Dancing Electrons Solve Long-Standing Puzzle of Oldest Magnetic Material

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An international team of experimental and theoretical researchers has discovered fingerprints of the quasiparticles that drive the phase transformation in magnetite, according to a paper published March 9 in Nature Physics.

A red laser beam triggers the dance of the newly discovered electronic waves in magnetite. Courtesy of Ambra Garlaschelli.

A red laser beam triggers the dance of the newly discovered electronic waves in magnetite. Courtesy of Ambra Garlaschelli.

Using an ultrashort laser pulse, the researchers were able to confirm the existence of peculiar electronic waves that are frozen at the transition temperature and start “dancing together” in a collective oscillating motion as the temperature is lowered.

When the temperature is lowered below 125 K, magnetite changes from a metal to an insulator, its atoms shift to a new lattice structure, and its charges form a complicated ordered pattern. This phase transformation, known as the Verwey transition, was the first metal-insulator transition ever observed, first discovered in the 1940s. For decades, researchers have not understood exactly how this phase transformation was happening.

“We were investigating the mechanism behind the Verwey transition and we suddenly found anomalous waves freezing at the transition temperature,” said MIT physics postdoc Edoardo Baldini, one of the lead authors of the paper. “They are waves made of electrons that displace the surrounding atoms and move collectively as fluctuations in space and time.”

Until now, no frozen waves of any kind had ever been found in magnetite. “We immediately understood that these were interesting objects that conspire in triggering this very complex phase transition,” said MIT physics Ph.D. student Carina Belvin, the paper’s co-author.

The objects that form the low-temperature charge order in magnetite are three-atom building blocks known as trimerons. “By performing an advanced theoretical analysis, we were able to determine that the waves we observed correspond to the trimerons sliding back and forth,” Belvin said. “The understanding of quantum materials such as magnetite is still in its infancy because of the extremely complex nature of the interactions that create exotic ordered phases.”

The researchers suggest that the larger significance of this finding will affect the field of fundamental condensed matter physics, advancing the comprehension of a conceptual puzzle that has been open since the early 1940s. Led by MIT professor of physics Nuh Gedik, the research was made possible by the use of ultrafast terahertz spectroscopy, an advanced laser apparatus based on ultrashort pulses in the extreme infrared.

“These laser pulses are as short as one-millionth of one-millionth of a second and allow us to take fast photographs of the microscopic world,” Gedik said. “Our goal now is to apply this approach to discover new classes of collective waves in other quantum materials.”

The measurements were performed with Ilkem Ozge Ozel in MIT’s Department of Physics. Gedik and his group also collaborated with a team of theorists including Gregory Fiete, an MIT affiliate professor, along with researchers from Poland and Italy. This work was supported in part by the U.S. Department of Energy and the Gordon and Betty Moore Foundation.

Published: March 2020
Terahertz (THz) refers to a unit of frequency in the electromagnetic spectrum, denoting waves with frequencies between 0.1 and 10 terahertz. One terahertz is equivalent to one trillion hertz, or cycles per second. The terahertz frequency range falls between the microwave and infrared regions of the electromagnetic spectrum. Key points about terahertz include: Frequency range: The terahertz range spans from approximately 0.1 terahertz (100 gigahertz) to 10 terahertz. This corresponds to...
Research & TechnologyMassachusetts Institute of Technologyultrashort laser pulsequasiparticlesmagnetitespectroscopyterahertzU.S. Department of EnergyGordon and Betty Moore Foundation of San FranciscoLasersTech Pulse

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