Superconductor classification

Superconductors can be classified in accordance with several criteria that depend on our interest in their physical properties, on the understanding we have about them, on how expensive is cooling them or on the material they are made of.

By their physical properties

* Type I superconductors: those having just one critical temperature, "T_c", changing abruptly from one state to the other.
* Type II superconductors: those having two critical temperatures, "T_c1" and "T_c2", being a perfect superconductor under "T_c1" and leaving completely the superconducting state above "T_c2", being in a mixed state when between both temperatures. These can alternatively/better be considered to have an upper and lower critical field.

By the understanting we have about them

* Conventional superconductors: those than can be fully explained with the BCS theory or related theories.
* Unconventional superconductors: those that failed to be explained using such theories.

This criterion is important, as the BCS theory is explaining the properties of conventional superconductors since 1957, but on the other hand we have no satisfactory theory to explain fully unconventional superconductors. In most of cases type I superconductors are conventional, but there are several exceptions as niobium, which is both conventional and type II.

By their critical temperature

* Low-temperature superconductors, or LTS: those whose critical temperature is below 77K.
* High-temperature superconductors, or HTS: those whose critical temperature is above 77K.

This criterion is used when we want to emphasize whether or not we can cool the sample with liquid nitrogen (whose boiling point is 77K), which is much more feasible than liquid helium (the alternative to achieve the temperatures needed to get low-temperature superconductors).

By material

* Some Pure elements, such as lead or mercury (but not all pure elements, as some never reach the superconducting phase).
** Some allotropes of carbon, as fullerenes and nanotubes (but never diamonds or graphite).::Most superconductors made of pure elements are type I (except niobium, technetium, vanadium and the abovementioned carbon).
* "'Alloys, such as
** Niobium-titanium (NbTi), whose superconducting properties where discovered in 1962.
** Gold-indium (AuIn), discovered in 1997.
** Uranium rhodium germanium (URhGe), which was discovered in 2005 to be a superconductor even under strong magnetic fields [cite journal | title = Magnetic Field-Induced Superconductivity in the Ferromagnet URhGe | author = F. Lévy, I. Sheikin, B. Grenier, A. D. Huxley | journal = Science | volume = 309 | pages = 1343–1346 | date = 26 Aug 2005 | doi = 10.1126/science.1115498] (however its critical temperature is very low, just 0.28K).
* "'Ceramics, which include
** The YBCO family, which are several yttrium-barium-copper oxides, especially YBa₂Cu₃O₇. They are the most famous high-temperature superconductors.
** Magnesium diboride (MgB₂), whose critical temperature is 39K [cite journal | format = letter | title = Superconductivity at 39 K in magnesium diboride | author = Jun Nagamatsu, Norimasa Nakagawa, Takahiro Muranaka, Yuji Zenitani and Jun Akimitsu | journal = Nature | volume = 410 | pages = 63–64 | date = 1 Mar 2001 | doi = 10.1038/35065039] , being the conventional superconductor with the highest known temperature.

See also

* Superconductivity

References