Topological magnetoelectric effects in magnetic topological insulator thin films via first-principles calculations

Broken time-reversal symmetry in topological insulators (TI) gives rise to novel quantum phases without the need of external magnetic fields. Most well-known is the Quantum Anomalous Hall (QAH) phase characterized by a non-zero Chern number and chiral edge-states localized at the boundary of the samples. Another important phase is the axion insulator (AI) phase, which has a zero Chern number and expected to display a quantized topological magnetoelectric effect (QTME). Recent experimental work on intrinsic antiferromagnetic TIs (such as MnBi₂Se₄/Te₄) suggests the possibility of realizing both the QAH and the AI phase in quasi–two-dimensional thin films of the same material by simply controlling the number of constituent septuple layers. Nevertheless, a direct verification of the elusive QTME in these systems has not yet been achieved due to the complexity and lack of precise control of the layered structure. To elucidate salient features of the AI phase, we present here a first-principles study of the crystal and electronic structure of multilayered magnetized TI thins films. The resulting microscopic tight-binding model is compared with a simplified effective model (1) based on coupled Dirac cone degrees of freedom on both surfaces of each septuple layer. We use these models to analyze theoretically the topological characteristics of these materials and investigate their response to external electric and magnetic fields.

(1) C. Lei, S. Chen, and A. H. MacDonald PNAS, 117(44), 27224–27230 (2020).