A “possible real solution to the energy crisis” that “could change everything”. That’s how recent headlines billed the mundane lumps of a dirty-looking material known as LK-99 reported by scientists in South Korea in July. Their findings were described in two papers (https://arxiv.org/abs/2307.12008 and https://arxiv.org/abs/2307.12037) posted to the arXiv preprint server – a website where researchers present work that has not yet been subjected to peer review. They said they had “for the first time in the world” made a superconductor that worked at room temperature and at everyday pressure.
A superconductor is a material that can conduct an electric current without any resistance, meaning that no energy is lost through heat. Superconductors have been known about for more than 100 years, but previous ones have worked only at extremely low temperatures or when under very high pressures. LK-99 on the other hand, the South Korean team said, was superconductive just sitting there on a benchtop. If they had been right, the discovery would genuinely have merited the word “revolutionary”.
But after weeks of feverish speculation and frantic attempts worldwide to make and test the new material, many experts in the normally recondite field of solid-state physics now think the claims were almost certainly wrong. There was reason to be sceptical from the outset: the South Korean scientists, Sukbae Lee and Ji-Hoon Kim of the Seoul-based startup company Quantum Energy Research Center had no track record in the field, and LK-99 – named after them and the year they began studying it – didn’t look much like high-temperature superconductors seen in the past.
A broad consensus is now emerging that the apparent signatures of superconductivity the Korean team reported – zero-resistance and a magnetic phenomenon called the Meissner effect – may have more mundane explanations. But even if LK-99 is a blind alley, the quest for a wonder material that is superconductive under everyday conditions will continue.
“It will happen,” says the physicist Jorge Hirsch of the University of California San Diego, “although it is hard to tell when.” But when it does, he says, it will result in “all sorts of incredible applications we haven’t even imagined yet”.
The holy grail of superconductors
Superconductivity was discovered in 1911 by the Dutch physicist Heike Kamerlingh Onnes, working in Leiden. He used liquid helium (which boils at -269C [-452F], just four degrees above absolute zero, the coldest temperature possible) to cool a wire of solid mercury, and found that at this frigid extreme its electrical resistance vanished. This meant that an electric current would lose no energy as it moved; theoretically, a current in a loop of a superconductive material would circulate for ever.
Electric currents in metals arise from electrons – the negatively charged fundamental particles in atoms – that are free to flow through the orderly array of atoms. Occasionally, a mobile electron will bounce off one of the vibrating atoms, losing some of its energy as heat – that’s the origin of electrical resistance. The farther the current travels, the more electrical energy it loses. As a result, about 5-10% of the electrical power generated in power plants is wasted as heat during its transmission to homes and industries along power lines.
Why all electrical resistance should suddenly disappear in mercury and many other metals at a superconducting critical temperature (known as Tc) was a mystery. But in 1957, three scientists explained that superconductivity arises when the mobile electrons pair up, thanks to the way the movements of one influence those of another. These “Cooper pairs” have a strange characteristic: they can all move as if they were one gigantic particle, too massive for any mere vibrating atom to disrupt.
For ordinary metals, this effect can happen only at very low temperatures, because the electron pairs are easily broken up by heat. But in the 1980s, solid-state physics was shaken by the discovery that a class of materials belonging to the family called cuprates – not metals, but brittle ceramic substances – could superconduct at higher temperatures than usual. The first of these did so at just -238C (35 Kelvin – absolute zero is -273C, or 0 Kelvin, with the size of a degree the same on the Celsius and Kelvin scales). Very soon such high-temperature superconductors were found with much higher superconducting critical temperatures, up to about 140K. This meant they could be cooled using liquid nitrogen (which boils at 77K), a much more abundant, cheap and convenient coolant than liquid helium.
The discovery won a Nobel prize for physics in 1987 and led to excited speculation about loss-free power lines and more. Because superconductors can carry high currents that would fry ordinary metal wires, they can be used to make very powerful electromagnets, generating strong magnetic fields. Such devices are now used in MRI scanners and in some prototypes for nuclear fusion reactors, where huge magnetic fields are needed to hold the very hot plasma. They might also be used for maglev trains, which are magnetically levitated above their rails to reduce friction and reach very high speeds.
But all such applications are still hampered by the need for cryogenic cooling. That’s hardly practical, for example, for power cables many miles long. The holy grail was a superconductor with a Tc of room temperature or higher. Was that even possible, given that the effect relied on electron behaviour usually evident only at low temperatures?
The excitement surrounding cuprate superconductors subsided. There has been renewed interest in recent years, however, with the discovery that some crystalline materials containing a lot of hydrogen show surprisingly high superconducting critical temperatures.
In 2015, a team in Germany reported superconductivity in a compound of hydrogen and sulphur at 203K – that’s just -70C. Four years later the same team described a compound of hydrogen and the metal lanthanum that showed signs of superconductivity, losing all resistance, at -23C, while a group in Washington DC earlier that year found the same material to be superconductive at just -13C. A team at Rochester University in New York caused great excitement in 2020 with a claim of superconductivity at almost 15C in a compound of carbon, hydrogen and sulphur – but they later had to retract the result amid allegations of misconduct.
