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Superconducting materials

69 sources, listed at the end · last revised 7 October 2026

Superconducting materials are the elements, alloys and compounds that, below a critical temperature (Tc, also called the transition temperature), conduct electricity with zero resistance and expel magnetic fields (the Meissner effect). They are sorted into families by chemistry and crystal structure: elemental metals and niobium alloys, magnesium diboride, copper oxides (cuprates), iron-based compounds, nickel oxides (nickelates), hydrogen-rich hydrides and several smaller groups. At ambient pressure the highest transition temperature of a stable phase is 133 to 138 K (−140 to −135 °C), in mercury cuprates.[1][2] Under pressure the highest reproduced value is about 250 K (−23 °C), in lanthanum hydride at about 170 GPa (about 1.7 million atmospheres).[3] No known material superconducts at room temperature and ambient pressure.

Superconductivity arises when electrons bind into pairs, and the main division among the materials is by what binds them. In a conventional superconductor the pairs are bound by vibrations of the crystal lattice, and Tc can be calculated from the crystal structure. In an unconventional superconductor the pairing has another origin, and no accepted theory predicts Tc. The elements, the niobium alloys, magnesium diboride and the hydrides are conventional. The cuprates and the iron-based compounds are unconventional, and the pairing in them and in the nickelates is not settled.

Most superconducting magnets are wound from two conventional materials, niobium-titanium (Tc 9.2 to 9.5 K) and niobium-tin (18 K), which cost far less as wire than any cuprate conductor.[4][5][6]

Conventional and unconventional pairing

The theory published by Bardeen, Cooper and Schrieffer in 1957 (BCS theory) explains superconductivity as the binding of electrons into pairs by phonons, the vibrations of the crystal lattice.[7] Materials that fit this description are called conventional. Their Tc can be computed from the crystal structure with no experimental input (see Superconductivity).[8][9]

For the sulfur hydride H3S a calculation published in November 2014 gave 191 to 204 K at 200 GPa, and an experiment published in 2015 found 203 K at 155 GPa.[10][11] The experimenters had started from an earlier prediction of about 80 K and state that the 2014 calculation did not guide their work.[9] Magnesium diboride was found by experiment and had not been predicted.[12]

An unconventional superconductor is one whose pairing does not appear to come from the interaction between electrons and the lattice.[13] The cuprates, the iron-based superconductors and the heavy-fermion compounds described below were each found by experiment without having been predicted.[12] For the cuprates, a 2024 computer study of a simplified model of interacting electrons found superconductivity and predicted no transition temperature.[14]

Elements, niobium alloys and compounds

Niobium has the highest transition temperature of any element at ambient pressure, about 9.2 K (−264 °C). About 50 elements superconduct, about 30 of them only under pressure, where scandium exceeds 30 K at 238 GPa.[15] Niobium is the usual material for the superconducting accelerating cavities of particle accelerators, which often run at about 2 K.[16]

Niobium-titanium (NbTi) is a ductile alloy with Tc between 9.2 and 9.5 K.[6][5] Like other magnet wires it is a type II superconductor: a magnetic field threads it in vortices, thin tubes of field, and it stays superconducting up to an upper critical field.[17] At 4.2 K (−269 °C), the boiling point of liquid helium, that field is about 11.5 tesla (T).[5] NbTi can be drawn directly into long wires in a copper matrix, and it is the cheapest practical superconductor for use in liquid helium.[5] Estimates of annual NbTi wire production range from hundreds to several thousand tons, and magnetic resonance imaging (MRI) scanners are the largest use.[4][5] The 1,232 main dipoles (bending magnets) of the Large Hadron Collider (LHC) are NbTi magnets cooled to 1.9 K and designed for a field of 8.3 T.[18][19]

Niobium-tin (Nb3Sn) was found to superconduct at 18 K in 1954.[20] It is an intermetallic compound, two metals combined in a fixed ratio, with the A15 crystal structure,[21] and its upper critical field is about 25 T at 4.2 K.[5] NbTi accelerator dipoles are limited to less than about 9 T, and magnet builders choose Nb3Sn for fields about 1.5 times higher.[4] Nb3Sn is brittle, so the wire is drawn from unreacted ingredients and the compound is formed afterward by heating above 600 °C, usually once the coil has been wound.[5][4] It is the usual material for high-field nuclear magnetic resonance (NMR) magnets, and about 500 metric tons of the wire were produced for two magnet systems of the ITER fusion experiment.[4][22] A related A15 compound, Nb3Ge, was reported in 1974 with an onset (where resistance starts to fall) above 23 K in sputtered films, the highest Tc known until 1986.[23][24]

A 2021 review put the price of conductor that carries 1,000 amperes over one meter, at 4.2 K in a 10 T field, at about 5 US dollars for Nb3Sn, 60 to 80 dollars for bismuth cuprates and 100 to 200 dollars for REBCO (rare-earth barium copper oxide), a cuprate tape described below.[5] Nb3Sn wire is itself several times more expensive than NbTi.[4] In conventional superconductors such as these, current passes between crystal grains without obstruction, so the grains need no alignment.[25]

