Magnesium protects tantalum, a promising material for making qubits

Scientists at the US Department of Energy’s (DOE) Brookhaven National Laboratory have discovered that adding a layer of magnesium improves the properties of tantalum, a superconducting material with great promise for building qubits, the basis of quantum computers. .

As described in an article published in the journal Advanced materials, a thin layer of magnesium prevents tantalum from oxidizing, improves its purity, and raises the temperature at which it functions as a superconductor. These three elements could increase tantalum’s ability to retain quantum information in the form of qubits.

This work builds on previous 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 tantalum’s tantalum characteristics, then worked with scientists at Brookhaven’s Department of Condensed Matter Physics and Materials Science (CMPMS) and theorists at DOE’s Pacific Northwest National Laboratory (PNNL) to reveal details of how the material oxidizes.

These studies showed why oxidation is a problem.

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

Although tantalum oxidation is generally self-limiting (a major reason for its relatively long coherence time), the team wanted to explore strategies to further restrict oxidation to see if they could improve the performance of tantalum. material.

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

All this work is carried out within the framework of the Co-design Center for Quantum Advantage (C²QA), a national research center in quantum information sciences led by Brookhaven. While ongoing studies explore different types of covering materials, the new paper describes a promising first approach: covering the tantalum with a thin layer of magnesium.

“When you make a tantalum film, it’s always in a high vacuum chamber, so there’s not a lot of oxygen to speak of,” Liu said. “The problem always occurs when you remove it. So we thought, without breaking the vacuum, after we put the tantalum layer down, maybe we could put another layer, like magnesium, on top to prevent the surface to interact with air.

Studies using transmission electron microscopy to image the structural and chemical properties of the material, atomic layer by atomic layer, showed that the strategy of coating the tantalum with magnesium was remarkably effective. The magnesium has formed a thin layer of magnesium oxide on the surface of the tantalum which appears to prevent oxygen from passing through.

“Electron microscopy techniques developed at Brookhaven Lab made it possible to directly visualize not only the chemical distribution and atomic arrangement within the thin magnesium layer and tantalum film, but also changes in their oxidation states,” said Yimei Zhu, co-author of the study. of the CMPMS. “This information is extremely valuable for understanding the electronic behavior of the material,” 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 a nanometer thick – remains confined directly beneath the magnesium/tantalum interface without disturbing 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 be significantly more disruptive to the electronic properties of tantalum,” said the study co-author. Andrew Walter, Soft’s principal beamline scientist. X-ray Scattering and Spectroscopy Program at NSLS-II.

The PNNL collaborators then used atomic-scale computer modeling to identify the most likely arrangements and interactions of 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, calculations revealed that magnesium has a greater affinity for oxygen than tantalum.

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

The scientists also demonstrated that the protection lasts a long time: “Even after a month, the tantalum is still in fairly good condition. Magnesium is a very good barrier against oxygen,” concluded Liu.

Magnesium had an unexpected beneficial effect: it “mopped up” unintended impurities from tantalum and, therefore, increased the temperature at which it functions as a superconductor.

“Even if we make these materials in a vacuum, there are still residual gases: oxygen, nitrogen, water vapor, hydrogen. And tantalum is very effective at sucking out these impurities,” Liu explained. “No matter how careful you are, you will still have these impurities in your tantalum.”

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

This could be very important for applications, because most superconductors must be kept very cold to work. In these ultracold conditions, most conducting electrons pair and move through the material without resistance.

“Even a slight increase 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.

“It will take follow-up studies to see if this material improves the performance of qubits,” Liu said. “But this work provides valuable insights and new materials design principles that could pave the way for realizing large-scale, high-performance quantum computing systems.”