July 10, 2015

Plutonium magnetism in constant flux making it nearly impossible to detect

Groundbreaking work at two Department of Energy national laboratories has confirmed plutonium’s magnetism, which scientists have long theorized but have never been able to experimentally observe. The advances that enabled the discovery hold great promise for materials, energy and computing applications. Plutonium is not devoid of magnetism, but in fact its magnetism is just in a constant state of flux, making it nearly impossible to detect.

Plutonium was first produced in 1940 and its unstable nucleus allows it to undergo fission, making it useful for nuclear fuels as well as for nuclear weapons. Much less known, however, is that the electronic cloud surrounding the plutonium nucleus is equally unstable and makes plutonium the most electronically complex element in the periodic table, with intriguingly intricate properties for a simple elemental metal.

While conventional theories have successfully explained plutonium’s complex structural properties, they also predict that plutonium should order magnetically. This is in stark contrast with experiments, which had found no evidence for magnetic order in plutonium.

Finally, after seven decades, this scientific mystery on plutonium’s “missing” magnetism has been resolved. Using neutron scattering, researchers from the Department of Energy’s Los Alamos and Oak Ridge (ORNL) national laboratories have made the first direct measurements of a unique characteristic of plutonium’s fluctuating magnetism.



Science Advances - The valence-fluctuating ground state of plutonium



Plutonium sort of exists between two extremes in its electronic configuration—in what we call a quantum mechanical superposition,” Janoschek said. “Think of the one extreme where the electrons are completely localized around the plutonium ion, which leads to a magnetic moment. But then the electrons go to the other extreme where they become delocalized and are no longer associated with the same ion anymore.”

Using neutron measurements made on the ARCS instrument at ORNL’s Spallation Neutron Source, a DOE Office of Science User Facility, Janoschek and his team determined that the fluctuations have different numbers of electrons in plutonium’s outer valence shell—an observation that also explains abnormal changes in plutonium’s volume in its different phases.



Abstract - The valence-fluctuating ground state of plutonium

A central issue in material science is to obtain understanding of the electronic correlations that control complex materials. Such electronic correlations frequently arise because of the competition of localized and itinerant electronic degrees of freedom. Although the respective limits of well-localized or entirely itinerant ground states are well understood, the intermediate regime that controls the functional properties of complex materials continues to challenge theoretical understanding. We have used neutron spectroscopy to investigate plutonium, which is a prototypical material at the brink between bonding and nonbonding configurations. Our study reveals that the ground state of plutonium is governed by valence fluctuations, that is, a quantum mechanical superposition of localized and itinerant electronic configurations as recently predicted by dynamical mean field theory. Our results not only resolve the long-standing controversy between experiment and theory on plutonium’s magnetism but also suggest an improved understanding of the effects of such electronic dichotomy in complex materials.



SOURCES - Science Advances, Oak Ridge National Lab, Youtube

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