LK-99 isn’t a superconductor — how science sleuths solved the mystery

Replications pieced together the puzzle of why the material displayed superconducting-like behaviours.

• Dan Garisto

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

The conclusion dashes hopes that LK-99 — a compound of copper, lead, phosphorus and oxygen — marked the discovery of the first superconductor that works at room temperature and ambient pressure. Instead, studies have shown that impurities in the material — in particular, copper sulfide — were responsible for the sharp drops in electrical resistivity and partial levitation over a magnet, which looked similar to properties exhibited by superconductors.

“I think things are pretty decisively settled at this point,” says Inna Vishik, a condensed-matter experimentalist at the University of California, Davis.

Claimed superconductor LK-99 is an online sensation — but replication efforts fall short

The LK-99 saga began in late July, when a team led by Sukbae Lee and Ji-Hoon Kim at the Quantum Energy Research Centre, a start-up firm in Seoul, published preprints1,2 claiming that LK-99 is a superconductor at normal pressure and temperatures up to at least 127 ºC (400 kelvin). All previously confirmed superconductors function only at extreme temperatures and pressures.

The extraordinary claim quickly grabbed the attention of the science-interested public and researchers, some of whom tried to replicate LK-99. Initial attempts did not see signs of room-temperature superconductivity, but were not conclusive. Now, after dozens of replication efforts, many experts are confidently saying that the evidence shows LK-99 is not a room-temperature superconductor. (Lee and Kim’s team did not respond to Nature’s request for comment.)

Accumulating evidence

The South Korean team based its claim on two of LK-99’s properties: levitation above a magnet and abrupt drops in resistivity. But separate teams in Beijing, at Peking University3and the Chinese Academy of Sciences4 (CAS), found mundane explanations for these phenomena.

Another study5, by US and European researchers, combined experimental and theoretical evidence to demonstrate how LK-99’s structure made superconductivity infeasible. And other experimenters synthesized and studied pure samples6 of LK-99, erasing doubts about the material’s structure and confirming that it is not a superconductor, but an insulator.

The only further confirmation would come from the Korean team sharing their samples, says Michael Fuhrer, a physicist at Monash University in Melbourne, Australia. “The burden’s on them to convince everybody else,” he says.

Perhaps the most striking evidence for LK-99’s superconductivity was a video taken by the Korean team that showed a coin-shaped sample of silvery material wobbling over a magnet. The team said the sample was levitating because of the Meissner effect — a hallmark of superconductivity in which a material expels magnetic fields. Multiple unverified videos of LK-99 levitating subsequently circulated on social media, but none of the researchers who initially tried to replicate the findings observed any levitation.

Half-baked levitation

Several red flags popped out to Derrick van Gennep, a former condensed-matter researcher at Harvard University in Cambridge, Massachusetts, who now works in finance but was intrigued by LK-99. In the video, the same edge of the sample seemed to stick to the magnet, and it seemed delicately balanced. By contrast, superconductors that levitate over magnets can be spun and even held upside-down. “None of those behaviors look like what we see in the LK-99 videos,” van Gennep says.

He thought LK-99’s properties were more likely the result of ferromagnetism. So he constructed a pellet of compressed graphite shavings with iron filings glued to it. A videomade by Van Gennep shows that his disc — made of non-superconducting, ferromagnetic materials — mimicked LK-99’s behaviour.

On 7 August, the Peking University team reported that this “half-levitation” appeared in their LK-99 samples because of ferromagnetism. “It’s exactly like an iron-filing experiment,” says Yuan Li, a condensed-matter physicist and study co-author. The pellet experiences a lifting force but it’s not enough to levitate — only enough to balance on one end.

Li and his colleagues measured their sample’s resistivity, and found no sign of superconductivity. But they couldn’t explain the sharp resistivity drop seen by the Korean team.

Impure samples

In their preprint, the Korean authors note one particular temperature at which LK-99’s showed a tenfold drop in resistivity, from about 0.02 ohm-centimetres to 0.002 ohm-cm. “They were very precise about it. 104.8ºC,” says Prashant Jain, a chemist at the University of Illinois Urbana–Champaign. “I was like, wait a minute, I know this temperature.”

The reaction that synthesizes LK-99 uses an unbalanced recipe: for every 1 part copper-doped lead phosphate crystal — pure LK-99 — it makes, it produces 17 parts copper and 5 parts sulfur. These leftovers lead to numerous impurities — especially copper sulfide, which the Korean team reported in its sample.

Jain, a copper-sulfide expert, remembered 104ºC as the temperature at which Cu2S undergoes a phase transition. Below that temperature, the resistivity of air-exposed Cu2S drops dramatically — a signal almost identical to LK-99’s purported superconducting phase transition. “I was almost in disbelief that they missed it.” Jain published a preprint7 on the important confounding effect.

