Why do some materials superconduct at high temperatures?
Some ceramics carry electricity with zero resistance far above the limits once thought possible. Forty years on, we still don't know how.
The question
In 1911 Heike Kamerlingh Onnes cooled mercury to about four degrees above absolute zero and saw its electrical resistance vanish completely. Superconductors carry current without losing energy and expel magnetic fields, which makes them essential for MRI scanners and particle accelerators. But for most of the 20th century they worked only at temperatures close to absolute zero, which requires expensive liquid helium.
In 1986 Georg Bednorz and Alex Müller found superconductivity at about 30 kelvin in a copper oxide ceramic — a material no one expected to conduct well at all. Within a year, a related compound was superconducting at 93 kelvin, above the boiling point of cheap liquid nitrogen. These "cuprates" set off a wave of excitement. Forty years later, the mechanism behind them is still unexplained.
What we know
- Ordinary superconductivity is understood. The BCS theory of 1957 explains it: vibrations of the crystal lattice bind electrons into pairs, and the pairs move together without resistance. Estimates based on it, such as William McMillan's in 1968, suggested that this mechanism could not work much above about 40 kelvin.
- The cuprates break this limit. At normal pressure, the record for an ordinary sample, held by a mercury-based cuprate since 1993, is about 133 kelvin. In 2026 the same compound, squeezed to very high pressure and released while deep-frozen, stayed superconducting up to 151 kelvin at normal pressure, but only in a fragile state that degrades when warmed above about 200 kelvin. The result has yet to be reproduced by other groups.
- Electrons in cuprates still form pairs, but of a different kind. Experiments in the 1990s showed that the pairs have "d-wave" symmetry, which suggests that the glue is not lattice vibrations but magnetic interactions between electrons.
- Cuprates are layered: superconductivity happens in planes of copper and oxygen atoms. Without added charge carriers, these materials are not metals at all but magnetic insulators. Superconductivity appears only in a range of "doping" between these extremes, and above it lies a strange state, the "pseudogap", that is itself not understood.
- New families have been found: iron-based superconductors in 2008, nickel oxides from 2019. Under enormous pressures, hydrogen-rich compounds superconduct at more than 200 kelvin, as shown for hydrogen sulphide in 2015 and lanthanum hydride in 2019. These hydrides seem to follow ordinary BCS physics, made extreme by light hydrogen atoms.
- The field has seen spectacular false claims. Two reports of room-temperature superconductivity published in Nature were retracted in 2022 and 2023. In 2023 the material LK-99 went viral; within weeks, laboratories showed that its sudden drop in resistance came from an impurity, copper sulphide, and that pure crystals of the material are insulators.
Main hypotheses
Magnetic pairing
Fluctuations of the electrons' magnetism, rather than lattice vibrations, bind electrons into pairs. This is the most widely favoured view, but it hasn't produced a theory that predicts the critical temperature.
Resonating valence bonds
Philip Anderson proposed in 1987 that electrons in the copper-oxygen planes form a quantum liquid of paired spins, which becomes a superconductor when charge carriers are added. The idea has been influential but remains unproven.
Competing orders
The pseudogap and other patterns found in cuprates, such as stripes of charge, may compete with superconductivity or may be part of its cause. Which, if either, is not settled.
Where understanding ends
The cuprates are a problem of strongly interacting electrons: each electron's behaviour depends on all the others. The standard methods of solid-state physics, which treat electrons almost independently, fail here, and exact calculations are impossible for realistic numbers of particles.
Even the simplest model thought to capture the essence of cuprates, the two-dimensional Hubbard model, is unresolved. Large numerical studies disagree on whether it superconducts at all, and in 2020 one careful study found that in its pure form it does not. In 2024 an overlapping team found that once electrons can also hop between next-nearest atoms, superconductivity does appear, alongside stripes of charge much like those seen in cuprates. The minimal model seems to need this extra ingredient.
The result is that we cannot predict which new materials will superconduct at high temperatures. Every major family was found by experiment, often by chance. And the ultimate question — whether superconductivity at room temperature and ordinary pressure is possible at all — has no theoretical answer.
