# Half-quantized layer hall effect as a probe of quantized axion field

## TL;DR

Researchers observed a half-quantized Hall effect in engineered magnetic topological insulator structures, providing direct evidence of a quantized axion field.

## Contribution

They demonstrated a reliable method to isolate and measure the half-quantized layer Hall effect in multiple devices.

## Key findings

- Half-quantized layer Hall effect of e²/2h was observed in over ten devices.
- The effect was consistent across both parallel and antiparallel magnetization configurations.
- The technique enables spatial engineering of quantized topological responses.

## Abstract

The axion insulator is a topological phase characterized by a quantized axion field, which manifests as a half-quantized anomalous Hall conductance (AHC) localized on individual surface layers. Experimental realization of this effect has remained elusive because opposite surface contributions typically cancel. We engineered magnetic axion insulator heterostructures using molecular beam epitaxy. By positioning the Fermi level asymmetrically, we isolated one topological surface state within the magnetic gap while displacing the other into a metallic regime. This technique reliably produced a half-quantized layer-resolved AHC of e²/2 h, termed the half-quantized layer Hall effect (LHE), across both parallel and antiparallel magnetization configurations in over ten devices. Our findings provide direct electrical evidence of the half-quantized LHE, a boundary signature of the quantized axion field in the bulk, which resolves a key challenge in the experimental verification of this topological quantum state and establishes a framework for spatially engineering quantized topological response.

The authors study a topological insulator (TI) sandwiched between two magnetic TIs. By keeping one of the magnetic TIs insulating, while tuning the other one into a metallic regime, they find half quantized anomalous Hall conductance, a boundary signature consistent with a quantized axion field.