Axion Insulator Phenomenon: Theoretical Marvels Meet Experimental Reality

An international team of physicists from China has successfully and experimentally observed the half-quantized Hall effect in an axion insulator – an exotic topological state of matter that had been predicted by theory but remained elusive for direct measurement.

The axion insulator belongs to the class of topological materials wherein quantum properties manifest not inside the crystal but at its boundaries. The key feature of such a state is the presence of a quantized axion field, which should lead to the emergence of half-quantized anomalous Hall conductivity on the material’s surface. Theory predicts that the contribution from each surface should equal e²/2h. However, in real-world samples, signals from opposite sides typically cancel each other out, keeping the effect “hidden” for years.

In the new study, scientists circumvented this issue through a specially designed heterostructure of a magnetic axion insulator, grown via molecular beam epitaxy. The key element was the asymmetric shifting of the Fermi level – the energy defining which electronic states in the material are available for conduction. As a result, one of the topological surface states became isolated within a magnetic gap, while another transitioned into a metallic regime and ceased providing a compensating contribution. This configuration enabled the direct measurement of the half-quantized laminar anomalous Hall conductivity of magnitude e²/2h. The authors term this the layered Hall effect (LHE).

It was consistently registered in more than ten devices and persisted under both parallel and antiparallel magnetization orientations of the layers, confirming its topological nature. The findings present direct evidence of the existence of the half-quantized boundary response associated with the quantized axion field within the material’s bulk. Thus, the work closes a key experimental gap in the physics of topological states and demonstrates the ability to spatially control the quantized topological response through heterostructure engineering. In condensed matter physics, axion insulators realize a mathematically analogous structure, allowing researchers to explore “axion physics” in solid state, paving the way for new types of quantum devices.

The practical significance of this result lies not in immediate technologies but in the expansion of tools for managing the quantum properties of matter. The half-quantized Hall effect represents a precisely defined, stable response, almost unaffected by defects and external perturbations. In the future, such effects may be used in ultra-stable quantum sensors, spintronic elements, and architectures of quantum electronics, where it is crucial not to amplify the signal but to guarantee its accuracy and reproducibility. The research demonstrates that complex quantum states, which once existed only in equations, can be not only realized in material but also “toggled” and “isolated” by layers – a fundamentally new level of control over quantum matter.