Half-Quantized Layer Hall Effect as a Probe of Quantized Axion Field

The layer Hall effect (LHE), an emergent Berry-curvature phenomenon encoded in the spatial degrees of freedom, establishes new transport paradigms in topological quantum matter ^1,2 . The axion insulator hosts a quantized axion field, which manifests through two intertwined phenomena ^3–7 : (i) a bulk topological magnetoelectric (TME) effect, and (ii) a surface-layer Hall effect with half-quantized anomalous Hall conductance (AHC). Hence, the magnetic axion insulator heterostructures ^8–10 provide an ideal condensed matter platform to investigate the quantization of such layer Hall effects; however, its unambiguous experimental detection remains a significant challenge. Here, we employ precision molecular beam epitaxy (MBE) of (Bi,Sb) ₂ Te ₃ topological insulator (TI) sandwich structures to construct prototypical axion insulators with atomically controlled magnetic interfaces. Through electrical gating, we demonstrate selective surface state manipulation via asymmetric Fermi level positioning. Crucially, by maintaining one surface within the magnetic gap while displacing the other into metallic conduction regimes, both parallel and antiparallel magnetization configurations exhibit half-quantized AHC s _xy = 0.5 e ² /h in over 10 devices (where e is the elementary charge, and h is the Planck’s constant). The observed layer-resolved half-quantization establishes two fundamental advances: (i) experimental realization of the half-quantized layer Hall effect, and (ii) direct electrical manifestation of a quantized axion field. Our findings not only resolve longstanding debates regarding experimental verification of the quantized axion field in magnetic TIs, but also establish a metrological framework for spatially resolved engineering of the quantized topological response.