Engineering axion insulator phase in superlattices with inversion symmetry breaking

TL;DR

This paper theoretically explores how magnetic configurations in HgTe/MnTe superlattices can induce various topological phases, including an axion insulator protected by magnetic symmetry, with potential for tunable topological properties.

Contribution

It demonstrates the realization of an axion insulator phase in superlattices through magnetic order and symmetry breaking, and maps phase transitions with layer thickness and magnetic orientation.

Findings

Axion insulator phase protected by magnetic symmetry observed.

Transition from topological to trivial insulator with increasing MnTe thickness.

Large anomalous Hall conductivity in ferromagnetic configurations.

Abstract

We study theoretically the interplay between magnetism and topology in three-dimensional HgTe/MnTe superlattices stacked along the (001) axis. Our results show the evolution of the magnetic topological phases with respect to the magnetic configurations. An axion insulator phase is observed for the antiferromagnetic order with the out-of-plane N\'eel vector direction below a critical thickness of MnTe, which is the ground state amongst all magnetic configurations. Defining $T$ as the time-reversal symmetry, this axion insulator phase is protected by a magnetic two-fold rotational symmetry $C_2{\cdot}T$. The axion insulator phase evolves into a trivial insulator as we increase the thickness of the magnetic layers. By switching the N\'eel vector direction into the $ab$ plane, the system realizes different antiferromagnetic topological insulators depending on the thickness of MnTe. These phases feature gapless surface Dirac cones shifted away from high-symmetry points on surfaces perpendicular to the N\'eel vector direction of the magnetic layers. In the presence of ferromagnetism, the system realizes a magnetic Weyl semimetal and a ferromagnetic semimetal for out-of-plane and in-plane magnetization directions, respectively. We observe large anomalous Hall conductivity in the presence of ferromagnetism in the three-dimensional superlattice.