Deyi Zhuo, Bomin Zhang, Humian Zhou, Han Tay, Xiaoda Liu, Zhiyuan Xi, Chui-Zhen Chen, Cui-Zu Chang
A single massive Dirac surface band is predicted to exhibit a half-quantized Hall conductance, a hallmark of the C=1/2 parity anomaly state in quantum field theory. Experimental signatures of the C=1/2 parity anomaly state have been observed in semimagnetic topological insulator (TI) bilayers, yet whether it supports a half-quantized chiral edge current remains elusive. Here, we observe a robust half-quantized Hall conductance plateau in a molecular beam epitaxy grown asymmetric magnetic TI trilayer under specific in plane magnetic field regimes, corresponding to the C=1/2 parity anomaly state. Within this state, both nonlocal and nonreciprocal transport signals are greatly enhanced, which we identify as direct evidence for a half-quantized chiral edge current localized at the boundary of the top gapped surface. Our numerical simulations demonstrate that this half-quantized chiral edge channel is the essential carrier of the observed half-quantized Hall conductance plateau. The half-quantized chiral edge channel emerges from the collective behavior of multiple metallic modes of massless Dirac electrons and is fundamentally distinct from the fully quantized chiral edge channel in the C=1 quantum anomalous Hall state. Our results provide experimental evidence for the half-quantized chiral edge transport in a C=1/2 parity anomaly state. This Letter establishes asymmetric magnetic TI trilayers as a platform for probing single Dirac fermion physics and paves the way to explore a series of exciting phenomena in the C=1/2 parity anomaly state, including the topological magnetoelectric effect and quantized magneto-optical response.