Universal role of S-symmetry in the antiferromagnetic topological insulators, GdBi and NdBi

RBi (R: Rare-earth elements) is a carrier-compensated semimetal with a NaCl structure and shows antiferromagnetic (AFM) order. Recently, the topological insulator (TI) phase with a negative band gap was established in this system, and the RBi system is gathering attention as a candidate material for AFM TIs. We performed angle-resolved photoemission spectroscopy (ARPES) measurements on type-I antiferromagnet NdBi and observed a massive Dirac cone in the AFM phase. We revealed that the combined symmetry S = ΘT_D (Θ: Time reversal, T_D: Transrational symmetry) plays an important role in protecting Dirac cones in the AFM TI phase. However, it is still unclear whether this S symmetry-protected Dirac cone state exists universally in other antiferromagnets with different magnetic structures.

Here, we report ARPES results of type-II antiferromagnet GdBi to investigate the universality of the combined symmetry S for the protection of the Dirac electronic state. We observed the gapless Dirac cone state in the AFM phase. We discuss the origin of this Dirac cone protection in terms of the observed pattern in ARPES spectra and crystal symmetry in the AFM state.