Superconductivity
Superconductivity is a phenomenon exhibited by certain materials — including some metals, alloys, and ceramic compounds — in which electrical resistance drops to exactly zero and magnetic fields are actively expelled from the material's interior (the Meissner effect), when the material is cooled below a characteristic critical temperature specific to that material.
Curious
Every ordinary conductor has some electrical resistance, meaning energy is continuously lost as heat when current flows through it — this is why wires warm up and why batteries eventually drain even in a simple closed circuit. Superconductors are fundamentally different: below their critical temperature, resistance vanishes completely and exactly, not merely becoming very small. A current set flowing in a closed superconducting loop has been observed to persist, essentially undiminished, for years, with no measurable decay — a genuinely striking macroscopic manifestation of an underlying quantum phenomenon.
Superconductors also expel magnetic fields from their interior when cooled below their critical temperature (the Meissner effect) — a property distinct from, and not simply explainable by, zero resistance alone, and one that makes magnetic levitation of a superconductor above a magnet a popular, visually striking classroom demonstration.
Exploring
| Material | Critical temperature (T_c) | Type |
|---|---|---|
| Mercury (first discovered) | 4.2 K | Conventional (Type I) |
| Niobium-titanium alloy | ~10 K | Conventional (Type II) — widely used in MRI magnets |
| YBCO (yttrium barium copper oxide) | ~92 K | High-temperature (unconventional) |
Deep Dive
Superconductivity was discovered by Heike Kamerlingh Onnes in 1911, while investigating the electrical resistance of mercury at extremely low temperatures — resistance dropped abruptly and completely to an unmeasurably small value at 4.2 K, a startling result Onnes had not anticipated. The full theoretical explanation for conventional superconductivity was not developed until nearly half a century later, with the 1957 BCS theory (named for its developers, John Bardeen, Leon Cooper, and John Robert Schrieffer), which showed that below the critical temperature, electrons can form loosely bound pairs (Cooper pairs) mediated by interactions with the crystal lattice's vibrations (phonons) — these paired electrons behave collectively as bosons rather than individual fermions, allowing them to condense into a single coherent quantum state that flows without scattering, and hence without resistance.
Type I and Type II superconductors: Superconductors are classified into two types based on how they respond to applied magnetic fields. Type I superconductors (mostly pure elemental metals) abruptly and completely lose superconductivity above a single critical magnetic field strength. Type II superconductors (mostly alloys and compounds, including virtually all practically useful superconducting magnet materials and all known high-temperature superconductors) instead allow magnetic field to partially penetrate in quantised bundles called vortices above a lower critical field, while retaining superconductivity in the surrounding material up to a much higher critical field — a property essential to their use in powerful superconducting magnets.
The discovery of high-temperature superconductivity in copper-oxide ceramic materials (cuprates) by Johannes Georg Bednorz and Karl Alex Müller in 1986 was a major, initially disbelieved surprise — these unconventional superconductors do not fit the standard BCS framework, and the precise mechanism enabling their considerably higher critical temperatures remains an active area of theoretical research, nearly four decades after their discovery, making high-temperature superconductivity one of condensed matter physics' most significant unsolved problems.
Concept Relationship Map
Requires: band theory
Enables: Meissner effect [PENDING FULL ENTRY], BCS theory [PENDING FULL ENTRY], Cooper pairs [PENDING FULL ENTRY]
Commonly confused with: assuming "high-temperature" superconductors work near room temperature (they operate far below it, just above liquid nitrogen temperature)
Sources
- Onnes, H.K. (1911). Communications from the Physical Laboratory of the University of Leiden, 12, 120.
- Bardeen, J., Cooper, L.N., Schrieffer, J.R. (1957). "Theory of Superconductivity." Physical Review, 108(5), 1175–1204.
- Bednorz, J.G., Müller, K.A. (1986). "Possible High T_c Superconductivity in the Ba-La-Cu-O System." Zeitschrift für Physik B, 64(2), 189–193.