What would settle it
- A theory that predicts the critical temperature of known cuprates from their composition and structure, and correctly predicts a new superconductor before it is made.
- Reliable solutions of models like the Hubbard model, from new numerical methods or quantum simulators, showing which ingredients produce superconductivity.
- A material that superconducts at room temperature and normal pressure, confirmed independently by several laboratories.
Sources
- Bardeen, J., Cooper, L. N., & Schrieffer, J. R. (1957). Theory of superconductivity. Physical Review, 108(5), 1175–1204.
- McMillan, W. L. (1968). Transition temperature of strong-coupled superconductors. Physical Review, 167(2), 331–344.
- Bednorz, J. G., & Müller, K. A. (1986). Possible high Tc superconductivity in the Ba−La−Cu−O system. Zeitschrift für Physik B Condensed Matter, 64(2), 189–193.
- Wu, M. K., et al. (1987). Superconductivity at 93 K in a new mixed-phase Y-Ba-Cu-O compound system at ambient pressure. Physical Review Letters, 58(9), 908–910.
- Anderson, P. W. (1987). The resonating valence bond state in La₂CuO₄ and superconductivity. Science, 235(4793), 1196–1198.
- Schilling, A., Cantoni, M., Guo, J. D., & Ott, H. R. (1993). Superconductivity above 130 K in the Hg–Ba–Ca–Cu–O system. Nature, 363(6424), 56–58.
- Tsuei, C. C., & Kirtley, J. R. (2000). Pairing symmetry in cuprate superconductors. Reviews of Modern Physics, 72(4), 969–1016.
- Kamihara, Y., Watanabe, T., Hirano, M., & Hosono, H. (2008). Iron-based layered superconductor La[O₁₋ₓFₓ]FeAs (x = 0.05–0.12) with Tc = 26 K. Journal of the American Chemical Society, 130(11), 3296–3297.
- Keimer, B., Kivelson, S. A., Norman, M. R., Uchida, S., & Zaanen, J. (2015). From quantum matter to high-temperature superconductivity in copper oxides. Nature, 518(7538), 179–186.
- Drozdov, A. P., Eremets, M. I., Troyan, I. A., Ksenofontov, V., & Shylin, S. I. (2015). Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system. Nature, 525(7567), 73–76.
- Drozdov, A. P., et al. (2019). Superconductivity at 250 K in lanthanum hydride under high pressures. Nature, 569(7757), 528–531.
- Li, D., et al. (2019). Superconductivity in an infinite-layer nickelate. Nature, 572(7771), 624–627.
- Qin, M., et al. (2020). Absence of superconductivity in the pure two-dimensional Hubbard model. Physical Review X, 10(3), 031016.
- Snider, E., et al. (2022). Retraction Note: Room-temperature superconductivity in a carbonaceous sulfur hydride. Nature, 610(7933), 804.
- Garisto, D. (2023). LK-99 isn't a superconductor — how science sleuths solved the mystery. Nature, 620(7975), 705–706.
- Puphal, P., et al. (2023). Single crystal synthesis, structure, and magnetism of Pb₁₀₋ₓCuₓ(PO₄)₆O. APL Materials, 11(10), 101128.
- Zhu, S., Wu, W., Li, Z., & Luo, J. (2023). First-order transition in LK-99 containing Cu₂S. Matter, 6(12), 4401–4407.
- Dasenbrock-Gammon, N., et al. (2023). Retraction Note: Evidence of near-ambient superconductivity in a N-doped lutetium hydride. Nature, 624(7991), 460.
- Xu, H., Chung, C.-M., Qin, M., Schollwöck, U., White, S. R., & Zhang, S. (2024). Coexistence of superconductivity with partially filled stripes in the Hubbard model. Science, 384(6696), eadh7691.
- Deng, L., et al. (2026). Ambient-pressure 151-K superconductivity in HgBa₂Ca₂Cu₃O₈₊δ via pressure quench. Proceedings of the National Academy of Sciences, 123(11), e2536178123.