Tunable topology via magnetic order as seen through exciton spectrum*

Detecting topological phase transitions in materials can be a challenge, typically requiring a detailed picture of the microscopic band structure, knowledge of wave function entanglement, as well as ground state degeneracy. However, it has been previously shown that examining the bulk exciton spectrum can be a means to read out a material's underlying topology. Here, we consider the effects of topology, in particular the momentum-space Berry curvature, on excitons in a magnetic topological material. By tuning the curvature of the material via magnetic order, which itself can be controlled through turning on an external magnetic field from zero to high field, we show how the curvature changes between ferromagnetic versus antiferromagnetic states. We propose an all-optical protocol for measuring changes to the topology of a magnetic material by probing its exciton spectrum. This is done via ultrafast photoluminescence spectroscopy which serves as a faithful proxy to quantify changes to the topology. By demonstrating the validity of our approach to tune the topology and detect it optically, we present an outlook on the use of this scheme to probe other topological phenomena, axion physics, availability of chiral edge states, and more.