First-Ever Evidence of 2D Material that Can Turn into a Magnetic Topological Insulator

A group of physicists from the United States and Korea has discovered the first evidence of a two-dimensional material that can transform into a magnetic topological insulator even if it is not placed in a magnetic field.

“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?’” stated Pengcheng Dai from Rice University, co-author of a study about the material reported in the American Physical Society journal PRX. Dai, whose group included researchers from Rice, Korea University, Oak Ridge National Laboratory (ORNL), and the National Institute of Standards and Technology, stated that the chromium triiodide (CrI₃) used in the new research “is just like the honeycomb of graphene, but it is a magnetic honeycomb.”

As part of experiments at ORNL’s Spallation Neutron Source, CrI₃ samples were bombarded with neutrons. A spectroscopic investigation during the tests unraveled the presence of collective spin excitations known as magnons. Spin, an inherent attribute of all quantum objects, plays a central role in magnetism, and the magnons represent a particular 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.

Pengcheng Dai, Professor of Physics and Astronomy, Member of Rice’s Center for Quantum Materials (RCQM)

Graphene as well as CrI₃ consist of Dirac points, which occur only in the electronic band structures of certain two-dimensional materials. Named after Paul Dirac, who in the 1920s helped reconcile quantum mechanics with general relativity, Dirac points are attributes where electrons move at relativistic speeds and behave as though they have zero mass. Dirac’s study had a crucial role to play in physicists’ understanding of both electron spin and electron behavior in 2D topological insulators, strange materials that attracted the 2016 Nobel Prize in Physics.

Although electrons cannot flow through topological insulators, they can zip around their one-dimensional edges on “edge-mode” superhighways. The materials get their name from a branch of mathematics called topology, which was used by 2016 Nobelist Duncan Haldane to offer an explanation for edge-mode conduction in a seminal 1988 paper that featured a 2D honeycomb model with a structure strikingly analogous 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. That’s the important point for dissipationless spintronic applications.

Jae-Ho Chung, Study Co-Author and Visiting Professor, Rice University; Professor of Physics, Korea University in Seoul

Spintronics is a gaining popularity within the solid-state electronics community to develop spin-based technologies for communication, computation, information storage, and much more. According to Chung, topological insulators that have magnon edge states are advantageous over those that have electronic edge states since the magnetic versions would not produce any heat.

Specifically, magnons are not particles but quasiparticles—or collective excitations that occur due to the behavior of a host of other particles. An analogy would be “the wave” sometimes performed by crowds in sports stadiums. While seeing a single fan, one would just see a person periodically standing, raising their arms, and sitting back down. One can see “the wave” only by looking at the entire crowd.

If you look at only one electron spin, it will look like it’s randomly vibrating. But according to the principals of solid-state physics, this apparently random wobbling is composed of exact waves, well-defined waves. And it doesn’t matter how many waves you have, only a particular wave will behave like a photon. That’s what’s happening around the so-called Dirac point. Everything else is just a simple spin-wave. Only around this Dirac point will the magnon behave like a photon.

Jae-Ho Chung, Study Co-Author and Visiting Professor, Rice University; Professor of Physics, Korea University in Seoul

According to Dai, the evidence for topological spin excitations in CrI₃ is specifically interesting since such evidence has been observed for the first time without the application of an external magnetic field.

“There was a paper in the past where something similar was observed by applying a magnetic field, but ours was the first observation in zero field,” he said. “We believe this is because the material has an internal magnetic field that allows this to happen.”

Dai and Chung stated that the internal magnetic field emerges from electrons moving at near relativistic speeds very close to the protons in the nuclei of the chromium and iodine atoms.

“These electrons are moving themselves, but due to relativity, in their frame of reference, they don’t feel like they are moving,” stated Dai. “They are just standing there, and their surroundings are moving very fast.”

According to Chung, “This motion actually feels the surrounding positive charges as a current moving around it, and that, coupled to the spin of the electron, creates the magnetic field.”

Dai stated that as part of the tests at ORNL, the CrI₃ samples were cooled to below 60 Kelvin and bombarded with neutrons, which also have magnetic moments. Neutrons that passed sufficiently close to an electron in the sample could subsequently excite spin-wave excitations that could be read with a spectrometer.

“We measured how the spin-wave propagates,” he said. “Essentially, when you twist this one spin, how much do the other spins respond.”

In order to ensure that a sufficient number of neutrons interact with the samples, Rice graduate student and study lead author Lebing Chen spent three months to come up with a perfect recipe for producing flat sheets of CrI₃ in a high-temperature furnace. The cooking time for each sample was around 10 days, and controlling changes in temperature within the furnace was found to be crucial. Once the recipe was perfected, Chen had to meticulously stack, align, and glue together 40 layers of the material. Since there was a need to precisely align hexagons in every layer, and the alignment could only be confirmed with Laue X-ray diffraction, each minute adjustment could take equal to or more than an hour.

“We haven’t proven topological transport is there,” stated Dai. “By virtue of having the spectra that we have, we can now say it’s possible to have this edge mode, but we have not shown there is an edge mode.”

According to the scientists, there will be a need for magnon transport experiments to prove the existence of edge mode, and they believe that the outcomes of the study will encourage other teams to attempt those experiments.