New topological properties found in the “old” cobalt disulfide material

Leading a collaboration of institutions in the US and abroad, the Department of Chemistry at Princeton University reports new topological properties of cobalt disulfide magnetic pyrite (CoS₂) which extend our understanding of electrical channels in this long-investigated material.

Using angle-resolved photoelectron spectroscopy and ab-initio calculations, researchers working with the Schoop laboratory discovered the presence of Weyl nodes in bulk CoS2, which allow them to make predictions about its surface properties. The material hosts Weyl-firm trusses and surface states of the Fermi arc in its band structure, which can allow it to serve as a platform for exotic phenomena and places it among the candidate materials for use in spintronic devices.

The research also establishes a long-running debate, demonstrating that CoS₂ it is not a true half-metal. A metal half is any substance that acts as a conductor to electrons with a centrifugal orientation, but as an insulator or semiconductor to those of the opposite orientation. Although all semimetals are ferromagnetic, most ferromagnets are not metals. This finding that CoS₂ not half the metal has important implications for the engineering of materials and devices.

Leslie Schoop, assistant professor of chemistry at Princeton Chemistry, called the work “a rediscovery of new physics in an old material.” The research was published this week in Scientific advances.

Cart₂ it has been a subject of study for decades due to its traveling magnetism and since the early 2000s – before topological isolators were predicted and discovered – due to its potential to be half a metal. The researchers were “happy” to put the latter discussion to rest.

Through Schoop research, the material was discovered to be a rare example of that group of topological magnetic metals proposed as charge-spin conversion agents. By dissolving the bulk and surface electronic structure of the CoS₂, researchers have shown that there is a relationship between electronic channels in the inner material that can predict other states on its surface. In a material, an electric current can pass through the bulk or flow along the surface. The researchers found that CoS in bulk₂ contains objects called Weyl nodes in its structure that serve as electronic channels that can predict other surface states.

“The beautiful physique here is that you have these Weyl nodes that require spin-polarized surface states. They can be harvested for spintronic applications,” Schoop said.

“These electronic states that exist only on the surface have chirality associated with them and, because of this chirality, electrons can also move in certain directions,” she added. “Some people are thinking of using these chiral states in other applications. There are not many magnetic materials in which they have been found before.”

Chirality refers to that property that makes an object or system indistinguishable from its mirror image — that is, it does not overlap — and is an important property in many branches of science.

Schoop added that electronic channels are polarized. This magnetism could be used to manipulate the material: scientists can change the direction of the magnetization, and the surface states could then be reconfigured in response to this applied magnetic field.

The co-author of the paper, Maia Vergniory, of the Donostia International Center for Physics in Spain, added: “There are very few magnetic materials that have been measured to have such surface states, or Fermi arcs, and this is like fourth, right? So it’s really amazing that we could measure and understand spiral channels from a material that has been known for so long. “

As colleagues in 2016, Schoop and Vergniory discussed investigating the material properties of CoS₂, especially if it could be classified as a true metal half. The survey went through several iterations after Schoop arrived at Princeton in 2017 and was worked on by Schoop and Vergniory graduate students in Donostia.

Niels Schröter, a colleague at the Paul Scherrer Institute in Switzerland and lead author of the paper, oversaw the team at the Swiss Light Source that mapped the Weyl material nodes.

“What we wanted to measure was not just the electronic surface structure,” Schröter said. “We also wanted to find out something about the electronic properties in bulk and, in order to obtain both complementary information, we had to use the specialized ADRESS beam line from the Swiss light source to test deep electrons in the bulk of the material. “

Schröter explained how engineers could build a device on the road using this material.

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

“The beauty of these topological materials is that these interfacial electrons that can be used for injection by rotation are very robust. You can’t get rid of them easily. These topology and spintronics fields can be encountered here, as topology is probably a way to make sure you have these spin-polarized interface states in contact with other magnetic materials that you want to control with currents or fields. “

Schoop added: “I think this kind of rediscovery in this very old and well-studied material is very interesting and I am glad to have these two amazing collaborators who helped her.