Researchers appear to have solved the LK-99 mystery. Scientific detective work has uncovered evidence that the material is not a superconductor and shed light on its actual properties.
This conclusion dashed hopes that LK-99 - a compound of copper, lead, phosphorus and oxygen - would be the first superconductor to function at room temperature and pressure. Instead, the study showed that impurities in the material, particularly copper sulfide, caused its electrical resistance to drop dramatically and sometimes exhibit levitating properties on magnets, properties similar to those of superconductors.
"I think things are pretty much settled at this point," says Inna Vishik, a condensed matter experimenter at the University of California, Davis.
The LK-99 saga began in late July when a team led by Sukbae Lee and Ji-Hoon Kim at the Center for Quantum Energy Research, a Seoul startup, released a preprint1,2It is claimed that LK-99 is a superconductor at atmospheric pressure and a temperature of at least 127 °C (400 Kelvin). All the superconductors demonstrated so far only work at very low temperatures and extreme pressures.
This remarkable claim quickly caught the attention of the scientific community and researchers, some of whom attempted to replicate LK-99.Preliminary experiments find no evidence of superconductivity at room temperature, but no conclusions were drawn. Now, after dozens of replication attempts, many experts say with confidence that the evidence shows LK-99 is not a room-temperature superconductor. (Lee and Kim's teams did not respondNaturePlease comment. )
The South Korean team's claim is based on two properties of LK-99: the levitation force above the magnet and the sudden drop in electrical resistance. But Peking University has a different team3and the Chinese Academy of Sciences4The Chinese Academy of Sciences (CAS) in Beijing has found a banal explanation for these phenomena.
another study5Researchers in the United States and Europe have combined experimental and theoretical evidence to show how the structure of LK-99 makes superconductivity impossible. and other experimenters synthesized and studied pure samples6Examination of LK-99 dispelled doubts about the structure of the material and confirmed that it is not a superconductor but an insulator.
The only further confirmation will come when the South Korean team shares their samples, says Michael Fuhrer, a physicist at Monash University in Melbourne, Australia. "It's their responsibility to convince other people," he said.
Perhaps the most compelling evidence for superconductivity in LK-99 isVideoPhotos by the South Korean team show a coin-shaped sample of the silver material dangling from a magnet. The researchers say the samples levitate because of the Meissner effect - a hallmark of superconductivity, in which the material repels a magnetic field. Several unconfirmed videos of LK-99's hovering subsequently circulated on social media, but none of the researchers who originally attempted to reproduce the discovery observed any hovering.
Derrick VanGennep, a former condensed matter researcher at Harvard University in Cambridge, Massachusetts who now works in finance but was intrigued by LK-99, encountered some red flags. In the video, one edge of the sample appears to be glued to a magnet and finely balanced. In contrast, a superconductor suspended from a magnet can rotate and even be turned upside down. "None of these behaviors resembled what we saw in the LK-99 video," VanGennep said.
He believes that LK-99's properties are more due to ferromagnetism. So he made a pellet from compressed graphite filings with iron filings glued to them. AVideoThe discs made by VanGennep show that his discs are made of non-superconducting ferromagnetic material and mimic the behavior of LK-99.
On August 7, the Peking University team reported3This “semi-levitation” effect occurs in our own LK-99 samples due to ferromagnetism. "It's like the iron file experiment," said team member Yuan Li, a condensed matter physicist. The particle experiences buoyancy, but not enough to keep it suspended—only balanced at one end.
Li and his colleagues measured the resistivity of the samples and found no evidence of superconductivity. But they could not explain the sharp drop in resistance observed by the South Korean team.
The Korean authors found that the resistivity of LK-99 dropped tenfold at a given temperature, from about 0.02 ohm-cm to 0.002 ohm-cm. "They calculated it very precisely: 104.8 °C," says chemist Prashant Jain of the University of Illinois at Urbana-Champaign. "I thought, wait a minute, I know the temperature."
The reaction to synthesize LK-99 followed an unbalanced recipe. For each copper-doped lead phosphate crystal (pure LK-99) produced, 17 parts copper and 5 parts sulfur are produced. These residues generate large amounts of impurities, especially copper sulfide (Cu).2S) the South Korean team reported finding the substance in their samples.
