  The kagome lattice based on 3d transition metals is a versatile platform for
novel topological phases hosting symmetry-protected electronic excitations and
exotic magnetic ground states. However, the paradigmatic states of the
idealized two-dimensional (2D) kagome lattice - Dirac fermions and topological
flat bands - have not been simultaneously observed, partly owing to the complex
stacking structure of the kagome compounds studied to date. Here, we take the
approach of examining FeSn, an antiferromagnetic single-layer kagome metal with
spatially-decoupled kagome planes. Using polarization- and
termination-dependent angle-resolved photoemission spectroscopy (ARPES), we
detect the momentum-space signatures of coexisting flat bands and Dirac
fermions in the vicinity of the Fermi energy. Intriguingly, when complemented
with bulk-sensitive de Haas-van Alphen (dHvA) measurements, our data reveal an
even richer electronic structure that exhibits robust surface Dirac fermions on
specific crystalline terminations. Through band structure calculations and
matrix element simulations, we demonstrate that the bulk Dirac bands arise from
in-plane localized Fe-3d orbitals under kagome symmetry, while the surface
state realizes a rare example of fully spin-polarized 2D Dirac fermions when
combined with spin-layer locking in FeSn. These results highlight FeSn as a
prototypical host for the emergent excitations of the kagome lattice. The
prospect to harness these excitations for novel topological phases and
spintronic devices is a frontier of great promise at the confluence of
topology, magnetism, and strongly-correlated electron physics.
