Visualizing local electronic properties of defects in magnetic topological insulators

Quantum anomalous Hall effect (QAHE) manifests as a quantized dissipationless Hall conduction due to chiral edge state circulating along the edge of 2D electron systems without external magnetic field. This effect relies on strong spin-orbit coupling and ferromagnetism. QAHE has been experimentally realized in both Cr-doped and V-doped (Sb$_{1−x}$Bi$_x$)$_2$Te$_3$, which are magnetic topological insulators (TIs). V-doped (Sb$_{1−x}$Bi$_x$)$_2$Te$_3$ exhibits higher Curie temperature and much less zero-field longitudinal resistance. Despite several earlier studies on these materials, it is still unclear why V is better than Cr for QAH effect. Here we present scanning tunneling microscopy and spectroscopy (STM/STS) studies in conjunction with first principle calculations on the local electronic properties in both Cr$_x$Sb$_2$Te$_3$ and V$_x$Sb$_2$Te$_3$ single crystals. Preliminary STM/STS results indicate the local properties of defects strongly influence the magnetic ordering in doped TIs, which is crucial for robust QAHE at elevated temperature.