The catch was that, to become superconducting, all these hydrogen-rich materials had to be squeezed between diamonds to tremendous pressures, comparable with those at the Earth’s core. That made such materials nonstarters for practical applications. But this March, the Rochester researchers made an even more startling claim: superconductivity at approximately 21C in yet another hydrogen-containing material that required only relatively mild squeezing. Solid-state physicists had barely recovered from that report – which no one has yet been able to reproduce – when along came LK-99, which purportedly needed no squeezing at all.
Social media buzz
LK-99 is decidedly weird for a putative high-temperature superconductor: it’s a greyish-black phosphate mineral called apatite containing copper and lead. Weirder still, while most superconductors are pretty good normal electrical conductors before they turn superconducting, LK-99 is an insulator above its purported Tc of 127C.
If the claims are true, LK-99 would therefore probably have an unprecedented way of becoming superconducting. The Korean team offered some theoretical arguments for how this might happen, but “these are not valid, in my opinion”, says Hirsch. The LK-99 Twittersphere (it’s now a thing) was set buzzing when the physicist Sinéad Griffin at the Lawrence Berkeley National Laboratory in California reported calculations of how the electrons in such a material are arranged in “flat bands” (an electronic feature associated with some high-temperature superconductors), which some interpreted as support for LK-99 as a superconductor. But Griffin herself says this isn’t necessarily the case. “Flat bands can mean superconductivity, but can also mean a wealth of other phenomena.”
Others have said the evidence about LK-99 provided by the original South Korean team is sloppy. Yes, their results showed a sudden drop in resistance below 127C – but experts say that this too can be produced by effects other than superconductivity. The resistance measurements are “not at all conclusive”, says Hirsch. The physicist Michael Fuhrer of Monash University in Australia says the electrical resistance doesn’t actually fall to zero, but to a value that is more than 1,000 times higher than the resistance of ordinary metals such as copper – it just looks like a huge drop to almost nothing because of the high value it starts from. “Overall, it doesn’t look like the kind of careful work you’d expect for a report of this impact,” he says.
Efforts to reproduce the LK-99 results in labs worldwide have instead eroded them. For example, a team at Southeast University in Nanjing, China, made a sample of LK-99 and reported on arXiv in August that they saw a drop to zero resistance only below about 110K – about -160C.
“Several labs have now synthesised LK-99 by different methods, and they’ve found that it’s not a superconductor,” says Fuhrer. He and Hirsch suspect that the large drop in resistance seen by the original team was caused by an impurity in the material.
One of the most compelling signs of superconductivity is that such materials will levitate above magnets, because of the way superconductivity “pushes” a magnetic field out of the substance itself: the Meissner effect. The Korean researchers said they saw it for LK-99, and on 3 August sent a video to the New York Times showing a speck of the stuff seemingly hanging suspended over a magnet. This followed a video posted on Chinese social media on 1 August by a team at Huazhong University of Science and Technology in Wuhan, China, professing to show a sample of magnetically levitated LK-99. But levitation can also be induced by ordinary magnetism, and another study posted to arXiv suggests that this is what’s happening with LK-99.
So it seems the party is over – for now. “There is a lot of evidence by now that it is not superconductivity,” says Hirsch.
Practical applications
All the same, there’s no reason to think room-temperature superconductivity is impossible. And if found, it could be a big deal. “A room-temperature superconductor that was a practical engineering material would be pretty transformative,” says John Durrell, a professor of superconductor engineering at the University of Cambridge. But, he adds: “There is a world of difference between a material that superconducts and a practical engineering superconductor.”
Applications might depend on how easily these brittle crystalline materials could be made into wires, and whether they would carry a big enough current without the superconductivity breaking down. Even if LK-99 had been as claimed, “It would probably take a lot of research to make it into a usable material,” says Durrell.
It is unlikely that the entire electricity grid would be rewired with it anyway, says Durrell. Rather, the main applications would be for making strong magnetic fields. MRI scanners could be cheaper and more compact – every GP surgery might have one. And we might make electromagnets that needed essentially no energy input once turned on, leading to maglev trains and more efficient motors, generators and wind turbines. Given the way existing strong magnets have enabled the powerful motors used by drones, Durrell says: “A practical room-temperature superconductor would potentially make electric aircraft with intercontinental range less of a dream and more of an imminent reality.”
But perhaps the real killer applications of such a material have yet to be imagined. After all, says Hirsch: “Imagine how different the world would be if semiconductors [such as the silicon used in all today’s microchips] would not work above liquid nitrogen temperature.”
All this remains hypothetical, but the quest will surely go on. Meanwhile, scientists in this normally arcane field have enjoyed a rare moment in the limelight. “It’s so exciting to see the interest in solid-state physics!” says Griffin.