Jain, an expert on copper sulfide, recalled that the temperature of copper was 104°C.2S undergoes a phase transition. Below this temperature, the specific resistance of Cu in air decreases2S drops sharply - a signal almost identical to the so-called superconducting phase transition of LK-99. "I can't believe they missed it," said Jain, who published the preprint7Regarding important disruptive effects.
On August 8, the CAS team reported4Effect of Cu2S impurity in LK-99. “Varying amounts of copper2"S can be synthesized through various processes," said team member Jianlin Luo, a physicist at the Chinese Academy of Sciences. The researchers tested two samples - the first, heated in a vacuum, produced 5 percent copper2The S content, second in air, gives 70% Cu2S content.
The resistivity of the first sample increased steadily as it cooled, as did the other samples in the replication experiment. However, the resistivity of the second sample dropped to around 112 °C (385 K), which agreed well with the Korean team's observations.
"So I said, 'Well, that obviously makes them think it's a superconductor,'" Führer said. "The nail in the coffin is that copper sulfide thing."
It is difficult to make conclusive statements about the properties of LK-99 because the material is unpredictable and samples contain various impurities. "Even due to our own growth, different batches may be slightly different," Li said. However, he believes a sample close enough to the original sample will be enough to verify that LK-99 is a superconductor under ambient conditions.
The sound explanation of the resistance drop and semi-levitation convinced many in the community that LK-99 is not a room temperature superconductor. But the mystery remains: what are the actual properties of this material?
Initial theoretical attempts to predict the structure of LK-99 using density functional theory (DFT) indicated interesting electronic features known as flat bands. Electrons in these regions move slowly and can be highly correlated. In some cases, this behavior leads to superconductivity. However, these calculations were based on untested assumptions about the structure of LK-99.
To better understand these materials, the US-European team5To calculate the structure of LK-99, a precise x-ray of the samples was performed. Crucially, the imaging allowed the team to perform rigorous calculations that clarified what was happening to the flat ribbons and showed them to be hostile to superconductivity. Instead, the flat bands in LK-99 come from highly localized electrons that cannot "jump" like in superconductors.
This was reported on August 14th by an independent team at the Max Planck Institute for Solid State Research in Stuttgart6Synthesis of pure LK-99 single crystals. Unlike previous synthesis attempts that relied on crucibles, this time they used a technique called floating zone crystal growth. This allowed the researchers to avoid introducing sulfur into the reaction, thereby eliminating copper2S impurities.
The result is clear purple crystals - pure LK-99, also known as Pb8.8copper1.2Phosphor6oxygen25. When separated from impurities, LK-99 is not a superconductor but an insulator with a resistance in the millions of ohms, too high for standard conductivity tests. It exhibits slight ferromagnetism and diamagnetism, but not enough to achieve partial levitation. "Therefore, we rule out the existence of superconductivity," the team concludes.
The team theorizes that the signs of superconductivity observed in LK-99 are caused by Cu2S impurities, these impurities are not found in its crystals. "This story shows exactly why we need single crystals," said Max Planck physicist Pascal Puphal, a crystal growth expert and Max Planck physicist who led the study. "Once we have a single crystal, we can clearly study the intrinsic properties of the system."
learn a lesson
Many researchers reflect on what they learned from the summer's superconducting phenomenon.
According to Leslie Schoop, a solid-state chemist at Princeton University in New Jersey and co-author of the Flat Band study, the lessons of hasty calculations are clear. "Even before LK-99 I was talking about using DFT carefully and now I have the best story for my next summer school," she said.
Jain points to the importance of old data that is often overlooked - the most important measurement of copper resistance on which he relies2S was released in 1951.
While some critics see the LK-99 saga as a paradigm of scientific reproducibility, others say it offered an unusually quick solution to an intriguing mystery. "Usually these things die very slowly, it's just a rumor and nobody has been able to reproduce it," Führer said.
When the copper oxide superconductor was discovered in 1986, researchers immediately began researching its properties. But nearly four decades later, the mechanism by which the material superconductors arise is still a matter of debate, Vishk said. Efforts to explain LK-99 came quickly. "The detective work covers all parts of the original observation - I think that's really great," she said. "And it's relatively rare."