This took us a Weyl: synthesis of a semimetallic Weyl ferromagnet with point Fermi surface

TL;DR

This paper reports the theoretical prediction and experimental observation of a semimetallic Weyl ferromagnet in (Cr,Bi)$_2$Te$_3$, characterized by a unique Fermi surface with two Weyl points and a large separation, distinct from conventional metallic Weyl materials.

Contribution

It introduces a new class of semimetallic Weyl ferromagnets with a point Fermi surface, expanding the understanding of topological materials and their electromagnetic responses.

Findings

Record bulk anomalous Hall angle > 0.5

Non-metallic conductivity regime observed

Weyl points separated by > 75% of the Brillouin zone

Abstract

Quantum materials governed by emergent topological fermions have become a cornerstone of physics. Dirac fermions in graphene form the basis for moir\'e quantum matter, and Dirac fermions in magnetic topological insulators enabled the discovery of the quantum anomalous Hall effect. In contrast, there are few materials whose electromagnetic response is dominated by emergent Weyl fermions. Nearly all known Weyl materials are overwhelmingly metallic, and are largely governed by irrelevant, conventional electrons. Here we theoretically predict and experimentally observe a semimetallic Weyl ferromagnet in van der Waals (Cr,Bi)$_2$Te$_3$. In transport, we find a record bulk anomalous Hall angle $> 0.5$ along with non-metallic conductivity, a regime sharply distinct from conventional ferromagnets. Together with symmetry analysis, our data suggest a semimetallic Fermi surface composed of two Weyl points, with a giant separation $> 75\%$ of the linear dimension of the bulk Brillouin zone, and no other electronic states. Using state-of-the-art crystal synthesis techniques, we widely tune the electronic structure, allowing us to annihilate the Weyl state and visualize a unique topological phase diagram exhibiting broad Chern insulating, Weyl semimetallic and magnetic semiconducting regions. Our observation of a semimetallic Weyl ferromagnet offers an avenue toward novel correlated states and non-linear phenomena, as well as zero-magnetic-field Weyl spintronic and optical devices.