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

翻译：

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

"LK-99并不是一个超导体 — 科学侦探是如何解开这个谜团的

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

复制实验拼凑出谜题的答案，解释了为什么这种材料显示出类似超导行为的特性。"

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.

研究人员似乎已经解开了LK-99的谜团。科学侦探工作揭示了证据，表明这种材料并不是一个超导体，并澄清了它的实际特性。

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.

这个结论打破了人们对LK-99的期望，这种由铜、铅、磷和氧组成的化合物曾被认为是第一个在室温和常压下工作的超导体的发现。相反，研究表明材料中的杂质，尤其是铜硫化物，是导致电阻率急剧下降和在磁铁上部分漂浮的原因，这些现象看起来类似于超导体所表现出的特性。

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

“我认为目前情况已经相当明确了，”加利福尼亚大学戴维斯分校的凝聚态实验学家Inna Vishik说道。

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.

LK-99的故事始于7月底，当时首尔一家初创公司量子能源研究中心的Sukbae Lee和Ji-Hoon Kim领导的团队发表了预印本1,2，声称LK-99在常压下，且在至少127℃（400开尔文）的温度范围内是一个超导体。所有之前确认的超导体只在极端温度和压力下运作。

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.)

这个非同寻常的说法迅速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。最初的尝试没有发现室温超导性的迹象，但并不具有决定性。现在，在经过数十次复制努力之后，许多专家都有信心地说，证据显示LK-99不是室温超导体。（李和金的团队未回应《自然》的置评请求。）

Accumulating evidence

积累的证据

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 University3 and the Chinese Academy of Sciences4 (CAS), found mundane explanations for these phenomena.

韩国团队基于LK-99的两个特性提出了他们的说法：在磁铁上方的悬浮和电阻率的急剧下降。但在北京，北京大学和中国科学院的两个独立团队找到了这些现象的平凡解释。

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.

另一项由美国和欧洲研究人员进行的研究结合了实验和理论证据，以证明LK-99的结构使得超导性不可行。其他实验者合成并研究了LK-99的纯样本，消除了对材料结构的疑虑，并确认它不是超导体，而是绝缘体。

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.

Monash大学物理学家Michael Fuhrer表示，唯一进一步的确认将来自于韩国团队分享他们的样本。“他们有责任说服其他人，”他说。

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.

对LK-99超导性最引人注目的证据可能是韩国团队拍摄的一段视频，显示了一个银色材料的硬币形状样本在磁铁上摇晃。团队表示，样本因梅森效应（超导体的一个标志，材料排斥磁场）而悬浮。随后，在社交媒体上传播了多个未经验证的LK-99悬浮视频，但最初尝试复制这些发现的研究人员没有观察到任何悬浮现象。

Half-baked levitation

不完全悬浮

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.

对于前哈佛大学凝聚态研究员、现在在金融界工作但对LK-99感到兴趣的Derrick van Gennep来说，有几个警示标志显现出来。在视频中，样本的同一边似乎粘在磁铁上，而且它看起来很平衡。相比之下，悬浮在磁铁上方的超导体可以旋转甚至倒挂。“LK-99的视频中没有一个表现像我们在超导体视频中看到的那样，” van Gennep说道。

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 video made by Van Gennep shows that his disc — made of non-superconducting, ferromagnetic materials — mimicked LK-99’s behaviour.

他认为LK-99的特性更有可能是铁磁性的结果。因此，他制作了一个由压缩石墨屑和粘在上面的铁屑组成的圆盘。Van Gennep制作的视频显示，他的盘子 — 由非超导性的铁磁性材料制成 — 模仿了LK-99的行为。

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.

8月7日，北京大学团队报告称，这种“半悬浮”现象出现在他们的LK-99样本中，因为它们具有铁磁性。“这就像一个铁屑实验一样，”凝聚态物理学家、研究合著者李元说。这个圆盘经历了一个抬升的力量，但不足以悬浮，只能保持在一端平衡。

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.

李和他的同事测量了样本的电阻率，并没有发现超导性的迹象。但他们不能解释韩国团队观察到的电阻率急剧下降。

Impure samples

不纯净的样本

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.”

在他们的预印本中，韩国作者注意到LK-99在一个特定温度下的电阻率出现了十倍的下降，从约0.02欧姆·厘米下降到0.002欧姆·厘米。“他们非常精确地指出了这一点，104.8℃，”伊利诺伊大学厄巴纳-香槟分校的化学家Prashant Jain说道。“我当时想，等一下，我知道这个温度。”

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.