On 8 August, the CAS team reported on the effects of Cu2S impurities in LK-99. “Different contents of Cu2S can be synthesized using different processes,” says Jianlin Luo, a CAS physicist. The researchers tested two samples — the first heated in a vacuum, which resulted in 5% Cu2S content, and the second in air, which gave 70% Cu2S content.

The first sample’s resistivity increased relatively smoothly as it cooled, and appeared similar to samples from other replication attempts. But the second sample’s resistivity plunged near 112 ºC (385K) — closely matching the Korean team’s observations.

“That was the moment where I said, ‘Well, obviously, that’s what made them think this was a superconductor,’” says Fuhrer. “The nail in the coffin was this copper sulfide thing.”

Making conclusive statements about LK-99’s properties is difficult, because the material is finicky and samples contain varying impurities. “Even from our own growth, different batches will be slightly different,” says Li. But Li argues that samples that are close enough to the original are sufficient for checking whether LK-99 is a superconductor in ambient coniditions.

Crystal clear

With strong explanations for the resistivity drop and the half-levitation, many in the community were convinced that LK-99 was not a room-temperature superconductor. But mysteries lingered — namely, what were the material’s actual properties?

Initial theoretical attempts using an approach called density functional theory (DFT) to predict LK-99’s structure had hinted at interesting electronic signatures called ‘flat bands’. These are areas where the electrons move slowly and can be strongly correlated. In some cases, this behavior leads to superconductivity. But these calculations were based on unverified assumptions about LK-99’s structure.

To better understand the material, the US–European group5 performed precision X-ray imaging of their samples to calculate LK-99’s structure. Crucially, the imaging allowed them to make rigorous calculations that clarified the situation of the flat bands: they were not conducive to superconductivity. Instead, the flat bands in LK-99 came from strongly localized electrons, which cannot ‘hop’ in the way a superconductor requires.

On 14 August, a separate team, at the Max Planck Institute for Solid State Research in Stuttgart, Germany, reported6 synthesizing pure, single crystals of LK-99. Unlike previous synthesis attempts that relied on crucibles, the researchers used a technique called floating zone crystal growth that allowed them to avoid introducing sulfur into the reaction, eliminating the Cu2S impurities.

The result was a transparent purple crystal — pure LK-99, or Pb8.8Cu1.2P6O25. Separated from impurities, LK-99 is not a superconductor, but an insulator with a resistance in the millions of ohms — too high to run a standard conductivity test. It shows minor ferromagnetism and diamagnetism, but not enough for even partial levitation. “We therefore rule out the presence of superconductivity,” the team concluded.

The team suggests that the hints of superconductivity seen in LK-99 were attributable to Cu2S impurities, which are absent from their crystal. “This story is exactly showing why we need single crystals,” says Pascal Puphal, a specialist in crystal growth and the Max Planck physicist who led the study. “When we have single crystals, we can clearly study the intrinsic properties of a system.”

Lessons learned

Many researchers are reflecting on what they’ve learned from the summer’s superconductivity sensation.

For Leslie Schoop, a solid-state chemist at Princeton University in New Jersey, who co-authored the flat-bands study, the lesson about premature calculations is clear. “Even before LK-99, I have been giving talks about how you need to be careful with DFT, and now I have the best story ever for my next summer school,” she says.

Jain points to the importance of old, often overlooked data — the crucial measurements that he relied on for the resistivity of Cu2S were published in 1951.

While some commentators have pointed to the LK-99 saga as a model for reproducibility in science, others say that it’s an unusually swift resolution of a high-profile puzzle. “Often these things die this very slow death, where it’s just the rumors and nobody can reproduce it,” says Fuhrer.

When copper oxide superconductors were discovered in 1986, researchers leapt to probe their properties. But nearly four decades later, there is still debate over the material’s superconducting mechanism, says Vishik. Efforts to explain LK-99 came readily. “The detective work that wraps up all of the pieces of the original observation — I think that’s really fantastic,” she says. “And it’s relatively rare.”