Magnesium diboride

Magnesium diboride (MgB2) was found to superconduct at 39 K in 2001.[26] It is conventional: a calculation based on phonon pairing reproduces the 39 K and the measured change of Tc with the mass of the boron isotope.[27] Two groups of its electrons couple to phonons with different strength, which gives the material two energy gaps where most superconductors have one: the gap parameter Δ, half the energy needed to break a pair, is calculated to be about 7 millielectronvolts (meV) for one group and 1 to 3 meV for the other.[28]

MgB2 wire is suited to use at around 20 K, a temperature reached with a cryocooler or with liquid hydrogen.[5] Its grain boundaries do not block current, and its upper critical field is about 18 T at 4.2 K.[5] For the High-Luminosity upgrade of the LHC, the first superconducting link built at the European laboratory CERN holds 19 MgB2 cables about 140 m long that together carry about 120,000 amperes at 25 K.[29]

Cuprates

Cuprates are layered copper oxides in which planes of copper and oxygen atoms (CuO2) are separated by layers of other elements that act as charge reservoirs.[30] Cuprate conductors are called high-temperature superconductors (HTS), and the niobium wires low-temperature superconductors (LTS).[5] The parent compounds are antiferromagnets, in which neighboring copper atoms are magnetized in opposite directions, and they do not superconduct. Superconductivity appears when the reservoir layers add electrons to the planes or remove them, a process called doping.[30]

The first cuprate superconductor, a lanthanum barium copper oxide, was reported in 1986 at about 35 K.[31] YBa2Cu3O7 followed in 1987 at 93 K, the first material to superconduct above 77 K (−196 °C), the boiling point of liquid nitrogen.[32] The bismuth cuprates, reported in 1988 at about 105 K,[33] have two main forms named for their ratios of bismuth, strontium, calcium and copper: Bi-2212 (Tc about 90 K) and Bi-2223 (108 K).[5] The mercury cuprate HgBa2Ca2Cu3O8+δ (Hg-1223) reached 133 K in 1993 and 138 K with partial thallium substitution in 1995, still the highest values for a stable phase at ambient pressure.[1][2]

Under 31 GPa the onset in Hg-1223 rises to 164 K.[34] In 2026 one group reported that a rapid release of pressure at low temperature (a pressure quench) left Hg-1223 in a metastable state with an onset of up to 151 K at ambient pressure. Zero resistance was not reported and the state degraded on heating above 200 K.[35] The result has not been reproduced (see Claims of room-temperature superconductivity).

Experiments that detect the sign of the pair state (phase-sensitive tests) have largely settled that the pairs in cuprates have d-wave symmetry, in which the pair state changes sign under a rotation of 90° in the copper-oxygen plane.[36] One line of theory starts from the strong repulsion between electrons, which makes the undoped parent an insulator.[37] A rigorous theory of the pairing has not been established,[13] and what sets the different maximum Tc of each cuprate family is not well understood; a 2016 study ties it to the share of charge on the oxygen atoms of the planes.[30]

In a cuprate the current that can cross a grain boundary falls exponentially as the misalignment between the two grains grows,[25] so a conductor needs aligned grains: within about 5° in REBCO tape.[5] REBCO, the family of YBa2Cu3O7, is deposited as a film 1 to 2 micrometers thick on a metal tape; the finished conductor is 0.1 to 0.2 mm thick.[4] The bismuth cuprates are packed as powder into silver tubes, then drawn into round wire (Bi-2212) or rolled into flat tape (Bi-2223).[5] All three keep upper critical fields above 100 T at 4.2 K.[5] At the US National High Magnetic Field Laboratory a REBCO test coil inside a 31 T resistive (non-superconducting) magnet reached a combined steady field of 45.5 T in 2017, a record published in 2019,[38][5] and a later version of the coil reached 48.7 T in 2025.[38] A model coil for the SPARC fusion project reached a peak field of 20.1 T on the conductor at 20 K, with 270 km of REBCO tape.[39] Bi-2223 tape has been used in demonstration power cables and in the current leads that feed the LHC magnets.[5][6]

Iron-based superconductors

Iron-based superconductors are layered compounds of iron with phosphorus, arsenic or selenium. LaOFeP was reported to superconduct at about 4 K in 2006,[40] and fluorine-doped LaFeAsO, from the same group, at 26 K in 2008.[41] Related compounds reached 55 K (SmFeAsO1−xFx) and 56 K (Gd1−xThxFeAsO), also in 2008,[42][43] which a 2021 review still gave as the highest values in the family.[5]

A calculation for LaFeAsO found a coupling between electrons and phonons that allows a Tc of at most 0.8 K, against the measured 26 K, so phonons alone cannot account for the superconductivity.[44] A pair state whose sign differs between two groups of electrons, driven by magnetic fluctuations, was proposed in 2008.[45] The mechanism is not settled. In a single layer of iron selenide (FeSe) on strontium titanate an energy gap closes near 65 K and one electrical measurement found a transition above 100 K, while others had found zero resistance only below 30 K; bulk FeSe superconducts at 9 K.[46][47] A 2017 review weighs phonon and magnetic pairing for this system without deciding between them.[47]

Several iron-based compounds keep upper critical fields above 80 T at 4.2 K, which makes them candidates for high-field magnets, and tapes about 100 m long had been made in laboratories by 2021.[5]

Nickelates

Nickelates are nickel oxides built, like the cuprates, from planes of metal and oxygen atoms. Two groups reach the highest transition temperatures.

