Experimental verification of Weyl nodes in cobalt disulfide, compared with the theoretical prediction. Image: Princeton Department of Chemistry, Schoop Lab.Leading a collaboration of institutions in the US and abroad, Princeton University's Department of Chemistry is reporting new topological properties of the magnetic pyrite cobalt disulfide (CoS2) that expand science's understanding of electrical channels in this long-investigated material.
Using angle-resolved photoelectron spectroscopy and ab-initio calculations, researchers working within the Schoop Lab at Princeton discovered the presence of Weyl nodes in bulk CoS2 that allow predictions to be made about its surface properties. The material hosts Weyl-fermions and Fermi-arc surface states within its band structure, which may allow it to serve as a platform for exotic phenomena and potentially find use in spintronic devices.
The research also settles a long-standing debate, by proving that CoS2 is not a true half-metal. A half-metal is any substance that acts as a conductor to electrons of one spin orientation but as an insulator or semiconductor to those of the opposite orientation. Although all half-metals are ferromagnetic, most ferromagnets are not half-metals. This finding that CoS2 is not a half-metal has important implications for materials and device engineering.
Leslie Schoop, assistant professor of chemistry at Princeton Chemistry, called the work "a rediscovery of new physics in an old material". Schoop and her colleagues report their findings in a paper in Science Advances.
CoS2 has been a subject of study for many decades because of its itinerant magnetism. Since the early 2000s – before topological insulators were predicted and discovered – it has also been investigated for its potential to be a half-metal. Th researchers were "happy" to put the latter discussion to rest.
Through the Schoop research, CoS2 was discovered to be a rare example of a group of magnetic topological metals proposed as agents of charge-to-spin conversion. By disentangling the bulk and surface electronic structure of CoS2, the researchers have demonstrated that there is a relationship between electronic channels in the inner material that can predict other states at its surface.
An electrical current can go through the bulk of a material or flow along its surface. Researchers found that bulk CoS2 contains objects called Weyl nodes within its structure that serve as electronic channels that can predict 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," said Schoop.
"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."
Chirality refers to the property that makes an object or system distinguishable from its mirror image – i.e. not superimposable – and is an important property in many branches of science.
Schoop added that the electronic channels are polarized. This magnetism could potentially be used to manipulate CoS2: scientists could switch the magnetization direction and surface states could then be reconfigured as a response 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 spinchannels in a material that was known for so long," said Maia Vergniory from the Donostia International Physics Center in Spain, who is a co-author of the paper.
As colleagues in 2016, Schoop and Vergniory discussed investigating the material properties of CoS2, particularly whether it could be classified as a true half-metal. Their investigation went through several iterations after Schoop arrived at Princeton in 2017, and was worked on by graduate students under Schoop and under Vergniory at 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 out the material Weyl nodes.
"What we wanted to measure was not just the surface electronic structure," said 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 explained how engineers might build a device down the road using CoS2. "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.
"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," added Schoop.
This story is adapted from material from Princeton University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.