Topological behavior of electrons in 3-D magnetic material

Topological behavior of electrons in 3-D magnetic material
Researchers in the laboratory of M. Zahid Hasan (second from left). Photo by Denise Applewhite, Princeton University

An international team of researchers led by scientists at Princeton University has found that a magnetic material at room temperature enables electrons to behave counterintuitively, acting collectively rather than as individuals. Their collective behavior mimics massless particles and anti-particles that coexist in an unexpected way and together form an exotic loop-like structure. The study was published this week in the journal Science.

The key to this behavior is topology—a branch of mathematics that is already known to play a powerful role in dictating the behavior of electrons in crystals. Topological materials can contain massless particles in the form of light, or photons. In a topological crystal, the electrons often behave like slowed-down light yet, unlike light, carry electrical charge.

Topology has seldom been observed in magnetic materials, and the finding of a magnetic topological material at room temperature is a step forward that could unlock new approaches to harnessing topological materials for future technological applications.

“Before this work, evidence for the topological properties of magnets in three dimensions was inconclusive. These new results give us direct and decisive evidence for this phenomenon at the microscopic level,” said M. Zahid Hasan, the Eugene Higgins Professor of Physics at Princeton, who led the research. “This work opens up a new continent for exploration in topological magnets.”

Hasan and his team spent more than a decade studying candidate materials in the search for a topological magnetic quantum state.

“The physics of bulk magnets has been understood for many decades. A natural question for us is: Can magnetic and topological properties together produce something new in three dimensions?” Hasan said.

Thousands of magnetic materials exist, but most did not have the correct properties, the researchers found. The magnets were too difficult to synthesize, the magnetism was not sufficiently well understood, the magnetic structure was too complicated to model theoretically, or no decisive experimental signatures of the topology could be observed.

Then came a lucky turning point.

“After studying many magnetic materials, we performed a measurement on a class of room-temperature magnets and unexpectedly saw signatures of massless electrons,” said Ilya Belopolski, a postdoctoral researcher in Hasan’s laboratory and co-first author of the study. “That set us on the path to the discovery of the first three-dimensional topological magnetic phase.”

The exotic magnetic crystal consists of cobalt, manganese and gallium, arranged in an orderly, repeating three-dimensional pattern. To explore the material’s topological state, the researchers used a technique called angle-resolved photoemission spectroscopy. In this experiment, high-intensity light shines on the sample, forcing electrons to emit from the surface. These emitted electrons can then be measured, providing information about the way the electrons behaved when they were inside the crystal.

“It’s an extremely powerful experimental technique, which in this case allowed us to directly observe that the electrons in this magnet behave as if they are massless. These massless electrons are known as Weyl fermions,” said Daniel Sanchez, a Princeton visiting researcher and Ph.D. student at the University of Copenhagen, and another co-first author of the study.

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