What Is Superconductivity? Zero Resistance and a Floating Magnet

Heike Kamerlingh Onnes at Leiden was the first person to liquefy helium, in 1908, which gave him access to temperatures within a few degrees of absolute zero, and he used it to settle an argument about what happens to the resistance of metals as they get very cold. The expectation, depending on whose theory you preferred, was either a gradual decline or a rise as electrons froze in place. What he measured in mercury in 1911 was neither: at 4.2 kelvin the resistance dropped abruptly to a value indistinguishable from zero. He initially suspected a short circuit. The effect was real, it appeared in lead and tin as well, and it won him a Nobel Prize in 1913. The second signature effect was found by Walther Meissner and Robert Ochsenfeld in 1933, who showed that a superconductor actively expels a magnetic field from its interior as it cools through the transition, rather than merely trapping whatever field was there, which established that the state is a distinct thermodynamic phase rather than just a perfect conductor.

The explanation came from John Bardeen, Leon Cooper and Robert Schrieffer in 1957 and is known by their initials. In an ordinary metal, resistance arises because electrons scatter off vibrations and defects in the crystal lattice, losing energy as heat. In a superconductor, an electron moving through the lattice pulls the positive ions slightly towards it, creating a brief region of excess positive charge that attracts a second electron, so the lattice mediates a weak attraction between two electrons that would otherwise repel. Paired this way, into what are called Cooper pairs, the electrons behave as composite particles that can all occupy the same quantum state, and the whole population condenses into a single coherent quantum state described by one wavefunction across the entire sample. Scattering a single electron out of that state requires a minimum energy, the energy gap, and below the critical temperature there is not enough thermal energy available to do it, so there is no mechanism for losing energy and no resistance.

The 1957 theory implied an upper limit to the transition temperature of around thirty kelvin, and in 1986 Georg Bednorz and Alex Muller found superconductivity in a copper oxide ceramic at thirty-five, which won them a Nobel Prize the following year and started an extraordinary scramble. Within months a related compound superconducted at ninety-two kelvin, above the boiling point of nitrogen, which mattered enormously in practice because liquid nitrogen is cheap and liquid helium is not. Records since have reached around 138 kelvin at ordinary pressure in the copper oxides, and much higher in hydrogen-rich compounds under pressures of millions of atmospheres, where a hydrogen sulphide compound reached 203 kelvin in 2015. The mechanism in the copper oxides is still not settled after nearly forty years, with electron pairing apparently mediated by magnetic interactions rather than lattice vibrations, and it remains one of the major open problems in condensed matter physics. The field has also had a run of retracted claims of room-temperature superconductivity, including a high-profile case in 2023, which has made the community appropriately sceptical.

Despite the cooling requirement, the technology is embedded in modern infrastructure:

• •Magnetic resonance imaging, the largest application by value, where superconducting coils generate the strong stable fields that make the scan possible, and there are tens of thousands of such machines in hospitals
• •Particle accelerators, including the magnets that steer beams around the Large Hadron Collider, which is cooled with superfluid helium to 1.9 kelvin
• •Magnetically levitated trains, in commercial operation in Japan and China
• •SQUIDs, superconducting quantum interference devices, the most sensitive magnetometers known, used to measure magnetic fields from the brain and heart and in geological survey
• •Superconducting qubits, the leading approach to quantum computing
• •Fusion reactors, where high-temperature superconducting tape has made compact designs plausible, and power cables in a handful of city centres where space is too tight for conventional conductors

Three constraints keep it from replacing ordinary wiring. The obvious one is temperature, since even the high-temperature materials need liquid nitrogen and the cooling plant costs money, space and energy. The second is that superconductivity is destroyed by a sufficiently strong magnetic field and by a sufficiently large current, and since a current generates its own field, there is a maximum current any given wire can carry, which is the binding constraint in magnet design. The third is mechanical: the copper oxide superconductors are brittle ceramics, and turning a ceramic into a flexible kilometre-long wire that can be wound into a coil took two decades of materials engineering and remains expensive. A genuine room-temperature, ambient-pressure superconductor would change electrical transmission, transport and computing, which is why the claims keep coming and why each one is now examined very carefully.

Below a critical temperature certain materials lose all electrical resistance and expel magnetic fields entirely, effects found in 1911 and 1933 and explained in 1957 by electrons pairing through lattice distortions and condensing into a single quantum state with an energy gap that blocks scattering. Copper oxide ceramics discovered in 1986 work above the boiling point of liquid nitrogen by a mechanism still not understood. The technology underpins medical scanners, particle accelerators and quantum computers, and is limited by cooling, critical currents and brittleness.