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Glossary

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

This glossary defines the terms used in the articles on room-temperature superconductivity and links each to the article that treats it in full. Throughout, Tc is the critical temperature, K the kelvin, GPa the gigapascal and nm the nanometer.

A15 compound. One of more than 60 compounds sharing one cubic crystal structure. Niobium-tin (Nb3Sn), with a Tc of 18 K, is used in high-field magnets; the family held the Tc record from 1954 to 1986.[1][2]

Ambient pressure. The pressure of the surrounding air, taken as one standard atmosphere: 101,325 pascals, about 0.0001 gigapascal (GPa).[3] It is part of the definition of a room-temperature superconductor.

BCS theory. The 1957 theory of superconductivity by John Bardeen, Leon Cooper and J. Robert Schrieffer: vibrations of the crystal lattice bind electrons into Cooper pairs, which carry current without resistance.[4][5]

Coherence length (ξ). The shortest distance over which the density of superconducting electrons can change: 83 nanometers (nm) in lead and, along the copper-oxide layers, 1.6 nm in the cuprate YBa2Cu3O7 (YBCO).[6][7][2] A vortex core is two coherence lengths across.[8]

Conventional superconductor. A superconductor whose electrons are paired by phonons, as BCS theory describes.[4] Among such materials the highest Tc at ambient pressure is 39 K, in magnesium diboride (MgB2); the hydride H3S, at 203 K under pressure, is also conventional.[9][10][11]

Convex hull. In stability calculations, the lower boundary of a plot of energy against composition, joining the most stable compounds of a set of elements; a compound above it can lower its energy by decomposing into those on it.[12] Li2AuH6, calculated to superconduct at 91 to 116 K, lies 172 millielectronvolts (meV) per atom above it; nine in ten known metastable compounds lie within 67 meV per atom of it.[10][12]

Cooper pair. Two electrons of opposite spin bound by an attraction that, in a conventional superconductor, the crystal lattice provides.[5] Breaking a pair costs the energy gap. See Superconductivity.

Critical current (Ic). The largest current a superconductor carries before resistance returns.[2] A superconducting magnet runs below it: the dipole magnets of CERN's Large Hadron Collider need 11,850 amperes to reach their full field of 8.3 T.[13] See Applications of superconductors.

Critical current density (Jc). The critical current per unit of cross-section: about 4 × 105 amperes per square centimeter (A/cm2) for niobium-titanium wire at 4.2 K in a 5 tesla field. Practical applications need 105 A/cm2 under operating conditions.[2]

Critical field (Hc). The magnetic field above which a material stops superconducting: 0.08 tesla (T) for lead at absolute zero, and about 25 T at 4.2 K for the upper critical field of niobium-tin (Nb3Sn), a type II superconductor.[5][2]

Critical temperature (Tc), also called the transition temperature. The temperature below which a material superconducts: 4.2 K for mercury, the first superconductor found, and 133 to 138 K for the mercury cuprates that hold the record at ambient pressure.[4][14][15]

Cryocooler. A cryogenic refrigerator, used in place of a liquid coolant in applications such as holding magnesium diboride (MgB2) at about 20 K.[16][2] A cryocooler for superconducting electronics at 4 K needs about 1 kilowatt of input power for 0.1 watt of cooling.[17]

Cuprate. A copper oxide superconductor. The first was reported in 1986 at about 35 K, and cuprates have held the ambient-pressure record since, now 133 to 138 K.[18][2][14][15] YBa2Cu3O7 (YBCO), with a Tc of 93 K, is made into tape as REBCO.[19][2] See Superconducting materials.

Density functional theory (DFT). A quantum-mechanical method that calculates a material's properties from the density of its electrons.[20] It underlies the prediction of Tc, in one survey for more than 20,000 metals.[10]

Diamagnetism. A material's opposition to an applied magnetic field, which makes a magnet repel it: weak in ordinary matter such as water, complete in a superconductor in a weak field, whose interior field is then zero (see Meissner effect).[21][5]

Diamond anvil cell (DAC). A device, invented in 1958, that squeezes a sample between two diamond anvils.[22] The hydride LaH10 was measured in such cells at about 170 GPa (about 1.7 million atmospheres); in one study the samples measured up to 10 by 20 micrometers.[23][24]

Electron-phonon coupling (λ). The strength of the interaction between conduction electrons and phonons, a dimensionless number. Tc rises with it in a conventional superconductor; one calculation for the hydride H3S at 200 GPa gives 1.84.[25][26]

Eliashberg theory, also called Migdal-Eliashberg theory. The theory, formulated in 1960, that calculates the Tc of a conventional superconductor from its phonon spectrum and its coupling to the electrons.[25][27] For H3S at 200 GPa one such calculation, with an assumed value for the repulsion between electrons, gave 194 K against 190 K measured.[26]

Energy gap (2Δ). The minimum energy needed to break a Cooper pair: about 1 millielectronvolt (meV) in the elemental superconductors and about 60 meV in the hydride H3S under pressure.[5][28] See Superconductivity.

