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[guide to Xin Zhiyuan] is it really a stone hammer this time? Nature published articles combing through the evidence that LK-99 is not a superconductor, and the scientific community has begun to sum up lessons learned.

The doubt cloud of room temperature superconductivity for many days seems to have come to an end.

Yesterday, Nature posted: "LK-99 is not a room temperature superconductor-how do scientific detectives solve this mystery?" Dan Garisto is a science writer with a degree in physics and used to work in Fermi Laboratories.

Article address: https://www.nature.com/articles/d41586-023-02585-7

On August 16, the Institute of Physics of the Chinese Academy of Sciences conducted a more detailed study of LK-99 samples from three different research groups. It was considered that the electromagnetic properties of the three independent samples were derived from cuprous sulfide, which denied the room temperature superconductivity of LK-99.

Paper address: https://arxiv.org/abs/2308.07800

In the latest article on the 16th, Dan Garisto summarizes the recent more than ten days of room temperature superconducting reversal events, piecing together the research results of various institutions, and reveals the mystery of why LK-99 shows similar superconducting behavior.

Scientific detectives have found evidence that LK-99 is not a superconductor-the sharp decline in the resistivity of the sample and the partial suspension of magnets are due to impurities in the material, especially cuprous sulfide.

This conclusion completely breaks the hope of LK-99 as "the first room temperature and atmospheric pressure superconductor in history".

"I think it's over," said Inna Vishik, a condensed matter experimenter at the University of California, Davis. "

The LK-99 scandal began in late July, when a team led by Sukbae Lee and Ji-Hoon Kim of the Quantum Energy Research Center, a start-up in Seoul, published two preprinted papers on arXiv, claiming that LK-99 was a superconductor at an atmospheric temperature higher than 127C (400K) and instantly attracted the world's attention.

Because all previously identified superconductors work only at extreme temperatures and pressures.

The two papers have triggered a frenzy of recurrence experiments by academic institutions and even amateurs all over the world. Since August, there has been a steady stream of repetition experiments and papers every day.

On August 2, Huake team successfully reproduced the maglev phenomenon of the sample. On August 3, the team of Southeast University successfully measured the zero resistance phenomenon of LK-99. These repetition experiments once pulled the excitement of the public and academia to the highest.

Unfortunately, after many repeated experiments, many institutions around the world have come to the conclusion that LK-99 is not a room temperature superconductor.

A series of evidence is in the false that LK-99 is the two basic reasons why the South Korean team believes that LK-99 is a superconductor: suspension above a magnet and a sudden drop in resistivity, both of which have been convincingly explained by the research results of two independent teams from Peking University and the Chinese Academy of Sciences (CAS).

A study by Princeton University in the United States and Mapu Institute in Germany combined experimental and theoretical evidence to prove why the structure of LK-99 could not be a superconductor.

The researchers of Mapu Institute in Germany synthesized and studied the pure sample of LK-99, dispelled any doubts about the structure of the material before, and confirmed that LK-99 is not a superconductor, but an insulator.

The pure crystal of LK-99 synthesized by ▲ 's team at Max Planck in Germany and the only evidence that further support that LK-99 may be a room temperature superconductor during this period came from another video shared by the Korean team. Now the eyes of the world are focused on the Korean team.

So far, the most convincing evidence to support LK-99 superconductivity is the video shot by the South Korean team. No suspension or superconductivity was observed in subsequent reproduction attempts.

The unverified suspended former condensed matter researcher at Harvard University named Derrick van Gennep, who was interested in LK-99, made a video similar to the LK-99 sample shown by the Korean delegation.

Like the LK-99 video, the edge of the sample seems to be glued to the magnet and appears to be subtly balanced. By contrast, a superconductor suspended on a magnet can rotate or even float on the other side.

He believes that the nature of LK-99 is more likely to be the result of ferromagnetism. So he made the sample out of particles made of compressed graphite shavings glued to iron shavings and imitated the suspension video of LK-99.

On August 7, the team of Peking University also concluded that the suspension of their LK-99 samples was due to ferromagnetism.

They say in the paper that the particles in the sample can only be balanced at one end because of their ferromagnetism, but not enough to levitate.

Li and his colleagues measured the resistivity of the sample and found no sign of superconductivity. But they could not explain the sharp decline in resistivity seen by the South Korean team.

In the preprint submitted by the Korean team, the impure sample showed an obvious resistance jump of LK-99: the resistivity dropped from 0.02ohms / cm to 0.002 ohms / cm at 104.8 ℃.

However, the reaction formula for the synthesis of LK-99 is uneven, which leads to 17 phr of copper and 5 phr of sulfur in pure LK-99, that is, every phr of copper-doped lead phosphate crystal.

These residues become impurities in LK-99, especially copper-sulfur compounds account for a large proportion.

These impurities were also reported in the samples of the Korean team. After that, these copper-sulfur compound impurities become the key to solve the mystery of LK-99 "superconductivity".

Jain, an expert on copper sulphides, remembers that 104 ℃ is the temperature at which the phase transition of Cu2S occurs. Below this temperature, the resistivity of Cu2S exposed to air drops sharply, which is almost the same as the signal of LK-99 's so-called superconducting phase transition.

After studying the South Korean team's paper, Jain said, "I can hardly believe they missed the analysis of Cu2S. "

On August 9, Jain published a preprinted paper on this important confusing effect on arXiv.

