Plutonium is one of the most complex elements in the periodic table. First synthesized and isolated in 1940 by scientists at the University of California, Berkeley, plutonium has been studied closely for more than eight decades. It’s most often associated with its role in nuclear security, but it’s also vital to nuclear power, where it is produced in reactors and can be recycled as fuel. Despite plutonium’s importance, some of its most fundamental behaviors remain a mystery.
Scientists at the Idaho National Laboratory (INL) have made an important discovery: A compound called plutonium hexaboride (PuB₆) exhibits a one-of-a-kind quantum property known as a topological Kondo insulating state. Published in Physical Review Research, this finding marks one of only a handful of times such behavior has been observed in plutonium material — opening a new window for research into how some of nature’s most complex elements actually work.
Understanding the discovery
A topological Kondo insulating state sounds complex, but the core idea is surprisingly intuitive.
Most materials on Earth fall into one of two camps: They either conduct electricity (such as copper wiring) or they don’t allow electricity to pass through easily (like rubber insulation). Topological insulators break this mold in a fascinating way. They have special properties that block electrical current within their interior while allowing it to flow freely along their exterior surfaces. The surface conductivity of topological insulators is unusually strong; it can’t be easily disrupted by impurities or physical defects.
The “Kondo” part refers to a specific quantum effect where electrons inside a material interact so strongly with one another that they create entirely new collective behaviors — ones that can’t be predicted by looking at individual atoms in isolation. Plutonium is a striking example. It contains 5f electrons, which are especially prone to these intense interactions, making it one of the most dramatic and complex materials known.
“Plutonium is defined by the unusual dual nature of its 5f electrons,” said INL scientist Krzysztof Gofryk, who led the study. “This makes it difficult to understand, but scientifically fascinating. Plutonium hexaboride gives us a rare opportunity to see how strong correlations and topology work together in actinide materials.”
80 years later, plutonium still has surprises
Actinides are in the family of elements that include plutonium and uranium. Their electrons govern critical properties like magnetism, electrical conductivity and how materials hold up under extreme radiation and temperature. It’s necessary to understand those properties at the quantum level, the scale of atoms and electrons, to predict how nuclear materials will age, how to improve reactor safety and how to design future energy systems.
Actinides are notoriously difficult to study, and progress on this front has been gradual. Plutonium compounds are extraordinarily difficult to handle, synthesize and measure. Only a handful of facilities in the world can do it safely, and INL is one of them. INL is home to specialized infrastructure that includes plasma focused ion beam techniques used to prepare micro-size plutonium samples for ultra-cold quantum measurements, the most accurate way to see the quantum mechanics without interference from heat. These capabilities made this latest discovery possible.
“These advanced preparation techniques allow us to study plutonium at very low temperatures,” said INL researcher Daniel Murray. “INL is the only facility with the expertise and infrastructure to efficiently and safely perform this kind of research on transuranium materials.”
Charting new territory in actinide science
The INL team didn’t stop its work on plutonium hexaboride at lab measurements. In collaboration with Columbia University, INL paired experimental results with advanced computer modeling to better understand what plutonium hexaboride is doing at the quantum level.
“Our calculations capture the essential electronic and structural properties of plutonium hexaboride,” said INL researcher Shuxiang Zhou. “They provide strong support for its topological nature and offer an efficient path for studying similar actinide materials.”
The combination of carefully conducted experiments and rigorous theory gives these findings credibility among scientists. It also provides a road map for studying other actinide materials that have historically been too difficult to explore.
One discovery, endless possibilities
When it comes to plutonium hexaboride’s practical applications, the research sits at the intersection of nuclear science and quantum physics. On the nuclear side, this research will advance the practical and high-stakes work of keeping reactors safe while extending the life of nuclear materials to help secure the country’s energy future. On the quantum side, the research has potential applications in quantum computing, advanced sensing and frontier technologies that could fundamentally reshape how researchers model nuclear systems and materials.
The finding supports the U.S. Department of Energy’s recent $625 million push to advance quantum science as a pillar of U.S. technological leadership. Understanding how topological quantum states emerge in actinide materials could inform how researchers simulate complex nuclear behavior, helping industry design longer-lasting reactor materials and develop technologies that don’t yet exist.
Further, the plutonium hexaboride research underscores INL’s role as a national scientific asset with a one-of-a-kind capability to safely design, fabricate and study plutonium-based quantum materials.
The paper, “Electronic correlations and topology in Kondo insulator plutonium hexaboride,” was published as a letter in Physical Review Research. Available at: https://journals.aps.org/prresearch/abstract/10.1103/hwpn-gll9
This research was conducted at Idaho National Laboratory, in collaboration with Columbia University, and was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.
