Magnetic Proximity Effect in the Surface of 3D Topological Insulators

Recent experiments have focused on proximity-coupling ferromagnets with 3D topological insulators (TIs) to open a time-reversal-breaking mass gap in the surface state dispersion and generate a quantum anomalous Hall (QAH) effect. Little, however, is theoretically understood about the magnetic proximity effect in this system. In this talk, we investigate this proximity effect by modeling the aforementioned heterostructure within a tight-binding framework. We analytically derive a contact self-energy for the ferromagnet that, when added to the surface state Hamiltonian, breaks time-reversal symmetry. By examining the spectral function, we see that the resultant surface state gap depends non-linearly on both the hopping parameters and exchange interaction strength within the ferromagnet. For large exchange interaction strength in the ferromagnet, we find that the surface state mass gap, and thus the QAH conductivity, has the opposite sign as the exchange field in the ferromagnet, contrary to naïve expectations. Based on these results, we propose a simple experimental setup by which a topological phase transition can be realized in this heterostructure by varying the angle of a small external magnetic field.