A team of U.S. and Korean physicists has found the first evidence of a two-dimensional material that can become a magnetic topological insulator even when it is not placed in a magnetic field.
Pengcheng Dai, Lebing Chen and Jae-Ho ChungLong Description
A team of physicists including (from left) Pengcheng Dai, Lebing Chen and Jae-Ho Chung has found the first evidence of a two-dimensional material that can become a magnetic topological insulator even when it is not placed in a magnetic field. (Photo by Jeff Fitlow/Rice University)
“Many different quantum and relativistic properties of moving electrons are known in graphene, and people have been interested, ‘Can we see these in magnetic materials that have similar structures?’” said Rice University’s Pengcheng Dai, co-author of a study about the material published in the American Physical Society journal PRX. Dai, whose team included scientists from Rice, Korea University, Oak Ridge National Laboratory (ORNL) and the National Institute of Standards and Technology, said the chromium triiodide (CrI3) used in the new study “is just like the honeycomb of graphene, but it is a magnetic honeycomb.”
In experiments at ORNL’s Spallation Neutron Source, CrI3 samples were bombarded with neutrons. A spectroscopic analysis taken during the tests revealed the presence of collective spin excitations called magnons. Spin, an intrinsic feature of all quantum objects, is a central player in magnetism, and the magnons represent a specific kind of collective behavior by electrons on the chromium atoms.
“The structure of this magnon, how the magnetic wave moves around in this material, is quite similar to how electron waves are moving around in graphene,” said Dai, professor of physics and astronomy and a member of Rice’s Center for Quantum Materials (RCQM).
Both graphene and CrI3 contain Dirac points, which only exist in the electronic band structures of some two-dimensional materials. Named for Paul Dirac, who helped reconcile quantum mechanics with general relativity in the 1920s, Dirac points are features where electrons move at relativistic speeds and behave as if they have zero mass. Dirac’s work played a critical role in physicists’ understanding of both electron spin and electron behavior in 2D topological insulators, bizarre materials that attracted the 2016 Nobel Prize in Physics.
Electrons cannot flow through topological insulators, but can zip around their one-dimensional edges on “edge-mode” superhighways. The materials draw their name from a branch of mathematics known as topology, which 2016 Nobelist Duncan Haldane used to explain edge-mode conduction in a seminal 1988 paper that featured a 2D honeycomb model with a structure remarkably similar to graphene and CrI3.
“The Dirac point is where electrons move just like photons, with zero effective mass, and if they move along the topological edges, there will be no resistance,” said study co-author Jae-Ho Chung, a visiting professor at Rice and professor of physics at Korea University in Seoul, South Korea. “That’s the important point for dissipationless spintronic applications.”
Spintronics is a growing movement within the solid-state electronics community to create spin-based technologies for computation, communicate and information storage and more. Topological insulators with magnon edge states would have an advantage over those with electronic edge states because the magnetic versions would produce no heat, Chung said.
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