Researchers Identify Novel Topological Properties in Cobalt Disulfide

The Schoop Lab, which is heading an association of institutions both in the United States and overseas, has reported unexpected new topological characteristics of cobalt disulfide (CoS₂)—a magnetic pyrite—expanding one’s interpretation of electrical channels in this extensively researched material.

The researchers used ab-initio calculations and angle-resolved photoelectron spectroscopy and observed that bulk CoS₂ contains Weyl nodes. These Weyl nodes enabled the team to make predictions about the surface properties of this material.

Within its band structure, the material contains Weyl-fermions and Fermi-arc surface states, which may allow it to act as a platform for unusual phenomena and also places it among materials candidates for applications in spintronic devices.

The new study also resolves an age-old debate, demonstrating that CoS₂ is not a real half-metal. In this context, a half-metal refers to any substance that serves as a conductor to electrons of a single spin orientation but acts as a semiconductor or insulator to those of the reverse orientation.

While all half-metals are known to be ferromagnetic in nature, a majority of the ferromagnets are actually not half-metals. This latest discovery that CoS₂ is not a half-metal holds crucial implications for designing devices and materials.

Leslie Schoop, assistant professor of chemistry, refers to the study as “a rediscovery of new physics in an old material.” The study was recently published in the Science Advances journal.

For many years, CoS₂ has been a topic of study due to its itinerant magnetism and since the early 2000s—before scientists predicted and identified topological insulators—due to its ability to be a half-metal.

Investigators were 'happy' to settle the latter discussion. The study partly spurred to link both these concepts by looking for a novel type of topological magnet in an extensively researched material.

Thanks to the Schoop study, the material was found to be an exceptional example of that class of magnetic topological metals that were suggested as agents of charge-to-spin conversion.

By unraveling the surface and bulk surface electronic structure of CoS₂, investigators have shown that a link exists between electronic channels in the internal material that can estimate other states at its surface.

An electrical current in a material can either pass through the bulk, or travel along the surface. Scientists have observed the presence of certain objects, known as Weyl nodes, within the structure of bulk CoS₂. These nodes act as electronic channels that can estimate other states at the surface.

The beautiful physics here is you have these Weyl nodes that demand spin-polarized surface states. These may be harvested for spintronic applications.

Leslie Schoop, Assistant Professor of Chemistry, Princeton University

 “These electronic states that only exist at the surface have chirality associated with them, and because of that chirality the electrons can also only move in certain directions. Some people think about using these chiral states in other applications. There aren’t many magnetic materials where these have been found before,” added Schoop.

Chirality is the property that renders a system or object indistinguishable from its mirror image, that is, not superimposable, and is a major characteristic in several branches of science.

Schoop further added that the electronic channels are actually polarized. A magnetism like this could be applied to exploit the material—investigators can swap the direction of magnetization and the surface states could be subsequently reconfigured as a reaction to this applied magnetic field.

There are just a very few magnetic materials that have been measured to have such surface states, or Fermi arcs, and this is like the fourth, right? So, it’s really amazing that we could actually measure and understand the spin channels in a material that was known for so long.

Maia Vergniory, Study Co-Author, Donostia International Physics Center

Both Schoop and Vergniory, who were collaborators in 2016, discussed studying the material characteristics of CoS₂, specifically whether it could be categorized as a real half-metal.

The study went through a number of iterations after Schoop joined Princeton University back in 2017 and this was further studied by graduate students under Schoop and Vergniory at Donostia International Physics Center.

Niels Schröter, a collaborator from the Paul Scherrer Institute in Switzerland and the lead author of the study, supervised the group at the Swiss Light Source that plotted the Weyl nodes in the material.

“What we wanted to measure was not just the surface electronic structure,” stated Schröter. “We also wanted to learn something about the bulk electronic properties, and in order to get both of these complementary pieces of information, we had to use the specialized ADRESS beamline at the Swiss Light Source to probe electrons deep in the bulk of the material.”

Schröter elaborated on how engineers might construct a device in the days to come with the help of this material.

“You would put this material in contact with another material, for instance with a magnetic insulator or something like that in which you then want to create magnetic waves by running an electric current through it,” Schröter added.

“The beauty of these topological materials is that these interfacial electrons that may be used for spin-injection, they are very robust. You cannot easily get rid of them. This is where these fields of topology and spintronics may meet, because topology is maybe a way to ensure that you have these spin-polarized interface states in contact with other magnetic materials that you would like to control with currents or fields.”

I think that this kind of rediscovery in this very old and well-studied material is very exciting, and I’m glad I have these two amazing collaborators who helped nail it down.

Leslie Schoop, Assistant Professor of Chemistry, Princeton University

The study was funded by the National Science Foundation via the Princeton Center for Complex Materials, a Materials Research Science and Engineering Center DMR-1420541; by Princeton University via the Princeton Catalysis Initiative; and by the Gordon and Betty Moore Foundation through Grant GBMF9064 to L.M.S.

Journal Reference

Schröter, N. B. M., et al. (2020) Weyl-fermions, Fermi-arcs, and minority-spin carriers in ferromagnetic CoS₂. Science Advances. doi.org/10.1126/sciadv.abd5000.