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arXiv · 2606.15371

Topological Surface Charge Detection via Terahertz Time-domain Spectroscopy

Tong Shen·Yiyang Xie·Yucheng Dai·Yifan Zhang·Yuanze Li·Xufeng Kou·Tian Liang
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Abstract

The topological magnetoelectric effect (TME) in three-dimensional topological insulators manifests as a quantized surface charge accumulation proportional to an applied magnetic field. Here we demonstrate an optical method using terahertz time-domain spectroscopy (THz-TDS) to detect surface charge accumulation in a chromium-doped (Bi,Sb)$_2$Te$_3$ thin film under oblique incidence, achieving sub-milliradian Faraday rotation precision. Unlike transport probes that require ultralow longitudinal conductivity, this optical technique is robust against finite $\sigma_L$, degrading by less than $0.3\%$ even when $\sigma_L \sim \sigma_T$. We extract the charge accumulation $\eta/B_z$ from the measured Faraday rotation and show results at $45^\circ$ and $60^\circ$ coincide within experimental uncertainty. Extending this to axion insulators, we predict that the TME produces an imaginary Faraday rotation linear in frequency, whose slope directly reflects the single-surface charge density. With improved sample thickness and precision, this optical scheme provides a viable pathway toward direct verification of the TME and four-dimensional quantum Hall effect.

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Tong Shen, Yiyang Xie, Yucheng Dai, Yifan Zhang, Yuanze Li, Xufeng Kou, Tian Liang. 2026-06-13. Topological Surface Charge Detection via Terahertz Time-domain Spectroscopy. https://arxiv.org/abs/2606.15371

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