INL reports findings on unusual quantum behavior of plutonium

Scientists at Idaho National Laboratory have discovered that plutonium hexaboride (PuB₆) displays a type of unusual quantum property called a topological Kondo insulating state. Materials with this property are neither typical electricity conductors nor regular insulators. Rather, they have exterior surfaces that strongly conduct electricity and interiors that block electricity.

The findings of this research-which was published in the journal Physical Review Research and supported with funding from the Department of Energy's Office of Science-has implications for the nuclear power industry. According to the investigators, the study could lead to practical developments that improve the safety and security of nuclear reactors and extend the lives of nuclear materials.

Understanding actinides: Plutonium and uranium are both classified as actinides, which are the 15 radioactive metallic elements that have atomic numbers from 89 to 103. These elements have electron configurations that govern their properties of magnetism and conductivity, as well as how materials based on them can withstand extreme conditions of radiation and temperature.

A quantum-scale understanding of actinides at the electron level is necessary to fully understand how nuclear materials age under extreme radiation and temperatures. It could also inform the safer design of nuclear reactors. However, studying actinides at the quantum-scale is complex. While few laboratories have the required capabilities and technologies to do so, INL is one of them.

Dual nature of 5f electrons: INL physicist Krzysztof Gofryk, who lead the study, explained, "Plutonium is defined by the unusual dual nature of its 5f electrons [which interact very strongly to generate novel collective behaviors]. 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."

The researchers used INL's plasma-focused ion beam technology to prepare microscopic PuB₆ samples for quantum measurements under ultra-cold conditions, which is the most accurate way to observe the quantum behavior. In addition to these measurements, advanced computer modeling at Columbia University provided further insights into PuB₆ behavior at the quantum level.

The combination of techniques allowed the researchers to "capture the essential electronic and structural properties of plutonium hexaboride [and] provide strong support for its topological nature and offer an efficient path for studying similar actinide materials," in the words of INL materials scientist Shuxiang Zhou.

Nuclear power and quantum computing: This research could have applications in a number of technology areas. According to INL, it "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."