Nature LK-99 is not a superconductor

Researchers seem to have solved the mystery of LK-99. Scientific detective work has found evidence that the material is not a superconductor and clarified its actual properties.

This conclusion dashed hopes that LK-99 -- a compound of copper, lead, phosphorus, and oxygen (marking the discovery of the first superconductor to operate at room temperature and ambient pressure) -- was a superconductor. Instead, it was shown that impurities in the material: copper sulfide in particular, were responsible for the sharp drop in resistivity and partial suspension from the magnet, which appeared to be similar to the properties exhibited by superconductors.

Pure crystals of LK-99 synthesized by the team at the Max Planck Institute for Solid State Research in Stuttgart, Germany

In response, Inna Vishik, a condensed matter experimentalist at the University of California, Davis, said, "I think things are settled at this point."

The LK-99 saga began in late July, when a team led by Sukbae Lee and Ji-Hoon Kim of the Quantum Energy Research Center, a start-up company in Seoul, published a preprint claiming that LK-99 was a superconductor at atmospheric pressure and temperatures at least as high as 127 ºC (400 Kelvin). Until then, all proven superconductors had functioned only at extreme temperatures and pressures.

This extraordinary claim quickly attracted the attention of the scientifically interested public and researchers, some of whom attempted to replicate LK-99. Initial attempts found no sign of room-temperature superconductivity, but neither were they conclusive. Now, after dozens of replication efforts, many experts are confident that the evidence shows that LK-99 is not a room-temperature superconductor.

1) Accumulating Evidence

The Korean research team based their argument on two properties of LK-99: levitation on a magnet and a sudden drop in resistivity. But the Chinese Academy of Sciences team found a secular explanation for these phenomena.

On Aug. 8, the CAS team published a paper stating that no zero resistivity below the transition temperature was observed. "We believe that the so-called superconducting behavior in LK-99 is most likely due to a first-order structural phase transition of Cu2S around 385 K, which transforms from the β-phase at high temperatures to the γ-phase at low temperatures, leading to a decrease in resistivity."

Link:

https://arxiv.org/abs/2308.04353

Another study by American and European researchers combined experimental and theoretical evidence of how the structure of LK-99 makes superconductivity unfeasible. Other experimentalists synthesized and studied pure samples of LK-99, removing any doubt about the structure of the material: confirming that it is not a superconductor, but an insulator.

On August 9, U.S. and European researchers published a paper stating that the material is more likely to be a magnet than a room-temperature, room-pressure superconductor.

On Aug. 11, researchers posted a paper saying that the possibility of superconductivity in the crystal was ruled out.

At the time, Michael Fuhrer, a physicist at Monash University in Melbourne, Australia, commented, "Now, perhaps the only way to further confirm this is for the Korean research team to share their samples."

Perhaps the most telling evidence of LK-99's superconductivity is a video shot by the Korean team showing a coin-shaped sample of the silver material wobbling on a magnet. The Korean team said the sample was levitating because of the Meissner effect - one of the hallmarks of superconductivity - in which the material releases a magnetic field. Subsequently, several unconfirmed videos of LK-99 levitating circulated on social media, but none of the researchers who initially tried to replicate the discovery observed any levitation.

2) Only "half" levitated?

Something was discovered by Derrick van Gennep, a former condensed matter researcher at Harvard University in Cambridge, Massachusetts, who now works in finance but is interested in LK-99. In the video, the same edge of the sample appears to stick to the magnet, which appears to be delicately balanced. In contrast, a superconductor suspended on a magnet can rotate and even be held upside down.

Van Gennep concluded that LK-99's properties were more likely the result of ferromagnetism. So he made a pellet out of compressed graphite shavings with iron filings stuck to it, and Van Gennep made a video showing his disk (made of a non-superconducting ferromagnetic material) mimicking the behavior of LK-99.

Video address:

https://twitter.com/VanGennepD/status/1688052003216261120

On August 7, a team of researchers at Peking University reported that the "semi-levitation" of their LK-99 sample was due to ferromagnetism. Co-author Li Yuan, a condensed matter physicist, said, "It's like the filing iron experiment. The ball gets a lifting force, but it's not enough to levitate it, just enough to keep it balanced at one end."

Lee and his colleagues measured the resistivity of the samples and found no superconductivity. But they couldn't explain the sharp drop in resistivity seen by the Korean team.

3) Impure samples

In their preprint, the Korean authors note that the resistivity of LK-99 dropped tenfold, from about 0.02 ohm-cm to 0.002 ohm-cm, at a specific temperature; the temperature was 104.8ºC.

The reaction to synthesize LK-99 used an unbalanced recipe: for every 1 part of copper-doped lead phosphate crystals (pure LK-99) produced, 17 parts of copper and 5 parts of sulfur were produced. These residues produce a large number of impurities, notably copper sulfide, which the Korean research team found in their samples.

