一項(xiàng)令人矚目的超導(dǎo)體聲明正在成為新聞焦點(diǎn)

本周，社交媒體上熱議的一個(gè)聲明稱發(fā)現(xiàn)了一種新的超導(dǎo)體，不僅在室溫以上工作，而且在常壓下工作。如果屬實(shí)，這一發(fā)現(xiàn)將是凝聚態(tài)物理學(xué)史上最重要的發(fā)現(xiàn)之一，并可能引領(lǐng)各種技術(shù)奇跡，如磁懸浮交通工具和完美高效的電力網(wǎng)絡(luò)。然而，這兩篇相關(guān)論文于7月22日由韓國(guó)量子能源研究中心的Sukbae Lee和Ji-Hoon Kim及其同事在arXiv預(yù)印本服務(wù)器上發(fā)布，但論文缺乏細(xì)節(jié)，讓許多物理學(xué)家表示懷疑。這些研究人員未回應(yīng)《科學(xué)》雜志的置評(píng)請(qǐng)求。

“他們顯得非常業(yè)余，”阿貢國(guó)家實(shí)驗(yàn)室的理論學(xué)家Michael Norman表示。“他們對(duì)超導(dǎo)性了解不多，而且他們呈現(xiàn)的部分?jǐn)?shù)據(jù)看起來(lái)有些可疑?！比欢?，他表示，阿貢和其他地方的研究人員已在嘗試復(fù)制這個(gè)實(shí)驗(yàn)?！拔覀冞@里的人都在認(rèn)真對(duì)待，并盡力制造這種材料?！币晾Z伊大學(xué)厄巴納-香檳分校的凝聚態(tài)物理學(xué)家Nadya Mason表示：“我贊賞作者對(duì)數(shù)據(jù)的妥善處理，并對(duì)他們的制造技術(shù)進(jìn)行了清晰說(shuō)明?！比欢?，她警告說(shuō)：“數(shù)據(jù)似乎有些凌亂?！?/p>

什么是超導(dǎo)體？

超導(dǎo)體是一種能夠在電流傳導(dǎo)時(shí)不產(chǎn)生任何電阻的材料。如果您曾經(jīng)接受過(guò)核磁共振成像（MRI）檢查，您就曾躺在由超導(dǎo)線制成的大型電磁體內(nèi)。無(wú)阻力的電流傳導(dǎo)使其能夠產(chǎn)生非常強(qiáng)大的磁場(chǎng)，而無(wú)需加熱或消耗大量能源。超導(dǎo)體還有許多其他用途，例如制造無(wú)線電通信的頻率濾波器和在原子加速器中加速粒子。

聽(tīng)起來(lái)很奇怪。超導(dǎo)性是如何發(fā)生的？

通常，電子在晶體固體中無(wú)法輕易通過(guò)，因?yàn)樗鼈儠?huì)被晶格中振動(dòng)的原子反彈。然而，在一些材料中，當(dāng)溫度降低到足夠低時(shí)，電子形成了松散結(jié)合的重疊對(duì)，這些對(duì)在不分開(kāi)的情況下無(wú)法被偏轉(zhuǎn)。在低溫下，振動(dòng)不足以做到這一點(diǎn)。因此，這些電子可以暢通無(wú)阻地穿過(guò)材料。

是否有很多種類的超導(dǎo)體？

數(shù)十種元素金屬，如鉛、汞、鈮和錫以及它們的合金，在接近絕對(duì)零度時(shí)會(huì)變成超導(dǎo)體。在20世紀(jì)50年代，物理學(xué)家解釋了在這些傳統(tǒng)超導(dǎo)體中，晶格振動(dòng)也是創(chuàng)造電子對(duì)的“膠水”。在20世紀(jì)80年代，實(shí)驗(yàn)者們鑒定出了一種包含銅和氧層的復(fù)雜化合物，在高達(dá)133K的溫度下超導(dǎo)。二十年后，研究人員發(fā)現(xiàn)含鐵和砷層的化合物也可以在幾乎同樣高的溫度下超導(dǎo)?？茖W(xué)家認(rèn)為這些所謂的高溫超導(dǎo)體也依賴于電子配對(duì)，但是產(chǎn)生方式不同。最近，一組研究人員聲稱已在含氫、硫和碳化合物中以高壓形式實(shí)現(xiàn)了室溫超導(dǎo)性，但此項(xiàng)說(shuō)法備受爭(zhēng)議。

