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Symmetry principles play a critical role in formulating the fundamental lawsof nature, with a large number of symmetry-protected topological statesidentified 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 crystallinesymmetry, while evolving into Weyl fermions when either $P$ or $T$ is broken.Here, based on first-principles calculations, we reveal that massless Diracfermions are present in a layered FeSn crystal containing antiferromagneticallycoupled 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 withadditional non-symmorphic $S_{\rm{2z}}$ symmetry can be transformed to eithermassless/massive Weyl or massive Dirac fermions by breaking the $PT$ or$S_{\rm{2z}}$ symmetry. Our angle-resolved photoemission spectroscopyexperiments indeed observed the Dirac states in the bulk and two-dimensionalWeyl-like states at the surface. The present study substantially enriches ourfundamental understanding of the intricate connections between symmetries andtopologies of matter, especially with the spin degree of freedom playing avital role.

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