美国能源部(DOE)布鲁克海文国家实验室的科学家们近日发现，添加一层镁就可以改善钽的性能。作为超导材料的钽，在构建量子比特(量子计算机的基础)方面一直表现出巨大潜力。正如刚刚发表在Advanced Materials杂志上的论文所描述的，一层薄薄的镁可以防止钽氧化，提升其纯度，进一步提高其作为超导体的工作温度，这种方式最终可以提高钽在量子位中保存量子信息的能力。

团队使用该工具制备了净钽薄膜和存在镁保护层的钽薄膜，以此确定镁涂层是否会最大限度地减少钽氧化。(Jessica Rotkiewicz/布鲁克海文国家实验室)

这项工作建立在布鲁克海文功能纳米材料中心(CFN)、布鲁克海文国家同步加速器光源II(NSLS-II)和普林斯顿大学早期研究的基础上，他们尝试挖掘钽的优异特性，然后与布鲁克海文凝聚态物理与材料科学(CMPMS)部门的科学家和美国能源部太平洋西北国家实验室(PNNL)的理论学者合作，共同揭示材料如何氧化的细节。这些研究揭示了氧化为什么是一个问题。

同时参与了早期研究和这项新工作的主要作者CFN科研工作者刘明钊（音译）解释说：“当氧气与钽发生反应时，它会形成一个无定形的绝缘层，从通过钽晶格的电流中吸收微弱的能量。这种能量损失破坏了材料在相干状态下保持量子信息的能力，也就是量子相干性。”

虽然钽的氧化通常会自我限制，这是其可以保持较长相干时间的关键原因，但研究小组想要探索进一步抑制氧化的策略，看看它们是否能改善材料的性能。

上图对无涂层钽(Ta)的氧化(左)和涂有超薄镁层(Mg)钽的氧化(右)进行了比较，其中氧化物穿透Ta晶格。镁作为氧屏障，有效地抑制了Ta的氧化，并从Ta中滤出杂质，两者都提高了底层Ta薄膜的超导性能，使其在更高的温度下表现出超导性。(布鲁克海文国家实验室)

刘明钊继续解释道:“钽氧化的主要原因是因为必须在空气中对它进行加工，空气中的氧气会与其表面发生反应。所以，作为化学家，我们能做些什么来阻止这个过程吗？其中一种策略便是找些东西来掩盖它。”

所有这些工作都是在量子优势协同设计中心(C2QA)中进行的，这是布鲁克海文领导的国家量子信息科学研究中心。虽然正在进行的研究探索了不同种类的覆盖材料，但这篇新论文描述的在钽表面涂上一层薄薄的镁这种方法极具潜力。

刘明钊补充说:“当你制作钽薄膜时，它一般都是在高真空的房间里，所以没有多少氧气可言。问题都是在你把它拿出来时发生的。所以，我们认为，在不打破真空的情况下，我们把钽层放下去后，也许我们可以在上面再覆盖一层，比如镁，来阻止表面与空气相互作用。”

利用透射电子显微镜对材料的结构和化学性质一层一层进行了研究，结果表明，在钽上涂上镁的策略是非常成功的。镁在钽表面形成了一层薄薄的氧化镁，似乎可以阻止氧气通过。

CMPMS的研究合著者朱一梅（音译）指出：“布鲁克海文实验室开发的电子显微镜技术不仅可以直接看到薄镁涂层和钽膜内的化学分布和原子排列，还可以看到它们的氧化态变化，这些信息对于理解这种材料的电子特性非常有价值。”

NSLS-II的X射线光电子能谱研究揭示了镁涂层对限制氧化钽形成的影响。测量结果表明，一层极薄的氧化钽(厚度小于1纳米)仍然被限制在镁/钽界面的正下方，而不会破坏钽晶格的其余部分。

NSLS-II X射线散射和光谱学计划的首席光束线科学家、研究合著者Andrew Walter表示：“这与未涂覆的钽形成鲜明对比，其中超过三纳米厚的氧化钽层对钽的电子特性具有更大的破坏性。”

然后，PNNL的合作者使用原子尺度的计算模型，根据原子的结合能和其他特征，确定最可能的排列和原子的相互作用。这些模拟帮助研究小组对镁为何如此有效有了一个机制上的理解。计算表明镁对氧的亲和力比钽高。

PNNL的理论家之一Peter Sushko说道:“虽然氧对钽有很高的亲和力，但它更喜欢和镁在一起，而不是和钽在一起。因此，镁与氧气反应可以形成保护性的氧化镁层。你甚至不需要那么多镁来完成这项工作，只要两纳米厚度的镁几乎就完全阻止了钽的氧化。”

科学家们还证明了这种保护可以持续很长时间。即使一个月后，钽仍然处于相当良好的状态，镁确实是一种很好的氧气屏障。镁还有一个意想不到的优势：它像海绵一样吸掉了钽中难以发现的杂质，最终提高了钽作为超导体工作的温度。

刘明钊表示：“尽管我们是在真空中制造这些材料，但总会有一些残余气体，比如氧气、氮气、水蒸气和氢气。而钽很擅长吸收这些杂质，因此不管你多么小心，你的钽里总是会有这些杂质。”

但当科学家们添加了镁涂层后，他们发现镁对杂质的强亲和力将它们拉了出来。所得钽的纯度较高，超导转变温度也更高。这对于应用来说是非常重要的，因为大多数超导体必须保持在非常低的温度下才能运行。在这些较低温度的条件下，大多数导电电子配对后在材料中可以无阻力地移动。

刘明钊总结道：“即使只提高一点转变温度，也可以大大减少剩余的未配对电子的数量，这使得材料可以成为更好的超导体，并增加其量子相干时间。我们必须进行后续研究，看看这种材料是否能提高量子比特的性能。但这项工作提供了非常有价值的理论和新的材料设计原则，这为实现大规模、高性能量子计算系统铺平了道路。”

Ultrathin Magnesium-Based Coating becomes an Efficient Oxygen Barrier for tantalum superconducting materials

By Brookhaven National Laboratory，February 5, 2024.

