### What Has Been Achieved

The strongly spin-momentum coupled electronic states in topological insulators (TI) are created by a combination of spin-orbit coupling and time-reversal symmetry. The surface conductance associated with these states is expected to be quantized and has a mathematical description identical to that used for fundamental particles known as ‘axions’. The axion description can be robustly proven by interfacing the two opposite surfaces of a TI thin film with magnetic moments that can be independently reoriented with respect to each other. We have used this property to demonstrate axion physics by measuring the Hall effect and electric resistance of such a TI heterostructure. When the magnetization on the opposite surfaces is in the same direction, the Hall conductance is quantized to e^{2}/h, where e is electron charge and h is Planck’s constant, while the electrical conductivity is zero. This is known as the quantum anomalous Hall (QAH) insulator. When the magnetizations are oppositely oriented, the Hall and electrical conductivity both vanish. This is the ‘axion insulator.’

### Importance of Achievement

Our results provide a robust model system for studying fundamental concepts in physics that cut across subfields, including condensed matter, field theory and particle physics.

### Unique Features of the MIP That Enabled Project

MBE growth of high quality topological insulator thin films on GaAs substrates.

### Publications

Xiao, D., Jiang, J., Shin, J.-H., Wang, W., Wang, F., Zhao, Y.-F., Liu, C.-X., Wu, W., Chan, M. H. W., Samarth, N., Chang C.-Z.

(2018) Room-temperature spin-orbit torque switching induced by a topological insulator. Phys. Rev. Lett. 056801.