What Is Superconductivity? Current That Flows Without Losing Anything

Cooled below a temperature characteristic of the material, a superconductor undergoes a sharp transition in which its electrical resistance drops to zero, and the word zero is meant literally rather than as an approximation, since currents set circulating in superconducting loops have been observed to persist without measurable decay over periods of years. The second defining property is that the material expels magnetic fields from its interior rather than merely preventing them from changing, which is distinct from what a perfect conductor would do and which is why superconductivity is a genuine phase of matter rather than an extreme of ordinary conduction. A sufficiently strong magnetic field, a sufficiently large current or a rise in temperature will destroy the state, and those three limits determine what any given material can be used for.

The applications in service are substantial and mostly involve magnets:

• •Medical imaging scanners, which are by far the largest use and which rely on strong stable fields
• •Particle accelerators, whose bending magnets could not be built with ordinary conductors
• •Experimental fusion devices, which need very strong confining fields
• •Extremely sensitive magnetic sensors, capable of detecting the fields produced by brain activity
• •Research magnets for materials science and chemistry
• •Maglev transport and some power cables, which exist in operation but remain limited

The explanation for the conventional form took nearly fifty years after the discovery and is genuinely subtle. Electrons repel each other, so the idea that they pair up requires an attractive interaction, and the one identified operates through the lattice of atoms, with a passing electron distorting the lattice slightly and the distortion attracting a second electron, producing a weak attraction mediated by the material itself. Paired electrons behave collectively rather than individually and occupy a single coherent state, which means scattering an individual electron out of it is no longer possible without disrupting the whole, and that is why resistance vanishes. The theory was published in 1957 and explains the conventional materials well. It does not explain the higher-temperature materials discovered later, whose mechanism remains an open problem in physics.

The demonstration in which a magnet hovers above a cooled sample is genuine and involves more than the expulsion of magnetic fields. Many practical superconductors allow the field to penetrate at isolated points while remaining superconducting elsewhere, and those points become pinned to defects in the material, which locks the magnet in place rather than merely pushing it away. The result is stable in a way simple repulsion is not, so the magnet can hang below the sample as readily as above it and stays put if nudged, which is why the effect looks so unlike ordinary magnetic levitation. The same pinning is what allows superconducting magnets to carry enormous currents without the internal field disrupting them, so a laboratory demonstration and the operation of a hospital scanner rest on the same property.

The history is one of a series of surprises. The phenomenon was found in 1911 in mercury cooled with liquid helium, and for decades the highest transition temperatures crept upwards slowly and stayed far below anything convenient. In 1986 a class of copper oxide ceramics was found to superconduct at much higher temperatures, and within months materials working above the boiling point of liquid nitrogen were reported, which mattered enormously because nitrogen is cheap and helium is not. Progress since has been slower than the excitement of that period suggested, the ceramics are brittle and awkward to make into wire, and no mechanism explaining them commands agreement. Materials superconducting at room temperature under enormous pressure have been reported, with several high-profile claims subsequently retracted, and the field treats new announcements cautiously for good reason.

Below a characteristic temperature resistance drops to zero literally, with currents persisting for years, and the material also expels magnetic fields, which makes it a distinct phase rather than an extreme of ordinary conduction. Electrons pair through a distortion of the atomic lattice and behave collectively, which is why scattering stops. Medical imaging scanners are by far the largest use.