Time-Reversal-Breaking Weyl Fermions in Magnetic Heusler Alloys

TLDR

Weyl fermions have been observed in time‑reversal‑invariant semimetals and photonic materials with broken inversion symmetry, where they are expected to exhibit exotic transport such as the chiral anomaly, but most known materials contain many Weyl nodes near the Fermi level, complicating transport behavior. Here we predict, for the first time, a new family of Weyl systems defined by broken time‑reversal symmetry, namely, Co‑based magnetic Heusler materials XCo₂Z (X = IVB or VB; Z = IVA or IIIA). To search for Weyl fermions in the centrosymmetric magnetic systems, we recall an easy and practical inversion invariant, which has been calculated to be –1, guaranteeing the existence of an odd number of pairs of Weyl fermions. The predicted Co‑based magnetic Heusler alloys exhibit only two Weyl nodes at the Fermi level—the minimum possible—protected by rotational symmetry along the magnetic axis, separated by a large Brillouin‑zone distance, with calculated Fermi arcs, offering a realistic platform for experimental studies of magnetic Weyl physics.

Abstract

Weyl fermions have recently been observed in several time-reversal-invariant semimetals and photonics materials with broken inversion symmetry. These systems are expected to have exotic transport properties such as the chiral anomaly. However, most discovered Weyl materials possess a substantial number of Weyl nodes close to the Fermi level that give rise to complicated transport properties. Here we predict, for the first time, a new family of Weyl systems defined by broken time-reversal symmetry, namely, Co-based magnetic Heusler materials XCo2Z (X = IVB or VB; Z = IVA or IIIA). To search for Weyl fermions in the centrosymmetric magnetic systems, we recall an easy and practical inversion invariant, which has been calculated to be -1, guaranteeing the existence of an odd number of pairs of Weyl fermions. These materials exhibit, when alloyed, only two Weyl nodes at the Fermi level - the minimum number possible in a condensed matter system. The Weyl nodes are protected by the rotational symmetry along the magnetic axis and separated by a large distance (of order 2$\pi$) in the Brillouin zone. The corresponding Fermi arcs have been calculated as well. This discovery provides a realistic and promising platform for manipulating and studying the magnetic Weyl physics in experiments.