Direct measurement of the exchange gap in a 3D topological insulator/2D magnet heterostructure

We demonstrate direct chemical potential sensing of the gapped Dirac surface states of 3D topological insulator (TI) in proximity with a ferromagnet insulator. In the dual gated heterostructures with 2D magnet Cr2Ge2Te6 and topological insulator BiSbTeSe2, created through exfoliation and assembly of van der Waals layers, we measure the chemical potential of the top gapped surface by using the bottom surface state as a sensor layer. These two surface states are capacitively coupled via the bulk, and the chemical potential of the top surface can be measured directly from the position of the Dirac point of the bottom surface state as a function of the applied top gate voltage. We observe a jump in the chemical potential of 12 ± 4 meV close to the Dirac point of the top surface indicating the formation of an exchange gap. Our results provide a direct method for quantitively measuring exchange gap strengths in magnetic topological systems.