Jain, an expert on copper sulfide, remembers 104ºC as the temperature at which the phase transition of Cu2S occurs. Below this temperature, the resistivity of Cu2S exposed to air drops dramatically - a signal almost identical to the superconducting phase transition claimed by LK-99.

On August 9, Prashant K. Jain wrote that "Copper(I) sulfide is known to undergo a phase transition at 104 degrees Celsius from an ordered low-temperature phase to a high-temperature superionic phase. As a result of this phase transition, Cu(I) sulfide exhibits a sharp shift in resistivity and heat capacity, which is expected to coincide with the temperature-induced transition of LK-99. This means that LK-99 must be synthesized without any Cu2S to definitively verify the superconducting properties of LK-99."

In the August 8 article, the CAS team reported on the effect of Cu2S impurities in LK-99. At that time, CAS physicist Luo Jianlin said, "Different levels of Cu2S can be synthesized by different processes. The researchers tested two samples: the first was heated in vacuum with 5 percent Cu2S; the second was heated in air with 70 percent Cu2S."

The resistivity of the first sample increased relatively smoothly during cooling, similar to samples in other replication attempts. However, the resistivity of the second sample dropped sharply as it approached 112 ºC (385 K), in close agreement with the Korean team's observations.

Temperature dependence of the resistivity of the two samples (S1 and S2).

It is difficult to make a definitive statement about the properties of LK-99 because the material is subtle and the samples contain different impurities. Therefore, a sample close enough to the original sample is sufficient to test whether LK-99 is a superconductor under ambient conditions.

4) "Crystal clear" LK-99

With a strong explanation for the resistivity drop and half warp, many were convinced that LK-99 was 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) have hinted at an interesting electronic signature, known as the "flat band". In these regions, electrons move slowly and may be strongly correlated. In some cases, this behavior can lead to superconductivity. But these calculations are based on unproven assumptions about the structure of LK-99.

To better understand the material, the U.S.-European research team performed precise X-ray imaging of their samples to calculate the structure of LK-99. Most importantly, the imaging allowed them to perform rigorous calculations that clarified the case of flat bands: they are not conducive to superconductivity; on the contrary, the flat bands in LK-99 originate from strongly localized electrons, which are unable to "hop" in the way required for a superconductor.

On August 14, another team at the Max Planck Institute for Solid State Research in Stuttgart, Germany, reported synthesizing pure LK-99 single crystals. In contrast to previous crucible-dependent synthesis attempts, the researchers used a technique known as floating zone crystal growth, which avoids the introduction of sulfur into the reaction and eliminates Cu2S impurities.

The result is a transparent purple crystal, pure LK-99, Pb8.8Cu1.2P6O25. LK-99, isolated from the impurities, is not a superconductor, but an insulator, with a resistance of several million ohms, which is too high for standard conductivity tests. It showed slight ferromagnetism and diamagnetism, but not enough for partial levitation.

-- "Therefore, we rule out the possibility that superconductivity exists."

The team believes that the signs of superconductivity seen in LK-99 can be attributed to Cu2S impurities, which are not present in their crystals.

a) Magnetization versus temperature for the 2nd batch of crystals combining the cryogenic and heater options, measured in field strength cooling (FC) and zero field strength cooling (ZFC) modes at 7 T. b) Magnetization versus field strength for the same crystals measured at 2, 300 and 800 K. c) Magnetization versus field strength for the same crystals, measured at 2, 300 and 800 K. d) Magnetization versus field strength for the same crystals measured at 2, 300 and 800 K.

This series of "oopses" has led us to reflect on what we have learned from the superconductivity phenomenon that was a sensation this summer, and even spread to the stock market crash. Some critics see the LK-99 saga as a model of scientific reproducibility, while others see it as an unusually rapid solution to a high-profile problem.

Beyond its scientific importance, room-temperature superconductivity will be disruptive in terms of the technological applications and economic and social impact it will drive - which is why there has been a "rush" of enthusiasm from academia and industry to solve the puzzle.

In 1911, the Dutch physicist Heinrich Conrad Onnes first discovered the phenomenon of superconductivity. He found in his experiments, when certain metals cooled to a very low temperature, the resistance suddenly dropped to zero. This discovery opened a new era in the study of superconductivity.

Since then, three scientists, Bardeen, Cooper and Scholliver, proposed the BCS theory in 1957; Karl and Alexander Miller discovered a copper oxide material in 1986 that could achieve superconductivity at liquid nitrogen temperatures, known as a high-temperature superconductor; and in 2020, in collaboration with scientists from the U.S., Germany, and France, a team of researchers reported that sulfur hydride was realized at very high pressures in the Room temperature superconductivity breakthrough ......

Research organizations in the United States, Europe, Japan, China, Canada, Australia, and South Korea have also made important advances in superconductivity research.