Dirac Fermions in Antiferromagnetic FeSn Kagome Lattices with Combined Space Inversion and Time Reversal Symmetry

TL;DR

This study reveals the presence of symmetry-protected Dirac fermions in antiferromagnetic FeSn kagome lattices, demonstrating how combined symmetries influence topological states, with experimental verification via ARPES.

Contribution

It uncovers stable Dirac fermions protected by combined $PT$ and non-symmorphic symmetries in FeSn, expanding understanding of symmetry-topology relationships in quantum materials.

Findings

Dirac fermions are present in FeSn with broken $P$ and $T$ but preserved $PT$ symmetry.

Dirac states observed in bulk and surface via ARPES.

Symmetry breaking can transform Dirac fermions into Weyl or massive Dirac fermions.

Abstract

Symmetry principles play a critical role in formulating the fundamental laws of nature, with a large number of symmetry-protected topological states identified in recent studies of quantum materials. As compelling examples, massless Dirac fermions are jointly protected by the space inversion symmetry $P$ and time reversal symmetry $T$ supplemented by additional crystalline symmetry, while evolving into Weyl fermions when either $P$ or $T$ is broken. Here, based on first-principles calculations, we reveal that massless Dirac fermions are present in a layered FeSn crystal containing antiferromagnetically coupled ferromagnetic Fe kagome layers, where each of the $P$ and $T$ symmetries is individually broken but the combined $PT$ symmetry is preserved. These stable Dirac fermions protected by the combined $PT$ symmetry with additional non-symmorphic $S_{\rm{2z}}$ symmetry can be transformed to either massless/massive Weyl or massive Dirac fermions by breaking the $PT$ or $S_{\rm{2z}}$ symmetry. Our angle-resolved photoemission spectroscopy experiments indeed observed the Dirac states in the bulk and two-dimensional Weyl-like states at the surface. The present study substantially enriches our fundamental understanding of the intricate connections between symmetries and topologies of matter, especially with the spin degree of freedom playing a vital role.