The wave of excitement caused by LK-99 — the purple crystal that was going to change the world — has now died down after studies showed it wasn’t a superconductor. But a question remains: would a true room-temperature superconductor be revolutionary?
The answer is that it depends — on the application, and on whether the hypothetical material also has other crucial qualities. But at least in some scientific fields, in particular those that use strong magnetic fields, better superconductors would be likely to have a huge impact.
LK-99 isn’t a superconductor — how science sleuths solved the mystery
Superconductors are materials that, at a certain temperature, begin to carry electric currents without resistance — and therefore without producing waste heat. But all confirmed superconductors exhibit the property only at low temperatures or under extreme pressures, or both. Some scientists are seeking materials in which the transition to superconductivity occurs in normal conditions, at room temperature and ambient pressure.
Although the low temperature requirements of today’s superconductors severely limit their use in everyday applications, the materials have become ubiquitous in the laboratory, where researchers can use a range of techniques to lower their temperatures. This is doable, but often adds cost and complexity to an experiment.
An extreme example is the Large Hadron Collider (LHC), the accelerator at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland. To keep protons moving in a 27-kilometre circle, the LHC generates strong magnetic fields with superconducting coils kept at a temperature of just 1.9 kelvin (–271.25 ºC). Doing so requires a cryogenic system containing 96 tonnes of liquid helium, the largest of its kind in the world. “If you didn’t need extreme temperatures, the engineering would be simplified,” says Luca Bottura, a nuclear engineer who is a magnet researcher at CERN.
So, it stands to reason that a superconductor that works at room temperature, or close to it, would quickly revolutionize many fields of science, right? Not so fast.
Take quantum computers, the nascent technology that promises to solve certain tasks that are beyond the reach of classical computers. One of the leading approaches for building quantum computers is to store information in loops of superconducting material. These are cooled to near absolute zero (−273.15 ºC) inside expensive Russian-doll-like devices called dilution refrigerators.
In a superconductor-based quantum computer, performance quickly degrades when the temperature rises even by a fraction of a degree — for reasons that have nothing to do with superconductivity. Quantum calculations are extremely sensitive to any kind of noise, and thermal vibrations are a major enemy, producing spurious ‘quasiparticles’, says Yasunobu Nakamura, a co-inventor of superconducting quantum computing. “At around 100–150 millikelvin, we already start seeing the adversarial effect of thermally excited quasiparticles,” says Nakamura, who is a physicist at RIKEN in Wako, Japan.
In other cases, the experiment itself might not require extreme cold, but the superconductor could still need to be kept much colder than the temperature at which it transitions to superconductivity, known as Tc. Superconductors vary in their physical properties, and in many applications — especially for high-field magnets — two other properties are crucial. These are called critical current and critical magnetic field. Superconductivity is lost not only when temperatures rise, but also when a material is either pushed to carry more than a certain amount of current or exposed to a high enough magnetic field.
Crucially, both critical field and critical current are temperature-dependent: the lower the temperature, the higher the current and magnetic field the material can withstand. So, just because a superconductor has a high Tc, that doesn’t mean that it will be possible to use it at any temperature below Tc. In many applications, a superconductor’s performance will improve as the system gets colder.
Fortunately, the best superconductors discovered so far, including a class called copper-oxide (or cuprate) superconductors, can also withstand very high magnetic fields — when kept cold enough.
In the field
Four years ago, one cuprate was used to obtain a record for the strength of a steady (rather than pulsed) magnetic field at the US National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida. The superconducting coils at the NHMFL produced a magnetic field of 45.5 tesla, but only if they were kept in liquid helium, so below 4.2 kelvin. “We’re not using high-Tc superconductors because the Tc is high — we’re using them because [their critical magnetic field] is high,” says physicist Laura Greene, chief scientist at the NHMFL.
“If you want a high-field magnet, you want to run this at as low a temperature as possible, because that’s where you get the real power of superconductivity,” says Yuhu Zhai, a mechanical and electrical engineer at another US national lab, the Princeton Plasma Physics Laboratory (PPPL) in New Jersey.
CERN is exploring options for a future particle collider that would eventually smash protons with energies seven times greater than at the LHC — a range in which physicists hope they can discover new elementary particles. To reach those higher energies, particles must be accelerated using higher fields or along a longer accelerator loop, or both. To build such a machine, physicists dream of digging a circular tunnel up to 100 kilometres long, next to that of the LHC. But even with such a large loop, superconducting magnets like the LHC’s — 8-tesla monsters with niobium–titanium coils — could not generate the required fields, estimated at 16 to 18 tesla. “At this point, it’s clear that we have to turn to other materials,” says Bottura.
Current high-Tc superconductors could get there — but probably only if they are kept at liquid-helium temperatures. A similar accelerator proposal in China, the Circular Electron–Positron Collider, would also use high-Tc magnets. “We have been considering high-temperature superconducting materials for quite some time, mainly cuprates and iron-based,” says Wang Yifang, head of the Institute of High Energy Physics in Beijing.
Copper-oxide-based superconductors have other disadvantages, however: they are brittle ceramic materials that are expensive to produce and to engineer into cables. Their critical currents are also still too low, Wang says. Another class of superconductors that are iron-based could, in principle, perform better while being half the cost of copper oxides, he adds.
Bottura and others are researching the feasibility of a completely new type of accelerator. By replacing protons with muons — particles similar to electrons but 207 times more massive — a collider could study the same type of physics as a 100-km proton–proton collider, but in a much smaller ring, perhaps even one that could fit in the existing LHC tunnel. Making muons go in a circle would not involve magnetic fields of particularly intense strength. But the catch is that producing a muon beam with the right properties might require magnets of as much as 40 tesla.
At that strength, says Bottura, “the problem is no longer the superconductor — it’s to keep the coils in place”. The currents inside electromagnetic coils tend to push the magnet apart. At 40 tesla, even the strongest steel could not withstand the mechanical stress. Instead, the magnets might have to be harnessed using stronger materials, such as carbon fibres. (Strength requirements are not as stringent for the NHMFL magnets, which need to produce a high field in a space only a few centimetres wide.)
So, in both proton and muon colliders, a superconductor with vastly better performance than anything discovered so far could make a huge difference, but other engineering challenges would arise.
Journey to fusion
Structural strength already poses serious constraints in another class of machines — those that aim to harness the energy of nuclear fusion. A long-established approach to fusion attempts to confine a plasma using magnets arranged in a doughnut shape called a tokamak. The plasma is heated to millions of degrees to smash various isotopes of hydrogen together. The largest experimental tokamak in the world, called ITER, is being built in southern France and will use massive liquid-helium-cooled magnets to produce fields of nearly 12 tesla.
But both industrial and publicly funded labs are pushing to design tokamak magnets based on high-Tc superconductors for multiple reasons, says Zhai. Higher fields could drastically raise the rate at which a fusion reactor burns its fuel, and therefore increase the energy that can be produced — at least in principle, because many of the crucial steps towards extracting energy from fusion have yet to be demonstrated. One positive outcome of the industrial effort to increase production of high-Tc magnetic materials is that their cost has dropped. (They are still much more expensive than niobium–titanium ones, however.)
In addition, tokamaks should eventually forgo liquid helium cooling, Zhai says, and not just because cooling systems are complex to build. Helium is a scarce resource, and it would not be feasible to build hundreds of ITER-sized reactors that use it.
The search for better superconducting materials is a high-risk task, says Greene, because the successes have so far been few and far between. Nevertheless, she adds, “It’s hard work, and it’s exciting work, and it’s making changes in the world.”