The newly discovered Fermi arcs that can be controlled through magnetism could be the future of electron spin-based electronics.
These new Fermi arcs were discovered by a team of researchers from Ames Laboratory and Iowa State University, as well as by collaborators from the United States, Germany and the United Kingdom. During their investigation of the rare earth NdBi (neodymium-bismuth) monopnictide, the research team discovered a new type of Fermi arc that appeared at low temperatures when the material became antiferromagnetic, i.e. neighboring spins point in opposite directions. .
Fermi surfaces in metals are a boundary between occupied and unoccupied energy states by electrons. Fermi surfaces are normally closed contours that form shapes such as spheres, ovoids, etc. The electrons on the Fermi surface control many properties of materials such as electrical and thermal conductivity, optical properties, etc. On extremely rare occasions, the Fermi surface contains disconnected segments which are known as Fermi arcs and are often associated with exotic states such as superconductivity.
Adam Kaminski, leader of the research group, explained that the recently discovered Fermi arcs are the result of the division of the electron band, which results from the magnetic order of the Nd atoms that make up 50% of the sample. However, the electron splitting that the team observed in NdBi was not typical band splitting behavior.
There are two established types of gang splitting, Zeeman and Rashba. In both cases the bands retain their original shape after the split. The cleavage of the bands observed by the research team produced two bands of different shapes. As the temperature of the sample decreased, the separation between these bands increased and the shapes of the bands changed, indicating a change in the mass of the fermion.
“This split is very, very unusual, because not only does it increase the separation between those bands, but they also change the curvature,” Kaminski said. “This is very different from anything that people have observed to date.”
Previously known cases of Fermi arcs in Weyl semimetals persist because they are caused by the crystalline structure of the material which is difficult to control. However, the Fermi arcs the team discovered in NdBi are induced by the magnetic ordering of the Nd atoms in the sample. This order can be easily changed by applying a magnetic field and possibly changing the Nd ion with another rare earth ion such as Cerium, Praseodymium or Samarium (Ce, Pr or Sm). As Ames Lab is a world leader in rare earth research, such changes in composition can be easily explored.
“This new type of Fermi arc appears every time the sample becomes antiferromagnetic. So when the sample develops a magnetic order, these arcs seemingly appear out of nowhere, ”Kaminski said.
According to Kaminski, another important feature of these new Fermi arcs is that they have what is called a spin plot. In normal metals, each electronic state is occupied by two electrons, one spin up, one spin down, so there is no net spin. The newly discovered Fermi arcs have a unique rotation orientation at each of their points. Since they exist only in a magnetically ordered state, the arcs can be turned on and off very quickly by applying a magnetic pulse, such as from an ultrafast laser.
“Having such a decoration or a spinning texture is important because one of the missions in electronics is to move away from charge-based electronics. Everything you use now relies on the movement of electrons in the wires and that causes dissipation,” he said. said Kaminski.
The ability to control the spin of electrons refers to a new branch of information technology called spintronics, which relies on the spin of electrons rather than charges moving along wires.
“Instead of moving a charge, we reverse the orientation of the spin or cause the spin to propagate along the wire,” Kaminski explained. “These rotational changes are technically not supposed to dissipate energy, so it doesn’t cost a lot of energy to store information like rotation or move information like rotation.”
Kaminski stressed the importance of this discovery in the field, but said there is still a lot of work to be done before these discoveries can be used in new technologies.
Reference: “Emergence of Fermi arcs due to magnetic cleavage in an antiferromagnet”, by Benjamin Schrunk, Yevhen Kushnirenko, Brinda Kuthanazhi, Junyeong Ahn, Lin-Lin Wang, Evan O’Leary, Kyungchan Lee, Andrew Eaton, Alexander Fedorov, Rui Lou, Vladimir Voroshnin, Oliver J. Clark, Jamie Sánchez-Barriga, Sergey L. Bud’ko, Robert-Jan Slager, Paul C. Canfield and Adam Kaminski, 23 March 2022, Nature.
DOI: 10.1038 / s41586-022-04412-x
The growth and characterization of the crystals were supported by the Center for the Advancement of Topological Semimetals (CATS), an Energy Frontier Research Center funded by the US DOE, Office of Basic Energy Sciences.
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