Investigating the ferromagnetism-induced surface gap formation and effects of optical excitations in Cr-doped topological insulators (TIs)*

Despite the exciting observation of the quantum anomalous Hall effect (QAHE) in Cr-doped (Bi_xSb_1-x)₂Te₃ compounds, the microscopic origin of ferromagnetism (FM) and the temperature (T) evolution of the anomalous Hall (AH) resistance in these magnetic TIs remain poorly understood. We perform scanning tunneling spectroscopic and electrical transport studies on MBE-grown magnetic TIs, including uniformly 10% Cr-doped (Bi_xSb_1-x)₂Te₃ and bilayer systems consisting of a layer of pure (Bi_xSb_1-x)₂Te₃ on top of a 10% Cr-doped (Bi_xSb_1-x)₂Te₃ layer. In the STS studies on the bilayer system, a surface gap Δ opens up at T_C^2D ~ 180 K, much higher than the bulk Curie temperature T_C^3D ~ 35 K determined from the onset of AHE. The spatial distribution of Δ is largely homogeneous, and Δ increases with decreasing T, reaching Δ = (59±7) meV at 147K. Additionally, longitudinal (R_xx) and AH (R_xy) resistance are measured both with and without light. An increase in R_xy and decrease in R_xx is observed in the bilayer system under circularly polarized (CP) light (wavelengths λ =1600 ~ 1700 nm). In contrast, both R_xx and R_xy are suppressed under CP light for uniformly Cr-doped (Bi_xSb_1-x)₂Te₃. The physical implications of these findings will be discussed.