Glossary
The words this field uses.
Every term that keeps turning up around superconductors, defined the way we would explain them out loud. Search them, or cut the list by topic.
39 terms
The basics
Superconductor
- A material that carries electric current with no resistance, so none of the energy turns into heat on the way. Every material does this only below some temperature, and for most of them that temperature is close to absolute zero.
Critical temperature (Tc)
- The temperature a material has to be colder than before it superconducts. It is the number the field has chased for a century, and it is quoted in kelvin.
Kelvin (K)
- The temperature scale that starts at absolute zero. Room temperature is about 293 K, liquid nitrogen boils at 77 K, and liquid helium at 4.2 K. A degree of change is the same size as a degree Celsius.
Ambient pressure
- Ordinary atmospheric pressure, the pressure everything a person can build actually operates at. A material that only superconducts under enormous pressure is a real result and not yet a usable one.
Meissner effect
- A superconductor pushes magnetic fields out of its interior, which is why a magnet floats above one. It is the definitive test: zero resistance can be imitated by a very good ordinary conductor, and this cannot.
Cooper pair
- Two electrons bound together so they move through the material as a single unit. Superconductivity is current carried by these pairs, which travel without scattering their energy away as heat.
BCS theory
- The 1957 theory that explained every superconductor known at the time: vibrations of the crystal lattice bind electrons into pairs. It earned a Nobel Prize, and it does not explain the copper oxides that hold every ambient-pressure record.
Unconventional superconductor
- A superconductor whose pairing BCS theory does not explain. The cuprates are the famous family, and the central fact of the field sits here: the materials with the highest temperatures are the ones with the least settled physics.
High-temperature superconductor (HTS)
- The families that superconduct above roughly 30 K, which in practice means the ones that can run in liquid nitrogen instead of liquid helium. That one change of refrigerant changes the economics of everything built on them.
Type II superconductor
- A superconductor that admits a strong magnetic field as discrete threads of flux instead of failing outright. Every practical conductor is Type II, because anything wound into a magnet has to survive the field it creates.
Transition temperature record
- The highest critical temperature anybody has published under a given condition. At ambient pressure the durable record has stood since 1993, which is the fact this company exists because of.
The numbers that matter
Critical current density (Jc)
- How much current a superconductor carries per unit of cross-section before it stops superconducting. A material with a high transition temperature and a low critical current is not a conductor anybody can build with.
Engineering current density (Je)
- The same measure taken across the whole wire, including everything in it that is not superconductor. It is the number a magnet designer uses, because the copper, steel and insulation in a real conductor take up room and carry nothing.
Critical field
- The magnetic field a superconductor tolerates before it gives up. It matters because most of the things people want superconductors for are magnets, which means the material has to survive its own field.
Critical surface
- Temperature, magnetic field and current, drawn as one boundary. A conductor works only inside the volume the three enclose, and every application is a point in that space, so quoting a transition temperature alone describes one edge of a three-dimensional object.
Cost per kiloamp-metre ($/kA·m)
- The price of moving a thousand amps across one metre of conductor. It is the number that decides whether a magnet gets built, and it only means something at a stated temperature and field: a $/kA·m with no operating point beside it is not a specification.
Cryocooler
- A closed-cycle refrigerator that holds a superconductor at operating temperature without liquid refills. The colder the conductor, the larger the machine and its power draw, so much of the case for a warmer superconductor is really a case for a smaller cryocooler.
The materials
Cuprate
- A family of copper-oxide ceramics that includes almost every high-temperature superconductor found so far. They were discovered in 1986 and they still hold the ambient-pressure record.
REBCO
- A cuprate written as rare-earth barium copper oxide, and the conductor most high-field magnets are wound from today. It is not sold as a wire but as a coated tape, which is part of why it costs what it does.
Iron-based superconductor
- The second family of high-temperature superconductors, found in 2008 in layered compounds of iron with arsenic or selenium. Their discovery proved the cuprates were not a one-off, and their pairing is not explained by BCS theory either.
