Theoretical aspects of type-II ferromagnetic Weyl phases and their role in 3D quantum Hall phenomena.

Weyl semimetals (WSMs), a subset of topological semimetals, are distinguished by the presence of Weyl fermions as their low-energy excitations. The discovery of new types of WSMs is highly desirable, as they could provide platforms for exploring novel and exotic topological quantum states. In this talk, I will present our theoretical analysis to support the experimental discovery of a ferromagnetic (FM) type-II WSM in Sb-doped MnBi₄Te₇, exhibiting an extraordinary three-dimensional (3D) quantum Hall effect (QHE). Our theory suggests that this Weyl state evolves from a nodal ring state in the k.p type effective model in the presence of mirror-z symmetry, where higher-order k-terms break the mirror-z symmetry and split the nodal line into two sets of type-II Weyl nodes, each containing six nodes connected by three-fold rotation and inversion symmetry. This Weyl state displays pronounced anisotropy, with a substantial reduction in Fermi velocity along the k_z​ axis, which likely explains the observed 3D QHE. This work presents a significant step toward understanding and harnessing topological quantum states in FM WSMs.