Symmetry-Indicated Time-Reversal-Doubled Axion Insulators

TL;DR

This paper identifies a new topological phase called time-reversal-doubled axion insulator (T-DAXI) in nonsymmorphic topological crystalline insulators, demonstrating unique spin-related effects and proposing real material realizations through ab initio calculations.

Contribution

It reveals the existence of symmetry-enforced T-DAXI phases in nonsymmorphic topological insulators and explores their spin magnetoelectric effects and material realizations.

Findings

T-DAXI phase is hosted by nonsymmorphic topological crystalline insulators.

Partial axion angles are quantized to π, leading to half-quantized quantum spin Hall states.

External magnetic fields induce detectable spin polarization and alternating spin currents.

Abstract

The axion insulator exhibits a topological magnetoelectric effect characterized by an axion angle $\theta=\pi$, while the time-reversal-doubled axion insulator (T-DAXI) can be viewed as two copies of an axion insulator related by time-reversal symmetry. In this work, we show that a topological crystalline insulator with nonsymmorphic glide or screw symmetry hosts the T-DAXI phase. The spin-resolved topology of the T-DAXI phase is guaranteed by the nonsymmorphic symmetry invariant $\delta_g=1$ or $\delta_s=1$ in certain spin directions. In this phase, the partial axion angles are quantized to $\pi$, and the gapped surfaces realize half-quantized quantum spin Hall states. By applying an external magnetic field along the $z$ direction, electrons with opposite spins accumulate on opposite $(001)$ surfaces, producing a topological spin polarization in real space. When the magnetic field is time-periodic, this leads to an alternating spin current detectable in experiment. Using $\mathrm{\textit{ab initio}}$ calculations, we demonstrate that mixed bismuth monohalides Bi4Br3I and Bi4BrI3 realize the nonsymmorphic T-DAXI with $\delta_g=\delta_s=1$. Our findings not only reveal the symmetry-enforced T-DAXIs in nonsymmorphic topological crystalline insulators, but also introduce the spin magnetoelectric effect as a novel topological spin response.