doi: https://doi.org/10.1038/d41586-023-02585-7

References

1. Lee, S. et al. Preprint at arXiv https://doi.org/10.48550/arXiv.2307.12037 (2023).

2. Lee, S. et al. Preprint at arXiv https://doi.org/10.4855arXiv.2307.12008 (2023).

3. Guo, K., Li, Y. & Jia, S. Sci. China Phys. Mech. Astron. https://doi.org/10.1007/s11433-023-2201-9 (2023).

   Article Google Scholar 

4. Zhu, S. et al. Preprint at arXiv https://arxiv.org/abs/2308.04353 (2023).

5. Jiang, Y. et al. Preprint at arXiv https://arxiv.org/abs/2308.05143 (2023).

6. Puphal, P. et al. Preprint at arXiv https://arxiv.org/abs/2308.06256 (2023).

7. Jain, P. Preprint at arXiv https://arxiv.org/abs/2308.05222 (2023).

中文翻译

（由AI翻译生成）

LK-99不是超导体——科学侦探如何解开这个谜团

复制拼凑了为什么材料表现出超导样行为的谜题。

• 丹·加里斯托

研究人员似乎已经解决了LK-99的谜题。科学侦探工作发现了该材料不是超导体的证据，并澄清了其实际特性。

结论破灭了希望LK-99——一种由铜、铅、磷和氧组成的化合物——标志着第一个在室温和环境压力下工作的超导体的发现。相反，研究表明，材料中的杂质——特别是硫化铜——是电阻率急剧下降和磁铁上部分悬浮的原因，这看起来与超导体表现出的特性相似。

加州大学戴维斯分校的凝聚物质实验家Inna Vishik说：“我认为在这一点上，事情已经非常果断地解决了。”

声称超导体LK-99在网上引起了轰动——但复制工作没有达到要求

LK-99传奇始于7月下旬，当时首尔一家初创公司量子能源研究中心由Sukbae Lee和Ji-Hoon Kim领导的团队发表了预印本1,2，声称LK-99在正常压力和温度高达至少127oC（400开尔文）下的超导体。所有先前确认的超导体仅在极端温度和压力下工作。

这一非同寻常的主张很快引起了对科学感兴趣的公众和研究人员的注意，其中一些人试图复制LK-99。最初的尝试没有看到室温超导的迹象，但并不确定。现在，经过数十次复制工作，许多专家自信地说，证据表明LK-99不是室温超导体。（Lee和Kim的团队没有回应Nature的置评请求。）

积累证据

韩国团队的主张基于LK-99的两个属性：磁铁上方的悬浮和电阻率的突然下降。但北京、北京大学3和中国科学院4（CAS）的独立团队为这些现象找到了平凡的解释。

美国和欧洲研究人员的另一项研究5结合了实验和理论证据，以证明LK-99的结构如何使超导不可行。其他实验者合成并研究了LK-99的纯样品6，消除了对该材料结构的怀疑，并确认它不是超导体，而是绝缘体。

澳大利亚墨尔本莫纳什大学的物理学家Michael Fuhrer说，唯一的进一步确认来自韩国团队分享他们的样本。他说：“他们有责任说服其他人。”

也许LK-99超导性最引人注目的证据是韩国团队拍摄的一段视频，该视频显示，一枚硬币形状的银色材料样本在磁铁上摇晃。该团队表示，由于迈斯纳效应，样品正在悬浮——这是材料排出磁场的超导性标志。LK-99悬浮的多个未经验证的视频随后在社交媒体上流传，但最初试图复制这些发现的研究人员都没有观察到任何悬浮。

半烤的悬浮

马萨诸塞州剑桥哈佛大学前凝聚态研究员Derrick van Gennep发出了几面红旗，他现在从事金融工作，但对LK-99很感兴趣。在视频中，样品的同一边缘似乎粘在磁铁上，似乎微妙地平衡。相比之下，悬浮在磁铁上的超导体可以旋转，甚至可以倒置。van Gennep说：“这些行为看起来都不像我们在LK-99视频中看到的那样。”

他认为LK-99的特性更可能是铁磁性的结果。因此，他用铁屑粘在上面制作了一块压缩石墨刨花颗粒。Van Gennep制作的一段视频显示，他的圆盘由非超导铁磁性材料制成，模仿了LK-99的行为。

8月7日，北京大学团队报告说，由于铁磁性，这种“半悬浮”出现在他们的LK-99样本中。凝聚态物理学家、研究合著者袁立说：“这就像一个铁归档实验。”弹丸受到提升力，但不足以悬浮——只够在一端保持平衡。

李和他的同事测量了样本的电阻率，没有发现超导性的迹象。但他们无法解释韩国队看到的电阻率急剧下降。

不纯的样品

在他们的预印本中，韩国作者注意到一个特定温度，LK-99的电阻率下降了十倍，从大约0.02欧姆-厘米下降到0.002欧姆-厘米。“他们对此非常精确。104.8oC，”伊利诺伊大学厄巴纳-香槟分校的化学家Prashant Jain说。“我想，等一下，我知道这个温度。”

合成LK-99的反应使用不平衡的配方：每制造1份铜掺杂磷酸铅晶体——纯LK-99，它生产17份铜和5份硫。这些残留物会导致许多杂质——特别是硫化铜，韩国团队在其样本中报告了这一点。

硫化铜专家Jain记得104oC是Cu2S发生相变的温度。在该温度以下，空气暴露的Cu2S的电阻率急剧下降——这个信号几乎与LK-99所谓的超导相变相同。“我几乎不敢相信他们错过了。”Jain发表了一份关于重要混淆效应的预印本7。