合成LK-99的反应使用了一个不平衡的配方：对于每1部分掺铜铅磷酸盐晶体（纯LK-99），它生成17部分铜和5部分硫。这些剩余物导致了众多杂质 — 尤其是韩国团队在样本中报道的铜硫化物。

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.

Jain, 一位铜硫化物专家，记得Cu2S在104℃发生相变的温度。在这个温度以下，暴露在空气中的Cu2S的电阻率急剧下降 — 几乎与LK-99声称的超导相变的信号相同。“我几乎不敢相信他们错过了这一点。” Jain在一个重要的混杂效应方面发表了一篇预印本7。

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.

8月8日，中国科学院团队报告了LK-99中Cu2S杂质的影响。“不同的Cu2S含量可以使用不同的工艺合成，”中国科学院物理学家Jianlin Luo表示。研究人员测试了两个样本 — 第一个在真空中加热，导致Cu2S含量为5%，第二个在空气中加热，导致Cu2S含量为70%。

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.

第一个样本的电阻率在冷却时相对平稳增加，与其他复制尝试的样本类似。但第二个样本的电阻率在112℃附近（385K）急剧下降 — 与韩国团队的观察非常接近。

“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.”

“那是我说‘显然，这就是使他们认为这是一个超导体的原因’的时刻，”Fuhrer说。“铜硫化物是锤子。”

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.

对于LK-99的性质做出确凿的陈述是困难的，因为这种材料很难操作，样本含有不同的杂质。“即使从我们自己的培养中，不同的批次也会略有不同，”李说。但李认为，接近原始的样本足以检查LK-99是否在环境条件下是一个超导体。

Crystal clear

清晰无误

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?

在对电阻率下降和半悬浮现象有强有力的解释后，许多人都确信LK-99不是一个室温超导体。但是仍然存在一些谜团 — 即，这种材料的实际特性是什么？

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.

最初的理论尝试使用一种称为密度泛函理论（DFT）的方法来预测LK-99的结构，这些尝试暗示了有趣的电子特征，称为“平带”。这些区域中的电子移动缓慢，可以强相关。在某些情况下，这种行为会导致超导。但这些计算是基于对LK-99结构的未经验证的假设。

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.

为了更好地了解这种材料，美国-欧洲团队5对样本进行了精密的X射线成像，以计算LK-99的结构。关键是，成像使他们能够进行严格的计算，澄清了平带的情况：它们不利于超导性。相反，LK-99中的平带来自于强烈定位的电子，这些电子不能像超导体那样“跳跃”。

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.

8月14日，德国斯图加特马克斯·普朗克固态研究所的一个独立团队6报告称合成了LK-99的纯净单晶。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，使他们能够避免在反应中引入硫，消除了Cu2S杂质。

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.

结果是一个透明的紫色晶体 — 纯LK-99，或Pb8.8Cu1.2P6O25。分离出杂质后，LK-99不是超导体，而是一个电阻百万欧姆的绝缘体 — 这个值太高，无法进行标准的电导测试。它表现出轻微的铁磁性和抗磁性，但甚至没有足够的部分悬浮。团队得出结论：“因此我们排除了超导性的存在。”

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.”

团队表示，LK-99中看到的超导性迹象可以归因于不存在于他们晶体中的Cu2S杂质。“这个故事完全说明了我们为什么需要单晶，”普朗克物理学家Pascal Puphal说道，他是晶体生长专家，领导了这项研究。“当我们拥有单晶时，我们可以清楚地研究系统的固有特性。”

Lessons learned

吸取的教训

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

许多研究人员正在反思他们从夏季的超导现象中学到了什么。

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.

对于新泽西州普林斯顿大学的固态化学家Leslie Schoop来说，她是平带研究的合著者，关于过早计算的教训是明确的。“甚至在LK-99之前，我就一直在讲述如何小心处理DFT，现在我有了最佳的故事来参加我的夏季学校，”她说。

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.

Jain强调了旧数据的重要性 — 他对于Cu2S电阻率所依赖的关键测量数据是在1951年发表的。

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.

虽然一些评论人士把LK-99的故事视为科学中可重复性的一个模型，但也有人表示，这是一个异常迅速解决一个备受关注的难题的方式。“通常这些事情会以非常缓慢的方式消失，只会变成谣言，没有人能够复制出来，”Fuhrer说。

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.”

当铜氧化物超导体于1986年被发现时，研究人员迅速着手探索它们的性质。但近四十年过去了，关于这种材料的超导机制仍然存在争议，Vishik说道。解释LK-99的努力来得很容易。“把所有原始观察的片段都拼凑在一起的侦探工作 — 我认为这真是太棒了，”她说。“而且这种情况相对较少见。”