Topological switch in a near room temperature van der Waals ferromagnet*

Two-dimensional (2D) van der Waals magnets have offered a large playground for exploring exotic correlated phenomena emerging from the interplay between dimensionality, magnetism, and topology. Their versality also allows a stronger tunability under various experimental tuning knobs. We will present the evidence for a switchable topological phase in a metallic near room temperature van der Waals ferromagnet from high resolution angle-resolved photoemission spectroscopy measurements supported by a combination of other experimental techniques and theoretical models. We will demonstrate the topological switching to hinge upon a symmetry change that results in a switching of the electronic state between a magnetic Weyl nodal line phase and a geometrical flat bands phase. 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.