網(wǎng)址：https://www.nature.com/articles/d41586-023-02585-7

翻譯：

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

"LK-99并不是一個(gè)超導(dǎo)體 — 科學(xué)偵探是如何解開這個(gè)謎團(tuán)的

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

復(fù)制實(shí)驗(yàn)拼湊出謎題的答案，解釋了為什么這種材料顯示出類似超導(dǎo)行為的特性。"

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.

研究人員似乎已經(jīng)解開了LK-99的謎團(tuán)?？茖W(xué)偵探工作揭示了證據(jù)，表明這種材料并不是一個(gè)超導(dǎo)體，并澄清了它的實(shí)際特性。

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.

這個(gè)結(jié)論打破了人們對(duì)LK-99的期望，這種由銅、鉛、磷和氧組成的化合物曾被認(rèn)為是第一個(gè)在室溫和常壓下工作的超導(dǎo)體的發(fā)現(xiàn)。相反，研究表明材料中的雜質(zhì)，尤其是銅硫化物，是導(dǎo)致電阻率急劇下降和在磁鐵上部分漂浮的原因，這些現(xiàn)象看起來類似于超導(dǎo)體所表現(xiàn)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.

“我認(rèn)為目前情況已經(jīng)相當(dāng)明確了，”加利福尼亞大學(xué)戴維斯分校的凝聚態(tài)實(shí)驗(yàn)學(xué)家Inna Vishik說道。

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，當(dāng)時(shí)首爾一家初創(chuàng)公司量子能源研究中心的Sukbae Lee和Ji-Hoon Kim領(lǐng)導(dǎo)的團(tuán)隊(duì)發(fā)表了預(yù)印本1,2，聲稱LK-99在常壓下，且在至少127℃（400開爾文）的溫度范圍內(nèi)是一個(gè)超導(dǎo)體。所有之前確認(rèn)的超導(dǎo)體只在極端溫度和壓力下運(yùn)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.)

這個(gè)非同尋常的說法迅速引起了對(duì)科學(xué)感興趣的公眾和研究人員的注意，其中一些人試圖復(fù)制LK-99。最初的嘗試沒有發(fā)現(xiàn)室溫超導(dǎo)性的跡象，但并不具有決定性?，F(xiàn)在，在經(jīng)過數(shù)十次復(fù)制努力之后，許多專家都有信心地說，證據(jù)顯示LK-99不是室溫超導(dǎo)體。（李和金的團(tuán)隊(duì)未回應(yīng)《自然》的置評(píng)請(qǐng)求。）

Accumulating evidence

積累的證據(jù)

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.

韓國(guó)團(tuán)隊(duì)基于LK-99的兩個(gè)特性提出了他們的說法：在磁鐵上方的懸浮和電阻率的急劇下降。但在北京，北京大學(xué)和中國(guó)科學(xué)院的兩個(gè)獨(dú)立團(tuán)隊(duì)找到了這些現(xiàn)象的平凡解釋。

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.

另一項(xiàng)由美國(guó)和歐洲研究人員進(jìn)行的研究結(jié)合了實(shí)驗(yàn)和理論證據(jù)，以證明LK-99的結(jié)構(gòu)使得超導(dǎo)性不可行。其他實(shí)驗(yàn)者合成并研究了LK-99的純樣本，消除了對(duì)材料結(jié)構(gòu)的疑慮，并確認(rèn)它不是超導(dǎo)體，而是絕緣體。

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大學(xué)物理學(xué)家Michael Fuhrer表示，唯一進(jìn)一步的確認(rèn)將來自于韓國(guó)團(tuán)隊(duì)分享他們的樣本?！八麄冇胸?zé)任說服其他人，”他說。

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.

對(duì)LK-99超導(dǎo)性最引人注目的證據(jù)可能是韓國(guó)團(tuán)隊(duì)拍攝的一段視頻，顯示了一個(gè)銀色材料的硬幣形狀樣本在磁鐵上搖晃。團(tuán)隊(duì)表示，樣本因梅森效應(yīng)（超導(dǎo)體的一個(gè)標(biāo)志，材料排斥磁場(chǎng)）而懸浮。隨后，在社交媒體上傳播了多個(gè)未經(jīng)驗(yàn)證的LK-99懸浮視頻，但最初嘗試復(fù)制這些發(fā)現(xiàn)的研究人員沒有觀察到任何懸浮現(xiàn)象。

Half-baked levitation

不完全懸浮

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.