Flux pinning. The trapping of vortices on crystal defects, which lets a wire carry current without loss in a magnetic field, because a moving vortex dissipates energy. The highest critical current densities reached are 25 to 30% of the theoretical maximum.[8] See Superconductivity.

Four-probe measurement. A resistance measurement with four contacts: two pass a current through the sample and two read the voltage, so the resistance of wires and contacts stays out of the result.[29] It showed zero resistance in H3S in 2015 and reproduced the 250 K transition of LaH10 in 2020.[11][24] See Verifying a superconductor.

Gigapascal (GPa). One billion pascals of pressure: 10,000 bar, or about 9,869 standard atmospheres.[30][31] The hydride H3S reaches its Tc of 203 K at 155 GPa, about 1.5 million atmospheres.[11]

High-temperature superconductor (HTS). A superconductor with a Tc well above those of the metals and alloys known before 1986, often one that works above the 77 K boiling point of liquid nitrogen. The name arose for the cuprates and is also used for hydrides.[2][4][32]

Hopfield parameter (η). A measure of how strongly electrons scatter when one kind of atom in a crystal moves. Divided by the atom's mass and the square of its vibration frequency, it gives that atom's share of the electron-phonon coupling; one calculation for the hydrogen in H3S at 220 GPa gives 10.1 electronvolts (eV) per square angstrom (1 angstrom is 0.1 nm).[33]

Hydride. A compound of hydrogen with other elements. Lanthanum hydride (LaH10), at about 250 K and about 170 GPa, has the highest reproduced Tc of any material.[23][24] See High-pressure hydrides.

Iron-based superconductor. One of a family of layered iron compounds. The first was reported in 2006 at about 4 K; in 2008 others reached 26 K and then 55 to 56 K, still the highest in a 2021 review.[34][35][36][37][2]

Irreversibility field (Hirr). The magnetic field above which pinning fails and the critical current density is zero, the upper limit of field for applications.[2][38] In YBCO at 77 K it is about half the upper critical field.[38]

Isotope effect. A shift in Tc when an element is replaced by an isotope of different mass, evidence that lattice vibrations take part in the pairing. In H3S above 170 GPa, replacing hydrogen with deuterium, which has twice the mass, lowers Tc: it varies as the mass to the power of about −0.3, against −0.5 in simple BCS theory.[11] See Verifying a superconductor.

Josephson junction. Two superconductors separated by a thin insulating barrier through which electron pairs tunnel, as Brian Josephson predicted in 1962.[39] It is the working element of the superconducting quantum interference device (SQUID) and of voltage standards.[40] See Applications of superconductors.

Kelvin (K). The unit of temperature in the International System of Units (SI), counted from absolute zero in steps the size of a degree Celsius. Zero degrees Celsius is 273.15 K and room temperature, 20 °C, about 293 K.[41]

Liquid helium. Helium below its boiling point, about 4.2 K (−269 °C) at atmospheric pressure, the coolant for niobium-titanium and niobium-tin magnets.[13][2] See Applications of superconductors.

Liquid nitrogen. Nitrogen below its boiling point of 77 K (−196 °C) at atmospheric pressure, a cheaper and more abundant coolant than liquid helium.[42][5] YBCO, with a Tc of 93 K, was in 1987 the first superconductor it could cool.[19]

Magnetic susceptibility (χ). The ratio of the field a material adds by its own magnetization to the field applied: about −10−5 for copper or water.[21] In a superconductor that keeps the field out entirely the added field cancels the applied one, so the ratio is −1.[5] A sharp drop on cooling is one test in verification.

Megabar (Mbar). One million bar: 100 GPa, or about 987,000 atmospheres.[30][31] The superhydrides with the highest Tc, such as LaH10, have been made only at pressures of this order.[43][23]

Meissner effect. The expulsion of magnetic field from the interior of a material when it becomes superconducting, first observed in 1933.[4] A perfect conductor would keep whatever field it held when cooled, so expulsion is a test separate from zero resistance.[44][5] Keeping out a field applied after cooling is called shielding or screening. In a 2022 SQUID study of the hydrides H3S and LaH10, where pinning is strong, shielding was clear and expulsion barely detectable;[32] a 2026 imaging study reported both in lanthanum hydride.[45]

Metastable. Higher in energy than the stable state but long-lived, because an energy barrier slows the change to that state. Half of 29,902 known inorganic crystalline phases in one survey are metastable, by a median of 15 millielectronvolts (meV) per atom.[12] See Predicting superconductors.

