Researchers at the DOE’s Brookhaven National Laboratory in the United States have found that a coating of magnesium enhances the characteristics of tantalum, a superconducting material with a lot of potential for creating qubits, the building blocks of quantum computers.
A small coating of magnesium prevents tantalum from oxidizing, enhances its purity, and increases the temperature at which it functions as a superconductor, as detailed in a study published in the journal Advanced Materials. Tantalum’s capacity to retain quantum information in qubits could be improved by all three.
This study expands upon previous research conducted by a team from the Center for Functional Nanomaterials at Brookhaven, the National Synchrotron Light Source II (NSLS-II) at Brookhaven, and Princeton University to better understand the intriguing properties of tantalum.
The team collaborated with researchers from DOE’s Pacific Northwest National Laboratory (PNNL) and the Condensed Matter Physics & Materials Science (CMPMS) Department at Brookhaven to uncover details regarding the material’s oxidation process.
These studies demonstrated the problem of oxidation.
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.
Mingzhao Liu, Study Lead Author and Staff Scientist, Interface Sciences/Catalysis, Center for Functional Nanomaterials, Brookhaven National Laboratory
Even though tantalum’s oxidation is often self-limiting, which contributes to its comparatively lengthy coherence time, the researchers sought to investigate ways to further restrict oxidation to see if they might enhance the material’s functionality.
Liu added, “The reason tantalum oxidizes is that you have to handle it in air and the oxygen in air will react with the surface. So, as chemists, can we do something to stop that process? One strategy is to find something to cover it up.”
The Co-design Center for Quantum Advantage (C2QA), a national quantum information science research center managed by Brookhaven, is the umbrella organization for all of this activity. The current study proposes a possible initial technique: covering the tantalum with a thin layer of magnesium. While ongoing investigations examine several sorts of cover materials, this strategy is described as promising.
“When you make a tantalum film, it is always in a high-vacuum chamber, so there is not much oxygen to speak of. 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,” Liu added.
Studies utilizing transmission electron microscopy to study the material’s structural and chemical characteristics, atomic layer by atomic layer, revealed that the approach of coating tantalum with magnesium was extremely effective. The magnesium deposited a thin coating of magnesium oxide on the tantalum surface, which appeared to prevent oxygen from passing 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. This information is extremely valuable in comprehending the material’s electronic behavior.
Yimei Zhu, Study Co-Author and Senior Advisor, Brookhaven National Laboratory
Studies using X-Ray photoelectron spectroscopy at NSLS-II demonstrated how the magnesium coating affected tantalum oxide formation inhibition. According to the measurements, the magnesium/tantalum interface is surrounded by a minuscule layer of tantalum oxide that is less than one nanometer thick and does not disturb the remainder 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.
Andrew Walter, Study Co-Author and ARI/SXN Lead Beamline Scientist, Brookhaven National Laboratory
PNNL collaborators then employed atomic-scale computer modeling to determine the most likely configurations and interactions of the atoms according to their binding energies and other properties. The group was able to gain a mechanistic understanding of magnesium's exceptional performance thanks to these simulations.
The calculations showed that, at the most basic level, magnesium has a greater affinity for oxygen than tantalum.
Peter Sushko, one of the PNNL theorists, added, “While oxygen has a high affinity to tantalum, it is ‘happier’ to stay with the magnesium than with the tantalum. 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 researchers also showed that the protection lasts for an extended time.
“Even after one month, the tantalum is still in pretty good shape. Magnesium is a really good oxygen barrier,” Liu further stated.
The magnesium had an unexpectedly positive effect: it “sponged out” accidental impurities in the tantalum, raising the temperature at which it acts as a superconductor.
Liu added, “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. No matter how careful you are, you will always have these impurities in your tantalum.”
However, the scientists found that the magnesium coating’s significant attraction to the contaminants caused them to be drawn out when applied. There was an increase in the superconducting transition temperature of the resulting purer tantalum.
Given that most superconductors require extremely low temperatures to function, that might be crucial for applications. The majority of conducting electrons couple up in these extremely low temperatures and pass through the material with little resistance.
Liu noted, “Even a slight elevation in the transition temperature could reduce the number of remaining, unpaired electrons. There will have to be follow-up studies to see if this material improves qubit performance. 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.”
The Office of Science at DOE provided funding for this study. The National Institute of Standards and Technology (NIST) operated Spectroscopy Soft and Tender Beamlines (SST-1 and SST-2) at NSLS-II; CFN's Materials Synthesis & Characterization, Proximal Probe, and Electron Microscopy facilities; CMPMS’s Advanced Energy Materials Group and CMPMS’s Electron Microscopy and Nanostructure Group facilities; and DOE’s Lawrence Berkeley National Laboratory’s National Energy Research Scientific Computing Center (NERSC)’s computational resources were utilized by the scientists. NERSC, CFN, and NSLS-II are user facilities of the DOE Office of Science. Additional co-authors from Princeton University, Stony Brook University, NIST, CFN, CMPMS, NSLS-II, and PNNL also contributed to the study.
Zhou, C., et. al. (2024). Ultrathin Magnesium-Based Coating as an Efficient Oxygen Barrier for Superconducting Circuit Materials. Advanced Materials. doi:10.1002/adma.202310280