對(duì)于前哈佛大學(xué)凝聚態(tài)研究員、現(xiàn)在在金融界工作但對(duì)LK-99感到興趣的Derrick van Gennep來說，有幾個(gè)警示標(biāo)志顯現(xiàn)出來。在視頻中，樣本的同一邊似乎粘在磁鐵上，而且它看起來很平衡。相比之下，懸浮在磁鐵上方的超導(dǎo)體可以旋轉(zhuǎn)甚至倒掛?！癓K-99的視頻中沒有一個(gè)表現(xiàn)像我們?cè)诔瑢?dǎo)體視頻中看到的那樣，” van Gennep說道。

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.

他認(rèn)為L(zhǎng)K-99的特性更有可能是鐵磁性的結(jié)果。因此，他制作了一個(gè)由壓縮石墨屑和粘在上面的鐵屑組成的圓盤。Van Gennep制作的視頻顯示，他的盤子 — 由非超導(dǎo)性的鐵磁性材料制成 — 模仿了LK-99的行為。

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，北京大學(xué)團(tuán)隊(duì)報(bào)告稱，這種“半懸浮”現(xiàn)象出現(xiàn)在他們的LK-99樣本中，因?yàn)樗鼈兙哂需F磁性?！斑@就像一個(gè)鐵屑實(shí)驗(yàn)一樣，”凝聚態(tài)物理學(xué)家、研究合著者李元說。這個(gè)圓盤經(jīng)歷了一個(gè)抬升的力量，但不足以懸浮，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.

李和他的同事測(cè)量了樣本的電阻率，并沒有發(fā)現(xiàn)超導(dǎo)性的跡象。但他們不能解釋韓國(guó)團(tuán)隊(duì)觀察到的電阻率急劇下降。

Impure samples

不純凈的樣本

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.”

在他們的預(yù)印本中，韓國(guó)作者注意到LK-99在一個(gè)特定溫度下的電阻率出現(xiàn)了十倍的下降，從約0.02歐姆·厘米下降到0.002歐姆·厘米?！八麄兎浅＞_地指出了這一點(diǎn)，104.8℃，”伊利諾伊大學(xué)厄巴納-香檳分校的化學(xué)家Prashant Jain說道?！拔耶?dāng)時(shí)想，等一下，我知道這個(gè)溫度。”

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的反應(yīng)使用了一個(gè)不平衡的配方：對(duì)于每1部分摻銅鉛磷酸鹽晶體（純LK-99），它生成17部分銅和5部分硫。這些剩余物導(dǎo)致了眾多雜質(zhì) — 尤其是韓國(guó)團(tuán)隊(duì)在樣本中報(bào)道的銅硫化物。

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, 一位銅硫化物專家，記得Cu2S在104℃發(fā)生相變的溫度。在這個(gè)溫度以下，暴露在空氣中的Cu2S的電阻率急劇下降 — 幾乎與LK-99聲稱的超導(dǎo)相變的信號(hào)相同?！拔?guī)缀醪桓蚁嘈潘麄冨e(cuò)過了這一點(diǎn)?！?Jain在一個(gè)重要的混雜效應(yīng)方面發(fā)表了一篇預(yù)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，中國(guó)科學(xué)院團(tuán)隊(duì)報(bào)告了LK-99中Cu2S雜質(zhì)的影響?！安煌腃u2S含量可以使用不同的工藝合成，”中國(guó)科學(xué)院物理學(xué)家Jianlin Luo表示。研究人員測(cè)試了兩個(gè)樣本 — 第一個(gè)在真空中加熱，導(dǎo)致Cu2S含量為5%，第二個(gè)在空氣中加熱，導(dǎo)致Cu2S含量為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.

第一個(gè)樣本的電阻率在冷卻時(shí)相對(duì)平穩(wěn)增加，與其他復(fù)制嘗試的樣本類似。但第二個(gè)樣本的電阻率在112℃附近（385K）急劇下降 — 與韓國(guó)團(tuán)隊(duì)的觀察非常接近。

“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.”

“那是我說‘顯然，這就是使他們認(rèn)為這是一個(gè)超導(dǎo)體的原因’的時(shí)刻，”Fuhrer說?！般~硫化物是錘子?！?/p>

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.

對(duì)于LK-99的性質(zhì)做出確鑿的陳述是困難的，因?yàn)檫@種材料很難操作，樣本含有不同的雜質(zhì)。“即使從我們自己的培養(yǎng)中，不同的批次也會(huì)略有不同，”李說。但李認(rèn)為，接近原始的樣本足以檢查L(zhǎng)K-99是否在環(huán)境條件下是一個(gè)超導(dǎo)體。

Crystal clear

清晰無誤

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?

