Piers COLEMAN : Axionic tunneling from a topological insulator

Our current understanding of topological materials is incomplete, and might be likened to that of Bose–Einstein condensation (BEC) prior to the discovery of superfluidity. Just as interactions are required to stabilize superfluidity in a BEC,  we expect that interactions in topological insulators will stabilize new emergent properties. 

As a concrete example of such challenging new developments, I will present  recent Scanning tunneling results[1], that demonstrate that the surface of the topological insulator Samarium Hexaboride has a voltage-driven spin sensitivity  which develops via a surface phase transition that stabilizes axionic properties at low temperatures – with a surface magnetization tuned by the electric field on the tunneling tip [2].  Remarkably similar physics has recently been observed in (BiBr)_4[3]. 

I will discuss how these new results can not be understood as the result of helical tunneling, arguing instead that they derive from the development of axionic surface properties.  From a mathematical perspective, 3D topological insulators as unitary rotations of conventional insulators.  Our current understanding of topological insulators treats the surface as an Ising domain wall between a conventional and topological insulators.  The remarkable spin-sensitivity of SmB6 and (BiBr)_4 suggest a new perspective:  that the surfaces of topological insulators form a Bloch domain wall in which the anomalies associated with a continuous axial rotation between normal and topological matter create an axionic surface state with a voltage-tuned magnetization.

[1] Work supported by the U.S. Department of Energy (DOE)

under Contracts No. DE-FG02- 99ER45790 and DE-FG02-84ER45118 .

[2] Saikat Banerjee, Anuva Aishwarya, Fei Lei, Lin Jiao, Vidya Madhavan,

Eugene Mele and Piers Coleman, arXiv 2512.05057  (2025).

[3] Zhuying Wang,  Xianhui Chen et al, arXiv 2512.23290 (2025).