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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 and Hc2, being a perfect superconductor under the
lower critical field (Hc1) and leaving completely the superconducting state to a normal conducting state above the
upper critical field (Hc2), being in a mixed state when between the critical fields.
Type-1.5 superconductor – Multicomponent superconductors characterized by two or more coherence lengths
This criterion is important, as the BCS theory has explained the properties of conventional superconductors since 1957, yet there have been no satisfactory theories to explain unconventional superconductors fully. In most cases, type I superconductors are conventional, but there are several exceptions such as
niobium, which is both conventional and type II.
By their critical temperature
Low-temperature superconductors, or LTS: those whose critical temperature is below 77 K.
77 K is used as the split to emphasize whether or not superconductivity in the materials can be achieved with
liquid nitrogen (whose
boiling point is 77K), which is much more feasible than
liquid helium (an alternative to achieve the temperatures needed to get low-temperature superconductors).
By material constituents and structure
Some
pure elements, such as
lead or
mercury (but not all pure elements, as some never reach the superconducting phase).
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)
The
YBCO family, which are several
yttrium-
barium-
copper oxides, especially YBa2Cu3O7. They are the most famous high-temperature superconductors
Nicklates (RNiO2R=Rare earth ion) where Sr-doped infinite-layer nickelate NdNiO2[1] undergo a superconducting transition at 9-15 K. In the family of
Ruddlesden-Popper phase analogon Nd6Ni5O12 (n=5) becomes superconducting at 13 K[2] This is not a complete list and topic of current research.