這個(gè)韓國(guó)團(tuán)隊(duì)在主張什么？

不僅僅是超級(jí)超導(dǎo)體。在尚未經(jīng)過(guò)同行評(píng)審的預(yù)印本中，研究人員認(rèn)為在用銅摻雜后，由常見(jiàn)元素鉛、氧和磷組成的材料能夠在常壓和至少400K的溫度下超導(dǎo)，這比水的沸點(diǎn)還要高。他們基本上在說(shuō)，您可以制備這種材料的樣品，從烤箱里拿出來(lái)，在您的實(shí)驗(yàn)室臺(tái)上就可以不產(chǎn)生任何電阻地導(dǎo)電。他們展示了數(shù)據(jù)，不僅顯示零電阻，還表明材料似乎排斥磁場(chǎng)，這是超導(dǎo)性的關(guān)鍵特征。

對(duì)于懷疑的原因是什么？

有幾個(gè)原因，Norman解釋道。首先，未摻雜的材料，磷酸鉛礦石，不是金屬，而是一種非導(dǎo)電的礦物質(zhì)。這對(duì)于制造超導(dǎo)體來(lái)說(shuō)并不是一個(gè)有前途的起點(diǎn)。此外，鉛和銅原子具有相似的電子結(jié)構(gòu)，因此將一些鉛原子替換為銅原子不應(yīng)該大大影響材料的電學(xué)性質(zhì)。Norman表示：“您有一塊巖石，而最終還是得到一塊巖石?！贝送猓U原子非常重，這應(yīng)該抑制振動(dòng)，使得電子配對(duì)更加困難。

作者對(duì)于發(fā)生了什么有解釋嗎？

這些論文沒(méi)有提供有關(guān)所發(fā)生物理現(xiàn)象的確切解釋。但研究人員猜測(cè)，在他們的材料內(nèi)部，摻雜略微扭曲了長(zhǎng)鏈鉛原子的結(jié)構(gòu)。他們稱超導(dǎo)性可能沿著這些1D通道發(fā)生。但Norman表示這將令人驚訝，因?yàn)橐痪S系統(tǒng)通常不會(huì)產(chǎn)生超導(dǎo)性。此外，摻雜引入的混亂應(yīng)該進(jìn)一步抑制超導(dǎo)性。Norman表示：“您有一個(gè)維度是壞的，而且您有混亂，這也是壞的?！盡ason則不太確定。她指出，李和金還表示，鏈條中可能存在一種電荷的波動(dòng)，而類似的電荷模式在高溫超導(dǎo)體中也有所見(jiàn)。她說(shuō)：“也許這種材料確實(shí)處于強(qiáng)相互作用的非常規(guī)超導(dǎo)體的理想狀態(tài)?！?/p>

這將如何得到驗(yàn)證？

最關(guān)鍵的問(wèn)題將是是否有人能夠復(fù)制這些觀測(cè)結(jié)果。Norman表示，這不應(yīng)該太難，因?yàn)榱姿徙U礦石是一種眾所周知的材料，其他人應(yīng)該能夠合成。然而，這并不像一些社交媒體上的旁觀者所認(rèn)為的那樣簡(jiǎn)單。“一般公眾似乎對(duì)于這種‘4天、多步驟、小批量、固態(tài)合成’有多么‘容易’感到非常興奮，”SLAC國(guó)家加速器實(shí)驗(yàn)室的凝聚態(tài)物理學(xué)家Jennifer Fowlie在Twitter上說(shuō)，“有些人從未因過(guò)度使用研缽而起水泡，這一點(diǎn)真的很明顯?！北M管如此，物理學(xué)家們將迅速對(duì)這一聲明進(jìn)行測(cè)試，Norman預(yù)測(cè)：“如果這是真的，我們將在一周內(nèi)得知。”

A spectacular superconductor claim is making news.?

Here's why experts are doubtful?