Paper address: https://arxiv.org/abs/2308.05222

In conclusion, the paper states that in order to clearly verify the superconductivity of LK-99, it must be under the condition that LK-99 does not have any Cu2S.

Coincidentally, the day before the publication of the Jain paper, the Institute of Physics of the Chinese Academy of Sciences confirmed the effect of Cu2S impurities on LK-99.

The research team believes that the "superconducting" behavior in LK-99 is probably due to the decrease of resistivity caused by the first-order structural γ phase transition from high-temperature β phase to low-temperature phase of Cu2S at 385k.

Paper address: https://arxiv.org/abs/2308.04353

The team synthesized LK-99 samples with different levels of Cu2S, the first of which was heated in a vacuum with 5 per cent Cu2S. The second sample was heated in air and had a Cu2S content of 70%.

When testing the resistivity of the two samples, it was found that:

The resistivity of the sample with 5% Cu2S increases relatively smoothly during cooling, which appears to be similar to that of other replication attempts.

However, the resistivity of the sample containing 70% Cu2S dropped sharply around 112 ℃ (385K), which was very close to the observation of the Korean team.

But the team also said it was difficult to draw firm conclusions about the properties of LK-99 because there were so many impurities in the material that different batches of samples were synthesized.

But the team also believes that if you can synthesize a sample close enough to the original sample of the South Korean team, you can test whether LK-99 is a superconductor at room temperature.

Pure LK-99 crystal with the strong explanation of resistivity decline and other non-superconductors in semi-suspension, it seems that LK-99 is not a room temperature superconductor has gradually become the mainstream view. However, the actual nature of LK-99 has not been confirmed by scientists.

Because it is very difficult to separate and purify LK-99, and the samples synthesized each time are very different, scientists initially tried to use a method called density functional theory (DFT) to predict the structure of LK-99.

Under the guidance of this theory, the structure of the predicted LK-99 shows interesting electronic characteristics, which is called "flat band" (flat bands).

A "flat band" refers to an area where electrons move slowly and may be strongly related. In some cases, the electronic characteristics of the "flat band" can lead to superconductivity.

However, this is only a calculation based on the unconfirmed structural hypothesis of LK-99.

To better understand the material, the US-European team performed single crystal X-ray diffraction (SXRD) imaging of their samples to calculate the structure of the LK-99.

Paper address: https://arxiv.org/abs/2308.05143

Imaging allows the research team to make rigorous calculations, illustrating the case of flat belts: they are not conducive to superconductivity. In contrast, the flattening in LK-99 brings in strongly localized electrons, which do not "transition" as superconductors need.

The team ruled out the possibility that LK-99 is a superconducting material, arguing that it is more likely to be a ferromagnet.

So far, many studies have denied the possibility that LK-99 is a room temperature superconductor.

However, some people still think that LK-99 is a supporter of room temperature superconductivity, and they think that none of the studies that negate LK-99 have synthesized LK-99 with up to the standard purity, which is the reason for the failure of previous experiments.

But on August 14, an independent team at the Max Planck solid State Research Institute in Stuttgart, Germany, reported that pure single crystals of LK-99 had been synthesized.

Paper address: https://arxiv.org/abs/2308.06256

Unlike previous crucible-dependent synthesis methods, the team used a technique called floating region crystal growth (floating zone crystal growth).

They avoid the introduction of sulfur into the reaction during the synthesis of LK-99, thus eliminating Cu2S impurities. A transparent purple crystal, pure LK-99, was obtained from the experimental synthesis.

However, the results show that after the separation of impurities, LK-99 is not a superconductor, but an insulator with a resistance of millions of ohms. Its resistance is so high that it cannot even be tested for standard conductivity.

At the same time, although pure LK-99 shows slight ferromagnetism and diamagnetism, it can not show suspension effect at all.

"for these reasons, we ruled out the existence of LK-99 superconductivity," the team concluded.

It is also confirmed that the superconductivity reflected in LK-99 basically comes from Cu2S impurities, but there is no such impurity in the pure crystal of LK-99.

Pascal Puphal, a scientist at the Max Planck Institute of Physics who led the study, said: "this event explains the importance of single crystals. When we have single crystals, we can clearly study the inherent characteristics of a system." "

Lessons learned many researchers are reflecting on what they can learn from this superconducting storm.

Leslie Schoop, a solid-state chemist at Princeton University, believes that scientists need to learn from immature calculations.

"before the LK-99 incident, I thought it was necessary to use DFT seriously," she said. Now, I have prepared a wonderful case for the next summer project. "

Jain pointed out the importance of the data in previous studies, such as the Peking University paper that cuprous sulfide is the cause of resistance jump is the key to deny LK-99 superconductivity, and the measurement results of cuprous sulfide resistivity were published as early as 1951.

While some commentators see the LK-99 incident as a model of repeatability in scientific research, others say it is an example of an unusually quick solution to a high-profile problem.

A scientist said: "this kind of thing is often difficult to falsify in the past, because most similar materials are very difficult to copy." "

For example, after the copper oxide superconductor was discovered in 1986, researchers began to explore the properties of the material. Now nearly forty years later, the superconducting mechanism of this material is still controversial.

But the study of LK-99 is going well, although it is really not very common.

Reference:

Https://www.nature.com/articles/d41586-023-02585-7

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