"Visualization of an axion insulating state at the transition between 2 chiral quantum anomalous Hall states"

Monica Allen: Yongtao Cui, Eric Yue Ma, Masataka Mogi, Minoru Kawamura, Ion Cosma Fulga, David Goldhaber-Gordon, Yoshinori Tokura, and Zhi-Xun Shen; Proceedings of the National Academy of Sciences, 07/16/19.

Additional Authors: Yongtao Cui, Eric Yue Ma, Masataka Mogi, Minoru Kawamura, Ion Cosma Fulga, David Goldhaber-Gordon, Yoshinori Tokura, and Zhi-Xun Shen

Abstract:

Quantum-relativistic materials often host electronic phenomena with exotic spatial distributions. In particular, quantum anomalous Hall (QAH) insulators feature topological boundary currents whose chirality is determined by the magnetization orientation. However, understanding the microscopic nature of edge vs. bulk currents has remained a challenge due to the emergence of multidomain states at the phase transitions. Here we use microwave impedance microscopy (MIM) to directly image chiral edge currents and phase transitions in a magnetic topological insulator. Our images reveal a dramatic change in the edge state structure and an unexpected microwave response at the topological phase transition between the Chern number N=1 and N=−1 states, consistent with the emergence of an insulating N=0 state. The magnetic transition width is independent of film thickness, but the transition pattern is distinct in differently initiated field sweeps. This behavior suggests that the N=0 state has 2 surface states with Hall conductivities of  ½e2/h but with opposite signs.