This week, social media has been aflutter over a claim for a new superconductor that works not only well above room temperatures, but also at ambient pressure. If true, the discovery would be one of the biggest ever in condensed matter physics and could usher in all sorts of technological marvels, such as levitating vehicles and perfectly efficient electrical grids. However,?the two related papers, posted to the arXiv preprint server by Sukbae Lee and Ji-Hoon Kim of South Korea’s Quantum Energy Research Centre and colleagues on 22 July, are short on detail and have left many physicists skeptical. The researchers did not respond to?Science’s request for comment. “They come off as real amateurs,” says Michael Norman, a theorist at Argonne National Laboratory. “They don't know much about superconductivity and the way they’ve presented some of the data is fishy.” On the other hand, he says, researchers at Argonne and elsewhere are already trying to replicate the experiment. “People here are taking it seriously and trying to grow this stuff.” Nadya Mason, a condensed matter physicist at the University of Illinois, Urbana-Champaign says, “I appreciate that the authors took appropriate data and were clear about their fabrication techniques.” Still, she cautions, “The data seems a bit sloppy.” What is a superconductor? A superconductor is a material that can convey an electrical current without any resistance at all. If you’ve ever had an MRI, you’ve lain inside a big electromagnet made of superconducting wire. The resistance-less flow enables it to make a very strong magnetic field without heating up or consuming enormous energy. Superconductors have myriad other uses, from making frequency filters for radio communications to accelerating particles in atom smashers. Sounds weird. How does superconductivity happen? Ordinarily, electrons cannot pass easily through a crystalline solid because they bounce off vibrating atoms in the crystal lattice. However, in some materials, at low enough temperatures, the electrons form loosely bound, overlapping pairs–which can’t be deflected without breaking the pair. And at low temperatures, the vibrations aren’t strong enough to do that. So these electrons glide through the material unimpeded. Are there lots of superconductors? Dozens of elemental metals—lead, mercury, niobium, tin—and alloys of them become superconductors when chilled to near absolute zero. In the 1950s, physicists explained how in these conventional superconductors, lattice vibrations also supply the glue that creates the electron pairs. In the 1980s, experimenters identified complex compounds containing layers of copper and oxygen that superconduct at temperatures as high as 133 K. Twenty years later, researchers found that compounds containing layers of iron and arsenic could superconduct at temperatures almost as high. Scientists think these so-called high-temperature superconductors also rely on electron pairing, but created through a different mechanism. Recently, one group has made controversial claims of achieving?superconductivity at room temperature—albeit at high pressure—for compounds containing hydrogen, sulfur, and carbon. What is the South Korean group claiming? Nothing less than the ultimate superconductor. In the preprints, which have not been peer-reviewed, the researchers argue that when seasoned, or “doped” with copper, a material made of the common elements lead, oxygen, and phosphorus superconducts at ambient pressure and temperatures at least as high as 400 K—higher than the boiling point of water. Essentially, they’re saying you can bake up a sample of this stuff, pop it out of the oven, and just sitting there on your lab bench it will conduct electricity without any resistance. They present data that show not only zero resistance, but also that the material appears to expel a magnetic field, a key signature of superconductivity. What are the reasons for skepticism? There are several, Norman says. First, the undoped material, lead apatite, isn’t a metal but rather a nonconducting mineral. And that’s an unpromising starting point for making a superconductor. What’s more, lead and copper atoms have similar electronic structures, so substituting copper atoms for some of the lead atoms shouldn’t greatly affect the electrical properties of the material, Norman says. “You have a rock, and you should still end up with a rock.” On top of that, lead atoms are very heavy, which should suppress the vibrations and make it harder for electrons to pair, Norman explains. Do the authors have an explanation for what’s going on? The papers don’t provide a solid explanation of the physics at play. But the researchers speculate that within their material, the doping slightly distorts long, naturally occurring chains of lead atoms. They say the superconductivity might occur along these 1D channels. But that would be surprising, Norman says, because 1D systems don’t generally produce superconductivity. What’s more, the disorder introduced by the doping ought to further suppress superconductivity. “You have one dimension, which is bad, and you have disorder, which is also bad,” Norman says. Mason isn’t so certain. She notes that Lee and Kim also suggest that a kind of undulation of charge might exist in the chains and that similar charge patterns have been seen in high-temperature superconductors. “Maybe this material really just hits the sweet spot of a strongly interacting unconventional superconductor,” she says. How will this be sorted out? The big question will be whether anybody can reproduce the observations. That shouldn’t be too hard, Norman says, as lead apatite is a well-known material that others should be able to synthesize. However, doing that isn’t quite as simple as some spectators on social media have made it out to be. “The general public seems oddly pumped about how ‘easy’ the 4-day, multistep, small batch, solid state synthesis is,” Jennifer Fowlie, a condensed matter physicist at SLAC National Accelerator Laboratory, quipped on Twitter. “Some of you haven't had blisters from overusing your pestle and it shows.” Nevertheless, physicists will put the claim to the test very quickly, Norman predicts: “If this is real, we’ll know within a week.”