Lab research team solves condensed matter physics puzzle

Anne M Stark, LLNL, (925) 422-9799, stark8@llnl.gov

Joe Bradley adjusts the miniXES at the Advanced Photon Source in Chicago.
Led by LLNL scientist Magnus Lipp and former LLNL scientist Joseph Bradley, researchers at the Lab, the University of Washington, Stanford and the Carnegie Institute have answered a longstanding, much debated question in condensed matter physics.

The question has to do with the rare earth element cerium (Ce), which undergoes a surprising, large isostructural volume collapse at high pressure. Since the 1970s, scientists have been arguing about this fascinating behavior in the most widely read and high-impact physics journals, but the LLNL team has found an experimental signature that strongly favors one model for the collapse, termed the "Kondo Volume Collapse."

Cerium is a chemical element that can be used for catalysts and fuel additives. The work appears in the Nov. 9 edition of Physical Review Letters.  "We have developed a very powerful methodology for studying rare earth systems at high pressure using X-ray spectroscopy," Bradley said.

The team discovered that researchers can directly probe quantum mechanical observables using these techniques, which makes them a powerful, direct test of theory. The team has developed instrumentation in collaboration with Jerry Seidler's group at the University of Washington (Pacold et al., "A miniature X-ray emission spectrometer (miniXES) for high-pressure studies in a diamond anvil cell", J. Synchrotron Rad. 19, 245 (2012)) that speeds up data collection by a factor of 100 or more.

"This means that we can not only answer the Ce question, but we can study many systems and gain some real understanding about f electron delocalization in general, which is a 'holy-grail' question in condensed matter physics," Bradley said. "In other words, It is one that can be directly transferred to the 5f electron in actinides."

Other key Lab experimental contributors include Kevin Moore, William Evans, Hyunchae Cynn and Brian Maddox. Adam Sorini performed supporting calculations before also leaving the Lab earlier in the year.

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