: having two critical fields,Hc1 andHc2, being a perfect superconductor under thelower critical field (Hc1) and leaving completely the superconducting state to a normally conducting state above theupper critical field (Hc2), being in a mixed state when between the critical fields.
This criterion is useful as BCS theory has successfully explained the properties of conventional superconductors since 1957, yet there have been no satisfactory theories to fully explain unconventional superconductors. In most cases conventional superconductors are type I, but there are exceptions such asniobium, which is both conventional and type II.
77 K is used as the demarcation point to emphasize whether or not superconductivity in the materials can be achieved withliquid nitrogen (whoseboiling point is 77K), which is much more feasible thanliquid helium (an alternative to achieve the temperatures needed to get low-temperature superconductors).
Niobium-titanium (NbTi), whose superconducting properties were discovered in 1962.
Ceramics (often insulators in the normal state), which include
Cuprates i.e. copper oxides (often layered, not isotropic)
TheYBCO family, which are severalyttrium-barium-copper oxides, especially YBa2Cu3O7. They are arguably the most famous high-temperature superconductors.
Nickelates (RNiO2R=Rare earth ion) whereSr-doped infinite-layer nickelate NdNiO2[1] undergo a superconducting transition at 9-15 K. In the family ofRuddlesden-Popper phase analog Nd6Ni5O12 (n=5) becomes superconducting at 13 K.[2] Note that this is not a complete list and is a topic of current research.
