19 February 2016

New research offers exciting answers to a physics mystery

Team opens new path for understanding hidden order.

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TALLAHASSEE, Fla. — A team of researchers at the National High Magnetic Field Laboratory (National MagLab) have uncovered a new way to unravel a puzzle that has baffled physicists worldwide for more than 30 years: What is the nature of the peculiar ordered state in the crystalline metal, uranium-ruthenium-silicide? Their work is published today in Nature Communications.

Working with a homegrown uranium-based compound that exhibits an unexplained state of matter known as "hidden order," the research team learned that even a small amount of chemically-driven electronic tuning profoundly affects this strange ordered state and the superconductivity that occurs within the compound. Their research is offering new insights into what may cause this unknown ordered state.

"Hidden order is a mystery that physicists have been working on for decades," explains Ryan Baumbach, research faculty at the National MagLab and lead researcher in this work. "There is an obvious change in the material's behavior at very cold temperatures, but nobody knows exactly what is ordering or why."

Baumbach's research team is the first to alter the chemical structure of URu2Si2 by replacing silicon with phosphorus using a specialized material synthesis technique called molten metal flux growth. This technique is key to creating a high-quality single crystal samples and allows for the introduction of elemental phosphorous into the melt, an important technical step that enabled this work.

Silicon and phosphorus are adjacent to each other in the periodic table and share many properties, but phosphorus has one additional electron. This type of chemical substitution does not disrupt the delicate relationship between uranium and ruthenium that uniquely produces hidden order and superconductivity.

"We didn't really expect any dramatic changes because of how similar phosphorus is to silicon and because we weren't directly affecting the electronics thought to play the most important role in hidden order and superconductivity," said Baumbach. "But it turns out that the details of the electronic structure, as controlled by the delocalized ligand electrons, are extremely important to the physics of this material."

Working in high magnetic fields at extremely low temperatures (below -428 degrees Fahrenheit) within the MagLab's DC Field Facility (SCM2 magnet system), the research team learned that the chemical substitution has a drastic effect, rapidly destroying hidden order and superconductivity with only a few percent of phosphorous. After hidden order and superconductivity disappear, f-electron lattice physics persists without an ordered state. Magnetism abruptly appears at large phosphorous concentrations.

A long-standing question has been to what extent hidden order is a local effect associated with the uranium f-electrons. This research offers new insights into what might cause the ordered state behavior by highlighting the importance of the itinerant (non-localized) ligand electrons.

"This research is an exceptional example of the interdisciplinary nature of the work performed at the National MagLab. It is only possible because of the vision of world-class in-house faculty like Dr. Baumbach, whose exceptional knowledge and ability to coalesce resources at our multiple campuses (Florida State University, Los Alamos National Laboratory and University of Florida) open new landscapes for investigation," said MagLab Chief Scientist Laura Greene. "It requires a convergence of physics, materials research and chemistry to create a groundbreaking result like this and to answer some of science's biggest and most important questions. The work of the Baumbach group is uniquely suited to driving the fundamental and applied research across each of the MagLab's seven user facilities and brings the Physics Department at FSU to new heights."

Baumbach and his team are looking forward to continuing to study the physics of this material using high magnetic fields and other experimental techniques with the goal of understanding how phosphorus substitution controls certain electrons and their relationship to the ordered state. They also hope this work is just the beginning to a better understanding of hidden order and will stimulate a variety of new investigations to help understand the parent compound and uncover exciting new physics.

"One hope out of this work is that other researchers will apply their unique advanced measurement techniques to this new substitution series to further explain what the boundary conditions for hidden order could be," said Baumbach. "This approach could also be useful for studying other puzzling materials such as the related cerium-copper-silicon (CeCu2Si2) and ytterbium-rhodium-silicon (YbRh2Si2) which demonstrate unique behaviors at certain conditions."

Collaborators on this work included Andrew Gallagher, Kuan-Wen Chen, David Graf, Scott Riggs and Arkady Shekhter from the National MagLab; Thomas Albrecht-Schmitt from Florida State University, and Naoki Kikugawa from the National Institute for Materials Science in Tsukuba, Japan. The work was supported by the National Science Foundation (DMR-1157490), the State of Florida and the Department of Energy. Part of the work was performed under NHMFL-UCGP funding.

Story by Kristin Roberts. Photo by Stephen Bilenky. Illustration by Caroline McNiel.

The National High Magnetic Field Laboratory is the world’s largest and highest-powered magnet facility. Located at Florida State University, the University of Florida and Los Alamos National Laboratory, the interdisciplinary National MagLab hosts scientists from around the world to perform basic research in high magnetic fields, advancing our understanding of materials, energy and life. The lab is funded by the National Science Foundation (DMR-1157490) and the state of Florida. For more information, visit us online at nationalmaglab.org or follow us on Facebook, Twitter, Instagram and Pinterest at NationalMagLab.