Niobium alloys (NbTi and Nb₃Sn)
- The workhorse superconductors of the last sixty years, wound into nearly every MRI magnet and accelerator dipole in service. They run in liquid helium near 4 K, and they are the incumbents any new conductor gets priced against.
Magnesium diboride (MgB₂)
- A simple two-element compound that sat in chemical catalogues for half a century before anyone cooled it below 39 K and found it superconducting, in 2001. Its late discovery says less about the material than about how unsystematic the search has been.
Hydride superconductor
- A hydrogen-rich compound that superconducts at temperatures close to a warm room, but only under pressures found inside a diamond anvil cell. The pressures are around a hundred and fifty gigapascals, which is not a condition anything can be built in.
Metastable
- Holding a state it will eventually leave. A metastable superconducting phase can be real, measured, and published, and still be unusable because it does not last or cannot be made in a size worth using.
Doping
- Adjusting a recipe to add or remove charge carriers, usually by substituting a few percent of one element for another. An undoped cuprate does not superconduct at all; doped, it holds the record. The distance between nothing and the record is a few atoms per hundred.
Making and surviving
Synthesis route
- The recipe for actually making a material: what goes in, at what temperature, in what atmosphere, for how long. A predicted compound with no synthesis route is a suggestion rather than a candidate.
Single crystal
- A sample whose atoms line up in one unbroken arrangement, with no boundaries between differently oriented grains. Measurements that mean anything about a material's intrinsic behaviour are made on these, which is why crystal growth is its own discipline.
Coated conductor
- A superconductor grown as a thin film on a metal tape rather than drawn as a wire. It is how brittle ceramics are made into something that bends, and the process is most of the price.
Pressure-quenched
- Squeezed hard enough to force a material into a structure it would not otherwise take, then released fast enough that it stays in that structure after the pressure is gone. What comes out is metastable: it relaxes back given time or warmth.
Flux pinning
- Defects engineered into a conductor to hold magnetic flux lines in place, because flux that moves dissipates energy and spoils the zero resistance. It is why how a conductor is processed matters as much as what it is made of.
Persistent current
- A current started in a closed superconducting loop and simply left running. MRI magnets operate this way for years with the power supply disconnected, and the measured stability of such loops is the cleanest demonstration that the resistance really is zero.
Quench (in a magnet)
- The moment a superconducting magnet suddenly stops superconducting and its stored energy turns into heat all at once. It is a different word from pressure-quenching a sample: this one is the failure a magnet designer spends most of the design surviving.
The search and the proof
In silico
- Done in a computer rather than at a bench. Screening candidates in silico is what makes it possible to consider millions of materials, only a handful of which are worth anyone's furnace time.
Density functional theory (DFT)
- The standard tool for computing a material's properties from first principles. It finds ground-state energies and structures well enough to run at industrial scale, and it fails, in a way the field has documented for decades, for the strongly interacting electrons in the materials that hold every ambient-pressure record.
Retrodiction
- Testing a method against something already known by hiding the answer, running the method, and comparing. It shows agreement with the published record. It is not the same as predicting something nobody has measured yet, and we are careful not to describe it as though it were.
Diamond anvil cell
- A small press that squeezes a sample between two diamond tips to reach pressures found deep inside planets. Samples are tens of microns across, which is why a result from one is a physics result rather than a material.
Scanning tunnelling microscope (STM)
- An instrument that maps a surface by holding a sharp tip a fraction of a nanometre above it and measuring the current that crosses the gap. It resolves single atoms, and it is one of the instruments our partner labs use to check what the platform predicts.
Josephson junction
- Two superconductors separated by a barrier a few atoms thick, which current crosses with no voltage across it. It is the working element of the most sensitive magnetic detectors ever built and of most superconducting quantum processors.
SQUID
- A superconducting loop containing Josephson junctions, and the most sensitive detector of magnetic fields known. It can read the field produced by a working human brain from outside the skull.