The infinite-layer nickelates, which share the crystal structure of a group of copper oxides, superconduct as thin films: Nd0.8Sr0.2NiO2 was reported in 2019 at 9 to 15 K.[48] In 2025, films of (Sm,Eu,Ca,Sr)NiO2 approached 40 K at ambient pressure, with zero resistance at 31 K.[49]

The second group is the family of La3Ni2O7, in which the nickel-oxygen layers come in pairs. Single crystals of La3Ni2O7 showed signatures of superconductivity under pressure in 2023, with a maximum Tc of 80 K between 14.0 and 43.5 GPa;[50] zero resistance was reported in 2024.[51] Replacing part of the lanthanum with samarium gave an onset of 92 K, with zero resistance at 73 K at 21.6 GPa, and an onset of 96 K at a higher samarium content (2026).[52] Thin films of the same family superconduct at ambient pressure when the substrate they are grown on compresses them by about 2% in the plane.[53] The first reports, published from December 2024, gave onsets of 26 to 45 K with zero resistance only near 2 K and 9 K,[54][53] and a 2026 report gave about 63 K with zero resistance at about 37 K.[55] A three-layer relative, La4Ni3O10, superconducts at about 30 K at 69 GPa (2024).[56]

Theoretical work describes the infinite-layer and two-layer nickelates with the models of mutually repelling electrons used for cuprates,[57][58] and the pairing mechanism is not settled.

Hydrides

For the hydrides in detail, see High-pressure hydrides.

Hydrides are compounds of hydrogen with other elements. Hydrogen-rich hydrides made and measured inside a diamond anvil cell have the highest reproduced transition temperatures: 203 K in H3S near 155 GPa,[11] and about 250 K in LaH10 near 170 GPa.[3] They are conventional: in both compounds Tc falls when hydrogen is replaced by its heavier isotope deuterium, the isotope effect expected when phonons bind the pairs.[11][3] Samples are small, up to 10 by 20 micrometers in one LaH10 experiment,[59] and no hydride superconductor is used in a device.

A 2025 preprint reported onsets up to 298 K in LaSc2H24 at 260 GPa. It has not been reproduced, and a second group did not obtain the compound in seven attempts (see Claims of room-temperature superconductivity).[60][61]

Other families

In heavy-fermion superconductors, compounds of elements such as cerium and uranium, the conduction electrons behave as if they were heavier than free electrons, about 200 times heavier in the first one found, CeCu2Si2, which superconducts at about 0.5 K (1979).[62][63] The plutonium compound PuCoGa5, placed by its discoverers between these compounds and the cuprates, superconducts above 18 K (2002).[64] An organic salt, di-(tetramethyltetraselenafulvalene) hexafluorophosphate or (TMTSF)2PF6, was found to superconduct at 0.9 K under 1.2 GPa (12 kilobar) in 1980.[65] Fullerides are solids of C60 molecules combined with alkali metals: potassium-doped C60 superconducts at 18 K (1991),[66] and Cs3C60, an insulator at ambient pressure, reaches 38 K near 0.7 GPa (7 kilobar), reported in 2008 as the highest value for a molecular material.[67] A 2016 review attributes fulleride superconductivity to phonons acting together with the repulsion between electrons.[68] Two sheets of graphene stacked with a twist of about 1.1° superconduct at up to 1.7 K when an applied voltage adds or removes electrons (2018).[69]

Families compared

Family Example Highest Tc Pressure Pairing Used in
Elements Niobium 9.2 K; above 30 K (scandium)[15] ambient; 238 GPa phonon accelerator cavities
Niobium alloys and compounds NbTi, Nb3Sn onset above 23 K (Nb3Ge film)[23] ambient phonon MRI, NMR, accelerator and fusion magnets
Magnesium diboride MgB2 39 K[26] ambient phonon accelerator current links
Cuprates Hg-1223 (highest Tc); REBCO, Bi-2223 (conductors) 133 K; 138 K with thallium substitution; 164 K onset[1][2][34] ambient; ambient; 31 GPa not settled high-field magnets, demonstration cables, current leads
Iron-based Gd1−xThxFeAsO 56 K (bulk)[43] ambient not settled none
Nickelates La3Ni2O7 family 92 to 96 K onset (crystal); 63 K onset (film)[52][55] 21.6 GPa (92 K crystal); ambient not settled none
Hydrides LaH10 about 250 K[3] about 170 GPa phonon none
Fullerides Cs3C60 38 K[67] about 0.7 GPa phonon, aided by electron repulsion[68] none

See also

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