What Is a High-Temperature Superconductor?

Published July 2, 2026

Part 1 of our five-part Superconductor 101 series. The other parts cover the main superconductor materials, how a superconducting cable is built, how the tape itself is manufactured, and where superconductors are used today.

Every project in our tracker depends on one strange physical phenomenon: below a certain temperature, some materials conduct electricity with exactly zero resistance. The kilometre-long power cables under city streets, the fusion magnets and the wind turbine generators all trace back to that single fact.

This article explains what zero resistance means in practice, what the catches are, and why the word high-temperature in “high-temperature superconductor” (HTS) refers, somewhat confusingly, to temperatures around −200 °C.

Resistance, and what it costs us

In an ordinary metal like copper, electric current is a flow of electrons through a lattice of metal atoms. The electrons constantly collide with vibrating atoms and impurities, and every collision converts a little electrical energy into heat. This is electrical resistance. It is why cables get warm, why long transmission lines lose a meaningful share of the power fed into them, and why there is a limit to how much current a copper wire can take before it melts.

Engineers fight resistance the only way copper allows: with more copper. Higher power means thicker conductors, wider cable trenches, bigger transformers and more cooling. A large share of the cost and physical bulk of an electricity grid is, at root, the cost of resistance.

Zero resistance, literally

In 1911 the Dutch physicist Heike Kamerlingh Onnes cooled mercury with liquid helium and watched its resistance do something nobody expected. At 4.2 kelvin (−269 °C) it dropped abruptly to zero. Not to some very small value, but to zero, as precisely as anyone has been able to measure in the century since.

The consequences are startling. A current started in a closed superconducting loop keeps circulating on its own, with no power source, essentially forever. Experiments have run such persistent currents for years without measurable decay. The current generates no heat and loses no energy inside the superconductor.

Why this happens took physicists nearly half a century to explain. The short version of the BCS theory of 1957 is that at low temperature, electrons pair up and the pairs move through the metal collectively, in a shared quantum state that ordinary collisions cannot disturb. For this series, the practical takeaway matters more than the mechanism: below a critical temperature, the material carries current for free.

The catch: three critical limits

Superconductivity comes with conditions. Every superconductor operates inside a boundary defined by three limits, and crossing any one of them switches the material back into an ordinary (usually rather poor) conductor:

Critical temperature (T꜀). Warm the material past this point and superconductivity disappears. Each material has its own T꜀: about 9 K for the niobium-titanium alloy in MRI machines, about 90 K for the REBCO compounds used in most modern HTS projects.
Critical current density (J꜀). Push too much current through a given cross-section and superconductivity breaks down. This is the number engineers care about most, because it sets how much power a cable of a given size can carry.
Critical magnetic field (B꜀). Expose the material to a strong enough magnetic field and it also gives up. Since currents create magnetic fields of their own, this limit and the current limit are intertwined.

The three limits trade off against each other. Run a superconductor well below its T꜀ and it tolerates more current and more field. That is why some projects in the tracker operate REBCO, a material that superconducts at 90 K, down at 20 K instead: the extra cold buys a great deal of current-carrying headroom.

An abrupt, unplanned exit from the superconducting state is called a quench. The material suddenly develops resistance while carrying a huge current, and all the heating it was spared arrives at once. Detecting and safely managing quenches is a central engineering discipline in every superconducting system, from hospital MRI magnets to fusion reactors.

The Meissner effect

Superconductors do something else besides conducting perfectly: they actively expel magnetic fields from their interior. This behaviour was discovered in 1933 and is called the Meissner effect. It is the physics behind the famous photograph of a small magnet hovering above a nitrogen-cooled black disc. The superconductor refuses to let the magnet’s field pass through it, so the magnet floats on its own repelled field.

The Meissner effect matters beyond demonstrations. It is the definitive test that a material is a true superconductor rather than merely a very good conductor, and in modified form it underpins magnetic levitation concepts for transport.

Type I and Type II

The first superconductors discovered, pure metals like mercury, lead and tin, expel magnetic fields completely but surrender to quite weak ones. These are called Type I, and they are of little use for power engineering.

Every superconductor used in real projects is Type II. Above a first threshold field, these materials let magnetic field lines thread through them in discrete bundles called vortices, while the bulk of the material stays superconducting up to vastly higher fields. Keeping those vortices anchored in place is called flux pinning, and it is engineered deliberately by building nanoscale defects into the material for the vortices to snag on. Good flux pinning is what allows a Type II superconductor to carry large currents in strong magnetic fields. Materials scientists spend entire careers on it; cable buyers experience it simply as a better critical current.

Why 1986 changed everything

For 75 years after Onnes, every known superconductor needed extreme cold. Critical temperatures sat below about 23 K, reachable in practice only with liquid helium at 4.2 K. Helium is scarce and expensive, and it demands complex refrigeration plants and specialist skills. Superconductivity worked (MRI machines and particle accelerators prove it) but only in applications valuable enough to justify a helium habit.

Then in 1986, Georg Bednorz and Alex Müller at IBM Zurich found superconductivity in a copper-oxide ceramic at around 35 K, smashing the old ceiling and earning a Nobel Prize within a year. Researchers pushed the new family further within months, and in early 1987 yttrium barium copper oxide (YBCO) was shown to superconduct at about 93 K.

That specific number was the revolution, because of a mundane fact of cryogenics: liquid nitrogen boils at 77 K (−196 °C). Nitrogen makes up 78% of air. Liquid nitrogen is produced industrially by the tanker-load, costs less per litre than milk, and is routinely handled in food plants and hospitals. A superconductor that works above 77 K can be cooled with it.

So the term high-temperature superconductor means one that superconducts at temperatures reachable with liquid nitrogen rather than liquid helium. By everyday standards it is still brutally cold (“high-temperature” is physicist humour), but it moved superconductivity from exotic laboratory cryogenics into the realm of industrial refrigeration.

The copper-oxide ceramics brought a hard problem of their own. They are brittle ceramics rather than ductile metals, and turning them into kilometre-long flexible conductors took two more decades of materials engineering. Part 4 of this series tells that story.

What zero resistance makes possible

Combine zero resistance with practical cooling and you get capabilities copper cannot match:

Far more power through the same space. An HTS cable can carry several times the power of a conventional cable of the same duct size. In Shenzhen, China, a 400-metre HTS cable operating at just 10 kV delivers the capacity of a conventional 110 kV line. It is one of several such projects in our tracker.
Magnets stronger than iron and copper allow. Superconducting coils carry currents that would vaporize copper windings, producing the 20-tesla-class fields behind compact fusion reactor designs like SPARC.
Smaller, lighter machines. A superconducting wind turbine generator (the EcoSwing project in Denmark) came out roughly 40% lighter than its permanent-magnet equivalent.
Devices with no conventional equivalent, such as superconducting fault current limiters, which exploit the sharp transition out of superconductivity to act as self-resetting circuit breakers.

Where to go next

The rest of this series builds up the practical picture:

And for the state of the industry right now, the project tracker on our homepage lists real HTS installations worldwide: what they are made of, how they are cooled, and whether they are running today.

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