Nickelate. A nickel oxide; some with structures like those of the cuprates superconduct. The first, a thin film reported in 2019, did so at 9 to 15 K.[46][47] Under pressure, single crystals showed signs of superconductivity near 80 K above 14 GPa (2023), and samarium-substituted crystals reached onsets of 92 to 96 K (2026).[48][49] See Superconducting materials.

Onset. The temperature at which a sample's resistance begins to fall steeply on cooling, taken as the start of a superconducting transition.[50] It can lie well above the temperature of zero resistance: about 63 K against 37 K in a nickelate film reported in 2026.[51] See Verifying a superconductor.

Penetration depth (λL). The depth to which a magnetic field reaches into the surface of a superconductor: 37 nm in lead and about 140 nm in YBCO.[6][7]

Phase coherence. The sharing of one quantum state by all the Cooper pairs in a sample, which superconductivity needs as well as pairing. The stiffness that keeps the pairs in step, expressed as a temperature, is estimated at about 600,000 K in lead (Tc 7.2 K), where pairing therefore sets Tc, and at 130 to 190 K in the record cuprate, close to its Tc.[52]

Phonon. A quantum of vibration of a crystal lattice.[53] Phonons pair the electrons in a conventional superconductor, and the high vibration frequencies of hydrogen favor a high Tc in hydrides.[4][11]

Preprint. A research paper made public before peer review, typically on arXiv, which hosts more than three million articles.[54] The 2025 claim of 298 K in LaSc2H24 at 260 GPa is a preprint and has not been reproduced.[55][56]

Quench. A pressure quench is the rapid release of pressure on a cold sample, which can leave a high-pressure state in place at ambient pressure. In 2026 one group reported a 151 K onset in a cuprate treated this way, a result not independently reproduced.[50] In a magnet, a quench is the sudden return of part of the winding to the resistive state, which in a helium-cooled magnet boils off the liquid.[57]

REBCO. Rare-earth barium copper oxide, the cuprate family of YBa2Cu3O7 (YBCO, Tc 93 K), made as a film on metal tape.[2][19] Among its applications, a model coil for a fusion project reached 20.1 T at 20 K.[58]

Resistivity (ρ). A material's opposition to electric current, in ohm meters (Ω m), independent of the size and shape of the sample: 1.68 × 10−8 Ω m for copper and 109 to 1014 Ω m for glass at 20 °C.[59] See Superconductivity.

Retraction. The formal withdrawal of a published paper by its authors, their institution or the journal's editor, after which indexes label it as retracted.[60] Nature retracted claims of room-temperature superconductivity in 2022 and 2023.[61][62]

SQUID. A superconducting quantum interference device: a superconducting loop interrupted by one or two Josephson junctions, among the most sensitive magnetometers, with a threshold near 10−14 T.[63][40] In verification it detects the diamagnetism of samples inside pressure cells.[32]

Superhydride. A hydride unusually rich in hydrogen, such as LaH10, in which the hydrogen atoms, ten to each lanthanum atom, form a cage around each metal atom.[43][23]

Tesla (T). The SI unit of magnetic flux density, the usual measure of magnetic field strength.[30] The Earth's field at the surface is about 0.00005 T and that of a dipole magnet of CERN's Large Hadron Collider 8.3 T.[64][65] See Applications of superconductors.

Type I superconductor. A superconductor that keeps a magnetic field out entirely up to a single critical field and is normal above it. Most are pure elements such as aluminum and lead, with critical fields well below 1 T.[5]

Type II superconductor. A superconductor with two critical fields, between which it stays superconducting while the field threads it in vortices.[5][8] The upper field can exceed 100 T, as in YBCO, so magnet wire is of this type.[2][5]

Unconventional superconductor. A superconductor whose pairing is thought to come from something other than the interaction of electrons with the lattice. The cuprates are among such materials, and no agreed theory of them exists.[66][4]

Vortex. A thread of magnetic field through a type II superconductor: a non-superconducting core ringed by circulating current, carrying one quantum of magnetic flux, about 2.07 × 10−15 weber (tesla square meters).[8][67][30] See Superconductivity.

Zero resistance. The absence of resistance to a steady electric current below Tc.[68] An experiment can only set an upper bound: for the hydride H3S in 2015 the resistivity was below about 10−11 Ω m, roughly a hundred times lower than that of pure copper at the same temperature.[11] See Verifying a superconductor.

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