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 magnetic properties

• Type I superconductors: those having just one critical field, H_c, and changing abruptly from one state to the other when it is reached.
• Type II superconductors: having two critical fields, H_c1 and H_c2, being a perfect superconductor under the lower critical field (H_c1) and leaving completely the superconducting state above the upper critical field (H_c2), being in a mixed state when between the critical fields.

By the understanding we have about them

• Conventional superconductors: those that can be fully explained with the BCS theory or related theories.
• Unconventional superconductors: those that failed to be explained using such theories, e.g.:
  • Heavy fermion superconductors

This criterion is important, as the BCS theory is explaining the properties of conventional superconductors since 1957, but on the other hand there have been 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 30 K.
• High-temperature superconductors, or HTS: those whose critical temperature is above 30 K.

Some now use 77 K as the split 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, such as fullerenes, nanotubes, or diamond.

Most superconductors made of pure elements are type I (except niobium, technetium, vanadium, silicon, and the abovementioned Carbon allotropes)

• Alloys, such as
  • Niobium-titanium (NbTi), whose superconducting properties were discovered in 1962.
• Ceramics, which include
  • Cuprates i.e. copper oxides
    • The YBCO family, which are several yttrium-barium-copper oxides, especially YBa₂Cu₃O₇. They are the most famous high-temperature superconductors.
  • Iron-based superconductors, including the oxypnictides
  • Magnesium diboride (MgB₂), whose critical temperature is 39K,^[1] being the conventional superconductor with the highest known temperature.
• other

eg the "metallic" compounds Hg
3NbF
6 and Hg
3TaF
6 are both superconductors below 7 K (−266.15 °C; −447.07 °F).^[2]

See also

References