UPTON, NY—Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have discovered that adding a layer of magnesium improves the properties of tantalum, a superconducting material that shows great promise for building qubits, the basis of quantum computers. As described in a paper just published in the journal Advanced Materials, a thin layer of magnesium keeps tantalum from oxidizing, improves its purity, and raises the temperature at which it operates as a superconductor. All three may increase tantalum’s ability to hold onto quantum information in qubits.

This work builds on earlier studies in which a team from Brookhaven’s Center for Functional Nanomaterials (CFN), Brookhaven’s National Synchrotron Light Source II (NSLS-II), and Princeton University sought to understand the tantalizing characteristics of tantalum, and then worked with scientists in Brookhaven’s Condensed Matter Physics & Materials Science (CMPMS) Department and theorists at DOE’s Pacific Northwest National Laboratory (PNNL) to reveal details about how the material oxidizes.

Those studies showed why oxidation is an issue.

“When oxygen reacts with tantalum, it forms an amorphous insulating layer that saps tiny bits of energy from the current moving through the tantalum lattice. That energy loss disrupts quantum coherence—the material’s ability to hold onto quantum information in a coherent state,” explained CFN scientist Mingzhao Liu, a lead author on the earlier studies and the new work.

While the oxidation of tantalum is usually self-limiting—a key reason for its relatively long coherence time—the team wanted to explore strategies to further restrain oxidation to see if they could improve the material’s performance.

“The reason tantalum oxidizes is that you have to handle it in air and the oxygen in air will react with the surface,” Liu explained. “So, as chemists, can we do something to stop that process? One strategy is to find something to cover it up.”

All this work is being carried out as part of the Co-design Center for Quantum Advantage (C2QA), a Brookhaven-led national quantum information science research center. While ongoing studies explore different kinds of cover materials, the new paper describes a promising first approach: coating the tantalum with a thin layer of magnesium.

“When you make a tantalum film, it is always in a high-vacuum chamber, so there is not much oxygen to speak of,” said Liu. “The problem always happens when you take it out. So, we thought, without breaking the vacuum, after we put the tantalum layer down, maybe we can put another layer, like magnesium, on top to block the surface from interacting with the air.”

Studies using transmission electron microscopy to image structural and chemical properties of the material, atomic layer by atomic layer, showed that the strategy to coat tantalum with magnesium was remarkably successful. The magnesium formed a thin layer of magnesium oxide on the tantalum surface that appears to keep oxygen from getting through.

“Electron microscopy techniques developed at Brookhaven Lab enabled direct visualization not only of the chemical distribution and atomic arrangement within the thin magnesium coating layer and the tantalum film but also of the changes of their oxidation states,” said Yimei Zhu, a study co-author from CMPMS. “This information is extremely valuable in comprehending the material’s electronic behavior,” he noted.

X-ray photoelectron spectroscopy studies at NSLS-II revealed the impact of the magnesium coating on limiting the formation of tantalum oxide. The measurements indicated that an extremely thin layer of tantalum oxide—less than one nanometer thick—remains confined directly beneath the magnesium/tantalum interface without disrupting the rest of the tantalum lattice.

“This is in stark contrast to uncoated tantalum, where the tantalum oxide layer can be more than three nanometers thick—and significantly more disruptive to the electronic properties of tantalum,” said study co-author Andrew Walter, a lead beamline scientist in the Soft X-ray Scattering & Spectroscopy program at NSLS-II.

Collaborators at PNNL then used computational modeling at the atomic scale to identify the most likely arrangements and interactions of the atoms based on their binding energies and other characteristics. These simulations helped the team develop a mechanistic understanding of why magnesium works so well.

At the simplest level, the calculations revealed that magnesium has a higher affinity for oxygen than tantalum does.

“While oxygen has a high affinity to tantalum, it is ‘happier’ to stay with the magnesium than with the tantalum,” said Peter Sushko, one of the PNNL theorists. “So, the magnesium reacts with oxygen to form a protective magnesium oxide layer. You don’t even need that much magnesium to do the job. Just two nanometers of thickness of magnesium almost completely blocks the oxidation of tantalum.”

The scientists also demonstrated that the protection lasts a long time: “Even after one month, the tantalum is still in pretty good shape. Magnesium is a really good oxygen barrier,” Liu concluded.

The magnesium had an unexpected beneficial effect: It “sponged out” inadvertent impurities in the tantalum and, as a result, raised the temperature at which it operates as a superconductor.

“Even though we are making these materials in a vacuum, there is always some residual gas—oxygen, nitrogen, water vapor, hydrogen. And tantalum is very good at sucking up these impurities,” Liu explained. “No matter how careful you are, you will always have these impurities in your tantalum.”

But when the scientists added the magnesium coating, they discovered that its strong affinity for the impurities pulled them out. The resulting purer tantalum had a higher superconducting transition temperature.

That could be very important for applications because most superconductors must be kept very cold to operate. In these ultracold conditions, most of the conducting electrons pair up and move through the material with no resistance.

“Even a slight elevation in the transition temperature could reduce the number of remaining, unpaired electrons,” Liu said, potentially making the material a better superconductor and increasing its quantum coherence time.

“There will have to be follow-up studies to see if this material improves qubit performance,” Liu said. “But this work provides valuable insights and new materials design principles that could help pave the way to the realization of large-scale, high-performance quantum computing systems.”