Highly anisotropic type-II ferromagnetic Weyl states exhibiting a three-dimensional quantum Hall effect

Topological semimetals, especially Weyl semimetals (WSMs), provide an important platform for exploring novel quantum phenomena, thanks to their distinctive electronic structures and their potential to transition into various topological phases. The compound (MnBi₂Te₄)(Bi₂Te₃)ₘ (m = 0, 1, 2) has garnered significant attention recently due to its capacity to host a range of intriguing topological quantum states, such as quantum anomalous Hall insulators (QAHI), axion insulators, higher-order topological insulators, and ideal Weyl semimetals. In this presentation, I will focus on the WSM in (MnBi₂Te₄)(Bi₂Te₃)ₘ and show our recent discovery of a ferromagnetic (FM) type-II WSM in Sb-doped Mn(Bi,Sb)₄Te₇. Through Sb doping, we achieved precise tuning of both the chemical potential and magnetic phases. In the sample doped with approximately 27% Sb, where the chemical potential is near the charge neutrality point, we observed transport signatures characteristic of WSMs, including the chiral anomaly, a large anomalous Hall effect, and Fermi surface reconstruction. Remarkably, this WSM state exhibits a 3D quantum Hall effect due to its significantly reduced Fermi velocity along the z-axis. Theoretical analysis suggests that this WSM phase evolves from a nodal ring state, where higher-order k-terms split the nodal line into type-II Weyl nodes. These findings highlight the extraordinary tunability of the Mn(Bi₁₋ₓSbₓ)₄Te₇ system, where fine control over chemical potential and magnetic properties opens pathways to new quantum phases.