8月8日，CAS团队报告了Cu2S杂质在LK-99中的影响。CAS物理学家Jianlin Luo说：“Cu2S的不同内容可以用不同的过程合成。”研究人员测试了两个样本——第一个在真空中加热，产生了5%的Cu2S含量，第二个在空气中产生了70%的Cu2S含量。

第一个样品的电阻率随着冷却而相对平稳地增加，并且看起来与其他复制尝试的样品相似。但第二个样本的电阻率在112摄氏度（385K）附近暴跌——与韩国团队的观察结果非常吻合。

“那是我说，'嗯，显然，这就是让他们认为这是一个超导体'的时刻，”Fuhrer说。“棺材里的钉子是这个硫化铜的东西。”

很难对LK-99的特性做出结论性陈述，因为材料很挑剔，样品中含有不同的杂质。李说：“即使从我们自己的成长来看，不同的批次也会略有不同。”但李认为，与原件足够接近的样品足以检查LK-99在环境条件下是否是超导体。

晶莹剔透

由于对电阻率下降和半悬浮的有力解释，社区中的许多人确信LK-99不是室温超导体。但谜团挥之不去——即，材料的实际特性是什么？

使用一种称为密度泛函理论（DFT）的方法来预测LK-99结构的最初理论尝试暗示了被称为“扁平带”的有趣电子签名。这些是电子移动缓慢且可以强烈相关的地方。在某些情况下，这种行为会导致超导性。但这些计算是基于对LK-99结构的未经验证的假设。

为了更好地了解材料，美欧组5对其样本进行了精确的X射线成像，以计算LK-99的结构。至关重要的是，成像使他们能够进行严格的计算，以澄清扁平带的情况：它们不利于超导性。相反，LK-99中的扁平带来自强局部电子，这些电子不能像超导体要求的那样“跳跃”。

8月14日，德国斯图加特马克斯·普朗克固态研究所的一个单独团队报告了6合成LK-99的纯单晶。与之前依赖坩埚的合成尝试不同，研究人员使用了一种称为浮区晶体生长的技术，使其能够避免将硫引入反应中，从而消除Cu2S杂质。

结果是透明的紫色晶体——纯LK-99，或Pb8.8Cu1.2P6O25。与杂质分离的LK-99不是超导体，而是电阻为数百万欧姆的绝缘体——太高了，无法进行标准的电导率测试。它表现出轻微的铁磁性和变磁性，但甚至不足以进行部分悬浮。“因此，我们排除了超导性的存在，”团队总结道。

该团队认为，在LK-99中看到的超导性暗示可归因于晶体中没有的Cu2S杂质。晶体生长专家、领导这项研究的马克斯·普朗克物理学家Pascal Puphal说：“这个故事恰恰说明了为什么我们需要单晶。”“当我们有单晶时，我们可以清楚地研究一个系统的内在属性。”

吸取的教训

许多研究人员正在反思他们从夏天的超导感觉中学到的东西。

对于新泽西州普林斯顿大学固态化学家Leslie Schoop来说，他共同撰写了扁平带研究，关于过早计算的教训是明确的。她说：“甚至在LK-99之前，我一直在谈论你需要如何小心DFT，现在我的下一个暑期学校有了有史以来最好的故事。”

Jain指出了经常被忽视的旧数据的重要性——他依赖的Cu2S电阻率的关键测量结果于1951年发布。

虽然一些评论家指出LK-99传奇是科学中可重现性的模型，但其他人表示，这是一个高调谜题的异常快速的解决。Fuhrer说：“通常这些东西会死得非常缓慢，这只是谣言，没有人可以重现它。”

当1986年发现氧化铜超导体时，研究人员跳来探索它们的特性。但近四十年后，关于这种材料的超导机制仍然存在争议，Vishik说。解释LK-99的努力是很容易的。她说：“侦探工作总结了原始观察的所有部分——我认为这真的很棒。”“而且这相对罕见。”

doi：https://doi.org/10.1038/d41586-023-02585-7

参考文献

1. Lee，S.等人。在arXiv https://doi.org/10.48550/arXiv.2307.12037 (2023)预印本。

2. Lee，S.等人。预印本在arXiv https://doi.org/10.4855arXiv.2307.12008 (2023)。

3. 郭，K.，李，Y。& Jia, S.科学。中国物理机甲。Astron。https://doi.org/10.1007/s11433-023-2201-9 (2023)。

   文章 谷歌学术 

4. Zhu，S.等人。在arXiv https://arxiv.org/abs/2308.04353 (2023)预印本。

5. Jiang，Y.等人。arXiv https://arxiv.org/abs/2308.05143 (2023)上的预印本。

6. Puphal，P.等人。arXiv https://arxiv.org/abs/2308.06256 (2023)上的预印本。

7. Jain，P。在arXiv https://arxiv.org/abs/2308.05222 (2023)预印本。

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