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Zusammenfassung:
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 nonsymmorphic S-2z symmetry, can be transformed to either massless/massive Weyl or massive Dirac fermions by breaking the PT or S-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 paper 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.
