Weyl-fermions, Fermi-arcs, and minority-spin carriers in ferromagnetic CoS2

TL;DR

This study reveals Weyl fermions, Fermi-arc surface states, and minority-spin carriers in ferromagnetic CoS2, highlighting its topological properties and implications for spintronics, while clarifying its electronic structure and half-metallicity status.

Contribution

It provides the first direct observation of Weyl cones, Fermi-arc surface states, and a minority-spin electron pocket in CoS2, combining experimental and theoretical insights into its topological and magnetic properties.

Findings

Discovery of Weyl cones at the Fermi level

Observation of topological Fermi-arc surface states

Identification of a minority-spin bulk electron pocket

Abstract

The pyrite compound CoS2 has been intensively studied in the past due to its itinerant ferromagnetism and potential for half-metallicity, which make it a promising material for spintronic applications. However, its electronic structure remains only poorly understood. Here we use complementary bulk- and surface-sensitive angle-resolved photoelectron spectroscopy and ab-initio calculations to provide a complete picture of its band structure. We discover Weyl-cones at the Fermi-level, which presents CoS2 in a new light as a rare member of the recently discovered class of magnetic topological metals. We directly observe the topological Fermi-arc surface states that link the Weyl-nodes, which will influence the performance of CoS2 as a spin-injector by modifying its spin-polarization at interfaces. Additionally, we are for the first time able to directly observe a minority-spin bulk electron pocket in the corner of the Brillouin zone, which proves that CoS2 cannot be a true half-metal. Beyond settling the longstanding debate about half-metallicity in CoS2, our results provide a prime example of how the topology of magnetic materials can affect their use in spintronic applications.