High-temperature superconductivity is superconductivity occurring at transition temperatures substantially above those of traditional metallic superconductors. Below a critical temperature, , a material can carry electrical current without resistance and exhibit the Meissner effect, the expulsion of magnetic flux under suitable conditions. “High-temperature” is a relative designation: many such materials still require cryogenic cooling. The field encompasses both unconventional superconductors, especially copper oxides and iron-based compounds, and hydrogen-rich materials whose high transition temperatures require extreme pressure. (energy.gov)
Definition and historical development
There is no universally applicable numerical boundary separating high- and low-temperature superconductivity. Historically, the designation became associated with materials exceeding the approximately 23 kelvin transition temperatures reached by conventional compounds before 1986. An important practical benchmark is 77 K, the boiling temperature of liquid nitrogen at atmospheric pressure, because superconductors operating above this temperature can use nitrogen cooling rather than colder helium-based systems. Materials below 77 K nevertheless remain part of high-temperature-superconductivity research. (nature.com)
In 1986, J. Georg Bednorz and K. Alexander Müller discovered superconductivity near 35 K in a barium–lanthanum–copper oxide system at IBM. Their discovery established ceramics as an important class of superconducting materials and earned them the 1987 Nobel Prize in Physics. In 1987, researchers led by Maw-Kuen Wu and Ching-Wu Chu reported superconductivity up to approximately 93 K in a yttrium–barium–copper oxide system, crossing the liquid-nitrogen threshold. Subsequent discoveries substantially expanded the available material families. (nobelprize.org)
Principal material families
Copper-oxide superconductors, or cuprates, contain copper–oxygen planes that are central to their electronic properties. Representative families include yttrium barium copper oxide, bismuth-based compounds, and mercury-based compounds. Their superconductivity depends strongly on chemical composition and carrier concentration, often controlled through substitution or oxygen content. Many parent compounds are antiferromagnetic insulators; introducing mobile charge carriers produces superconductivity over a limited composition range. Plots of against carrier concentration commonly show a superconducting “dome.” (nature.com)
Iron-based superconductors attracted widespread attention following the 2008 report of a 26 K transition in fluorine-doped LaFeAsO. Related bulk compounds reached approximately 55 K. They contain iron combined with elements such as arsenic, selenium, or tellurium and have several electronically active bands. Magnetism, structural changes, and superconductivity are closely connected, although their relationship differs among compounds. Their inclusion illustrates why high-temperature superconductivity cannot simply mean superconductivity above 77 K. (nature.com)
Hydrogen-rich superconductors provide a distinct route to high . In 2019, lanthanum hydride, approximately LaH₁₀, was reported to superconduct near 250 K under pressures around 170 gigapascals. Its behavior is consistent with strong electron–lattice coupling, unlike the usual interpretation of cuprates. These results concern microscopic samples compressed to extreme pressures, not materials operating under ordinary ambient conditions. (nature.com)
Microscopic physics
Superconductivity is a collective quantum state involving paired electrons and coherent ordering. In conventional BCS theory, an effective attraction mediated by phonons, the quantized vibrations of a crystal lattice, forms Cooper pairs. Pair formation and coherence are also central to unconventional superconductivity, but the interaction responsible for pairing need not be the conventional phonon mechanism. (energy.gov)
Cuprates have predominantly d-wave pairing symmetry. Their superconducting order parameter changes sign with direction in momentum space, and the superconducting energy gap has nodes where it vanishes. Strong electron interactions and magnetic fluctuations are important ingredients in theoretical descriptions, but a complete, universally accepted microscopic explanation remains unresolved. Iron-based compounds can exhibit different gap structures, so a single pairing symmetry does not characterize every high-temperature superconductor. (nature.com)
The nonsuperconducting states are also significant. Many cuprates exhibit a pseudogap, a partial suppression of low-energy electronic states that can persist above . Its relationship to superconductivity and other electronic orders is a major research question. Angle-resolved photoemission spectroscopy measures momentum-dependent electronic excitations, while tunneling and scattering experiments provide complementary information about energy gaps and correlations. (nature.com)
Magnetic behavior and experimental evidence
Technologically important cuprates are type-II superconductors. Within a range of applied magnetic fields, flux penetrates as quantized vortices rather than being completely excluded. Moving vortices cause electrical dissipation. Vortex pinning immobilizes them at defects, enabling substantial currents without resistance. Consequently, usable performance depends on temperature, field, and current density, not alone. (nature.com)
Experimental identification combines electrical and magnetic measurements. A sharp resistance drop is important but is assessed alongside magnetic response, field dependence, and other evidence. The lanthanum-hydride study, for example, reported zero resistance, an isotope effect, and suppression of the transition temperature by an applied field. Such complementary tests help distinguish superconductivity from other changes in electrical transport. (journals.aps.org)
Conductors and applications
REBCO denotes rare-earth–barium–copper oxides, including the yttrium compound. Their coated conductors place a thin superconducting layer on a buffered metal substrate. Carefully aligned crystal growth limits detrimental grain-boundary effects, while engineered defects improve vortex pinning. Manufacturing must balance current capacity, mechanical strength, uniformity, and cost. High-field applications often operate well below liquid-nitrogen temperature to obtain better performance. (nature.com)
Applications include scientific magnets, nuclear-magnetic-resonance systems, power cables, and developing magnets for nuclear fusion. Proposed and demonstrated electrical uses also include motors, generators, and grid equipment. Although superconducting current avoids resistive losses in the superconducting material, refrigeration, alternating-current losses, joints, and protection systems still impose engineering requirements. A high transition temperature therefore does not by itself establish the efficiency or practicality of a complete device. (energy.gov)