在對(duì)電阻率下降和半懸浮現(xiàn)象有強(qiáng)有力的解釋后，許多人都確信LK-99不是一個(gè)室溫超導(dǎo)體。但是仍然存在一些謎團(tuán) — 即，這種材料的實(shí)際特性是什么？

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.

最初的理論嘗試使用一種稱為密度泛函理論（DFT）的方法來預(yù)測(cè)LK-99的結(jié)構(gòu)，這些嘗試暗示了有趣的電子特征，稱為“平帶”。這些區(qū)域中的電子移動(dòng)緩慢，可以強(qiáng)相關(guān)。在某些情況下，這種行為會(huì)導(dǎo)致超導(dǎo)。但這些計(jì)算是基于對(duì)LK-99結(jié)構(gòu)的未經(jīng)驗(yàn)證的假設(shè)。

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.

為了更好地了解這種材料，美國(guó)-歐洲團(tuán)隊(duì)5對(duì)樣本進(jìn)行了精密的X射線成像，以計(jì)算LK-99的結(jié)構(gòu)。關(guān)鍵是，成像使他們能夠進(jìn)行嚴(yán)格的計(jì)算，澄清了平帶的情況：它們不利于超導(dǎo)性。相反，LK-99中的平帶來自于強(qiáng)烈定位的電子，這些電子不能像超導(dǎo)體那樣“跳躍”。

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，德國(guó)斯圖加特馬克斯·普朗克固態(tài)研究所的一個(gè)獨(dú)立團(tuán)隊(duì)6報(bào)告稱合成了LK-99的純凈單晶。與之前依賴坩堝的合成嘗試不同，研究人員使用了一種稱為浮區(qū)晶體生長(zhǎng)的技術(shù)，使他們能夠避免在反應(yīng)中引入硫，消除了Cu2S雜質(zhì)。

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.

結(jié)果是一個(gè)透明的紫色晶體 — 純LK-99，或Pb8.8Cu1.2P6O25。分離出雜質(zhì)后，LK-99不是超導(dǎo)體，而是一個(gè)電阻百萬歐姆的絕緣體 — 這個(gè)值太高，無法進(jìn)行標(biāo)準(zhǔn)的電導(dǎo)測(cè)試。它表現(xiàn)出輕微的鐵磁性和抗磁性，但甚至沒有足夠的部分懸浮。團(tuán)隊(duì)得出結(jié)論：“因此我們排除了超導(dǎo)性的存在?！?/p>

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.”

團(tuán)隊(duì)表示，LK-99中看到的超導(dǎo)性跡象可以歸因于不存在于他們晶體中的Cu2S雜質(zhì)?！斑@個(gè)故事完全說明了我們?yōu)槭裁葱枰獑尉?，”普朗克物理學(xué)家Pascal Puphal說道，他是晶體生長(zhǎng)專家，領(lǐng)導(dǎo)了這項(xiàng)研究?！爱?dāng)我們擁有單晶時(shí)，我們可以清楚地研究系統(tǒng)的固有特性。”

Lessons learned

吸取的教訓(xùn)

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

許多研究人員正在反思他們從夏季的超導(dǎo)現(xiàn)象中學(xué)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.

對(duì)于新澤西州普林斯頓大學(xué)的固態(tài)化學(xué)家Leslie Schoop來說，她是平帶研究的合著者，關(guān)于過早計(jì)算的教訓(xùn)是明確的?！吧踔猎贚K-99之前，我就一直在講述如何小心處理DFT，現(xiàn)在我有了最佳的故事來參加我的夏季學(xué)校，”她說。

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強(qiáng)調(diào)了舊數(shù)據(jù)的重要性 — 他對(duì)于Cu2S電阻率所依賴的關(guān)鍵測(cè)量數(shù)據(jù)是在1951年發(fā)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.

雖然一些評(píng)論人士把LK-99的故事視為科學(xué)中可重復(fù)性的一個(gè)模型，但也有人表示，這是一個(gè)異常迅速解決一個(gè)備受關(guān)注的難題的方式。“通常這些事情會(huì)以非常緩慢的方式消失，只會(huì)變成謠言，沒有人能夠復(fù)制出來，”Fuhrer說。

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.”

當(dāng)銅氧化物超導(dǎo)體于1986年被發(fā)現(xiàn)時(shí)，研究人員迅速著手探索它們的性質(zhì)。但近四十年過去了，關(guān)于這種材料的超導(dǎo)機(jī)制仍然存在爭(zhēng)議，Vishik說道。解釋LK-99的努力來得很容易?！鞍阉性加^察的片段都拼湊在一起的偵探工作 — 我認(rèn)為這真是太棒了，”她說?！岸疫@種情況相對(duì)較少見?！?/p>