Electrons slow to a crawl in a strange new quantum state

Some of these materials can become superconductors, allowing electricity to flow without energy loss, while others develop charge orders, in which electrons settle into organized patterns instead of moving freely.

Researchers at the University of Chicago Pritzker School of Molecular Engineering (UChicago PME) have now uncovered an unexpected form of behavior in one such material, Fe₅GeTe₂. They found that it can enter a charge-ordered state where large numbers of electrons move together extremely slowly while still maintaining quantum coherence.

The findings, published in Science Advances, came from the laboratory of Asst. Prof. Shuolong Yang. The discovery challenges existing ideas about how the material behaves and may also point toward new technological uses.

"This is a fundamental discovery that deviates from theoretical predictions," Yang said. "We now have to go back and think about the magnetic interactions of this material from scratch, but it also leads to new possibilities in using this material for new kinds of memory devices."

Millions of Electrons Moving Together

Fe₅GeTe₂, discovered seven years ago, belongs to a family of materials known as van der Waals magnets. Because these materials can be formed into atomically thin layers, researchers are exploring whether they could enable memory technologies that differ from those built with conventional magnetic materials.

Yang and his colleagues, including postdoctoral scholars Gabriele Berruto and Qiang Gao, examined Fe₅GeTe₂ using angle-resolved photoemission spectroscopy (ARPES). The technique shines photons onto a material and ejects electrons from its surface, allowing researchers to map its electronic structure and magnetic states.

The team focused an ultraviolet laser onto an area just 10 micrometers across. What they saw was unexpected. The material displayed a flat electronic band, meaning that the range of electron energies associated with electrical conduction changed very little.

In a flat band, electrons do not move through the material as quickly as they normally would. Instead, they can become extremely slow while behaving collectively.

"We're not measuring one electron," Yang said. "We're measuring the interaction of thousands or millions of electrons, and they are all moving together in a coherent way. That's a quantum many-body phenomenon, and it's actually a very weird thing."

A Quantum Version of Slow Motion

Yang compares the behavior to water flowing over a waterfall. A steep slope makes water move quickly, while a shallow slope produces slower flow. In a similar way, the flat electronic band corresponds to electrons moving much more slowly.

The discovery changes how researchers understand Fe₅GeTe₂, a material that can exist in several different atomic arrangements.

"From a scientific perspective, it suggests that the magnetic interactions within the material are totally different from what theory predicts," Berruto said.

The unusual behavior could also have practical consequences. Because Fe₅GeTe₂ can occupy different magnetic states, those states may potentially be used to represent and store information.

Yang and his colleagues are now investigating whether a tightly focused laser can switch the material between this quantum many-body phase and other phases. Their work indicates that such switching is possible, raising the possibility that the effect could eventually be incorporated into a memory device.

Pushing Quantum Behavior Toward Room Temperature

Many unusual quantum effects only survive at temperatures close to absolute zero. In this case, however, the researchers found that the coherent behavior persisted up to 100 degrees above absolute zero.

That is still far below room temperature, but it is relatively high compared with similar quantum materials and makes the result especially promising for future applications.

"If we eventually want to use it in a memory device, it needs to work at room temperature," said Gao, who is now a research scientist at Lawrence Berkeley National Laboratory.

The team's next goal is to determine whether the same properties remain when Fe₅GeTe₂ is exfoliated until it is only a single atomic layer thick.

Remembering Physicist Peter Littlewood

The study was one of the final research publications involving Peter Littlewood, a distinguished UChicago physicist who died on June 15.

"He was a great theoretical physicist and a leader of quantum materials research at UChicago," Yang said. "We dedicate this paper to him."

Other authors on the paper include Khanh Duy Nguyen, Chaowei Hu, Paul Malinowski, Haoran Lin, Beomjoon Goh, Bo Gyu Jang, Xiaodong Xu, and Jiun-Haw Chu.

Funding: This work at the University of Chicago was supported by the U.S. Department of Energy, Grant No. DE-SC0022960, and partially supported by the Gordon and Betty Moore Foundation, Grant No. GBMF12763.