Topological Switch in a van der Waals Ferromagnet

Symmetries are fundamental to how we define phases of matter. More recently, symmetries and the active tuning of which have played an important role in realizing and tuning topological phases of matter. Previously, such tuning in quantum materials have been realized via the application of magnetic field, uniaxial strain, and pressure. Here we report the observation of quantum phase switching between a Weyl nodal line phase and a topological flat band phase in a van der Waals ferromagnet via a thermal annealing process. We further demonstrate that the switching of the electronic properties to be associated with a crystal symmetry change driven by an site ordering that is modified in the thermal annealing process. The presence or absence of the ordering modifes the global symmetry of the crystal and give rise to either magnetic Weyl nodal lines in one case and geometrically frustrated flat bands in the other. We further demonstrate that this topological switching is reverseably controlled via the thermal annealing and quenching method. Our work not only reveals a rich range of quantum phases emergent in 2D van der Waals ferromagnets, but also uncovers the potential of utilizing site occupancy as a novel degree of freedom for tuning symmetry and therefore topology in quantum materials for the realization of exotic emergent phases.