Emergent Topological and Magnetic Phases in 2D Van der Waals Materials

The recent discoveries of long-range magnetic order in van der Waals materials have established a new platform for probing emergent orders in correlated quantum materials. In this talk I will present our recent studies on the Fe_xGeTe₂ (x=3,5) family of 2D ferromagnets. In particular, I will report the observation of a quantum phase switching between a Weyl nodal line phase and a topological flat band phase in Fe₅GeTe₂ via a thermal annealing process. We further demonstrate that the switching of the electronic properties is associated with a crystal symmetry change driven by a Fe site ordering that is modulated by the thermal annealing process. The presence or absence of the ordering modifies the global crystal symmetry and gives rise to either magnetic nodal lines in one case or geometrically frustrated flat bands in the other. The modified crystal and electronic structures subsequently affect the magnetism of the two phases. We further demonstrate that this phase switching is reversibly 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.