Plutonium compound shows rare quantum state in actinide study

Researchers at Idaho National Laboratory have reported a rare quantum state in plutonium hexaboride, a compound made with one of the most difficult elements to study. The finding matters because it gives scientists another way to probe how plutonium and related nuclear materials behave at the level of electrons.

The work, published in Physical Review Research, identifies plutonium hexaboride, or PuB6, as showing a topological Kondo insulating state. INL said such behavior has been seen only a small number of times in plutonium-based materials.

Topological insulators are materials with an unusual split personality: their interiors resist electrical flow while their surfaces can conduct current. According to INL, that surface conduction can be unusually resistant to disruption from impurities or defects.

The Kondo part refers to strong interactions among electrons that create collective behavior beyond what scientists would expect from isolated atoms. INL said plutonium is a notable case because its 5f electrons have an unusual dual character and interact strongly, making the element hard to model and measure.

“Plutonium is defined by the unusual dual nature of its 5f electrons,” Krzysztof Gofryk, the INL scientist who led the study, said in the lab’s announcement. “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.”

Why actinides are hard to study

Actinides include uranium and plutonium. INL said their electrons shape properties such as magnetism, electrical conductivity and durability under radiation and high temperature, all of which are central to nuclear science.

Understanding those properties at atomic and electronic scales could help researchers predict how nuclear materials age, improve reactor safety and design future energy systems, according to the lab. But plutonium compounds are difficult to synthesize, handle and measure safely, limiting the number of facilities able to do this kind of work.

INL said its researchers used specialized infrastructure, including plasma focused ion beam methods, to prepare microscopic plutonium samples for ultracold measurements. The lab described those low-temperature experiments as a way to observe quantum behavior with less interference from heat.

“These advanced preparation techniques allow us to study plutonium at very low temperatures,” INL researcher Daniel Murray said. “INL is the only facility with the expertise and infrastructure to efficiently and safely perform this kind of research on transuranium materials.”

Experiments paired with modeling

The INL team worked with Columbia University to compare laboratory measurements with computer modeling. INL said the combined approach helped researchers assess the electronic and structural behavior of plutonium hexaboride.

“Our calculations capture the essential electronic and structural properties of plutonium hexaboride,” INL researcher Shuxiang Zhou said. “They provide strong support for its topological nature and offer an efficient path for studying similar actinide materials.”

The study’s publication lists the paper as “Electronic correlations and topology in Kondo insulator PuB6,” by K. Gofryk and colleagues. INL said the result offers a framework for examining other actinide compounds that have been difficult to explore.

The lab also linked the research to possible long-term applications in nuclear materials science, quantum computing, advanced sensing and modeling of nuclear systems. INL said better understanding of topological quantum states in actinides could help researchers simulate complex nuclear behavior and support work on longer-lasting reactor materials.

This story draws on original reporting from Phys.org.