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.

By their magnetic properties

Type I superconductor The interior of a bulk superconductor cannot be penetrated by a weak magnetic field, a phenomenon known as the Meissner effect. When the applied magnetic field becomes too large, superconductivity breaks down. Superconductors can be divided into ...

s: those having just one

critical field For a given temperature, the critical field refers to the maximum magnetic field strength below which a material remains superconducting. Superconductivity is characterized both by perfect conductivity (zero resistance) and by the complete expulsio ...

, ''H_c'', and changing abruptly from one state to the other when it is reached. *

Type II superconductor In superconductivity, a type-II superconductor is a superconductor that exhibits an intermediate phase of mixed ordinary and superconducting properties at intermediate temperature and fields above the superconducting phases. It also features the ...

s: 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 to a normal conducting state above the

upper critical field For a given temperature, the critical field refers to the maximum magnetic field strength below which a material remains superconducting. Superconductivity is characterized both by perfect conductivity (zero resistance) and by the complete expulsio ...

(''H_c2''), being in a mixed state when between the critical fields. *

By the understanding we have about them

Conventional superconductor Conventional superconductors are materials that display superconductivity as described by BCS theory or its extensions. This is in contrast to unconventional superconductors, which do not. Conventional superconductors can be either type-I or type ...

s: those that can be fully explained with the

BCS theory BCS theory or Bardeen–Cooper–Schrieffer theory (named after John Bardeen, Leon Cooper, and John Robert Schrieffer) is the first microscopic theory of superconductivity since Heike Kamerlingh Onnes's 1911 discovery. The theory describes sup ...

or related theories. *

Unconventional superconductor Unconventional superconductors are materials that display superconductivity which does not conform to either the conventional BCS theory or Nikolay Bogolyubov's theory or its extensions. History The superconducting properties of CeCu2Si2, a ty ...

s: those that failed to be explained using such theories, e.g.: **

Heavy fermion superconductor Heavy fermion superconductors are a type of unconventional superconductor. The first heavy fermion superconductor, CeCu2Si2, was discovered by Frank Steglich in 1978. Since then over 30 heavy fermion superconductors were found (in materials based ...

s 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 Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has sim ...

, 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 superconductor High-temperature superconductors (abbreviated high-c or HTS) are defined as materials that behave as superconductors at temperatures above , the boiling point of liquid nitrogen. The adjective "high temperature" is only in respect to previ ...

s, 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 Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wide ...

(whose

boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envir ...

is 77K), which is much more feasible than

liquid helium Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures. Liquid helium may show superfluidity. At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temp ...

(an alternative to achieve the temperatures needed to get low-temperature superconductors).

By material constituents and structure

* Some pure elements, such as

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

(but not all pure elements, as some never reach the superconducting phase). ** Some

allotropes of carbon Carbon is capable of forming many allotropy, allotropes (structurally different forms of the same element) due to its Valence (chemistry), valency. Well-known forms of carbon include diamond and graphite. In recent decades, many more allotrope ...

, such as

fullerene A fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule may be a hollow sphere, ...

nanotube A nanotube is a nanometer-scale hollow tube-like structure. Kinds of nanotubes * BCN nanotube, composed of comparable amounts of boron, carbon, and nitrogen atoms * Boron nitride nanotube, a polymorph of boron nitride * Carbon nanotube, includes g ...

s, or

diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the Chemical stability, chemically stable form of car ...

. ::Most superconductors made of pure elements are type I (except niobium,

technetium Technetium is a chemical element with the symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive. All available technetium is produced as a synthetic element. Naturally occurring technetium is a spontaneous ...

vanadium Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pas ...

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...

, and the above-mentioned Carbon allotropes) *

Alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...

s, such as ** Niobium-titanium (NbTi), whose superconducting properties were discovered in 1962. *

Ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...

s (often insulators in the normal state), which include **

Cuprates Cuprate loosely refers to a material that can be viewed as containing anionic copper complexes. Examples include tetrachloridocuprate ( uCl4sup>2−), the superconductor YBa2Cu3O7, and the organocuprates (e.g., dimethylcuprate u(CH3)2sup>� ...

i.e. copper oxides (often layered, not isotropic) *** The

YBCO Yttrium barium copper oxide (YBCO) is a family of crystalline chemical compounds that display high-temperature superconductivity; it includes the first material ever discovered to become superconducting above the boiling point of liquid nitrogen ...

family, which are several

yttrium Yttrium is a chemical element with the symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a "rare-earth element". Yttrium is almost always found in com ...

barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

oxides, especially YBa₂Cu₃O₇. They are the most famous high-temperature superconductors. **

Iron-based superconductor Iron-based superconductors (FeSC) are iron-containing chemical compounds whose superconducting properties were discovered in 2006. In 2008, led by recently discovered iron pnictide compounds (originally known as oxypnictides), they were in the firs ...

s, including the

oxypnictide In chemistry, oxypnictides are a class of materials composed of oxygen, a pnictogen (group-V, especially phosphorus and arsenic) and one or more other elements. Although this group of compounds has been recognized since 1995, interest in these com ...

s **

Magnesium diboride Magnesium diboride is the inorganic compound with the formula MgB2. It is a dark gray, water-insoluble solid. The compound has attracted attention because it becomes superconductor, superconducting at 39 K (−234 °C). In terms of its ...

(MgB₂), whose critical temperature is 39K, being the conventional superconductor with the highest known temperature. ** non-cuprate oxides such as BKBO * other : eg the "metallic" compounds and are both superconductors below .

References

{{Reflist Superconductivity

By their magnetic properties

By the understanding we have about them

By their critical temperature

By material constituents and structure

See also

References